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 A liquid-propellant rocket or liquid rocket utilizes a rocket engine that uses liquid rocket propellant, liquid propellants. Liquids are desirable because they have a reasonably high density and high Specific impulse, specific impulse (''I''sp). T ...

engine. Development began in the 1950s by the Reaction Motors Division of Thiokol Chemical Company to power the

North American X-15 The North American X-15 is a hypersonic rocket-powered aircraft. It was operated by the United States Air Force and the National Aeronautics and Space Administration as part of the X-plane series of experimental aircraft. The X-15 set speed an ...

hypersonic research aircraft. It could deliver up to of thrust with a specific impulse of or at sea level. Thrust was variable from 50 to 100

percent In mathematics, a percentage (from la, per centum, "by a hundred") is a number or ratio expressed as a fraction of 100. It is often denoted using the percent sign, "%", although the abbreviations "pct.", "pct" and sometimes "pc" are also use ...

, 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 applica ...

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 wast ...

, pumped into the engine by turbopumps at a mass flow rate of over per

minute The minute is a unit of time usually equal to (the first sexagesimal fraction) of an hour, or 60 seconds. In the UTC time standard, a minute on rare occasions has 61 seconds, a consequence of leap seconds (there is a provision to insert a nega ...

. 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

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