There is now steady progress in actual testing of hypersonic engines. In 2010, the unmanned X-51A flew for about 200 seconds at near hypersonic speed while accelerating during the test. Previous tests had lasted only a few seconds.
Reaction Engines, has recently achieved a go-ahead from the ESA for a major test of critical engine components of its Skylon vehicle concept. As air enters the intake, slows to subsonic speed and is compressed, it becomes very hot, which would reduce or eliminate engine power if it were not cooled. For that vehicle’s engine, which does not operate as a scramjet, the air is cooled to 133 kelvins (–140°C) using the liquid hydrogen fuel via a heat exchanger. After mach 5, they switch to a rocket using liquid oxygen. If the next test on the heat exchanger is successful, there is a good chance for the project to proceed to build a complete test engine.
There would seem to be five classes of potential air-breathing launchers based on oxidizer source, ignoring whether they are SSTO or TSTO, and whether the vehicle uses a ramjet or scramjet type engine during any part of its air-breathing phase.
1. Carries all the LOX at launch for use during the latter pure rocket phase, is boosted to hypersonic speeds by a rocket booster, and then uses atmospheric oxygen during the air-breathing (scramjet) phase. Example: conceptual launcher derived from current scramjet test vehicles.
2. Carries all LOX at launch for rocket phase but cools incoming air with LH2 during the air-breathing phase for use in a gaseous air-hydrogen rocket engine that then doubles up as a LOX/hydrogen rocket engine. Example: Skylon
3. Carries all LOX at launch for rocket phase, cools and condenses incoming air with LH2 during air-breathing phase and uses the liquefied air directly as the oxidiser during the same air-breathing phrase. Uses the same rocket engine during both flight phases. Example: Japanese vehicle powered by a LACE type engine system.
4. Carries no LOX at launch. Uses separate ram air intakes with embedded heat exchangers for ram air collection, liquefaction and LOX separation. This condenses the entire amount of the required incoming air, with LH2 in hypersonic flight, then separates and stores LOX on board for use during the rocket phase. The engines used are turbojet/turbo-ramjet, switching over to a scramjet in hypersonic flight and finally to a rocket engine. Example: Avatar/Hyperplane from India. The benefits of in-flight air liquefaction and lox separation/storage (said to yield payload fractions up to 30%) were reported earlier in mid-1960s from studies at the Applied Physics Laboratory of John Hopkins University.
5. A final variation. Carries no LOX at launch but with a separate intake provided for air collection, liquefaction, and LOX separation/on-board storage as in (4) above. Uses a single LACE (rocket) engine all the way to orbit, simplifying the propulsion system. The collected LOX is thus used for propulsion both in the atmosphere and above it. This is yet an unexplored concept, a possible integration of the Japanese LACE concept and the FLOX concept which emerged in India in the mid-1990s.
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