Liquid air cycle engine

A liquid air cycle engine (LACE) is a type of spacecraft propulsion engine that attempts to increase its efficiency by gathering part of its oxidizer from the atmosphere. A liquid air cycle engine uses liquid hydrogen (LH2) fuel to liquefy the air.

In a liquid oxygen/liquid hydrogen rocket, the liquid oxygen (LOX) 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 might dramatically lower the take-off weight of the spacecraft.

LACE was studied to some extent in the USA during the late 1950s and early 1960s, and by late 1960 Marquardt had a testbed system running, it labelled an ejector engine. However, as NASA moved to ballistic capsules during Project Mercury, funding for research into winged vehicles slowly disappeared, and LACE work along with it. In the 90s interest in Rocket-based combined cycle engines was revised, and tested on the NASA X-43 series.

LACE was also the basis of the engines on the British Aerospace HOTOL design of the 1980s.