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I’m sure many would argue that various car engines deserve the title “America’s engine,” but no matter what you call it, the Reaction Motors XLR11 rocket engine has done America proud – it’s taken the US to its current level of aerial superiority.

The XLR11 didn’t just spring into life. About 15 years before it was created a group of people in New Jersey formed the American Rocket Society. These guys investigated how to make a rocket on a budget. They would create their engine, find somewhere out of the way, test it (generating lots of smoke and noise), and leave as quickly as possible. Their success, partly as engineers and partly at not being arrested, led them to form Reaction Motors Inc.

In 1943, Reaction Motors began the development of the 6000C4, a liquid fueled, four-cylinder rocket. Each cylinder could be independently controlled, so that the rockets power could be stepped up and down while it was operating. At the time a rocket whose power output could be varied was difficult to engineer, so this was a practical solution to the problem. RMI also designed the rocket to be “man rated,” ie safe enough to be used in manned vehicles.

At the time Reaction Motors was developing their engine the Army Air Force had decided that if the sound barrier was going to be broken, a full on assault was required. This was a different attitude to much of the aircraft development before it. Typically, developers created a prototype, or even a completed article, then tried to interest the services in it. But breaking the sound barrier was a significantly larger challenge, in terms on money, knowledge and risk.

The Army Air Force contracted Bell to develop an aircraft, and the engine they chose to power the aircraft was not the up-and-coming jet , but a rocket engine, the 6000C4.

Within RMI, the engine was known as “Black Betsy,” because it was painted black. In operation it generated plenty of smoke and noise, which led the engineers to the nickname “the belching black bastard.” Military minds were not amused, so they named it the XLR11, the name which eventually stuck.

The photo above is a test firing of the XLR11 from Bob Holland’s website which has an amazing collection of photo’s about the various XLR engines and Reaction Motors in general.

The XLR11 used a mixture of ethyl alcohol and liquid oxygen, which had to be forced into the cylinders by a pressurized fuel system (something that carried significant risks). In later aircraft the fuel was fed by a turbopump.

Each cylinder generated 1,500lb of thrust, for a total of 6,000lb. The maximum weight of the X-1 was 12,250lb, so the X-1 wasn’t capable of accelerating vertically, but the engine was far from underpowered.

The XLR11 powered a long string of amazing aircraft:

  • All of the X-1 variants, from Glamorous Glennis through to the final X-1E variant.
  • The Douglas D-558 Phase II Skyrocket (as a booster rocket for the aircraft’s main jet engine)
  • The first flights of the X-15. This aircraft used two XLR11 engines which had been upgraded to 8,000lb of thrust. It was replaced in later flights by a 57,000lb thrust engine, the XLR99.
  • The Dryden lifting bodies that were used to explore the possibility of a wingless vehicle for return from space, which eventually led to the design of the space shuttle.

The XLR11 was a reliable workhorse (as rockets go) that saw America capture the record for Mach 1 and 2, helped the most successful of all X-planes, the X-15, into flight, and established the basis for the shape of the space shuttle.

Reaction Motors was dissolved in 1972, an unfitting end to a company that did so much to advance America’s knowledge of rocketry and high speed flight. It’s legacy hangs in pride of place in the National Air and Space Museum, and the successes it was part of have fueled the image of the X-planes to this day. But what worries me is what is going to keep the US at the top of the aerial superiority totem pole for the next 75 years?


(Images courtesy of Wikipeadia and


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