[55] No manned supersonic aircraft fought in World War II. In fact, the first manned supersonic flight had to wait until October 1O, 1947, when the Bell X-1 rocket plane exceeded the speed of sound. Nevertheless, the German V-2 ballistic missile penetrated the hypersonic range early in the war. The first V-2s fell on England in 1944 at speeds of Mach 5 (3400 mph) and were completely invulnerable to fighter interception. When the Allies captured the V-2 test facilities at Peenemunde on the Baltic, they discovered, to their surprise, a 0.4-meter (1. 2-foot) wind tunnel that could attain Mach 5 on an intermittent basis. In addition, a 1-meter (3.3-foot) continuous-flow tunnel capable of Mach 10 was under construction for the purpose of testing the German A-9 and A-10 intercontinental ballistic missiles destined for the bombardment of the United States. Hypersonic flight had thus leapfrogged the supersonic range. There was much debate about whether ballistic missiles would ever amount to much in a military sense, but the technically astounding V-2s made it imperative to at least explore this new range of flight.
There is no clear-cut beginning of the hypersonic range of flight. Generally, speeds above Mach 5 are considered hypersonic. This is the speed at which aerodynamic heating becomes important in aircraft design.
Hypersonic wind tunnels, like their supersonic cousins, employ the expanding nozzle principle to accelerate subsonic air to speeds faster than sound. Of course, the area ratio of the nozzle is much greater for hypersonic tunnels because the Mach 1 air at the nozzle throat must be accelerated so much more. To attain Mach 5, an area ratio (test section area divided by nozzle area) of 25 is required. The ratio jumps to [56] 536 for Mach 10. Consequently, hypersonic test sections are fed through tiny nozzles that expand into grossly larger test sections. Pressure ratios must rise dramatically too-from 1.1 at Mach 1, to 20 at Mach 5, and 350 at Mach 10. Such high-pressure ratios increase the number of compressor stages and naturally demand more power. The hypersonic power requirements for continuous operation are so large that intermittent wind tunnels are common. In the intermittent tunnel, energy is stored, usually as compressed air, and then released suddenly to force a large quantity of air through the diminutive throat of the nozzle in a short period of time.
So far, these considerations seem just simple extrapolations of supersonic tunnel design. But a new factor emerges as the tunnel air accelerates to hypersonic velocities: The air temperature drops dramatically as the air's latent heat is transformed into energy of motion. In a Mach 5 tunnel, for example, air at 200° F in the settling chamber before the nozzle will cool to - 350° F in the test section. This is close to the point at which air liquefies-not condensation of contained moisture but actual liquefaction of the air itself. To prevent liquefaction, tunnel air must be heated before it enters the settling chamber. In a Mach 10 tunnel, for example, the settling chamber air will typically be at a temperature of 3000 ° F and a pressure of 100 atmospheres. The hypersonic tunnel therefore resembles a rocket engine-a hot, highpressure one-although the ultimate energy source is not rocket fuel but stored compressed air.
