The Reaction Motors LR99 engine was the first large,

throttle A throttle is the mechanism by which fluid flow is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases (by the use of a throttle), but usually decreased. The term ''throttle'' ...

able, restartable liquid-propellant rocket engine. Development began in the 1950s by the

Reaction Motors Reaction Motors, Inc. (RMI) was an early American maker of liquid-fueled rocket engines, located in New Jersey. RMI engines with thrust powered the Bell X-1 rocket aircraft that first broke the sound barrier in 1947, and later aircraft such ...

Division of Thiokol Chemical Company to power the North American X-15 hypersonic research aircraft. It could deliver up to of thrust with a

specific impulse Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust. For engines whose reaction mass is only the fuel they carry, specific impulse is ...

of or at sea level. Thrust was variable from 50 to 100 percent, and the restart capability allowed it to be shut down and restarted during flight when necessary.

Design and development

The engine is propelled by

liquid oxygen Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries—is the liquid form of molecular oxygen. It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an app ...

and

anhydrous ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...

, pumped into the engine by

turbopump A turbopump is a propellant pump with two main components: a rotodynamic pump and a driving gas turbine, usually both mounted on the same shaft, or sometimes geared together. They were initially developed in Germany in the early 1940s. The purpo ...

s at a

mass flow rate In physics and engineering, mass flow rate is the mass of a substance which passes per unit of time. Its unit is kilogram per second in SI units, and slug per second or pound per second in US customary units. The common symbol is \dot ('' ...

of over per minute. After one hour of operation, the XLR99 required an overhaul. Operating times nearly twice that were recorded in tests, but declared largely unsafe. The basic X-15 aircraft carried fuel for about 83 seconds of full-powered flight, while the X-15A-2 carried fuel for just over 150 seconds. Therefore, each XLR99 was capable, in theory, of between 20 and 40 flights before an overhaul. Like many other liquid-fuel rocket engines, the XLR99s used

regenerative cooling Regenerative cooling is a method of cooling gases in which compressed gas is cooled by allowing it to expand and thereby take heat from the surroundings. The cooled expanded gas then passes through a heat exchanger where it cools the incoming comp ...

, in that the thrust chamber and nozzle had tubing surrounding it, through which the propellant and oxidizer passed before being burned. This kept the engine cool, and preheated the fuel. The basic engine has a mass of .

Operational history

The LR-99 was used exclusively to power the X-15 research aircraft after initial trials that used a pair of

Reaction Motors XLR11 The XLR11, company designation RMI 6000C4, was the first liquid-propellant rocket engine developed in the United States for use in aircraft. It was designed and built by Reaction Motors Inc., and used ethyl alcohol and liquid oxygen as propellan ...

Applications

* North American X-15

Variants

;XLR99-RM-1: prototype engines for initial testing and flight trials. ;YLR99-RM-1: Service test engines fitted to X-15s for later flights.

Specifications (YLR-99-RM-1)

References

External links

Reaction Motors XLR99 Rocket
– National Museum of the United States Air Force

{{DEFAULTSORT:XLR99 Aircraft rocket engines Rocket engines of the United States Rocket engines using the gas-generator cycle