The US Defense Advanced Research Projects Agency (DARPA) is to test a modified Pratt & Whitney F100 engine later this year - the powerplant selected for the Mach 4 reusable first stage of the RASCAL(responsive access small cargo affordable launch) space launcher - as recent combustion tests have provided positive results.

The F100 has a mass injection pre-compressor cooling (MIPCC) system that injects water and liquid oxygen into the inlet, boosting thrust at altitude and improving thrust-to-weight ratio. Speaking at the International Symposium on Air Breathing Engines, Vladimir Balepin, an MIPCC developer for the New York-based GASL test facility, says: "It fools the engine into believing it is flying at M1.64 when it is actually at M4, and when it is at around 88,000ft (26,840m) the engine feels it is at 24,000ft."

Recent tests at GASL have allayed concerns that the kinetic effects of pre-compression might cause flame-outs of the JP4-fueled engine or reduce thrust under certain conditions. The tests showed that with a maximum 11.5-13.5% mole percentage of water injection in front of the fan, the main combustor ignition time is delayed by 0.05-0.08s.

Balepin believes that if the combustor inlet temperature of the JP4/air/water mix exceeds 1,000¡C (1,830¡F), it will reform to produce hydrogen and other light, ignitable substances. Water in the afterburner will help JP4 reformation, improving engine performance.

Four MIPCC-modified F100s will power the Scaled Composites-designed MIPCC-powered vehicle (MPV) which forms RASCAL's manned first stage. The MPV will launch an expendable rocket second stage at 190,000ft to place a 115-180kg (250-395lb) payload into orbit, while the MPV re-enters and lands. First flight of RASCAL is planned for 2005.

Source: Flight International