Lately, a hot topic of conversation in military and technology circles is the development of practical hypersonic air vehicles. It is interesting to consider what makes a hypersonic aircraft or missile so much different than a simply supersonic vehicle. Supersonic flight speeds are airspeeds above the local speed of sound, which varies with altitude and temperature, but is roughly 1,100 fps (761 mph) at standard sea-level temperature and pressure. In general, the speed of sound decreases with altitude as the ambient air temperature drops, but Mach 1.0 is still generally in the neighborhood of 700 mph. Scientists and engineers have largely agreed that "hypersonic" flight begins at Mach 5.0 (roughly 3,500 mph) and above. To appreciate that speed, consider that the average hunting rifle bullet only travels around 2,000 mph.
When an object travels through the atmosphere faster than the local speed of sound, a shock wave forms either immediately in front of the object or obliquely along its surface, depending on the shape of the object. These "normal" or "oblique" shock waves change the local temperature and pressure of the airflow as it passes through the shock.
The traditional jet engine travelling at supersonic speeds must decelerate the air flowing into it to subsonic speed before fuel can be injected and burned. As the fuel burns the combustion gases are accelerated out the tailpipe and produce net thrust. Depending on the design of a supersonic fighter jet, the engine inlet air is decelerated to subsonic speed through a single normal shock wave or a mix of normal and oblique shock waves as the air enters the engine. This arrangement works up to about Mach 2.5, which is why so few aircraft fly much faster than that.
Beyond around Mach 3.2, it becomes impractical to decelerate the engine inlet air to subsonic speeds before mixing in fuel--the engine combustors must instead burn fuel within a locally supersonic airflow. This is the design challenge! It is very difficult to establish a stable and thorough fuel burn within supersonic flow, although recent technological breakthroughs have made it more practical. The engines using this new technology are called SCRamjets (Supersonic Combustion Ramjets) and can push aircraft well beyond Mach 5.0.
For most SCRamjet equipped aircraft, the maximum aircraft speed is limited not so much by engine thrust, but by aerodynamic heating of the airframe. Whenever air passes through a shock wave--normal or oblique--the local air pressure, density and temperature increase. The magnitude of the temperature increase is proportionate the speed of the airplane. Consider the windshield of the now-retired USAF SR-71 Blackbird reconnaissance plane, an aircraft with a maximum cruising speed of Mach 3.2 at high altitude. At Mach 2.0, aerodynamic heating raised the Blackbird's windshield temperature to over 250 degrees Fahrenheit. At Mach 3.2, the Blackbird's special quartz windshield temperature was over 600 degrees Fahrenheit, solely due to aerodynamic heating. Other parts of the airframe are subject to similar heating effects, with leading edges generally getting hottest. For a SCRamjet aircraft operating continuously at Mach 5.0, leading edge temperatures would soar to over 3,000 degrees Fahrenheit. Obviously special materials and cooling provisions are required to sustain flight at that speed for any length of time.
Hypersonic aircraft and missile design is currently thriving as engineers continue to overcome the technical, material, and thermodynamic challenges unique to hypersonic flight. It’s exciting to imagine the phenomenal aircraft that will be developed in the coming years! Thanks again for reading this blogpost!
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