The XLR-129 was an American

rocket engine A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordanc ...

design that would have used liquid hydrogen and liquid oxygen propellants. It was developed by

Pratt & Whitney Pratt & Whitney is an American aerospace manufacturer with global service operations. It is a subsidiary of Raytheon Technologies. Pratt & Whitney's aircraft engines are widely used in both civil aviation (especially airlines) and military aviat ...

and initially was to develop of thrust. It featured an

expanding nozzle The expanding nozzle is a type of rocket nozzle that, unlike traditional designs, maintains its efficiency at a wide range of altitudes. It is a member of the class of altitude compensating nozzles, a class that also includes the plug nozzle and ...

in order to tune performance over a wide range of altitudes. The XLR-129 was designed to be reusable and was initially paid for by the US Air Force, for a 1960s program called

ISINGLASS Isinglass () is a substance obtained from the dried swim bladders of fish. It is a form of collagen used mainly for the clarification or fining of some beer and wine. It can also be cooked into a paste for specialised gluing purposes. The E ...

, which was to be a manned rocket plane that was intended for surveillance overflights. For the

Space Shuttle The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program na ...

an attempt was made to increase the thrust to , but in the end Rocketdyne's

Space Shuttle Main Engine The Aerojet Rocketdyne RS-25, also known as the Space Shuttle Main Engine (SSME), is a liquid-fuel cryogenic rocket engine that was used on NASA's Space Shuttle and is currently used on the Space Launch System (SLS). Designed and manufacture ...

was used instead. The XLR-129 program was never completed, no complete engine was ever produced, but many systems were developed and tested.

References

Aircraft rocket engines Rocket engines using hydrogen propellant {{aircraft-engine-stub