In spite of being more advanced, Aerojet Rocketdyne’s XRS-2200 never made its maiden flight. This changes now.

Aerojet Rocketdyne Holdings, Inc., an aerospace and defense company based in the U.S. state of Florida, proposed in the mid-1990s an alternative type of rocket engine that competed with the traditional engine with bell-shaped nozzles. 

Aerojet Rocketdyne actually began testing linear aerospike engines in the 1970's with the Linear Test Bed engine program and resumed in the 1990's with the XRS-2200. Three XRS-2200 engines were built during the X-33 program and underwent testing at NASA's Stennis Space Center.

The XRS-2200 was designed for the Lockheed Martin X-33 space shuttle. During the tests, the XRS-2200 proved to be a highly efficient engine that could help launch a hypersonic aircraft fully into orbit with a vertical lift. The National Aeronautics and Space Administration (NASA) was thrilled about the technology.

And yet, XRS-2200 never made its maiden flight and soon was forgotten by the industry.

Although the single-engine tests were successful, the program was halted before the testing for the two-engine setup could be completed. 

The XRS-2200 produced 204,420 lbf (909,300 N) thrust with an Isp of 339 seconds at sea level, and 266,230 lbf (1,184,300 N) thrust with an Isp of 436.5 seconds in a vacuum.

When the launch site at Edwards AFB was completed and the X-33 vehicle was 98% assembled, a NASA official told U.S. Congress that “there was no reason to continue the program if Lockheed could not execute on the composite fuel tanks,” while Lockheed had come through successfully with aluminum.

The X-33 program was ultimately abandoned, and technical incompatibilities were cited as the official reason.

However, after decades of oblivion, Aerojet Rocketdyne – now a subsidiary of the American space and defense corporation L3Harris Technologies – is blowing the dust off the 70-year-old files amid realization that competitors are getting increasingly interested in the linear aerospike technology for the next-generation spacecraft. 

How it works

In a traditional engine, rocket engine bells aim to channel exhaust in one direction for thrust. The exhaust, a hot gas mix, tends to scatter randomly, reducing forward thrust efficiency.

Bells redirect misdirected exhaust, maintaining proper thrust. As the vehicle ascends, decreasing air pressure causes exhaust to expand beyond the bell, lowering engine efficiency.

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Aerospike engines address this by using a wedge-shaped "spike" on the outside edge, creating a virtual bell with ambient air on the other side, minimizing thrust loss.

So, again, instead of firing the exhaust out of a small hole in the middle of a bell, an aerospike engine avoids this random distribution by firing along the outside edge of a wedge-shaped protrusion, the "spike", which serves the same function as a traditional engine bell. The spike forms one side of a "virtual" bell, with the other side being formed by the outside air.

Side view and oblique top view of the XRS-2200 aerospike linear rocket engine. Credit: SecretProjects

The aerospike design utilizes ambient pressure at low altitude to compress exhaust against the spike. This causes the pressure in the base zone to approach ambient levels. As the pressure in front of the vehicle is also ambient, the exhaust at the spike's base balances out with the vehicle's drag. While this doesn't provide overall thrust, it prevents thrust loss due to the formation of a partial vacuum, making the base part's thrust negligible at low altitude.

As the vehicle climbs to higher altitudes, the air pressure holding the exhaust against the spike decreases, as does the drag in front of the vehicle.

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Pros and cons

The linear aerospike engine has a number of advantages over the bell-nozzled engine, according to aerospace experts: 

• Smaller nozzle: The truncated spike can be far smaller than a typical bell nozzle for the same performance, as shown below. In addition, a spike can give greater performance for a given length.
• Superior performance: Altitude compensation may result in greater installed performance.
• Less risk of failure: The aerospike engine uses a simple gas generator cycle with a lower chamber pressure than typical rocket engines reducing the risk of a catastrophic explosion. Although low chamber pressures result in reduced performance, the aerospike's high expansion ratio makes up for this deficiency.
• Lower vehicle drag: The aerospike nozzle fills the base portion of the vehicle thereby reducing a type of drag called base drag.
• Modular combustion chambers: The linear aerospike engine is made up of these small, easier to develop, less expensive thrusters that give the engine greater versatility.
• Thrust vectoring: Because the combustion chambers can be controlled individually, the vehicle can be maneuvered using differential thrust vectoring. This eliminates the need for the heavy gimbals and actuators used to vary the direction of traditional nozzles.
• Lower vehicle weight: Even though the aerospike tends to be heavier than the bell nozzle, it shares many major structural elements with the vehicle reducing overall weight.

On the other hand, there are disadvantages too:

• Cooling: The central spike experiences far greater heat fluxes than does a bell nozzle. This problem can be addressed by truncating the spike to reduce the exposed area and by passing cold cryogenically-cooled fuel through the spike. The secondary flow also helps to cool the body center.
• Manufacturing: The aerospike is more complex and difficult to manufacture than the bell nozzle. As a result, it is more expensive.
• Flight experience: No aerospike engine has ever flown in a rocket application. As a result, little flight design experience has been gained.

Whether Aerojet Rocketdyne succeeds to reanimate the XRS-2200 program remains to be seen. Competitors like Boeing, Arco Space or Polaris are busy experimenting with the linear aerospike engine, which means the technology does have a future.

See below a video explainer from Curious Droid.

…and a short documentary on this topic from the 1970s, thanks to a user called GandalfDDI.