Scramjets, the engines that burn fuel in air rushing faster than sound
A scramjet has no fans, no turbine and no moving parts at all. It relies on sheer speed to squeeze incoming air, then burns fuel in a stream still moving at supersonic pace. The catch is that it cannot start until something else has already hurled it past about Mach 4.
An ordinary ramjet uses shock cones to slow incoming air below the speed of sound before burning fuel in it. A scramjet skips that step, letting shockwaves from its own ignition do the braking, which keeps it efficient at extreme speeds. The layout is simple: a narrowing inlet compresses the air, a combustor burns gaseous fuel with atmospheric oxygen, and a widening nozzle accelerates the hot gas into thrust. Because it breathes air rather than carrying an oxidiser as rockets do, it only works inside the atmosphere.
Engineers expected hypersonic airliners soon after the Bell X-1 broke the sound barrier in 1947, yet most aircraft still top out between Mach 1 and Mach 3. Heat is a big reason. At these speeds air friction scorches the nose, wing edges and intakes, which demand costly special materials. In November 1964 Antonio Ferri got a ground-test engine to produce 517 pounds-force of thrust, about 80 percent of his target.
The first real flight came in 1991, a joint effort with NASA over the Soviet Union using a hydrogen engine built in Moscow and carried atop a converted missile to Mach 5.5. A British and Australian team proved combustion in the open atmosphere with HyShot on 30 July 2002. The X-43A flew under scramjet power in 2004, its final run touching Mach 9.6, and a Mach 10 flight followed on 15 June 2007 with DARPA and Australian scientists.
Duration came next. On 27 May 2010 the X-51A Waverider, dropped from a B-52 and boosted by a rocket, held Mach 5 for about 200 seconds, a record for sustained hypersonic flight, and a 2013 flight lasted six minutes. India's space agency ISRO fired twin scramjets for around 5 seconds at Mach 6 on 28 August 2016.
Source: Scramjet
