114 An uncomfortable realisation in the West that it is trailing both Russia and China in the technologies of flight at hypersonic speeds has goaded government agencies, giant aerospace corporations and even a few start-ups into renewed action, but they face formidable engineering challenges (writes Peter Donaldson). Development of hypersonic air vehicle concepts, ranging from airliners and runway-launched spacecraft to military reconnaissance vehicles and missiles, have been pursued in fits and starts for decades. Until very recently, however, flights at such speeds had lasted less than a handful of minutes and were made by small, experimental, uncrewed vehicles dropped from aircraft. That all this has changed became glaringly obvious last January, when Russia announced the entry into service of the 3M22 Zircon cruise missile, a weapon propelled by what is believed to be a supersonic combustion ramjet (‘scramjet’) engine and reportedly capable of Mach 8 or 9 with a range of up to 1000 km. There’s nothing like a bit of strategic panic to spur technological development. Hypersonic flight is usually considered to begin at Mach 5. While rockets achieve this easily, it is much harder for vehicles powered by air-breathing engines and lifted by wings, because of thermodynamic and aerodynamic effects that become increasingly dominant at this speed. One of these effects is aerodynamic heating, caused by the friction between the aircraft’s skin and the air, which can generate temperatures so high and temperature gradients so steep that only specially formulated alloys and composite materials can provide structural integrity. Even then, proper thermal management is essential to protect the airframe, engines, avionics and other systems. It is just as vital in the engine, as the air heats up when it is slowed and compressed by being forced into the intake, so much so that vital components in turbine engines, such as compressor blades begin to fail around Mach 3. Strategies to minimise heat build-up in engine intakes broadly fall into the camps of avoiding an excessive rise in pressure by keeping airflow though the engine supersonic, which happens in scramjets, and active cooling of the intake flow with a heat exchanger, which is what Reaction Jets does with its Synergetic Air Breathing Rocket Engine (SABRE). Generally, air-breathing hypersonic engines cannot produce thrust when stationary and do not start working until the aircraft is already at supersonic speed. Zircon gets around this by being launched by a rocket. For reusable vehicles with larger payloads, however, some of the R&D focus is on powerplants that produce sufficient thrust over the entire speed range, with variable or combined-cycle engines such as turboramjets and the above-mentioned SABRE, for example. Shockwaves and boundary-layer interactions become more complex at hypersonic speeds, with shockwaveinduced drag affecting aircraft performance, stability and control. Hypersonic speeds also present problems for sensors and communication systems through vibration, shock and heating, along with perhaps less obvious sources of trouble. Communications can be affected by Doppler shift, signal attenuation, plasma formation (which can cause comms to black out) and electromagnetic interference. Very high speeds also demand high-rate communication of sensor data and accurate antenna pointing/beam steering. Doppler shifts can also affect radar and electro-optical sensors, while turbulence, thermal effects, air compression, aerodynamic flow and shockwaves will cause optical aberrations, defocusing, secondary radiation and boresight errors if not properly accounted for. In all, hypersonic flight brings a feast of juicy problems for engineers to sink their teeth into. Proper thermal management is essential to protect the airframe, engines and avionics February/March 2024 | Uncrewed Systems Technology PS | Hypersonic flight Now, here’s a thing
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