A scramjet (supersonic combustion ramjet) is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to compress the incoming air forcefully before combustion (hence ramjet), but whereas a ramjet decelerates the air to subsonic velocities before combustion using shock cones, a scramjet has no shock cone and slows the airflow using shockwaves produced by its ignition source in place of a shock cone. This allows the scramjet to operate efficiently at extremely high speeds and does not require any moving parts. The operating regime and design of the scramjet results in great practical and technical challenges in their implementation. Because they can only operate in hypersonic airflow (>Mach 5), the aircraft to which they are attached must attain a hypersonic airspeed before the scramjet can be enabled. Any craft moving at such velocities is subject to a great deal of aerodynamic heating, necessitating the use of specialized (and expensive) materials in the construction of the craft, particularly at the nose, leading edges of the wings, air intakes, and within the engine. The engine's fuel has only a few milliseconds of residence time to complete combustion, foregoing the use of complex hydrocarbon fuels (like kerosene) which take a long time to completely burn. Not only must the fuel be chemically simple, it must also have a favorable energy-to-weight ratio, and preferably a high thermal capacity (so it can be used as an airframe coolant prior to combustion). The scramjet's operating environment exceeds the specifications of traditional jet engine control equipment, thus providing an additional design challenge. These challenges have hitherto prevented the widespread use of scramjets; they have so far mostly been demonstrated in research test articles, on experimental vehicles, and a handful of military aircraft. Proposed applications (if technical challenges were resolved) include high-speed intercontinental transport and space launch, to which they would be incredibly well-suited.
History
Before 2000 The Bell X-1 attained supersonic flight in 1947 and, by the early 1960s, rapid progress toward faster aircraft suggested that operational aircraft would be flying at "hypersonic" speeds within a few years. Except for specialised rocket research vehicles like the North American X-15 and other rocket-powered spacecraft, aircraft top speeds have remained level, generally in the range of Mach 1 to Mach 3. During the US aerospaceplane program, between the 1950s and the mid 1960s, Alexander Kartveli and Antonio Ferri were proponents of the scramjet approach. In the 1950s and 1960s a variety of experimental scramjet engines were built and ground tested in the US and the UK. In November 1964 Antonio Ferri successfully demonstrated a scramjet producing a net thrust of 517 pounds-force (2.30 kN), about 80% of his goal. In 1958, an analytical paper discussed the merits and disadvantages of supersonic combustion ramjets. In 1964 Frederick S. Billig and Gordon L. Dugger submitted a patent application for a supersonic combustion ramjet based on Billig's PhD thesis. This patent was issued in 1981 following the removal of an order of secrecy. In 1981 tests were made in Australia under the guidance of Professor Ray Stalker in the T3 ground test facility at ANU. The first successful flight test of a scramjet was performed as a joint effort with NASA over the Soviet Union in 1991. It was an axisymmetric hydrogen-fueled dual-mode scramjet developed by Central Institute of Aviation Motors (CIAM), Moscow, in the late 1970s, but modernized with a FeCrAl alloy on a converted SM-6 missile to achieve initial flight parameters of Mach 6.8, before the scramjet flew at Mach 5.5. The scramjet flight was flown captive-carry atop the SA-5 surface-to-air missile that included an experimental flight support unit known as the "Hypersonic Flying Laboratory" (HFL), "Kholod". Then, from 1992 to 1998, an additional six flight tests of the axisymmetric high-speed scramjet-demonstrator were conducted by CIAM together with France and then with NASA. Maximum flight speed greater than Mach 6.4 was achieved and scramjet operation during 77 seconds was demonstrated. These flight test series also provided insight into autonomous hypersonic flight controls.
2000s
In the 2000s significant progress was made in the development of hypersonic technology, particularly in the field of scramjet engines. The HyShot project demonstrated scramjet combustion on 30 July 2002. The scramjet engine worked effectively and demonstrated supersonic combustion in action. However, the engine was not designed to provide thrust to propel a craft. It was designed more or less as a technology demonstrator. A joint British and Australian team from UK defense company Qinetiq and the University of Queensland were the first group to demonstrate a scramjet working in an atmospheric test. Hyper-X claimed the first flight of a thrust-producing scramjet-powered vehicle with full aerodynamic maneuvering surfaces in 2004 with the X-43A. The last of the three X-43A scramjet tests achieved Mach 9.6 for a brief time. On 15 June 2007 the US Defense Advanced Research Project Agency (DARPA), in cooperation with the Australian Defence Science and Technology Organisation (DSTO), announced a successful scramjet flight at Mach 10 using rocket engines to boost the test vehicle to hypersonic speeds. A series of scramjet ground tests was completed at NASA Langley Arc-Heated Scramjet Test Facility (AHSTF) at simulated Mach 8 flight conditions. These experiments were used to support HIFiRE flight 2. On 22 May 2009 Woomera hosted the first successful test flight of a hypersonic aircraft in HIFiRE (Hypersonic International Flight Research Experimentation). The launch was one of ten planned test flights. The series of flights is part of a joint research program between the Defence Science and Technology Organisation and the US Air Force, designated as the HIFiRE. HIFiRE is investigating hypersonics technology and its application to advanced scramjet-powered space launch vehicles; the objective is to support the new Boeing X-51 scramjet demonstrator while also building a strong base of flight test data for quick-reaction space launch development and hypersonic "quick-strike" weapons.
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