dietz@SLB-DOLL.CSNET (Paul Dietz) (02/24/86)
As I understand it, the British vehicle, the HOTOL (Horizontal Takeoff and Landing) will not use scramjets; rather, it will use a combination of other engines. At low speeds (< mach 12?) it will use a LACE (liquid air cycle engine), a kind of airturboramjet that cools the incoming air with liquid hydrogen fuel before combustion. Some of the incoming air is actually liquified and mixed with the hydrogen to drive a small turbine that helps pump the fuel (a similar mechanism is used in LOX/LH2 rocket engines, except the LOX comes from the fuel tank). At higher speeds I believe the HOTOL will use an on-board supply of oxidizer and a rocket. This is not as bad as it may seem, since the fuel requirements for getting from 1/2 orbital velocity into orbit by rocket are far lower than going the whole way by rocket, and it lets the HOTOL avoid atmospheric heating at near orbital speeds (the power density of the air stream increases as the CUBE of velocity, at fixed air density, or as the square if the vehicle's altitude is increased to keep the mass flow rate of air through the engines constant). A good intermediate use for scramjets or LACE's might be in a reusable first stage for small nonreusable rockets. The hypersonic vehicle could lob a nonreusable booster out of the atmosphere at about mach 12. This second stage could have a very reasonable mass ratio (fueled/empty mass of perhaps 2.5).