Secret History

Dr. Richard P. Hallion is The Air Force Historian for the United States Air Force. Previously, Dr. Hallion was curator of science and technology at the Smithsonian Institution. NOVA spoke with him on the eve of the 50th anniversary of the breaking of the sound barrier.

NOVA: How early on did the Americans get involved in pursuing high-speed flight?

HALLION: The American experience in high speed flight is quite interesting. In the 1920s and '30s, what we often term as the Golden Age of Aviation we saw radical transformations in aircraft design. We went from the era of the wooden airplane to the all-metal airplane. We went from the era of the biplane to the era of the monoplane. We went from the era of relatively crude propulsion technology to the era of very reliable engine combinations. We went from an era in which people did not use the airplane for commercial purposes to an era by the early 1940s in which commercial aviation was very widespread. So this is a radically transforming time in aviation. In the 1920s, a great number of researchers started looking at the problems of flow around propellers, because propeller tips were beginning to approach the speed of sound as the propeller rotated. A propeller is, after all, a rotating wing. And the connection was made in the minds of researchers that if a propeller encounters disturbing flow conditions at high speeds, then obviously at high speeds the wing of an airplane would as well.

NOVA: Was the quest to break the sound barrier initially motivated by possible military applications?

HALLION: Absolutely not. In fact, in the early 1930s the initial thought on this was that this will improve the effectiveness of commercial aircraft. They will be able to operate at higher altitudes and speeds. At higher altitude, where your density is less, you can fly further or the same amount of energy, therefore, they will be more fuel efficient and they will be more payload efficient. And those are strictly commercial questions. Now you can turn them around and say, well, if you can carry greater payload and you can carry it more effectively that payload may be airmail or it may be bombs. So there's a military issue here obviously as well. But people, at least in the 1930s, were thinking less of military applications than commercial applications.

NOVA: At some point countries around the world began working in earnest on the problem of breaking the sound barrier. What prompted that stepped-up interest?

HALLION: The whole problem of high speed flight was of relatively academic interest until the mid 1930s. But in the mid 1930s two developments occurred that made it much more of a concern. First, we had the appearance of the jet engine. That opened up to us the potential of practical high speed flight. But the second event that occurred was the beginning of a series of mysterious and indeed alarming accidents involving high speed airplanes. And this started in 1937 with the accident of a experimental German fighter, a new German fighter in service, the Messerschmidt 109, which clearly came apart in flight because the pilot had experienced some major problem with aircraft control as he dove to higher and higher speeds. Very quickly those problems started to appear on other high performance military airplanes in the other nations of the world. Obviously what we were dealing here with was an international challenge; we're no longer confronting merely a barrier in the mind or a barrier that's a theoretical construct. Now we were seeing it as an actual physical barrier that would have to be overcome.

NOVA: The obstacles to breaking the sound barrier were the aircraft design itself and the building of a propulsion capable of pushing a plane that fast. Would you say that those were equal challenges?

HALLION: The propulsion challenge was not one that was as limiting as the aerodynamic challenge. The propulsion challenge was resolved by the development of the jet engine, and also by the availability of the rocket engine. And it was a matter from that point on of merely growing the maturity of those two systems. The real challenge, because there were many unknowns, and many grave difficulties, was the aerodynamic challenge—what actually happens to an airplane as it approaches closer and closer and hopefully eventually passes through the speed of sound. The flow conditions around the airplane change completely. It catches up in flight with its own pressure signals that are descending ahead of it, so to speak. It's like a ship catching up with its own bow wave and then passing through it. As a result of this, there was a very great interest in studying the phenomena of what was called trans-sonic flight—flight between the sub-sonic and super-sonic region. (See Sonic Boom)

NOVA: How did researchers go about investigating trans-sonic flight in those early days?

HALLION: The traditional tool for that was the wind tunnel. Unfortunately, wind tunnel technology at that time did not predict accurate and reliable measurement of flow conditions around aircraft at speeds in the region of the speed of sound. This was so because models in a wind tunnel would be exposed to air flow that would generate shock waves. These shock waves would reverberate and reflect across the test section of the tunnel and all subsequent measurements would be largely invalid. So there was a sizable gap that ran from approximately 75 percent of the speed of sound—what we refer to as mach .75—to about 1.25 times the speed of sound that was really an area that needed to be explored and understood. And it was to research that area that the British government and the United States government and indeed other countries as well, pursued the development of specialized research airplanes that would be instrumented to record and take data on flow conditions at those speeds. In effect these airplanes were to be research tools using the sky as a laboratory.

NOVA: In what ways did the success of the X-1 program give us an edge over other countries? (See Men of the X-1)

HALLION: The X-1 program, first of all, demonstrated that with appropriate design technology one could design a airplane that could pass through the speed of sound and not merely survive but be fully functional and fully controllable while doing so. Chiefly it demonstrated the following: that one needed to have a very smooth body shape, what we call a relatively high fineness ratio fuselage, a body that is long and slender. It's interesting that we saw that because if we look at the X-1 now, it has almost a pudgy look, if you will, compared to modern high performance airplanes. But for its time it was radically streamlined. It was based after all on the shape of a bullet. The other lesson that came out of the X-1 was that high performance wing design should emphasize thin wings that have what we call relatively low aspect ratios. In other words, the wings should be fairly short and thin. That enables you to fly faster and more effectively. Another design element that came out of the X-1 was that you needed to have large fully controllable tail surfaces. The ability to maneuver the entire tail surface of the X-1 played a key role in getting the X-1 through the speed of sound safely and controllably.

NOVA: At the time of the X-1 program, were there concerns that other countries were spying on us?

HALLION: During the second World War, even as the war was going on, we were already seeing some of the hallmarks of the Cold War. Namely we were seeing espionage directed against the United States by the Soviet Union, and we were seeing a counter-intelligence effort by the United States to try to find out what the Soviets were up to in terms of what they were trying to learn about us. So during the second World War we had the beginnings of a program that had some tremendous significance. It was called Venona, and it began in February 1943. It was run by the U.S. Army's Signal Intelligence Service. That's a forerunner of the present day National Security Agency. The purpose of Venona was to examine and possibly exploit encrypted Soviet diplomatic communications. Many messages were accumulated by the Venona team, but because these were encrypted it was very, very difficult to translate them. And many of the wartime messages were in point of fact not translated until after the war. From our translation activities of Soviet communications we learned that there was a very active effort by the Soviets to collect information on the United States.

NOVA: So we didn't find out about the Soviet espionage until after the war?

HALLION: Because of the volume and the nature of the traffic, many of these messages were not able to be broken until after the second World War, simply because the process of breaking them was so difficult. They were all encrypted in a cipher system. We had to break the cipher systems, and we had to find the keys in order to break those. Now what was very interesting, as we found out later, was that there were a number of people in various key government organizations that were targeted by the Soviets to be sources of information or who in fact, were themselves Soviet agents. One of these was an individual working in the National Advisory Committee for Aeronautics. His name was William Perl. William Perl was part of a spy ring established by Julius Rosenberg. Now the Rosenberg spy ring has always been thought of primarily as an atomic espionage ring. But in point of fact in its early years it was targeting the aeronautical industry and the electronics industry. As early as 1943, Perl was passing information to the Soviets on jet engine design. Perl later joined the Louis Research Center of the NACA, now NASA, and while there was engaged in the design of supersonic wind tunnel facilities, consulting on engine development and also did a lot of work related to the atomic airplane program. So Perl was a very highly placed source of information for the Soviets and was transmitting a great deal of information to them.

NOVA: How did the Soviets benefit from this information?

HALLION: There's a technology transfer that you see very clearly. The Mig fighter family is the classic example to use. If you take a look at when the designs of the Mig 15 and 17 are actually fixed (and they're developed in the immediate post World War II era) I think that the Soviets were not able to get the information that they needed in time to make those aircraft what they could have been. Instead where we see this [espionage] material radically transform Soviet military aviation is in the next generation of Mig aircraft, the Mig 19. The Mig 19 is the first Soviet supersonic jet fighter. It appears contemporaneously with the first American supersonic jet fighter, the F-100. They both appear in 1953. They both have roughly the same performance capabilities. In fact, one could argue that the Mig 19 actually had a slightly higher performance. And so what this shows is the gap closed.

NOVA: What was Britain's relationship with the Soviet Union during and after the war?

HALLION: Britain's relationship with the Soviets in the 1940s was a very interesting one. At the same time that we see Winston Churchill giving us the Iron Curtain metaphor, and we start seeing the emergence of the Cold War, we see the British government willy nilly selling high technology to the Soviets. And the classic example of what they sold were two high performance jet engines, the Rolls Royce Durwent and the Rolls Royce Nene. And the Nene engine, interestingly enough, in Korea, powered not only the Mig 15, but it also powered some of the American Airplanes (because it had been sold to the United States in license built form) that we were using against the Mig 15. For example, in Korea, in November, 1950, a U.S. Navy fighter airplane called the F 9 F Panther confronted the first production Soviet Mig 15's that were being flown in Korea—the two airplanes were flying using essentially the same engine. You could have interchanged the engines in these airplanes.

NOVA: Looking to the future, is it safe to say that the ability to fly faster than sound is, as far as modern military aircraft are concerned, a necessity?

HALLION: It's a sine quo non. It is absolutely critical. Supersonic flight, which has been very important to us in engagement since we first became a supersonic force, is as important now as it has ever been. Indeed, I would argue it is more important now. For rapid response, for getting into a hostile area and getting out, and for achieving military effects quickly, nothing beats supersonic speed. Indeed the tendency ultimately within the next 30 years will be for hypersonic speed.

NOVA: Define hypersonic.

HALLION: Moving on the order of five times the speed of sound. Now you may not be doing this with a piloted system or you may be using a supersonic airplane firing hypersonic weapons, but engagement now in the terminal arena where that engagement takes place—I think we will see engagement eventually moving into the hypersonic arena very, very quickly. Extremely quickly.

Photos: (1) U.S. Air Force; (2-3) UPI/Corbis-Bettmann; (4) Corbis-Bettmann; (5) Bell Helicopter Textron Inc.