The XLR-99 was the rocket engine that powered the X-15 planes to their record breaking speed and altitude. The XLR-99 was designed and manufactured by Reaction Motors Division of Thiokol Chemical Corporation.
The X-15 aircraft were ready before the XLR-99 engine and for a while the X-15's were powered by two XLR-11 engines. When the first one was delivered, ground tests on the airframe were begun. Because of the nature of the tests, a pilot was to be in the cockpit and in control of the engine during the tests. It was on it's first test, that the entire airplane exploded with pilot Scott Crossfield in it. Due to the exceptional design of the X-15, Scott Crossfield was able to walk away from the accident, although the cockpit was now located several yards from where it was originally. The cause of the failure was found to be unrelated to the XLR-99. When the second XLR-99 was delivered, it performed as expected.
There are several things that make the XLR-99 engine signifigant. It was the first man-rated (read not going to blow up) liquid fueled restartable rocket engine. The XLR-99 was remarkably small for the amount of power it developed; it only weighed 450 kg. The engine holds the world record for the most powerful engine ever used in an aircraft. It produced an amazing 57,850 pounds of thrust, generating 600,000 horsepower. For comparison, all of the Titanic's boilers produced only 40,000! It chewed through 4,500 kg its ammonia and liquid oxygen fuel per minute. It had a service life of only 1 hour. It only took 80 seconds of powered flight for it propell the X-15 from Idaho to California.
You can see a picture of the XLR-99 at:
http://www.astronautix.com/graphics/x/xlr99.jpg