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Liquid air cycle engine

A LACE, for Liquid Air Cycle Engine, is a type of spacecraft propulsion that attempts to gain efficiency by gathering part of its oxidizer from the atmosphere. In a LOX/LH2 bipropellant rocket the liquid oxygen needed for combustion is the majority of the weight of the spacecraft on lift-off, so if some of this can be collected from the air on the way, it can dramatically lower the overall size and weight of the spacecraft.

LACE works by compressing and then quickly liquifying the air. Compression is achieved through the ram-air effect in an intake similar to that found on a high-speed aircraft like the Concorde, where ramps in the intake create shock waves that compress the air. Instead of being mixed with fuel, the LACE design then blows the compressed air over a heat exchanger, in which the liquid hydrogen[?] fuel is flowing. This rapidly freezes the air, and the various constituents quickly liquify. By careful mechanical arrangement the liquid oxygen[?] can be removed from the other parts of the air, notably nitrogen and carbon dioxide, at which point it can be fed into the engine as normal. The hydrogen is so much lighter than oxygen that the now-warm hydrogen is often dumped overboard instead of being re-used as fuel, at a net gain.

The "trick" to the LACE system (and in rocket design, there's always a trick) is that in order to appreciably reduced the mass of the oxygen carried at launch, a LACE vehicle needs to spend more time in the lower atmosphere where it can collect enough oxygen to supply the engines. This leads to greatly increased drag, which may in fact offset the savings in weight. The advantages, or disadvantages, of the design continues to be a matter of some debate.

LACE was studied to some extent in the US during the late 1950s and early 1960s, where it was seen as a "natural" fit for a winged spacecraft project known as the Aerospaceplane. At the time the concept was known as LACES, for Liquid Air Collection Engine System, or ACES for Air Collection and Enrichment System. Both Marquardt[?] and General Dynamics were involved in the research, and by late 1960 Marquardt had a testbed system running that was capable of running a 275lbs thrust engine for minutes at a time. However, as NASA moved to ballistic capsules during Project Mercury, funding for research into winged vehicles slowly disappeared, and LACE along with it.

LACE then saw a brief re-emergence in England in 1982, when Alan Bond (formerly of the Blue Streak project) designed a new version of the LACE design he called SATAN. At the same time, John Scott and Bob Parkinson at British Aerospace had started some preliminary work on reusable launch systems. The two teams met and created HOTOL, which would use the BA designed airframe with a Rolls Royce version of Bond's engine, known as the RB454. In 1986 the project was given an official go-ahead to the tune of 2 million pounds for research, but the program was later killed in 1989 when the government refused further funding.

The principle designers then left to continue development on their own, but the RB454 had been classified top secret and could not be used. Instead Bond developed another version that was even more advanced, known as SABRE (ostensibly for Synergic Air Breathing Engine) which was placed in their Skylon airframe. Funding was not forthcoming and development tapered off.

See also:

air-augmented rocket

External links:

Liquid Air Cycle Rocket Equation (http://www.sworld.com.au/steven/space/lace.txt)
HOTOL (http://www.geocities.com/CapeCanaveral/Launchpad/6133/hotol)

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