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# Gary Flandro

### Professor Emeritus, University of Tennessee Space Institute

By David Zierler, Director of the Caltech Heritage Project

July 18, 2022

DAVID ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It is Monday, 18 July 2022. I am delighted to be here with Dr. Gary Flandro. Gary, it's great to be with you. Thank you so much for joining me today.

GARY FLANDRO: Thank you for giving me the opportunity to tell my stories.

ZIERLER: To start, would you please tell me your title and institutional affiliation?

FLANDRO: I am now an Emeritus Professor at the University of Tennessee Space Institute, Professor, where I held the Boling Chair of Excellence in Aeronautical and Mechanical Engineering until my retirement in 2009.

ZIERLER: What about Gloyer-Taylor Laboratories? Do you still retain an affiliation with them?

FLANDRO: Yes, I do. I'm a part owner and Chief Engineer of the company and a contributor to many GTL projects. If we have time, we might spend a little time discussing them, because GTL has been involved in some interesting and very important engineering work.

ZIERLER: Let's start on that now. Tell me a little bit about Gloyer-Taylor and what their mission is.

FLANDRO: Some history to begin with. The founder of GTL is Paul Gloyer who was one of my graduate students at UTSI. He is one of the most ambitious and creative individuals that I have ever known, a natural leader and organizer. In response to annual NASA student engineering design competitions, he led groups of UTSI students that won first prize awards against some stiff competition. I was the faculty advisor for these activities. Paul left UTSI to start his own aerospace propulsion company to design and build advanced rocket engines. The present Gloyer-Taylor Laboratories was the result of a 2004 merger of Pac Astro and Aspect Engineering (companies that Paul eventually purchased). He and his family lost their home and most of their belongings when Hurricane Katrina struck Louisiana in 2005. He then moved back to UTSI with the idea of completing his graduate studies and relocating his company. I offered to help him get started on this new venture.

From the outset we were focused on using analytical tools I had first developed in my research at Caltech and JPL, and continually improved throughout my various academic jobs and research programs. This was a continuation of my earlier rocket combustion instability work in Utah with the Thiokol and Hercules companies. Later, my research at UTSI led to a very detailed physical understanding of combustion instability, which led to reliable predictive and corrective capabilities. Paul Gloyer, my grad students and I developed and marketed computational tools to help the rocket industry solve their vibration problems. This led to what we later called the Universal Combustion Device Stability (UCDS) process for predicting the stability of liquid and solid rockets, jet engines, ramjets, and hypersonic scramjets.

One of the first opportunities for GTL to exercise our capabilities was a serious development problem NASA had encountered with the Ares launch vehicle, part of the Constellation program. Ares was to replace the Space Shuttle that was being phased out as the heavy lift vehicle for manned space missions. The Ares I (crew vehicle) and Ares V (cargo carrier) utilized solid rocket boosters (SRB) from the Space Shuttle program that were lengthened to provide more impulse needed to accommodate the payloads to be carried. In my earlier days as a Professor at the University of Utah, I worked with the Thiokol Corporation to help them with an unexpected vibration problem that first appeared in static tests of the Shuttle SRB motors. Low frequency (15 Hz) axial mode acoustic oscillations produced very large thrust oscillations called "thrust roughness" with magnitudes from 10,000 lbf at ignition growing to more than 50,000 lbf near the end of burn at 115 seconds. Fortunately, these disturbances were damped in flight by the Shuttle external fuel tank sufficiently to prevent a threat to the crew of the orbiter. Flight crews often complained of heavy vibration levels during the solid rocket burns that made it difficult to read instruments or operate controls. I was able to analyze the problem in detail and suggested several corrective procedures that were never implemented by Thiokol or NASA. The difficulty with the longer motors to be used in the Ares vehicles was that the vibration frequency was lowered to about 11 Hz. This posed a real threat to the crews, especially in the Ares I configuration utilizing a single SRB with the crew module mounted atop the rocket. Vibration levels during burn would exceed 10 g. At that low frequency and high amplitude crew capability and health would be seriously impaired (eyeballs resonate at the frequency), possibly leading to death of crew members.

When the NASA Ares Program problems arose, GTL submitted a proposal to NASA offering to help them confront the problem. They awarded us a contract, which also involved Dr. Vigor Yang (also a Fred Culick Caltech graduate student). In 2005 after an extensive study involving some detailed CFD (computational fluid dynamics modeling) coupled with our combustion instability software suite, we were able to demonstrate the exact cause of their SRB vibration problem. We provided to Thiokol's successor Alliant Technologies (ATK) several measures to eliminate the oscillations at their source. ATK incorporated a block upgrade into the Ares development cycle and awarded GTL a letter of commendation for our help with the problem.

Our corrective procedures were not implemented since they would require significant changes to the solid rocket motor design and production tooling. Therefore, NASA embarked on attempts to develop vibration mitigation schemes suggested by "tiger teams" hired to help with the problem. They came up with several unworkable methods for damping the vibrations. One proposal was to make the seats in the space capsule function as vibration absorbers. The necessary hardware would have taken up all the space in the spacecraft with no room left for the crew. Failure to identify workable fixes along, with an Augustine Commission analysis that indicated that the Ares I would cost over $1 billion per flight, led President Barack Obama to cancel the Constellation program in 2010. Since then, GTL has continued to receive significant funding from NASA, DARPA and DOD, in addition to many contracts from Aerospace companies. Recently, Gloyer-Taylor Laboratories LLC has received several very large contracts to build upon our innovative composite cryogenic fuel tank technology (based on the work of GTL Vice President Zachary Taylor). These light-weight tanks make single-stage-to-orbit operations possible. Such innovations will aid in bringing the cost of spaceflight to a more sustainable level and will play an essential role in development defensive strategies in response to recent threats of hypersonic weapons system deployment by China and Russia. GTL recently demonstrated that our cryogenic hydrogen tanks could have saved the innovative NASA X-33 single-stage-to-orbit launch vehicle program from cancellation. The X-33 was a technology demonstrator for what was to be the reusable Venture Star orbital spaceplane. They employed a honeycomb-walled composite liquid hydrogen tank that failed during fueling and pressure tests leading to the cancellation of the program after a huge investment of funds by NASA and Lockheed. Looking back, much was lost that would have greatly helped to provide practical and cost-effective access to space. Recently GTL was awarded a patent (US 10,876,732 B2, Scalable Acoustically Stable Combustion Chamber Design Methods) for a new liquid rocket propellant combustion chamber design strategy. The standard LRE combustion chamber geometry consists of a straight cylindrical duct leading to the entrance of the nozzle throat. We discovered that by changing the shape of the chamber we could eliminate combustion instability. We received funding from NASA to construct a test rocket engine to demonstrate this technology. The engine was built and successfully tested at UTSI. It is unfortunate that this simple corrective device did not exist at the time Rocketdyne was developing the large F-1 (1.5 million lb. thrust liquid engine) for the Saturn V heavy lift rocket. Five of these engines powered the first stage for the Apollo manned lunar program. In 1961 static tests of the engines exhibited severe combustion instability leading to heavy vibration and explosive failure. This threatened an early end to the Apollo manned lunar mission program. The data revealed that the instability involved high-frequency tangential mode spinning acoustic waves in the combustion chamber. These gas motions were much like the ones I had confronted in my work on the Sergeant rocket motor at JPL in 1958 (to be discussed later). A comprehensive theoretical understanding of combustion instability, like the one we had accomplished at GTL, did not exist. Thus started a frantic effort to solve the problem. Finally, after thousands of full-scale F-1 tests involving trial-and-error design changes the problem was solved. The F-1 engine development was the most expensive in history. Our simple combustion chamber geometry modification would have saved much money and heartache had we been there to help. ZIERLER: Let's move back now to UTSI, the University of Tennessee Space Institute. First, how long have you been with UTSI? FLANDRO: I've been there for about 30 years. ZIERLER: Tell me a little bit about its history and overall mission, where it finds itself with space science and planetary exploration. FLANDRO: UTSI is a result of Theodore von Kármán's interest in having an institution that could help train scientists working at the Arnold Air Force base here in Tullahoma, Tennessee. As you know von Kármán established the Caltech Guggenheim Aeronautics Laboratory (GALCIT) in 1930. He and his students also founded the Jet Propulsion Laboratory and the Aerojet Corporation. Dr. von Kármán and his friend General Hap Arnold are widely remembered as the "architects of American air supremacy" during WW II. After the war, General Arnold backed the idea that a special Air Force facility should be established for the development of jet and rocket propulsion. In 1949 Congress authorized$100 million to construct the facility. There were three candidate locations that had the necessary land, power, and water. Tullahoma, TN was one of them. Kármán as a member of the site selection committee insisted that there be not only an abundant supply of electric power and water but also a technical institute that could enable Air Force and civilian personnel to earn graduate degrees. Such a campus would also be an aid in attracting the best scientists and engineers to work at the facility. The state of Tennessee backed the proposal and UTSI was established as part of the University of Tennessee (Knoxville). Dr. B. H. Goethert, the chief scientist at the Air Force Systems Command (Andrews AFB), also backed the proposal and he became the first director of UTSI. Dr. Goethert was one of the Paperclip group from Germany, he was a well-known aeronautics expert and was a great organizer. All this was due to von Kármán; UTSI would not exist if it hadn't been for him. For many years, starting in 1956, UTSI has provided technical training and graduate degree programs for AEDC and students from many other countries.

ZIERLER: To what extent is the appointment academic? In other words, do you have an opportunity to teach and to supervise graduate students?

FLANDRO: My academic appointment as the Boling Chair of Excellence in Aerospace and Mechanical Engineering involved both teaching (I was awarded the UTSI Vice Presidents Award for Teaching Excellence in 1999) and research. I continued my research work in propulsion system development and space flight mechanics and was involved in numerous research efforts with the Arnold Engineering Development Center (AEDC).

ZIERLER: Just a snapshot in time, what are you currently working on, and what's interesting to you in the field of space science?

FLANDRO: My interactions with the propulsion industry, NASA, and DOD have continued through my involvement in GTL programs. UTSI offers my short courses in solid and liquid propulsion and hypersonic aerodynamics, vehicle design, performance analysis, and hypersonic airbreathing propulsion. Current concerns over the development of hypersonic weapons systems by China and Russia have kept me involved in this rapidly changing technology. I have recently presented these courses at NASA Redstone Arsenal (Huntsville, TN), Lockheed Martin (LMMFC), Joint Warfare Analysis Center (Dahlgren, VA), NASA Wallops Island Flight Facility (VA), and Orbital ATK (Allegany Ballistic Laboratory, VA).

ZIERLER: We're in an age now of commercial space flight. What do you see as some of the specific opportunities and pitfalls of having companies like Blue Origin and SpaceX now in the mix?

FLANDRO: I think giving private companies impendence in creating space related technologies and entire system development and operations is a healthy change from the old days. The great successes of SpaceX, for example, demonstrate what can be done. An important effect is that program costs and government related schedule impediments are reduced.

ZIERLER: One of many reasons I've been looking forward to speaking to you, of course—you were honored as a Distinguished Alumni recipient for Caltech in 2018. Tell me about that experience and how it gave you opportunity to reflect on what Caltech did for your career.

FLANDRO: This was a great honor and an opportunity to meet with many friends and teachers from my Caltech days. My three sons accompanied me to that event. We all enjoyed our stay at the Athenaeum, and I took the opportunity to show them my old stomping grounds at JPL, on the Caltech campus, and at the Huntington Library. In the past, I have had many opportunities to visit the campus and my inspiring teachers. I will never be able to thank them enough for the wonderful gift of knowledge they gave to me. While we were in Pasadena, several of my grad students (now working at JPL) arranged a special tour of JPL. We were able to visit the spacecraft assembly building and to view the Curiosity Mars Rover under construction. We had interesting discussions with Ed Stone and other colleagues at the luncheon hosted by Caltech President Rosenbaum.

In my brief acceptance talk at the award ceremony, I began by acknowledging Caltech trained individuals who had inspired me (a very long list). I started to describe Kip Thorne's influence, but hesitated, realizing that there would not be enough time to do that right. Instead, I began with Dr. Eugene Parker, who had been one of my first teachers at the University of Utah in 1952. Dr. Parker was a 1951 Caltech graduate, and was doing post-doctoral research and teaching in the U of U Mathematica and Physics departments. He was the finest instructors that I had ever had. I especially enjoyed his astronomy courses. When I later learned that Parker was a Caltech graduate, I knew that I must find a way to study Caltech someday. I also learned of the high standards required for admission to Caltech degree programs. So, at the time, I had no idea how I might ever qualify.

ZIERLER: Interacting with Eugene Parker—of course this was before Sputnik and before the possibilities of using space as a laboratory. Was your sense as an undergraduate that the Space Age was coming, that things like heliophysics and measuring the solar bubble were possibilities? Or did Sputnik and the Apollo program really make that a surprising new opportunity?

FLANDRO: In fact, anything to do with airplanes, space flight, robots, and astronomy had been on my mind from an early age. I built model airplanes starting at age 8 and have never stopped. I was fascinated by the glider developments that started in Germany after World War I (powered airplanes were forbidden by the Versailles Treaty). A young professor Theodore von Kármán and his student Wolfgang Klemperer at the Aachen University in Germany designed advanced monoplane gliders to compete in the Wasserkuppe glider competitions in 1921-22.

In the 1940s I learned of the V-2 rocket program and the jet and rocket propelled fighters in wartime Germany. In 1947, I began experimenting with pulsejet powered control line airplanes. One of my models attained a speed of over 150 mph, a very dizzying experience on 70 ft. long control lines.

My pulse jet engines (Dynajet and Minijet) were based on the German V-1 cruise missile propulsion systems used to bombard England in 1944-45. Incidentally, my Minijet model airplane jet engine was produced in 1946 at a facility a few blocks from Caltech on California Blvd. I have often wondered whether some Caltech folks might have been involved in that enterprise. It is interesting that the first series of JPL publications, written by Frank Malina, contained an in-depth analysis of the pulse jet engine as well as other promising propulsion concepts. These papers are no longer available, but I was fortunate enough to acquire a copy of the pulse jet report.

Allow me to relate one incident related to my pulsejet activities at the University of Utah. My friends and I came up with the idea of running a Dynajet powered airplane model along a cable stretched between the Engineering and Math buildings as a demonstration during the Engineering Week activities in 1955. You may know that an unfortunate feature of pulsejet engines is the very loud noise (typically over 140 decibels) they produce when running. It happened that the location of our demonstration was adjacent to the Kingsbury Hall auditorium where we had forgotten that Eleanor Roosevelt was addressing a large audience as we were making loud noises outside. As a result of this, our pulse jet demonstration was abruptly halted by the appearance of several FBI agents!

FLANDRO: I was in mechanical engineering, with an aeronautical option. The chairman of the Mechanical Engineering Department, Dr. Ralph Baker, was a Caltech grad. Another Caltech aeronautics graduate, Dr. Arlo Johnson, taught fluid dynamics, aerodynamics, and compressible flow theory. Under his direction I designed and built a small supersonic wind tunnel as a B.S. degree thesis project. I constructed a Schlieren imaging system to visualize the shock waves on small models suspended in the tiny test section. It worked beautifully. As you can see, Caltech was guiding me through my studies even before I got there.

ZIERLER: Tell me about coming to Caltech. What was exciting circa 1960? What were the professors working on? What did you want to join in with?

I came up with the idea that the oscillatory flow might create a spinning motion of the combustion gases in the Sergeant burning port. We tested this hypothesis by conducting a static test firing of the Sergeant rocket at the Thiokol facility in northern Utah. Assisted by my friend, Budd Love, another Sperry engineer, we instrumented one of the four the jet vanes suspended in the nozzle gas flow with strain gages to measure side loads on the jet vane. These jet vanes were like those used at Peenemunde for steering the V-2 flight path.

This verified that the culprit was indeed a spinning vortex gas flow generated during periods of intense combustion instability. I was not satisfied with our incomplete understanding of such oscillatory flow effects in rockets and vowed to work more on this problem in the future. In fact, it later became the topic of my Caltech Ph.D. thesis work.

The Sperry management were impressed that I had displayed some potential as a problem solver, so they sent me to the Jet Propulsion Laboratory to work on Sergeant missile problems in 1958. My first JPL boss was Tom Hamilton who assigned me the task of calculating the Sergeant standard trajectories using an ElectroData digital computer built by a company in Pasadena. This was my first interaction with a computer, having learned to depend on my slide rule for making mathematical calculations. The simple trajectory program was sophisticated for the time. It accounted for launch initial conditions and aerodynamic forces throughout flight. I carried out trajectory calculations for the first nine Sergeant test flights. This was a great experience and taught me valuable analytical techniques. This job required me to determine the timing of deployment of the missile drag brakes needed to control range. The drag brakes were required since the solid rocket thrust could not be terminated – once ignited it would burn until the all the propellant was used. The guidance system was set up to allow three vernier brake deployments during flight. The computer program was incapable of adjusting the length of each braking period needed to guide the missile to its selected target (range from 30 to 100 miles). Tom Hamilton taught me how to use the differential correction technique (based on Newton's iteration method) to solve this problem. I would run the program multiple times for a given initial condition with nominal second and third braking operations varying the first braking period until I was close to the desired range. This was a very useful analytical technique that I have used throughout my career. Once the trajectory parameters for a given flight were established, the guidance system was programmed and then was tested in the JPL Missile Building. The rocket was hung horizontally in a test bay, where I observed the simulated flight (no propulsion) taking place with the drag brakes phasing in and out and the jet vanes moving to compensate the missile attitude for simulated disturbances.

I was also introduced to many unsolved problems arising in the Sergeant rocket motor itself. That is where my deep involvement in the combustion instability phenomenon began. I was given the opportunity to work with several different JPL groups on a wide variety of problems. I once worked in an office with some of the engineers who had been involved with the famous Bumper-Wac flights launched at White Sands in 1949. This vehicle was a German V-2 first stage with a JPL Wac corporal upper stage. It reached an altitude of 250 miles in one flight and took the first pictures of the earth from space. Although were not allowed to discuss it, they were also involved in the earth satellite program sponsored by the Army. After the 1958 Explorer 1 flight, the group confronted by a mysterious anomaly with the vehicle orbital motion. Upon injection into its nearly circular orbit, the spacecraft was spinning about its longitudinal axis to provide gyroscopic stability. However, after a short time it transitioned into an end-over-end tumble. It was later discovered that this was caused by vibrations of the Explorer's four whip antenna wires leading to kinetic energy dissipation. Such energy dissipation leads to instability in spinning prolate bodies.

I met some great engineers. One of them, Dr. Jack Alford (yet another Caltech grad, who had left his job with the Technicolor company to work at JPL) suggested that I apply to Caltech Aeronautics graduate program. He had been impressed by my analysis of the Sergeant launch dynamics from its zero-length field launcher. I followed his advice, was accepted, and started work on my Caltech MS degree in 1959. Jack Alford introduced me to his friend Leonard Reid (a Naval carrier fighter pilot in WW II) who lived just a block south of the campus on Arden Road. Reid invited me to use a guest house above his garage while I was studying at Caltech. The only cost would be for me to mow their (very small) lawn and to keep the swimming pool clean. This was a great convenience, since it was only a short walk to and from the GALCIT building. It was also close to the Huntington Library where I spent much time enjoying the many gardens, and such displays as Newton's annotated personal copy of his Principia, and the Gainesborough paintings, Blue Boy and Pinkie, beloved of my mother.

My association with JPL never ended from that point on, and I often worked on projects on a part time basis. Such arrangements were encouraged by the Caltech faculty, many of whom had similar contacts at JPL. I have since taken every opportunity to continue working with these great people. I think I hold the record for being the most rehired person at JPL.

ZIERLER: [laughs]

FLANDRO: Of course, the most intriguing things had started in the late 1950s when the space program began in earnest. At that time most JPL programs were funded by the US army so there were no formal space related activities. However, there were signs of interesting things quietly going on. One day at the cafeteria I observed a group with non-other than Dr. Wernher von Braun sitting at the head of table. I hastened back to my office to collect my copy of his book, The Mars Project, that I had purchased while I was a student at the University of Utah. I asked Dr. von Braun if he would autograph it for me. He did so graciously. This book is still one of my treasures.

At the time, I had no idea why the Redstone group were at JPL. It later turned out that they were working on what was to become the Explorer earth satellite program, our answer to the Russian Sputnik. von Braun and his engineers from the Redstone Arsenal in Alabama were there because JPL was developing the upper stages for the Redstone liquid propellant rocket (a derivative of the German V-2) for the Explorer program. Explorer 1 was launched successfully from Cape Canaveral on Jan. 31, 1958, becoming the first United States satellite. Incidentally, the JPL spin stabilized "high-speed" upper stage used fifteen scaled down Sergeant solid rockets fired in three stages to provide the boost to orbital velocity of 18,000 mph.

By this time, I was fully engaged in many aspects of space flight. My ballistic missile trajectory computations and my experience with rocket motor combustion problems fascinated me. Many of the related problems were still unsolved. After completing my MS degree program in 1960, I returned to the University of Utah for a short time as an instructor in the Mechanical Engineering Department. While there, I met and married a wonderful girl. She encouraged me to continue my academic studies, so we moved to Pasadena in 1963 where I began my Caltech Ph.D. program. I also resumed my part time activities at JPL in the Advanced Projects Group. My wife was hired as a secretary at JPL.

The Advanced Projects Group supervisor was Joe (Elliott) Cutting. Joe, who is now 93 years old still lives in Pasadena. We had worked together on many different problems in my earlier days at JPL. In the summer of 1965, he assigned me the task of exploring outer planet mission opportunities. By this time, JPL had emerged as the center for unmanned space exploration. However, there was scant interest in the outer planets, mainly because of their great distances and long times-of-flight (Jupiter 3-5 years, Saturn 10-12 years, Uranus 20-28 years, Neptune 30-45 years) required for robotic exploration. At that time, it had proved difficult to build reliable spacecraft that could survive more than a year or two in the space environment. The focus at JPL was the inner planets, and missions to Mars and Venus absorbed much of the lab's resources. Nevertheless, I accepted the challenge and began searching for ways to make outer planet missions possible with existing rocket technology.

I had learned some useful tools that showed great promise for space mission design. When I first came to JPL in 1958, UCLA was offering a course that was later published as Howard Seifert's Space Technology textbook. The course was presented in weekly segments presented by experts in all aspects of space flight. JPL and Caltech personnel presented lectures covering liquid and solid rocket propulsion, inertial guidance, trajectory optimization, and lunar flight. All the seats at the UCLA campus were already filled when I first found out about this very popular course. Several JPL employees and I decided to access the course by attending the televised weekly lectures being shown in Lancaster, CA for the benefit of Edwards Air Force Base personnel and others who did not want to travel to Los Angeles. We took turns driving to Lancaster over the Angeles Crest Highway.

To me the most interesting part was Krafft Ehricke's lecture on interplanetary trajectories. Dr. Ehricke had been a colleague of Wernher von Braun in the Peenemunde days in Germany. He had become interested (as did von Braun) in space travel upon viewing the Fritz Lang movie Frau im Mond (Woman in the Moon) as a teenager. Ehricke had designed the V-2 trajectories and had also helped von Braun with trajectory calculations for his Mars Project book. He came to this country in 1947 with Project Paperclip and worked with the von Braun group in Huntsville and was later hired by Bell Aircraft. He moved to Convair in 1952 where he designed the Centaur upper stage booster that used liquid hydrogen and oxygen fuels. The Centaur plays a very important role later in this story; it was the second stage of the two Titan III rockets used to launch the Voyager missions.

In his Space Technology lecture, Ehricke introduced the idea of what he called "instrumented comets" for unmanned exploration of the solar system. His use of this description of an unmanned spacecraft clearly shows that he understood the manner in which comet orbits are modified by close passage by planets like Jupiter and Saturn. He recognized that the same mechanism could be used for interplanetary spacecraft trajectories. This mechanism was fully understood by astronomers in the late 1700's as they observed the dramatic changes in comet orbits as they were perturbed by planet flybys. The astronomer Leverrier described an exquisite example of a gravitational orbital perturbation of Comet 1886 III, which originally had an elliptical (nearly parabolic) solar orbit. This comet departed the solar system on a hyperbolic escape orbit with eccentricity e = 1.013 after an encounter with Jupiter. This mechanism is now called gravity assist or the "swing-by maneuver." The key characteristics of the gravity assist mechanism are:

1. Hyperbolic encounter with and intermediate gravitating body can increase (or decrease) the orbital energy (the size of the orbit) relative to the sun.
2. The eccentricity and inclination (shape and orientation) of the orbit are also modified.
3. The velocity vector relative to the sun can be redirected within limitations imposed by the approach conditions at the encounter the mass and orbit of the perturbing body.

Clearly, this makes available what amounts to free propulsive energy to supplement what can be provided by chemical rockets.

The application of gravity assist technology to augment chemical propulsion was used by a great many individuals after WWI. Much interest began in Russia and Germany. I published a paper in 2001, From Instrumented Comets to Grand Tours-On the History of Gravity Assist, that reviewed the history of gravity assist. I'll mention a few of the prominent users here: In 1928 Guido von Pirquet, founder of the Austrian Rocket Society (later shut down by the Anschluss), demonstrated the benefits of Jupiter gravity assist. Derek Lawden described gravity assist in a 1954 paper entitled Perturbation Maneuvers. G. A. Crocco presented a paper at the 1956 International Congress in Rome describing his one-year "Grand-Tour trajectory, Earth-Mars-Venus-Earth. Ehricke described his work on gravity assist in several papers and in his famous textbook, Space Flight (vol. 2) in 1959. I bought his books and used them at JPL to help me master the tools of astrodynamics.

A UCLA graduate student, Michael Minovitch later claimed to be the inventor of gravity assist (despite the history just reviewed and his access to Ehrike's work). Minovitch had been a grad student at UCLA in 1959 where Ehricke had described his gravity assist astrodynamics in the Seifert Space Technology course. Minovitch persuaded some friends to nominate him for a Nobel prize for this invention. He later learned that Dr. Richard Battin, a famous astrodynamicist (whose credentials included his directorships of the MIT Instrumentation Lab and the Apollo guidance, navigation, and control program) had published an article describing some gravity assist trajectory calculations he made in 1958 long before Minovitch's first paper in 1961. He then sued Battin, accusing him of stealing his work and causing him to lose the Nobel prize - he also lost the lawsuit! Minovitch sued many other individuals and groups (including JPL) over the following twenty years.

Other analysts at several institutions including JPL were already using the gravity assist trajectory design technique. I did not have access to computational tools suitable for multiple planet trajectories when I began my outer planet task in 1964. Attempting to use the JPL integrating trajectory programs would not have helped. This would have been a very costly and expensive way to work the problem. I realized that the extreme accuracy would not be important in the preliminary mission analyses, so I used simple two-body conic orbital calculations to get things started. Lockheed had already developed a computer program for JPL that did conic trajectory calculations, so I adapted that software to calculate the heliocentric trajectories for each leg of the multi-planet flight path. The most difficult task was to match trajectories across intermediate planet encounters. At the time there were no analytical tools available to me to handle this problem, so I invented my own. For example, if I needed an Earth-Jupiter trajectory for a specified launch date and flight duration (equivalent to the launch energy) I would compute the incoming hyperbolic encounter trajectory details at Jupiter encounter. If I wanted to continue this flight path to Saturn, say, I would need to match the energy on the outgoing hyperbolic orbit for the second leg. I did this by setting up multiple trajectory calculations each night for the IBM 7040 computer programmer to run. I would collect the output next morning, and by using graphical methods I could identify the matched trajectories and refined them by using the differential correction method I had learned from Tom Hamilton. It took many such calculations to map out the Earth-Jupiter-Saturn trajectories over the launch date and launch energy range I selected. The famed JPL "Rocket Girls" (Barbara Paulson, Helen Ling, and others) were a great help in my voluminous plotting and hand calculations. Following this procedure, I carried out the search for useful outer planet mission opportunities.

The first step was to examine the outer solar system geometry, so I made careful drawings showing planet locations as a function of time. This was in the late 1960s, so I decided to learn how the planets positions changed over the next ten to thirty years in the future. I was surprised to find that in the late 1970's, all of the outer planets were on the same side of the sun. It became clear that Jupiter was the key to the outer solar system. If you can reach Jupiter with the right encounter conditions, you can fly anywhere in the solar system or even escape it as Krafft Ehricke had already demonstrated. I began the search for the best possibilities for Jupiter gravity assisted orbits to the outer planets.

A little aside here: I already knew about this kind of thing, because when I was just an eight-year-old kid—you're not going to believe this—my mother gave me an astronomy book, Wonders of the Heavens by Arthur Draper, for Christmas in 1940. The book described what was known at the time about each of the planets. There was even a drawing of a spaceship suggesting that space travel might be a future possibility. All this appealed to me as a Buck Rogers and Flash Gordon comic book devotee. On the inside cover was a two-page diagram of the solar system showing drawings of all the planets. There were Mercury, Venus and the Earth at the bottom left, and proceeding clockwise, Jupiter appears over toward the right bottom side, and then Saturn waiting at the top right, followed (moving to the left) by Neptune and Uranus. The artist had conveniently drawn the solar system so that starting from the Earth the outer planets were arranged in the order Earth-Jupiter-Saturn-Uranus-Neptune. I couldn't resist the impulse to connect the planets by paths starting from the Earth. Maybe this early discovery was stored for later use in my memory and helped me in 1964, as I began connecting the planetary dots with real trajectories!

At JPL in 1965 I began very detailed mission analysis computations for all outer planet configurations of potential interest. I carried out the first Earth-Jupiter-Pluto study showing that 8 September 1977 was the best launch date with arrival at Pluto on 15 August 1983. Time of flight was 7 years. It is interesting to compare this result to the similar Jupiter-assisted trajectory used for the New Horizons Pluto Mission that required a 9-year flight time (because Pluto was farther from the sun at encounter). Too bad I couldn't get anyone at JPL interested in Pluto exploration.

For obvious reasons, I concentrated on the Earth-Jupiter-Saturn-Uranus-Neptune opportunity. and found that the best launch dates would be in September 1977 and October 1976. I liked the 1976 mission because it would be the fastest way to reach the outer planets, but it required passage through the Cassini division between the rings to produce an optimum gravity assist by passing closer to the planet. Flight time to Neptune was about eight years. That was my favorite trajectory.

Of course, that trajectory design had to be rejected because of the uncertainties about what might lie waiting in the region within the Cassini division. Later, when the Cassini Saturn orbiter made its close observation of the ring system, it was found that passage through the gap was not so very risky. I worked out how often the four-outer-planet "Grand Tour" would recur and discovered that if we did not use the 1976-77 opportunities, it would require a 175 year wait for another chance. At the time, I had the strong expectation that NASA would waste this chance, based on the political climate. In fact, there were rumors that NASA intended to shut down JPL.

ZIERLER: A bit of a chicken-and-the-egg question: when you started to think about these trajectory calculations, were there already discussions about a grand tour, what would eventually become the Voyager mission, or did these calculations really start those conversations?

FLANDRO: There was no discussion whatever about anything involving outer planet missions—nobody was yet interested in the outer planets. Even Mike Minovitch (the self-proclaimed inventor of gravity-assist), was doing inner planet trajectory calculations like those of Crocco and others. He also studied Jupiter solar system escape trajectories that had already been suggested by Krafft Ehricke. I was the only JPL analyst at the time conducting detailed outer planet mission studies. Joe Cutting and Francis Sturms carried out a mission study employing a Venus gravity assist to reach Mercury. This became the highly successful 1973 Mariner 10 mission. It was also the first flight demonstration to prove the utility of the gravity assist maneuver.

Joe Cutting and I showed our mission study results to Dr. Homer Joe Stewart, the chief scientist at JPL. With the help of the JPL "Rocket Girl computers," I had generated some detailed slides describing the four-outer-planet mission and showing how the flight paths at each of the planets would appear during encounter. Stewart immediately recognized the value that our findings would have for JPL. He said that we needed a catchy name for the four-planet mission. He recalled that in the 1950s Professor Crocco had named his Earth-Venus-Mars-Earth trajectories the "Grand Tour". Stewart suggested that the same name would be perfect for our outer planet mission opportunity. The very next day in the Pasadena newspaper, there appeared an article by Homer Joe Stewart saying that JPL had discovered a way to reach the planets of the outer solar system. He mentioned the interesting observation that "free" energy from the Jupiter encounter providing a massive velocity boost to the spacecraft comes at the expense of a loss of some of Jupiter's orbital energy. That is, the spacecraft speeds up, but Jupiter slows down. This alarmed readers, some of them Caltech students and some people working at JPL. They thought this called for a public outcry. How dare we mess up Jupiter's orbit! So, they gleefully organized the Pasadena Society for the Preservation of Jupiter's Orbit. Some of the students marched down Colorado Boulevard carrying signs calling for stopping JPL activities that affected Jupiter's Orbit. "We can't have people going around and messing up Jupiter's orbit." That was great fun, ranking right up there with the infamous Caltech undergraduate pranks.

ZIERLER: To clarify, when you say that nobody before these calculations was interested in the outer planets, is that simply because there was no feasible way to get to them?

FLANDRO: Partially that, but the scientific interest was simply not there at first. After we had discovered this mission design, I consulted experts at JPL to find out whether they thought we had a workable notion. They showed no interest in the outer planets and proceeded to explain the many impossibilities confronting such missions. The guidance guys said, "No, you can't guide accurately enough." The spacecraft design people said, "No, you can't build a spacecraft that would survive long enough to do this." The data people proclaimed that "You can't transmit any useful data over interplanetary distances (and think of the time delay between transmission and receipt of the signal)." Every expert I consulted had a negative response. You know, they said, "You can't get through the asteroid belt. A spacecraft cannot survive passing through the asteroid belt between Mars and Jupiter without colliding with something." Another problem voiced was that "spacecraft electronics cannot survive passage through Jupiter's magnetic field," On-and-on the negative responses built up. They declared, "You just cannot do that. Come on, kid don't bother us anymore." That inspired me to work a little bit harder on selling this whole thing.

ZIERLER: I wonder if you could give me a little Physics 101, the theories or even the Newtonian principles that gave you the guidance that these classical calculations would translate into a workable slingshot mission.

FLANDRO: Actually, the physics is quite simple. Many experts said, "this is just a pipe dream." One of the objections voiced, even by some JPL people was: "Gravity assist just doesn't work because of conservation of energy. If you fly past a planet, all the kinetic energy is conserved. What are you talking about? The energy doesn't change." But they forgot that the planets move in their orbit around the sun. Say the planet isn't moving, then it is true; the spacecraft arrives and leaves on a hyperbolic orbit, but the departure speed would be the same as the arrival speed. Therefore, spacecraft kinetic energy is conserved! But if the planet was stopped in its orbit, the centripetal force that balances the sun's gravitational force would disappear and the planet would drop into the sun! The answer is found in the classical work-energy method. Since the planet is moving relative to heliocentric space, its gravity pulls the spacecraft along while it is within the planet's gravitational sphere of influence. This pull does work on the spacecraft and thus changes its energy – this is the gravity assist mechanism You can either gain or lose energy depending upon whether you fly in front of or behind the planet in its motion around the sun. So, the physics is all there, and there's no question that it works.

Now that we knew that we had discovered real mission opportunities, the job of the Advanced Projects Group had become one of convincing the JPL management, the various vehicle design, guidance, and science teams that a successful spacecraft program could be built on our findings. By this time the Advanced Projects Group had a new supervisor, Dr. Roger Bourke. Roger realized that the next step would be to initiate a full mission study. We composed a memorandum in October 1966 requesting that such a study should begin. It worked! We were given to go-ahead to launch the Grand Tour mission study.

FLANDRO: Once we had submitted the letter, the science people finally became interested. Then things began to move quickly. We received little support from NASA Headquarters. They wanted the money for manned missions and for the space shuttle program. Robotic missions were way down on their list of important objectives. I should mention that Wernher von Braun was fully supportive of our proposal and began a Grand Tour spacecraft development in NASA Huntsville to utilize the Saturn V heavy lift rocket. By this time some NASA headquarters folks had already started their efforts to remove von Braun as a leader in the space program. There were also several individuals who tried to frustrate all JPL moves to begin an outer planet mission. The space shuttle was the main attraction at that time.

In the Spring of 1966 (I have forgotten the exact date, nor do I remember the names of all the attendees), JPL held a conference of academic groups and individuals using the gravity assist technology. The Illinois Institute of Technology Research (IITRI), Lockheed Missile and Space, and several other groups discussed their work. Cutting described the JPL Earth-Venus-Mercury study. The computational methods used for multi-planet studies were a main topic. It was at this meeting that I first publicly described our Grand Tour results. The attendees seemed startled that we had been able to accomplish the detailed mission analysis without using a dedicated computer algorithm.

I had earlier suggested to my friend, Alva Joseph, a gifted JPL computer analyst, that we needed a fully automated precision search routine for multi-planet trajectories. He was heavily involved in some key projects such as the DPTRAJ, double-precision trajectory programs. His Fortran code to do the multi-planet job was a first step in several decades of development of great JPL trajectory analysis tools that supported all the spaceflight programs to come. Joseph later verified all our hand computations with his software. As a member of the JPL Mission Design and Navigation Software Group, my UTSI Ph.D. student, Steve Flanagan, was involved in developing an improved astrodynamics toolkit, MONTE (Mission Analysis, Operations and Navigation Toolkit Environment). It is an integrated development suite supporting trajectory optimization and operational orbit determination and flight path control. While Steve Flanagan was completing his doctoral program at UTSI, his wife baked me a mince pie for Thanksgiving. I had told Steve how much I missed my grandmother's mince pies, and that no such pies could be found in the modern world.

When it appeared that a real outer planet project was underway at JPL, Minovitch claimed that he had "invented" the Grand Tour Mission. This claim was voiced long after Cutting and I had published our detailed outer planet mission work. If his claim was valid, it begs the question, why didn't he say anything at the time?

Roger Bourke knew the answer, since he was familiar with the details of Minovitch's trajectory computations. They had been funded by his group. He explained to me that Minovitch's objective had been to find every possible multi planet trajectory for all possible combinations of target planets. This was totally different than my approach that was based on first identifying the best range of launch dates for a specific mission opportunity, and then conducting the detailed trajectory calculations for that mission profile.

Minovitch got into trouble with the JPL computer facility for using so much expensive computer paper to print out his results. After each nights runs, there would be a large stack of printout waiting for him in the morning. His computer output eventually filled his office. In any event, it would be difficult to find good mission possibilities by sifting through this mountain of data in a logical manner. It is certainly possible that somewhere in this enormous stack of information there existed an Earth-Jupiter-Saturn-Uranus-Neptune trajectory like mine. Minovitch must have looked for this case after we had already described it in our Grand Tour disclosure. He then made the claim that he had "discovered" the mission opportunity before us. In other words, we had in effect, shown him where to look.

ZIERLER: When you made this discovery, how far along were you in your graduate research at that point?

FLANDRO: I was in the final stages of my Ph.D. thesis writeup. Fred Culick and Frank Marble were my principal advisors. The subject was the Sergeant rocket combustion instability problem and the associated roll torque puzzle. I was dedicated to achieving a full understanding of the complicated fluid mechanics underlying these effects. Culick gave me valuable guidance along the way, and at the end of my labors he made it clear to the combustion instability community that no further work on the roll torque problem would be required.

I told Frank Marble about my work with the Advanced Projects Group. He invited me to discuss the JPL outer planet work at our weekly research seminar, and he suggested that I publish a paper describing my application of gravity assist. I submitted the manuscript to Astronautica Acta in 1966. The overly long title was: "Fast Reconnaissance Missions to the Outer Solar System Utilizing Energy Derived from the Gravitational Field of Jupiter." This was the first written disclosure of the Grand Tour opportunity and gave a detailed analysis of the gravity assist mechanism.

ZIERLER: You conveyed the initial excitement as you started to share these findings. What was the timing or your sense of how all of this ultimately would form the basis of making the Grand Tour to the outer planets a viable mission?

FLANDRO: We had already achieved full support from JPL. I knew that I would need to devote time and energy to describing our work to a larger audience. I started doing more detailed supportive calculations and devised a plan to advertise the value of the mission to our understanding of the solar system. I also concentrated on finishing my doctoral research and preparing for the final oral examination and thesis defense. My examiners were Professors Sechler, Roshko, Marble and Culick. They gave me a good workout, but I made it through.

At that point I had to make a career decision. I was about to get my PhD degree. I could either stay on at JPL or go on and do more academic things. I chose the latter, so I moved back to Utah to accept an offer to work as an assistant professor at the University of Utah. I maintained all my interest in my Caltech research. I continued writing papers on rocket propulsion problems and spaceflight matters. I got my students involved, and I enjoyed passing on my excitement about all the things I had experienced in my classes. Then I watched as an observer this whole Grand Tour mission as it developed. My coworkers at JPL took over and did all that was needed to make it work. They did a wonderful job of making this into a real mission. My friend Charlie Kohlhase got involved early on and worked the trajectory targeting to make it possible to collect images and data from the numerous moons of the outer planets. Ed Stone, of course, came into it later, and guided the program in a masterful way.

The spacecraft people were truly amazing. They were led by Harris "Bud" Schurmeier, Voyager's first Project Manager (In 1976 he became the Assistant Lab Director at JPL). This team took on this impossible-looking task and built a spacecraft that could do the job. But the funding they got from NASA was, of course, minimal. They were given only enough funding to accomplish a two-planet mission. One must keep in mind the political problems involved in securing funding for any mission requiring more than a four-year flight duration. Since any given presidential appointment is only guaranteed for four years, a president does not want to pay for a project that will only benefit the next administration. Thus began the two-planet Voyager project (originally called the MJS77 mission based on Mariner technology). And now these incredible spacecraft guys go to work. They designed and built the Voyager spacecraft, right from the beginning, with the capability to carry out the twelve-year, four-planet mission. JPL kept it close to the vest that they were building spacecraft to do the full Grand Tour mission on a two-planet budget. I knew many of these people from my days at JPL and they are incredibly gifted engineers. Of the design process, Schurmeier said, "One lesson is perseverance – when the original Grand Tour was cancelled by NASA, we didn't say ‘forget the whole thing'; we went back and tried to put something together that would sell and would be approved." One of the JPL engineers, Ray Heacock said "What we did was to use conservative design practices, reliable components, conservative design margins, adequate testing, and as much redundancy as possible. We practically built two spacecrafts within each of the spacecraft structures. Most programs today are not able to be as conservative as Voyager and, as a result, are taking much more risk. Roger Bourke said, "The openness with which we have functioned has been a key, the openness to ideas, and let the best idea win out – that is an intrinsic quality of this place (JPL)." Voyager effectively rebuilt and revitalized JPL's place in solar system exploration. Dr. Pickering, the JPL Director once said that "we knew what we were doing but we didn't talk about it."

Bud Schurmeier, the Voyager Project Manager, passed away in 2013. I am compelled to describe his history in more detail. Schurmeier had been a Naval Pilot during World War II. He received his technical training at Caltech and joined JPL in 1949 to work in the wind tunnel section. In 1959 he became the deputy manager of the Sergeant battlefield missile project. In 1960 he organized the JPL Systems Division, after which he took over the Ranger lunar impact spacecraft program. He then became the project manager for the Mariner Mars 1969 project. Finally, he assumed leadership of the Voyager project. One of Schurmeier's responsibilities as the Voyager Project Manager was to select key personnel for the program. An important team member he selected was Ed Stone in the role of Project Scientist,

Like me, Bud Schurmeier built model airplanes, and later became an avid soaring and motor glider pilot. This brings-to-mind the last encounter I had with Bud. In 2004, we were both invited to dinner at the home of the Governor of Tennessee, Phil Bredesen (also a motor glider pilot who owned a beautiful Stemme two-place touring glider). I was accompanied by the UTSI Director John Caruthers. Bud was there to help Bredeson with some maintenance issues with his glider. After a great meal we began telling war stories (mostly about our glider flying). Bud recounted his glider accident that happened when he found himself at low altitude above mountainous terrain with no landing field in sight. Standard procedure in this circumstance is to deploy and start your motor in order to extend your flight to friendlier country. Bud's DG 400 glider was equipped with a 43 hp. Rotax engine. The worst happened – the engine failed to start after multiple attempts during which he became dangerously low. He made a forced landing in rough terrain and was injured but survived the crash.

Schurmeier was elected to the National Academy of Engineering in 1983 and became an AIAA Fellow in 1973. He received the NASA Exceptional Scientific Achievement Medal in 1965. He delivered the AIAA von Kármán Lecture in 1974.

ZIERLER: I wonder if you can explain some of the math or the planetary calculations where you realized that it was a now-or-never situation. And by never, I mean, "We do this now, or we have to wait another 175 years."

FLANDRO: That's it exactly. I was looking for the best launch years, so I was doing many thousands of computer calculations. Every day I would turn in a set of runs so I could explore the whole gamut of the possibilities. I was looking for the best launch dates and found that the fall of 1977 would be the best for any outer planet trajectory. I discovered that the best one, the one that would go Earth-Jupiter-Saturn-Uranus-Neptune, was a recurring possibility, but with a 175-year wait time, so if we didn't make it first time, which was only about ten years ahead then—if we don't do it in the next ten years, we're going to have to wait 175 more years for it to happen again. The news that there was a rare alignment of the outer planets as well as Venus, Mars and Mercury led to considerable furor. Some folks called this the "Jupiter Effect" that would lead to catastrophes such as a major San Andreas Fault earthquake and so on. This Jupiter Effect worry got into the news all over the world. I guess my work led to some unexpected turmoil.

ZIERLER: What was your sense of the chain of communication starting with you and your supervisors at JPL and ultimately leading to NASA headquarters in improving what would become the Voyager mission?

FLANDRO: There was no trouble at JPL. Dr. Stewart, the Chief Scientist at JPL, and Dr. Pickering the JPL Director were fully supportive right from the beginning (JPL at that time had become a NASA center). As you proceed to the upper echelons of NASA, it's a different matter. They did not want to support an unmanned mission of this magnitude. I've already explained what it was going on. I don't want to mention names, but these guys intended to cancel the Voyager project.

ZIERLER: The idea, Gary, that manned space flight was it. What about the Cold War? In what ways did national security implications loom large for what NASA's priorities were at this time?

FLANDRO: I think that they were very concerned about that, of course. I can't speak to their basic worries, but of course they were there. But in terms of the space program, I think that it was so important from the standpoint of keeping pace with the Russians that it would keep going despite all the problems with the Cold War. We didn't back off at all, in part because the Apollo lunar program was going on and we wanted to beat the Russians to the Moon.

This reminds me of an incident in about 1972, I got a call from my son-in-law. He said, "Gary, do you know that you were in Playboy?" I said, "What? You mean I'm the centerfold?" [laughs] He said, "No. There was an article in Playboy in which you are mentioned by name." He lent me his copy of Playboy and there, lo and behold, is this article, "Dark Was the Night" by Richard Powers. It's a fiction story, about a guy who's not me, but is supposed to be someone like me, who works at JPL. My name comes up as this graduate student that found a new mission possibility. The writer describes how the mission came about. He even discusses the conflict between the manned space mission proponents and the unmanned guys. He then describes the history of the mission using his imaginary JPL employee, takes it all the way to the end and describes what is going to happen. The hero dies, but the mission story proceeds through solar system escape. In 2029, the last remining Voyager project member shuts down all the systems and JPL "becomes a sarcophagus." In the year 294231 "Voyager 2 sails four light-years from Sirius, the brightest star in the sky, Earth has long since gone dim." In the year 577256880, some intelligent life form finds the Voyager spacecraft and the Golden Record mounted on it and deciphers it using the instructions printed on the cover. The title of the Playboy article is "Dark Was the Night," which was the name of one of the songs on the Voyager Golden Record sung by Blind Willie Nelson. Anyway, the aliens have got all this information, pictures of the earth, human life, and its music, millions of years in the future. A beautiful story. I can send you a copy if you want it.

ZIERLER: Oh, I'd love it. Given NASA's initial reluctance to pursue an outer planet mission, what changed? Who were some of the heroes of the story at Caltech and JPL that convinced HQ otherwise?

FLANDRO: I think the real heroes are people like Ed Stone and Bud Schurmeier (and many others) who saw the need for this mission and had the political savvy to do the marketing and organization of the project. That was a natural process. All the NASA naysayers were forced to back down.

ZIERLER: Gary, what aspects of your work at JPL were part of your thesis, and what parts were totally separate?

FLANDRO: Only my work related to combustion instability and to the unsolved Sergeant roll torque problem constituted the backbone of my Ph.D. thesis. The space flight work and a multitude of other tasks I worked on at JPL were totally separate matters.

ZIERLER: Tell me about working under the direction of Frank Marble. What was he like as a person?

FLANDRO: Marble was one of the best teachers that I have ever had and a beloved humanist. He had been a student of von Kármán and Hans Liepmann. Marble was a member of both the National Academy of Engineering and the National Academy of Sciences.

He was a close friend of the famous Hsue-Shen Tsien, (a member of the Caltech suicide squad and, much later, the founder of the Chinese Space program). Some history on Tsien must be inserted here to clarify his association with Marble and Caltech in general. After his adventures with the Caltech suicide squad and receiving his Ph.D. in 1939 Tsien, Malina, and Parsons wrote the first document to use the Jet Propulsion Laboratory name. He accompanied von Kármán after WW II to interview German scientists (including von Braun and the famous Dr. Ludwig Prandtl, von Kármán's doctoral advisor). Despite Tsien being wrongly accused of being a member of the Communist party, he became the Robert Goddard Professor of Jet Propulsion upon recommended by von Kármán, who described him as an "undisputed genius." He worked on high-speed flight (he coined the word "hypersonic") and designed an intercontinental rocket propelled space plane. His ideas led to the X-20 Dyna-Soar manned hypersonic glider under Air Force sponsorship. He was placed under house arrest by the government, but he continued his work at Caltech. He wrote the textbook Engineering Cybernetics in 1954, which was a major contribution to the theory of complex control systems. In a prisoner exchange (for the repatriation of American pilots captured during the Korean War), he was finally allowed to leave the country, returning to China after resigning from Caltech. Dr. Marble accompanied Tsien and his family to the Los Angeles Dock when they departed for China by ship in 1955. Tsien was awarded the Caltech Distinguished Alumni Medal in 1979, but he declined to come to America to receive the award. In 2021 Dr. Marble and his wife Ora Lee travelled to China and made a formal presentation of the Distinguished Alumni Award at Tsien's bedside twenty-three years after its announcement in 1979. The founder of the Chinese space program said many times that he had great respect for the American people, but not for its government. This impromptu award ceremony received widespread coverage in China.

I attended all of Marble's courses and learned much of importance to my career. He was the chairman of the Kármán Laboratory of Fluid Mechanics and Jet Propulsion located in the main GALCIT building. It had been constructed as a hydrodynamics lab in 1944. The Aerojet Company provided funds in 1960 for expansion of the laboratory and its research mission in fluid mechanics and jet propulsion. It was named in honor of von Kármán (the first director of the Caltech Graduate School of Aeronautics and the founder of Aerojet).

I had proposed to Marble that my work on the Sergeant rocket combustion instability and the unsolved roll torque problems would be a good Ph.D. research topic. He agreed and described his own combustion instability work with his Canadian student Captain Wilmot Brownlee. They had conducted a remarkable experimental study in 1958 using 6" solid rocket like the scaled-down Sergeant rockets used in the Explorer 1 project. Marble arranged for these motors to be built and tested at JPL. The goal of the study was to gain an understanding of the so-called "irregular burning" or "DC shift effect" in which the motor oscillations led to large increases in burning rate and chamber pressure that often led to explosive failure. An oft-observed side effect was the generation of a roll torque (as in the Sergeant rocket problem I had studied at Sperry and later at JPL). No successful theoretical models had yet been devised, so only experimentation would aid in making progress with the problem. Over 450 test firings were conducted varying only the geometry for a single polysulfide/ammonium perchlorate propellant (T-17). These were the first experiments with adequate instrumentation. The data set is still considered to provide one of the best experimental windows into this complex problem ever achieved. A startling set of observations demonstrated the roll torque phenomenon in which the motor oscillations produced torques estimated to be about 600 ft-lbf. My own Ph.D. thesis work led to an acceptable theoretical model for the roll torque effect. Much later, I used the insight gained in the GTL UCDS combustion instability software project to establish a comprehensive theoretical understanding of the Marble-Brownlee experimental results.

Along the way, Dr. Fred Culick, an MIT graduate, joined the Kármán Laboratory as an assistant professor in 1961. Since his main interest was rocket combustion instability, it was natural that Frank Marble should assign Fred to take over supervision of my thesis work. Fred gave me a copy of his new paper on combustion instability to review. In working through his analysis, I learned the keys to a good theoretical approach to the problem. He helped me acquire some essential mathematical tools, and I began applying them in my research. I finally understood how the Sergeant rocket roll torque problem came about because of spinning acoustic waves angular momentum transport.

ZIERLER: What were some of the main arguments or conclusions from your PhD thesis?

FLANDRO: First, that combustion instability was a very important problem and far more complicated than anybody realized. I made the point that problems of this sort are not anticipated at the system design stage, and that their unexpected appearance has often led to program cost overruns and cancellation. It involves a complex mix of gas dynamics, non-linear transition of acoustic waves into shock waves, combustion energy release, vortex shedding, and many other interactions. Second, it is vital to understand the physical mechanisms behind combustion instability side effects, such as unexpected forces and torques and the irregular burning DC pressure shift. We had made a good start, but the theoretical models needed much more work.

All this began with Frank Marble and Fred Culick starting me off on the right path. Fred and I became competitors later. I won't try here to go into all the details, but he came up with some ideas about the theory for combustion instability that I felt were incomplete, so we started arguing. He started treating me as his enemy, because I insisted, "No, Fred, you've got to account for vorticity in the problem." He had discovered a mechanism he called "flow turning" that had not been accounted for in combustion instability predictive algorithms. Flow turning represented a major damping effect on the wave system and predicted that all systems should be stable (despite not agreeing with experimental results). I published an AIAA paper in 1995 demonstrating the origin of the Fred's flow turning effect as an unsteady vorticity mechanism arising in the oscillating rocket flow field. I also found related phenomena that had not appeared in Fred's calculations. One of these mechanisms was a driving effect that cancelled the flow turning loss in many situations. He didn't like that at first, because he had missed some of the important things. Later he finally agreed that I was right. Much later, he said, "Okay, Gary, you were right after all." We became friends again.

ZIERLER: Circa 1967 when you were wrapping up at Caltech, what was the status of the negotiations between NASA and JPL? Were you confident at that point that a Grand Tour ultimately would happen?

FLANDRO: No, I had the distinct impression that we were still getting the run-around by NASA, so I was unsure where things were headed. I was sure that the Grand Tour opportunity would be wasted. After graduating from Caltech, I worked for a year at JPL and then accepted an academic position at the University of Utah as an assistant professor. I continued working on the outer planet mission design problem looking for ways to enhance payload without the use of large rockets. I had already done some work with solar electric low-thrust propulsion, and with the help of Dr. Carl Sauer at JPL I published a paper describing the use of a low thrust trajectory to Jupiter with a gravity assist continuation to the other planets. This resulted a significant payload benefit at the expense of a longer first-leg time of flight.

ZIERLER: With Apollo and the Moon landing in 1969, to what extent do you think that achievement opened NASA's vista about making this a possibility, now that this major milestone in manned space flight had already been accomplished?

FLANDRO: I think that it did have a direct effect, because we had by then demonstrated that our space programs had truly reached maturity, despite some early difficulties. That gave us the incentive to continue to push forward. I think that's what eventually made the Grand Tour a respected NASA program and enhanced JPL's image.

ZIERLER: In the early 1970s, when the Voyager mission is now committed, are you in Utah? Are you involved at all? Are you watching from the sidelines?

FLANDRO: I now became a very interested spectator. I did travel back and forth to JPL, and I worked summers several times on special projects such as low thrust trajectory optimization and solar sail missions. I had a team of students at the University of Utah designing and building a solar sail, as a project for their thesis work. We were encouraged to do this because my friends at Thiokol offered to pay the $10,000 for a NASA "Getaway special" payload cannister to be flown aboard the Space Shuttle. We proceeded to design a small sail that would fit into the cannister and would weigh less than 200 lb. We came up with a system that would eject the payload from the Shuttle cargo bay and deploy the aluminized mylar sail into low earth orbit where, hopefully, we could demonstrate the feasibility of this innovative spaceflight technology. After many months of intensive research and design work, our hopes were dashed when we discovered that NASA would not allow getaway special payloads to leave their cannisters! ZIERLER: Did you ever consider taking on full-time work at JPL, being more directly involved in Voyager? FLANDRO: Yes, many times. Dr. Marble was able to arrange a joint appointment where I would work at JPL and teach at Caltech. ZIERLER: Did you ever have opportunity to interact or work with Ed Stone, get a sense of his style leading the Voyager mission? FLANDRO: I did not work with him directly, but of course I had met him several times. When I received the NASA Exceptional Achievement Medal, Dr. Stone presented the award to me at JPL. After he gave me the medal, pointing up to a Building 180 third floor office window, I announced, "That is where I first realized that the four-outer planet mission could be accomplished." He was amused by that. This was my first contact with Ed. We had never worked together, but we crossed paths many times after. He was present in 2018 when I received the Caltech Distinguished Alumni Award. We both participated in the Voyager anniversary meeting held at the Smithsonian Air and Space Museum in Washington, where we had some interesting chats with Ann Druyan, the wife of Carl Sagan and one of the participants in the Voyager golden record project. I've always greatly admired Ed Stone in his several roles as a Caltech Professor, Voyager Project Leader, and later the Director of JPL. ZIERLER: I wonder if you see your calculations in forming the decision to have two spacecraft, for there to be Voyager 1 and Voyager 2 with different trajectories and different space flights. FLANDRO: I did not learn about the Voyager 1 trajectory decision until after both missions were underway. The space scientists were the ones that pushed that idea in order to enhance the mission data return. I saw that it would be a worthwhile option, with one spacecraft passing under Saturn to deflect the flight path to enable a close pass of its large moon Titan and to depart Saturn in a direction away from the plane of solar system. Earlier space flights had never ventured far out of the ecliptic plane. This was our opportunity to see the solar system from a new viewpoint. I thought that the sacrifice of Voyager 1 as a Grand Tour vehicle was well worth the cost. Keep in mind that no one outside of JPL realized at the time that Voyager 2 was prepared to fly the Grand Tour trajectory ZIERLER: How long were you teaching at the University of Utah? FLANDRO: I spent about 20 years there at the University of Utah. Then I went on to Georgia Tech and finally to UTSI. ZIERLER: Were you in Utah? Were you present for the launch of Voyagers 1 and 2? FLANDRO: My work commitments prevented me from attending the launches at Cape Kennedy. Rest assured; I watched the news coverage with great interest. ZIERLER: When it was finally time for launch, what were you most excited about in terms of possibility and discovery? FLANDRO: Just to watch those two Titan rockets carrying the two Voyager spacecraft on their way to the outer solar system was one of the most exhilarating moments of my life. I knew that atop the Titan III First Stage were the Centaur liquid propellant upper stages that had been designed by Krafft Ehricke. Remember that he had taught me how to use the gravity assist method to provide the energy to carry the precious payloads to their targets. I have thought many times since that Ehricke should have been remembered as a major contributor to the Voyager missions. I am now sorry that I did not push for such a recognition of this great man. ZIERLER: That's amazing! Did the spacecraft more or less perform exactly as you calculated? Were there any surprises in the notion of a slingshot or how powerful the gravitational assist would be? FLANDRO: No surprises at all. The gravity assist physics are so well-established that there was nothing there could result in problems other than failures in the guidance system. Precision guidance in the planetary approach and encounter are necessary to produce the required trajectory deflection and the resulting energy increment. There were some spacecraft surprises, as you may know. The Voyager 2 got shaken up during the trip to the Cape for launch and there was a problem with that. There were other unexpected events, but it was always possible to fix them because of the robustness of the spacecraft design. At one point in the Saturn encounter the scan cameras were fouled up. The mission team modified the programming, so they could get the pictures. They were able to do this because of the capabilities built into that spacecraft. Again, I've never seen such an amazing piece of engineering. It could repair itself when problems arose. Amazing Jet Propulsion Laboratory engineering. ZIERLER: What were the planetary encounters like, as a media event and as a scientific event? FLANDRO: Of course, everybody was fascinated by seeing, for example, the close-up images Jupiter's Great Red Spot and the rings of Saturn. The closeup views of the satellites of the outer planets were marvelous. One very important event was the discovery of the volcanic activity on Jupiter's moon Io. Although perhaps not as dramatic as the Apollo lunar landings, the public showed great interested in each of the Voyager planetary encounters. ZIERLER: Tell me about your decision to transfer from Utah to UTSI. FLANDRO: By 1974, I was an established full professor in the Mechanical Engineering Department. I had received numerous department teaching awards as well as the 1979 University of Utah Teaching Award. I had attracted significant research funding from the rocket propulsion companies (Hercules and Thiokol), from Aerojet, Lockheed, and the US Air Force. I had established a successful fluid-mechanics research laboratory, and a computer lab supported by the Hewlett Packard Company. I had received several awards such as the 1970 Golovine Award from the British Interplanetary Society (for my discovery of the Voyager opportunity). In 1974, I began a sabbatical year as Visiting Fellow at the University of Southampton (UK) Institute of Sound and Vibration Research. In addition to collaborating with world-class acousticians, I was able to pursue my study of the Supermarine Company, famous for winning the Schneider Cup seaplane races in the 1930s. They were designed by one of my heroes, Reginald Mitchell, who was also the designer of the Spitfire fighter that helped England win the Battle of Britain. The University of Southampton had some parts from Mitchell's airplanes stored in a basement, which I of course found very interesting. An international conference on aeroacoustics was held in Southampton, during which I encountered Dr. Anatol Roshko, one of my favorite teachers from Caltech. We discussed his latest work and compared notes on vortex flows. Upon my return, there were many problems developing in the University of Utah College of Engineering. There was pressure to scale down the traditional engineering departments such as Civil, Electrical, Chemical, and Mechanical Engineering to make room for Computer Science and a struggling Bioengineering Department. The Computer Science department had become very successful with pioneering work in computer graphics, computer architecture, digital audio, the development of the internet, and computer animation. The founder of Atari, Nolan Bushnell was a graduate of that department. These groups needed more space and they had targeted some of the extensive Mechanical Engineering Department laboratory space in the Merrill Engineering Building. The time came when a new Mechanical Engineering department chairman was needed. The College of Engineering Dean (who had been the Bioengineering Department Chair) attended a faculty meeting in which we were discussing the department chairman selection process. He indicated that he had already chosen our new department chairman. His candidate (a friend) had an industrial engineering background. He asked us to review his qualifications and interview him for the job. This we did. I interviewed the candidate accompanied by my colleague Bill Van Moorhem. We were startled when he insisted that he had a God-given ability as a leader. Afterward, all members of the ME faculty agreed that we did not want this person for our department chair because he had neither the right credentials nor the right vision to lead the department. When we had reported this information, the Dean stood up in the faculty meeting, declared that had decided to hire his friend despite our objections, and left the room. My reaction was to announce that I would immediately submit my letter of resignation, which I did. Although it was never made public, our new Chairman later stole money from the department and caused many other problems. He was quietly removed from the job to avoid publicity. I no longer had much respect for the leadership of the College of Engineering, so I began the search for a new job. Frank Marble offered to help. He said, "Why don't we arrange a full-time position at JPL and part-time appointment as an instructor at Caltech." That was attractive, but I didn't like the loss of my academic full professor rank. So, I looked for other opportunities. I had one from BYU in Provo, Utah, and an offer as a senior engineer at Thiokol in Brigham City, Utah. An offer of a full Professorship came from Georgia Tech. One of my good friends, Ed Price, a Distinguished Professor at Georgia Tech, wanted me to work with him on the combustion instability problem. I accepted the Georgia Tech offer. Ed and I wrote a combustion instability textbook that I used later in courses at UTSI. I very much enjoyed my teaching in the Aeronautics Department. The Tech students were outstanding. I taught fluid mechanics, propulsion systems, aerodynamics, aircraft stability and control, trajectory optimization, and astrodynamics. I continued the research I had started at the University of Utah (funded by the U.S. Air Force) on what was called the spinning upper stage nutation instability problem. I was awarded a research contract by the NASA Langley Research Center to study the effects of vehicle maneuvers on the operation of a hypersonic Scramjet propulsion system. This was in support of the National Aerospace Plane (NASP) project. I was hired by NASA Langley to help with a serious vibration problem they were experiencing with their 8-ft Mach 7 hypersonic wind tunnel. The tunnel was being used at the time in the development of the National Aerospace Plane Scramjet engine. This was a combustion-heated blow-down tunnel. The tunnel produced a loud low frequency howl when it was running. I was hired because of my experience with combustion instability problems. They believed that the oscillations might be driven by coupling of the flow field with oscillatory combustion near the propane injector system located at the upstream end of the tunnel. I thought something else was involved. The tunnel was cooled by air entering from the supply tank near the downstream end through the cylindrical gap between the outer surface of the tunnel and its interior. The air flow direction was abruptly deflected at the forward end and turned past the heater injector toward the test section. I realized that vortex shedding was the energy source and explained to them that in large ducts and rockets (like the Shuttle SRB) this was a common acoustic wave driving source. I showed them several techniques for solving the problem. I don't think they were ever used. I guess they just kept on disturbing people working in nearby buildings with their loud noises. The Voyager 2 Uranus encounter occurred in January 1986. I couldn't enjoy it because, unfortunately, the Space Shuttle Challenger Disaster took place at the same time. I became embroiled in the controversies over the causes of the failure of one of the four-segment solid rocket boosters leading to failure of the Shuttle hydrogen tank. CNN commentators (their office was a short distance from Georgia Tech) contacted me hoping that I would help them understand what had happened. It was clear to me that there had been a failure at one of the field joints where the motor segments are joined and sealed by O-rings. I thought that the freezing weather at the time of launch must be a factor. I suspected that the thrust oscillation problem I had previously studied for Thiokol might have played a role. To me, if the seals were already compromised by freezing conditions, then vibration of the motor case itself might lead to fracture of the seals. I voiced this concern in an interview at the CNN studio and then all hell broke loose. I began receiving telephone calls from all over the world. I received an invitation from television commentator Larry King to be interviewed on his program in New York City. I tried to explain that there were many things about the disaster that must be carefully evaluated before any final conclusions about the failure path could be reached. Later, I received a call from a Huntsville phone number. The caller did not identify himself, but he made it clear that I must make no more public statements about the SRB failure or face the consequences. I stopped responding to the incessant questions. You will recall that Caltech Physics Professor, Richard Feynman testified later to Rogers Committee investigating the failure, and demonstrated the failure sequence by dropping an O-ring sample (squeezed with a C-clamp to simulate the pressure load on the seal) into a glass of ice water. He showed that the rubber O-rings cannot properly expand in a 32-degree temperature environment as they must to seal the joints. That correctly placed the blame for the Challenger disaster on flight readiness decision made on the morning of the flight in freezing weather conditions. My friend, Allan McDonald, was the Thiokol engineer whose job it was to evaluate flight readiness and to sign-off on the launch decision. McDonald was the Thiokol liaison for the SRB motors at Cape Kennedy. I had worked with him in Utah when he was the Utah AIAA Section Chairman in 1968. My responsibilities as his vice-chairman included recruiting speakers for our monthly section meetings. He was quite pleased with my work, since one of my distinguished speakers was Edward Teller, inventor of the hydrogen bomb. I had fun introducing Teller at the meeting by using the famous poem by Prof. Harold Furth, in which Dr. Edward Anti-Teller meets a visitor from Earth (Edward Teller) – "Then shouting gladly o'er the sands, met two who in their alien ways, were like as lentils. Their right hands clasped, and the rest was gamma rays." Allan's job was to monitor all SRB system parameters and to make the go/no-go launch decision on the morning of the Challenger flight. There were numerous phone calls between NASA and Thiokol administrators regarding the cold weather threat to the launch. They knew of earlier signs of field joint damage on earlier Shuttle flights. One Thiokol engineer, Roger Boisjoly, who had studied the field joint anomalies in detail, said that the O-ring temperatures must always be greater than 53o F at launch. He recommended that the flight be postponed, and McDonald refused to sign the release paperwork that would allow the flight countdown to begin, despite orders from above that the launch must take place. The inevitable disaster followed. Allan McDonald was fired by the Thiokol management for not following their orders (he was subsequently rehired and managed the redesign project to correct the faulty field joint configuration). The Rogers Commission concluded that Thiokol Management had reversed its earlier position (following McDonald's advice) and recommended the Challenger launch at the urging of NASA Marshall despite the objections of its own engineers in order to accommodate a major customer. A sad commentary. I remained at Georgia Tech for six years and finally made the decision to leave. I was unable to move my family to Atlanta because of the terrible real-estate market brought about by the economic mess in the country. Interest rates had reached over 20%, and we were unable to sell our house in Salt Lake City. I learned that the University of Tennessee was seeking candidates for the Boling Chair of Excellence at the Space Institute. I applied for the job and after several interview trips to UTK in Knoxville and UTSI in Tullahoma and presentations on my research activities, I was awarded the UTSI Boling Chair. The original title was Boling Chair of Excellence in Space Propulsion. ZIERLER: Were you aware of UTSI? Were you aware of their reputation in the field, what they were doing? FLANDRO: Yes, I did know about UTSI, because of their connections to Caltech through von Kármán. I learned of the many attractive features of the campus, the staff, and the professors. I was given the use of a laboratory building with test bays in which propulsion system studies could be made. GTL used this facility to experimentally verify some of our theoretical work. We ran our liquid propellant rocket with the combustion chamber modifications there and proved that our patented design principle worked beautifully. ZIERLER: What was your research at the point you joined UTSI? What were you working on then? FLANDRO: Combustion instability was still a major part of it. I had also been doing work on solar sails and low thrust trajectories. I never lost interest in the trajectory work, so that was always going on. Many of my students at UTSI were interested in propulsion system problems. One of them, Paul Gloyer, later started the Gloyer-Taylor company. Several of my students became successful employees in academia, NASA, ATK, JPL and, of course, GTL. ZIERLER: As you well know, the gravitational assist research wasn't just useful for Voyager; it was also important for missions like Galileo and Cassini. Were you involved in any other NASA missions that needed gravitational assist? FLANDRO: Other people had taken over by that time. As a follow-on to the fly-by missions like the Voyagers, JPL proposed missions that would remain in orbit around a planet for detailed study. Both the Galileo and the Cassini missions would carry out this plan at Jupiter and Saturn respectively. John Casani, who had worked with Bud Schurmeier on the Voyager project was the first Galileo Project Manager. The Galileo Jupiter orbiter program was planned to make a continuous investigation of the Jovian moons by using gravity assist course corrections at each encounter. At that time, NASA had decreed that all future planetary missions would be launched by the Space Shuttle. The Galileo was designed to use the Centaur second stage (used for the Voyagers) to start the spacecraft from low earth orbit on its way to Jupiter. After several years of fabrication and testing (all the spacecraft components and spare parts received a minimum of 2000 hours of testing) the Galileo spacecraft left JPL on December 19, 1985, on its way to the Kennedy Space Center. It was scheduled for launch on the Space Shuttle Atlantis on May 20, 1986. The Challenger disaster destroyed that schedule because the decision had been made that no hydrogen fueled upper stages (like the Centaur) could be carried in the Shuttle. JPL was informed that Galileo could not be launched before October 1989, so it was shipped back to JPL. The cost to keep the Galileo in shape to fly was estimated at$50 million per year, so the cost of the mission would grow to \$1.4 billion.

At this point gravity assist came to the rescue. My friend Bob Mitchell (we used to play touch football outside our JPL office in building 125 during lunch) was now the Galileo Mission Design Manager and Navigation Team Chief. He began an effort to find a Jupiter transfer trajectory that would use a two-stage IUS (interim upper stage) that used propellants compatible with the Shuttle cargo bay. The necessary energy to reach Jupiter would be supplied by Venus-Earth-Earth Gravity assist (VEEGA) maneuvers. I had studied what I called VEGA (Venus-Earth Gravity assist trajectories) in 1980 in one of my summer sessions at JPL and hoped that there would be future uses for such a scheme. At that time, very few people had recognized that a swing-by at the Earth could be used to advantage.

The Galileo finally left the Kennedy Center on 17 October 1989 aboard Space Shuttle Atlantis. The first leg to Venus began and the Venus gravity assist was performed flawlessly. The successive Earth encounters followed, and the vehicle was its way to Jupiter. The mission was a great success, and the atmospheric probe yielded valuable information about the winds, temperatures, and chemistry of the Jovian atmosphere.

Earth-Jupiter-Earth-Venus multiple gravity assists were used for the Cassini Saturn orbiter mission, which was another great JPL success story. The Titan IVB with the Centaur upper stage was the launch vehicle – no Space Shuttle this time.

ZIERLER: What was it like when the Voyager spacecraft finally crossed out of the solar system, when they got beyond Pluto? What was that like for you, and what big question marks were in the field at that time?

FLANDRO: Everyone wondered where the solar system ended. This is where Eugene Parker, my teacher from long ago, comes into action. His work had led to a detailed understanding of the conditions at the edge of the solar system. I followed the stories of the approach of the two Voyagers to the heliopause, but things didn't happen very rapidly. There was no sudden change in physical conditions like one might expect in crossing the termination shock wave while moving toward the heliopause. Voyager 1 crossed the heliopause on 25 August 2012; Voyager 2 left the solar system on 5 November 2018.

Departing the solar system was uneventful. The JPL website has a Voyager Mission status display showing the changing speeds and distances of the spacecraft. A question frequently asked is when will the Voyagers approach another star? Some not very serious effort was given to targeting the Voyagers so that they would pass near as possible to other stars. Voyager 1 is expected to pass Ross 248, a red dwarf star in about 30,000 years. Our Voyagers will have died long since, but there may be someone waiting on a planet near Ross 248 who will intercept Voyager 1 and decipher its Golden Record (as Playboy described us in the Dark Was the Night article). Serius might be reached in 296,000 years (based on the 34,390 mph Voyager 2 velocity relative to the Sun). It is important to realize that the Voyager velocity relative to the galaxy must include a correction for the Sun's speed, which is 536,865 mph relative to the center of the Milky Way Galaxy. In 500 million years the Voyagers will complete a full orbit of the galaxy.

ZIERLER: I wonder if you have any strongly held views, based on what we learned about Pluto, whether it should be a planet or not.

FLANDRO: I do believe that Pluto is a planet. I don't think that endless arguments over the definition of the word planet accomplish anything. There is much to be learned about how Pluto and its companion Charon were formed.

ZIERLER: Now that the Voyagers are beyond the solar system, what role does gravity or gravitational assist play in their flight trajectory, where they're headed?

FLANDRO: None! They will not pass close enough to any other gravitating body (unless there's another massive planet hiding out there) with enough gravitational pull to excite any kind of a perturbation mechanism. At least not for the foreseeable future. It won't be an issue until they finally encounter other stars. By then there will be no one left on Earth to care.

ZIERLER: How are they moving now? What is the force that is propelling them?

FLANDRO: They are just coasting along on hyperbolic escape trajectories relative to the sun using the energy that been imparted to them by the Titan rockets and the subsequent planetary swing-by encounters. No more propulsion after the small midcourse nudges from the attitude control systems were completed. The gravitational force from the Sun is the only force affecting their motion.

If you compute a trajectory based on the Sun as the central body, you can accurately predict the spacecraft velocities and positions for a long time to come starting with the initial conditions at their last planet encounters. If you visit the JPL Voyager Mission Status website, you will find the spacecraft current speeds and their distances from the sun and the Earth. These are hyperbolic solar orbits. No other perturbations are going to affect them for a very long time to come. At their current distance the Sun's gravity is now so weak that their velocities are nearly constant. Recall that according to Newton's Law of Gravitation, the gravitational force is inversely proportional to the square of the radial distance, therefore their deceleration in practically zero. The current speeds of the two spacecraft are different because they started with different initial conditions after their final gravity assist maneuvers at Saturn and Neptune. Their current speeds are, Voyager 1: 38,026.77 mph; Voyager 2: 34,390.98 mph.

ZIERLER: Of course, the power systems for Voyagers 1 and 2 will not last forever. Is there any scientific value, is there anything that they could continue to teach us, even when we're no longer able to communicate with them?

FLANDRO: We require their radio signals to track them. If we can't communicate with them, then we're going to lose total contact with them. That will be in about two years or so from now. They're using a nuclear power source, and that will soon fail. When that happens, no further word will be heard from them unless they discover a new energy source that we don't know about. You never know about the Voyagers, they're pretty smart. [laughs]

ZIERLER: When the decision was made to put the Golden Records on the spacecraft, what did you think about that in terms of believing that there might be life forms that would grab them, and what was simply a message that we were sending to ourselves?

FLANDRO: The latter, despite the Playboy article, which predicted that they would be grabbed and deciphered. I thought when Ann Druyan and Carl Sagan came up with the Golden Record idea that it was a wonderful gimmick. If you think about it carefully, it is just a clever way for us to learn more about ourselves and to visualize what we might look like to some (nonexistent) alien observer.

ZIERLER: Is that because you don't tend to believe that there are life forms that are out there, or the physics of them traveling far enough to get to the spacecraft are simply not possible?

FLANDRO: The latter, but I do believe that life exists throughout the universe. I do not see how it could be otherwise. Just knowing what I do about the natural history of the Earth and the fact that any self-replicating molecule can be called a life form, makes it plain to me that life must exist elsewhere. Whenever suitable conditions exist anywhere in the universe, life will appear. Life is as much a part of the universe as stars and planets are. We may never be able to prove this.

ZIERLER: Returning to one of your main areas of research, what is the current state of play in combustion instability these days? What are the frontiers in that field?

FLANDRO: Despite almost a century of study and enormous expenditures (remember the Apollo F-1 engine problem) study of the combustion instability problem cannot, in my opinion, yet be described as a mature science. The UCDS software that we developed at GTL allows reliable prediction of system stability and guides the design of corrective procedures. These capabilities were missing only 20 years ago. We were successful only because we addressed important issues that were missed by others. To name a few, we realized that because of the nonlinear nature of the problem, acoustic waves transition into traveling shock waves, waves of vorticity and entropy must be accounted for, and vortex shedding effects must be accommodated. Nevertheless, as in all scientific fields, there is still very much to learn.

A very important question is, why do so many unsteady phenomena appear in rockets and jet engines? Why do unexpected effects such as spinning vortices and roll torques (as in my Ph.D. research) continue to catch us by surprise? One factor is that the energy density in a rocket motor is enormous. An exceedingly small fraction of that energy is sufficient to generate a multitude of unsteady flow mechanisms and oscillations. Vigor Yang (now the Chairman of the Georgia Tech Aerospace Engineering Department) showed that if only 0.01% of the energy contained in the combustion chamber at a given instant is converted to pressure oscillations, then the amplitude of the resulting oscillations may exceed 10% of the chamber pressure.

Clearly the energy released by combustion must be dissipated somehow. Flow of the hot combustion gases through the nozzle and heat transfer to the combustion chamber walls are two obvious energy dissipation mechanisms. Not long-ago rocket engine design was based on such simple steady-state notions as these. No attempt was made to address the possibility of combustion oscillations and related phenomena, mainly because no one had figured out exactly how they work.

What other mechanisms have we missed? Some recent progress has been made that will help us understand the plethora of unsteady flow effects in rockets. I believe that recent work by MIT Physicist, Dr. Jeremy England and others may show us the way. Surely one might argue that the laws of thermodynamics must hold all the answers. England has concentrated on understanding how the Second Law of Thermodynamics can often lead us to incorrect conclusions. A classical example is the conclusion that living organisms cannot appear in an isolated system because the system entropy must always increase. The creation of an organism requires that organized entities like DNA must somehow appear. Such events would lead to a decrease of the entropy in violation of the Second Law, which proclaims that left to itself the system must always become increasingly chaotic (disorganized). This is sometimes referred to as the arrow of time (the one-way direction or asymmetry of time). Creationists often appeal to this interpretation of the Second Law as irrefutable proof of the existence of God.

England's theory, Dissipative Self-Assembly, provides an extended interpretation of the Second Law. His theory is based on the concept that every process in nature is driven by the requirement that energy must be dissipated in the most efficient way. It does this by utilizing every possible physical or chemical mechanism that transports and dissipates energy. An important example being the creation of unsteady processes. This clarifies and extends the Second Law (the law of increasing entropy, or the arrow of time) in which entropy is the principal measure of any dissipative mechanism.

Let us test this idea by carrying out a thought experiment on a classical example of rocket combustion instability, the Rocketdyne F-1 engine used for our Apollo Lunar missions. The unexpected pressure oscillations experienced in the first F-1 tests starting in 1957 led to catastrophic failures. This almost led to cancellation of the Apollo program, since there was negligible theoretical understanding of such problems at the time. Thus, a long series of experimental firings was conducted to attempt gain enough physical understanding to correct the instability. Such a study was possible on liquid propellant rockets, since the engines can be shut down at the first sign of a problem detected by pressure sensors. Using information that was acquired through over 2500 full-scale test firings, there emerged an empirical picture of the phenomenon. In what follows, I will trace each phase of the extraordinarily complex set of interactions that caused the instability to see if we can clarify their origin as dissipative self-organizing events.

The cylindrical combustion chamber, with liquid oxygen and kerosene entering, mixing, and burning at the forward end of the chamber, with a choked nozzle at the aft end invites the creation of acoustic pressure fluctuations. These waves are a natural energy dissipating process. Due to the cylindrical geometry, transverse modes of oscillation are most easily excited. As energy passes to these waves from the propellant combustion their amplitude quickly grows causing nonlinear wave steepening to occur. These are shock-like traveling waves spinning around the chamber. Their presence was known both from the appearance of the wave forms detected by pressure transducers, but also from the evidence of enhanced heat transfer and shearing stresses at the injector interface. These burned, distorted, and even melted the injector face. All these features represent dissipative self-organization – a way for nature to dissipate some of the intense energy in the combustion chamber.

Another nonlinear effect of these high-amplitude waves is an increase in the engine chamber pressure that caused explosive failures in early tests before the engine shutdown equipment had been installed. This mechanism was first explored in the Caltech Brownlee-Marble experiments described earlier.

The physical evidence from the interrupted tests led the F-1 team to seek means to break-up the spinning wave motions. They did this welding baffles to the injector face to disrupt the organized unsteady flow. After 2500 trial-and-error tests they finally arrived at a baffle configuration that suppressed the instability, by the formation of vortices shed from the baffles by the flow induced across them by the spinning wave. Such vortex-shedding mechanisms are seen everywhere in nature as an energy dissipating phenomenon. The cost of making the F-1 engine work was enormous, but the Apollo program was saved.

ZIERLER: What are you most excited about for the future of space flight? What has Voyager and other missions done that allows us to think even bigger and beyond our solar system?

FLANDRO: I believe that Krafft Ehrike revealed the reasons that we continue pursue space flight when he wrote "The idea of traveling to other celestial bodies reflects to the highest degree the independence and agility of the human mind. It lends ultimate dignity to man's technical scientific endeavors. Above all it touches on the philosophy of his very existence."

I believe the topic of future space flight is currently in a state of confusion. We have watched while enormous resources and time have been wasted on space programs that are cancelled before they fly. The NASA Constellation debacle we discussed earlier is an example. After the successful Apollo missions, we were led to believe that lunar bases and trips to Mars would follow before the turn of the last century. That promise did not materialize. I would like to think that a manned Mars mission ought to be the next step. If you look closely, you will see that much of what drives space exploration has to do with the search for life in the cosmos. Robotic missions to Mars have put on the track to establishing whether life ever existed on Mars. The arguments for manned Mars missions include trying to establish a new place for mankind to live and to continue the in-person search for life there. The up-coming Europa robotic missions may lead to a more promising search for life in the oceans beneath Europa's icy surface. I don't think that establishing a base on Mars would be high on my list of priorities. But it is probably going to happen; SpaceX is committed to it. I think returning to the Moon and learning how to harvest its resources (for Helium-3 that is abundantly available in the lunar regolith) makes more sense than the Mars adventure. The Chinese are already well on the way to doing this.

ZIERLER: You mean the problems that we're facing right now suggest that we should be pouring resources into problems on Earth and not out in space?

FLANDRO: I think the way we use our resources, is embarrassingly bad. I think what we need to do is learn how to use all our resources more wisely, not just natural resources, but also political resources. Our interaction with other countries is terrible, and it's not getting any better. How can we exist as human beings in a culture like this that still can't even avoid war?

I've stayed with my technical work because I see it as one of the ways that we can go forward. But space flight itself isn't going to solve the problems. I used to think maybe it would. Krafft Ehricke sought ways to use space flight to solve societal problems. I very much liked his book Extraterrestrial Imperative. He believed that we should treat the Moon as a Seventh Continent. I think going back to the Moon (harvesting the lunar regolith for helium-3 so we can produce fusion energy and so on) might be a smarter way to use resources than going to Mars. It's popular to say, "Well, there's regolith on Mars, too." But wait! An important benefit of the Moon is that it is so close to us. Moving things back and forth to Mars is dangerous, expensive and time-consuming. Think about the logistics of supporting a base on Mars.

ZIERLER: All the problems that you listed here on Earth, as I'm sure you know, one of the arguments for colonizing Mars is it provides a Plan B for humanity. This is one of the guiding principles of SpaceX. What is your response to that?

FLANDRO: That has always been a compelling idea but making Mars into a habitable planet is a truly enormous undertaking. I won't use the word "impossible" to assess these Mars activities. I'm simply going to say that it is going to be very difficult to exploit Mars. If mankind can survive long enough, we might be able to do it. We'll make Mars a habitable planet, and could get its atmosphere built up again (Edgar Rice Burrough would be very pleased by the whole idea). Maybe SpaceX will make it work. I hope they will succeed. They are an extremely capable group of engineers.

ZIERLER: On the topic of impossible missions, I'm sure you've been following some of the amazing things that the James Webb Telescope is finding, including exoplanets.

FLANDRO: Absolutely. This kind of space exploration makes much sense.

ZIERLER: Do you ever envision a time when there could be a spacecraft that could be sent to an exoplanet?

FLANDRO: I can envision such a time, but we've got to achieve major breakthroughs in propulsion technology to make such things possible. The Voyagers have demonstrated to us the realities of interstellar flight. The truth is that if we decide to fly to an exoplanet, we will need to learn how to go much faster. The Voyagers hold the speed record for the fastest manmade vehicles. They are travelling through the galaxy at around 40,000 miles/hour. Unless you can accept flight durations of thousands of years, then you must travel at speeds that approach the speed of light, about 669,600,000 miles/hour (186,000 miles/second)! That is about 17,000 times faster than a Voyager interstellar spacecraft. Suppose you want to send a mission to Alpha Centauri (a triple star system closest to the sun). Proxima Centauri, the smallest star in the group, is a red dwarf with three known planets. One of these Proxima b (discovered in 2016) is about the size of the Earth and lies in the habitable zone, meaning it could harbor life. The distance is 4.24 light years So if you could travel at the speed of light, you could make the trip in 4.24 years. If a mission duration like that of Voyager 2 (12 years) is acceptable, then you can make the trip traveling at about 1/3 of light speed. That would be about 6,000 times the Voyager speed. This looks like a great exploratory mission if you can find a way to go that fast. The other problem is that if you are going that fast you will have some trouble trying take pictures as you flash on by. If you slow down to rendezvous with Proxima b, the flight duration will become about 25 years. Assume that we have somehow created the necessary propulsion technology. Say we want to look for life on Proxima b. The government will never support it. One thing I can safely predict is that the price-tag for light speed propulsion will be enormous. Interstellar flight will not be easy to accomplish. It's very much harder than colonizing Mars.

ZIERLER: Flying at the speed of light, what does the physics of that even look like?

FLANDRO: Talk to the black hole guys; they study things like this. This is Kip Thorne territory, a very well-known Caltech physicist who did a much work on such matters. He was born in Logan, Utah where my grandmother grew up. He is a 2017 Nobel Laureate. If you haven't already interviewed him, then he should be on your list for the Caltech Heritage Project. When it comes to interstellar flight, propulsion is the limiting technology. We simply have no idea how to do it. Maybe worm holes? Even Captain Kirk can't help with this. I am sure that the person who will make the necessary propulsion breakthrough has not yet been born. Rest assured she (or he) will arrive if humanity survives. For now, using the Webb Telescope, we can acquire much new and more detailed information about exoplanets without the use of human interstellar travel.

ZIERLER: On that note, I have two final retrospective questions that I'll ask you to put your thinking cap on. As you mentioned, war is such a problem right now. It's really an inhibiting factor to planetary science and to space science. But of course, you're a child of the Cold War, and so much of what was possible at NASA at JPL was through military funding because of the Cold War. I wonder if you could square that circle, how so much was possible as a result of the Cold War when you were coming of age, and yet you feel so negatively about the impact of war and international strife on current space flight prospects.

FLANDRO: The Cold War did provide us some incentive for space flight because the Russians were beating us at every turn. They built the first Earth orbiter; they made the first manned orbital flight. They took the first pictures of the far side of the Moon. They built the first spacecraft to land on Venus. They were ahead on about everything. They had a magnificent program going for a manned mission to the Moon, the N1 rocket program. There was a race ahead of us. President Kennedy accepted the challenge and committed us to go to the Moon. This is what gave NASA the funding that was needed to compete with the Russians. The competition aspect of the Cold War gave impetus to our space program. It worked beautifully.

It was a close thing, though. The Russians could have pulled it off. Their vehicle design was very good, and their liquid propellant rocket engines were very much better than ours. Russian NK-15, NK-33, and RD-180 staged combustion closed cycle engines are the best liquid propellant rockets engines ever made. Their program started four years after the American Apollo program. It was both rushed and underfunded. The death of Korolev, their "chief designer," led to many problems. Some difficulties arose because of the use of multiple rockets engines. There were 30 engines on their big N1 heavy-lift first stage (while we used five Rocketdyne F-1 engines on the Saturn V first stage). The use of multiple engines led to control system difficulties that were never solved. The Russian development philosophy was to get the hardware out for flight test without first conducting extensive component testing as is done in American projects. Thus, they began flight testing of the N1 before any full-scale static tests. In the first flight, in February 1969, a few seconds after liftoff the control system shut down one of the NK-15 engines leading to a complex series of failures that caused a fire and total loss of the control system. The first stage shut down and the vehicle exploded on the launch pad. The Russians had three more N1 failures not unlike the first one. This led the Kremlin to cancel their Moon program and ordered the destruction of their entire stockpile of beautiful rocket engines. It was much later discovered that Korolev's successor, Kuznetsov, had secretly moved many these engines to a secret site, which was revealed in 1990 to American visitors from the Aerojet General Company (the company founded by Theodore von Kármán). Thus began the sale of Russian rockets to the United States. Unfortunately, they have now stopped selling their engines to us in retaliation for our sanctions against them for Ukraine.

ZIERLER: For my last question, one that you might have thought about—it's impossible to answer, and that's why I'm going to ask you anyway. It's one that is always relevant in moments of great discovery. Going back to your work at JPL and the discovery of the gravitational assist, what aspects of it do you think in retrospect were so obvious that somebody would be bound to discover it, if not you, and what aspects were a combination of serendipity and good luck and being in the right place at the right time, that made it have to be you, at that place, at that time?

FLANDRO: Good question. I think that if I had not been there, it is certainly possible that that mission opportunity might have been missed. On the other side of the coin, eventually someone would most certainly have noticed the fortunate Grand Tour planetary configuration. I knew of several other groups working on Earth-Jupiter-Saturn trajectories, and it would have been a natural thing for them to find the extension to Uranus and Neptune. In fact, several of them came forward after Joe Cutting and I had announced our results. They claimed that they had thought of it too, but never got around to working out the details. We just happened to be the first to take it seriously enough to take on the hard work involved. I'm not saying that our contribution was any sort of special key to the whole thing, because someone else would surely have done it. Whether they would have done it in time is the essential question.

ZIERLER: Right. Because any more delay and it's possible that the window to build Voyager and launch it would have simply been too small, if it wasn't you.

FLANDRO: Yes, we had just the right amount of time to sell the mission to the scientific community, to get JPL interested in it as a project, and to get the spacecraft people at JPL motivated enough to design the spacecraft. Just enough time - just barely enough time.

ZIERLER: That certainly makes "timing is everything" a bit of an understatement, doesn't it?

FLANDRO: It does that indeed! Thank you for your insight.

ZIERLER: On that note, it has been a tremendous pleasure spending this time with you. I'm so glad we were able to do this and capture your memories for history. Thank you so much.

FLANDRO: Thank you for giving me the opportunity to participate.

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