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The linear aerospike engine is being developed by Rocketdyne as part of a cooperative agreement between the National Aeronautics and Space Administration (NASA) and Lockheed Martin to design and build a subscale X-33 test vehicle that will demonstrate the key technologies and lower costs that are needed for the next generation of Reusable Launch Vehicles (RLVs). NASA’s Marshall Space Flight Center in Huntsville, Ala. manages the X-33 program as part of its overall Reusable Launch Vehicle technology program. The aerospike allows the smallest, lowest cost RLV to be developed because the engine fills the base, reducing base drag, and is integral to the vehicle, reducing installed weight when compared to a bell-shaped engine.

How Rocket Engines Work

Rocket engines all perform the same basic function. They turn energy stored in propellants into thrust. Pump-fed liquid rocket engines performed this by ingesting the liquid propellants stored in the vehicle, increasing their pressure and flow rate by the use of turbine-driven pumps, delivering them to the combustion chamber for ignition, then exhausting the hot combustion gasses out a nozzle to produce thrust.

Why the Aerospike is Different

The difference between the linear aerospike and conventional rocket engines is the shape of the nozzle. Whereas the bell nozzle of a conventional engine expands the hot combustion gas on its inside surface, the aerospike nozzle expands the gas on its outside surface. And the linear aerospike nozzle is not a bell shape at all, but the shape of a “V” called a ramp. This unusual shape enhances performance and allows a more optimum vehicle design. Aerospike nozzles can be circular or linear with the latter being ideal for the X-33/RLV application.

Aerospike Provides Higher Performance

The enhanced performance of the aerospike is due to the external expansion of combustion gasses. In a bell nozzle engine a particular nozzle shape and length will expand its combustion gasses outward only as far as the nozzle allows. They are designed to be the best compromise of shape and length for a particular vehicle and flight path. At higher altitudes, gasses could expand farther if there were more nozzle length, thereby improving performance. But since the nozzle cannot easily change shape--that is, growing longer--it essentially gives up some performance.

The aerospike offers a solution to this problem. It’s plume is open to the atmosphere on one side and free to move, allowing the engine to operate at its optimum expansion at all altitudes. It compensates for decreasing atmospheric pressure as the vehicle ascends, keeping the engine’s performance very high throughout the entire trajectory. While the free side blooms outward to adjust the combustion gas pressure to the local atmospheric pressure, it is pushing against the nozzle ramp surface on the other side, producing thrust. This altitude compensating feature also allows a simple, low-risk gas generator cycle to be used. Instead of recirculating and burning the turbine exhaust gasses, as in the Space Shuttle Main Engine, or dumping the exhaust, as with most other bell-nozzle engines, the aerospike is designed to exhaust the gasses through the truncated end of the spike, creating additional thrust.

Aerospike Provides Optimum Vehicle Design

The linear aerospike allows the smallest, lowest cost vehicle to be developed because the engine fills the base, reducing base drag, and is integral to the vehicle, reducing installed weight when compared to a bell-shaped engine. A rectangular-shaped (“linear”) aerospike engine fits nicely into a vehicle with a rectangular back end. The conformal ability of the aerospike reduces its installed weight when compared to a bell engine, thereby reducing the vehicle’s size and cost. In additional, since the aerospike is conformal, it produces a hot gas plume which nearly fills the base area of the vehicle, thereby reducing “base drag” cause by open area at the base of the vehicle. The base drag reduction allows the vehicle to be smaller than a similar vehicle with a bell engine.

X-33 and RLV Linear Aerospikes

Like the vehicle itself, the linear aerospike engine designed for the X-33 is subscale. Four engines will be built for the X-33. Two will be installed on the vehicle, and two used for testing with one of the two engines to be rebuilt and used as a spare. Each of the two engines on the X-33 will have a series of 20 combustion chambers - ten aligned along the forward end of each nozzle ramp--and will produce 206,400 pounds of thrust at sea level. The full-scale aerospike for the RLV will produce 431,000 pounds of thrust with 14 combustion (seven per side). Propellants for both the subscale and prototype engines are liquid oxygen and liquid hydrogen. The only combustion by-product from burning these chemicals together is steam. The X-33 and RLV vehicles will be steered by using differential thrust--that is, varying the thrust of the aerospike engine segments to produce pitch, roll and yaw--opposed to moving or gimballing the entire engine to change direction. This feature also reduces mechanical systems and contributes significantly to a lighter weight vehicle when compared to one with bell engines. The X-33 engine can throttle from 40 to 119 percent of its designed thrust level, while the RLV engine will throttle from 18 to 100 percent for the RLV prototype. Engine life of the RLV linear aerospike is expected to be 100 missions.

Design, Assembly and Testing

Design work for the four linear aerospikes for the X-33 has already begun at Rocketdyne. The design phase should be completed by summer, 1997. Fabrication, component testing and engine assembly are scheduled for completion by mid-1997, with engine ground testing to begin in 1998. The first flight of the X-33 will be in 1999. The flight test series is scheduled to end in December 1999. Design work on the full-scale aerospike prototype begins in mid-1997. The 15 month design phase ends in mid-1998. After fabrication and component testing, engine assembly starts in early 2000. Engine ground tests take place in the second half of 2000.

Engine assembly will be done at Rocketdyne. Component testing will be at both Rocketdyne and NASA’s Marshall Space Flight Center. Engine testing will be at NASA’s Stennis Space Center in Mississippi.

Source: NASA