Breaking the barrier

“You’ve never been lost until you’ve been lost at Mach 3.” – Paul F. Crickmore

(test pilot)

Until about the start of the Second World War the strange phenomena which develop at transonic speeds were academic, since the propeller aircraft of the day did not fly that fast. But starting in 1937 mysterious accidents began to occur at high speeds. An experimental early version of what was to become Germany’s most potent fighter of the war, the Messerschmitt Bf 109, disintegrated as its pilot lost control in a fast dive. Pretty soon other new, high performance military airplanes were running into similar difficulties. For these fast, propeller-driven fighters the airflow over the wings could achieve Mach 1 in a dive, making air compressibility a real rather than a theoretical issue.

In the US, the aeronautics community was rudely awakened to the realities of this unknown flight regime in November 1941 when Lockheed test pilot Ralph Virden was unable to pull his new P-38 fighter out of a high-speed dive and crashed (due to the problem of ‘Mach tuck’ described in a previous chapter). It became clear that any propeller fighter pilot who inadvertently pushed his fast plane into a steep dive was risking his life. Aggravating this problem was that bombers were flying ever higher, which meant that in order to reach their prey interceptors had to venture into the thin, cold air of the stratosphere in which the speed of sound was lower, and thus issues of air compressibihty occurred at slower flight speeds than they did when flying nearer the ground.

A really thorough understanding of high-speed aerodynamics was initially not necessary, because measures to prevent control problems focused on limiting the dive speed and temporarily disrupting the airflow to prevent shock waves from forming on the wings and controls. Due to the limitations of propellers and piston engines, it was accepted that conventional aircraft would never be able to fly faster than sound.

Then jet and rocket aircraft appeared. These were quickly realized to require the potential to fly at Mach 1 or even faster for extended durations, so a real understanding of transonic and supersonic aerodynamics rapidly became a ‘hot’ issue that promised real military advantages. The name ‘sound barrier’ had been coined by a journalist in 1935 when the British aerodynamicist W. F. Hilton explained to him the high-speed experiments he was conducting. In the course of the conversation Hilton showed the newsman a plot of airfoil drag, explaining: “See how the resistance of a wing shoots up like a barrier against higher speed as we approach the speed of sound.” The next morning, it was incorrectly referred to in the newspaper as “the sound barrier”. The name caught on because the issues which conventional high-speed aircraft invariably encountered on reaching transonic speeds gave the impression that the magic Mach number was indeed a barrier that would need to be overcome.

It not only represented a physical barrier, but also a psychological one: there were many skeptics who said supersonic flight was impossible because aerodynamic drag increased exponentially until a veritable wall of air emerged. They pointed to the loss of the second prototype of the de Havilland DH 108 Swallow on 27 September 1946. This high-speed jet disintegrated while diving at Mach 0.9, killing pilot Geoffrey de Havilland Jr., and crashing into the Thames Estuary. Shock stall had pitched the nose downwards, and the resulting extreme aerodynamic loads on the aircraft cracked the main spar and rapidly folded the wings backwards. Others, however, noted that rifle bullets could fly at supersonic speeds, so the sound barrier was not an impenetrable wall. Indeed, during the war it had been realized that streamlined bullet shapes were ideal for supersonic speeds. This is how rockets such as the German А4/ V2 got their familiar shape. The V2 achieved Mach 4 as it fell from the sky towards London. In fact, because it fell faster than the speed of sound there was no audible warning of the imminent danger until the impact reduced whole blocks of houses to rubble; only afterwards did the sound arrive. But aircraft need wings, rudders, ailerons and other devices to develop lift and facilitate control, making their aerodynamics much more complicated than those of bullets and rockets. The traditional tool for gathering aerodynamic data and developing new aircraft and wing shapes was the wind tunnel. However, the technology available at that time did not permit accurate and reliable measurement of airflow conditions at transonic speeds: the aircraft models placed in the wind tunnels would generate shock waves in the high-speed air flowing around them, and these in turn would reverberate and reflect across the test section of the tunnel. As a result, there was a lot of interference and the measurements of the model did not correlate to the real world in which aircraft flew in the open air rather than in an enclosed tunnel. Also, you can scale an aircraft but you cannot scale the air, so air flowing around a small-scale model does not necessarily behave in the same way as air flowing around a real airplane.

The least understood area was from about Mach 0.75 to 1.25, the transonic regime where the airflow would be unstable and evolve quickly, and for which no accurate aerodynamic drag measurements and theoretical models were available. It was called the ‘transonic gap’; the aerodynamicists nightmare equivalent to the ‘sound barrier’ so dreaded by pilots. The aerodynamic drag is especially high in this range of speeds, peaking at just below Mach 1. However, it actually diminishes considerably at higher supersonic speeds (which is why modern aircraft either fly well below or well above Mach 1, spending as little time and fuel as possible at transonic speeds). Drag occurs at transonic speeds for two reasons: firstly as a result of the build-up of shock waves where the airflow reaches Mach 1 (typically over the wings), and also because the air behind the shock waves often separates from the wing and creates a high-drag wake. At even higher speeds the shock waves move to the trailing edge of the wing and the drag-inducing air-separation diminishes and finally vanishes, leaving only the shock waves.

All major military powers realized that if their aircraft were to remain state-of- the-art and competitive, then transonic aerodynamics was an area that really needed to be explored and mastered. Specialized and heavily instrumented research aircraft would be needed, speeding through the real atmosphere rather than a wind tunnel. In effect these airplanes were to be flying laboratories. In the US, work was started on the Bell X-l, the first of the famous X-plane series and the first aircraft to break the dreaded sound barrier. The Russians initiated their transonic research using the captured German DFS 346, but soon moved on to designing their own aircraft.

The UK started development of the Miles M.52 research aircraft in 1943. It was to be powered by an advanced turbojet (because the British had considerable experience on such engines, and little on rockets). The jet’s fuel economy meant it would be able to take off using its own power. The M.52 might have become the first plane ever to exceed Mach 1 if the secret project hadn’t been canceled by the new government in early 1946, weeks before completion of the first prototype for subsonic testing. Apart from dramatic government budget cutbacks, one reason for the cancellation was that, based on captured German research, it was feared that the M.52’s razor sharp but straight wings were unfit for high-speeds and that swept-back wings were a must for supersonic flight. The Miles engineers had thought about a delta wing for the M.52 during the war, but discarded it as being too experimental for their short-term project. However, the Bell X-l did not have swept wings either, and both aircraft used all-moving horizontal stabilizers to preclude shock-stall problems (interestingly, the Bell engineers got the idea for the special tailplane from the M.52 team during a visit to Miles Aircraft in 1944). Had the British continued their ambitious project, they could have beaten the US in breaking the sound barrier: in 1970 a review by jet engine manufacturer Rolls Royce concluded that the M.52 would probably have been able to fly at supersonic speeds in level flight.

A 30%-scale radio-controlled model of the original M.52 design powered by an Armstrong Siddeley Beta rocket engine and launched from a de Havilland Mosquito did reach a speed of Mach 1.38 on 10 October 1948. This was quite an achievement, but by then the manned X-l had stolen the show with its record-braking Mach 1- plus flight over the desert of California about a year earlier. In spite of the UK’s prowess in aeronautical design and records, there never would be a British counterpart to the X-plane series of the Unites States.

The development of specialized rocket aircraft purely to reach extreme speeds and altitudes went in parallel with that of the rocket/mixed-power operational interceptor. However, where the interceptors needed only to go as high as the maximum altitude that enemy bombers could achieve, there were no limits for the experimental aircraft: they were meant to provide information on entirely new areas of aerodynamics and aircraft design, and their designers and pilots kept on coaxing ever more impressive performance from them. Rocket engines proved to be very appropriate for propelling research aircraft up to extreme speeds and altitudes, since

endurance was not of great importance. Rocket engines were light and relatively simple compared to jet engines of similar thrust, and because they did not need air intakes this made it much easier to design airframes suitable for supersonic flight. From the 1940s through to the late 1960s the rocket propelled X-planes achieved velocities and altitudes unrivaled by contemporary jet aircraft, with some of their pilots gaining ‘astronaut wings’. The rocket powered interceptor turned out to be a dead end but the early rocket research aircraft led to the Space Shuttle and the current designs for future spaceplanes. New experimental rocket planes are still being developed, although when they fly they are often unmanned.

If flying mixed-propulsion interceptor prototypes was a risky business, then the pilots of early experimental research rocket aircraft had a truly dangerous job. These aircraft were, by definition, going beyond the known boundaries of velocity, altitude and aerodynamics; what pilots refer to as “pushing the envelope”. Such planes had to incorporate new, often hardly tested, technology such as experimental wing designs, powerful rocket engines and innovative control systems. Unsurprisingly, whilst being tested several of these experimental aircraft crashed, blew up, or were ripped apart by aerodynamic forces. There were no accurate computer simulations and knowledge databases to warn of design errors, incorrect assumptions and unexpected situations that are nowadays resolved long before a new airplane makes its first test flight. In fact, the research aircraft of the 1940s, 1950s and 1960s were providing the data required to set up such models, and they had to obtain it the hard way. Modern aerodynamic design tools still depend on the experience gained in those years.

In addition, the means of escaping from a plane heading for disaster were much more limited than for today’s test pilots, who have sophisticated avionics on board to tell them what is happening to their aircraft, and reliable ejection seats which permit a bail-out at any speed and altitude. In a recent interview for NOVA Online, Chuck Yeager, the first man to break the sound barrier in the X-l, summarized the test pilot philosophy of time as follows: “Duty above all else. See, if you have no control over the outcome of something, forget it. I learned that in combat, you know… you know somebody’s going to get killed, you just hope it isn’t you. But you’ve got a mission to fly and you fly. And the same way with the X-l. When I was assigned to the X-l and was flying it I gave no thought to the outcome of whether the airplane would blow up or something would happen to me. It wasn’t my job to think about that. It was my job to do the flying.” The urgency of Cold War developments, as well as an acceptance of loss of life ingrained into pilots and aircraft developers during the Second World War, meant high risks were taken and many test pilots perished as their new aircraft succumbed to some overlooked detail in the design.