One method of utilizing the atmosphere for propulsion has to do with liquefying the air and using it as an oxidizer-propellant in the engine. By using a cryogenic fuel such as liquid hydrogen, air brought into the vehicle can be rapidly cooled until it condenses as a liquid. This liquefied air would then be massively injected into the combustion chamber, along with the onboard hydrogen, and used to propel the vehicle up to Mach 6 or 7.
The disadvantage of these LACE (liquid air cycle engine) techniques is that it not only requires heavy condenser units, but also works only inside the atmosphere. It also requires a much larger fuel flow than do other air-breathing engines, for complicated reasons having to do with temperature pinch points, latent heats of vaporization, and hydrogen embrittlement, all of which are beyond the scope of this book. LACE engines promise specific impulses of some 800 s, compared to a / of 400 s for a pure rocket, or 10,000 s for a conventional subsonic air-breathing turbojet.
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