Much of NASA's investment in the X-33 demonstrator will be in the development of its linear 'aerospike' engines. The aerospike "-is tremendously efficient because it is simpler than the current bell-nozzle rocket engine", says Micky Blackwell, president and chief operating officer of Lockheed Martin Aeronautics. "It automatically adjusts itself to perform with maximum efficiency at all altitudes, and the entire propulsion system is an integral part of the vehicle, just as it is on an aircraft. There are no throw-away components".
The liquid-oxygen/liquid-hydrogen cryogenic power plant has been under development at Rockwell's Rocketdyene division since the late 1960s, and over $1 billion has been spent on the project. The engine has never been tested in flight, although the latest full-scale version has been tested on over 70 occasions, for a total of 4,400s running time.
The main benefit of the engine is its ability to compensate for decreasing atmospheric pressure as it operates at ever-higher altitudes during the ascent, while maintaining high engine performance. Conventional bell nozzles are optimised for a specific altitude and ambient pressure, and perform non-optimally elsewhere, but the aerospike engine automatically adjusts for optimum performance at all altitudes.
In a bell nozzle, engine exhaust gases flow through a constriction or throat and then the expansion away from the centerline is contained by the nozzle. Careful design is needed to achieve desired high-altitude performance, while avoiding flow separation from the walls of the nozzle near the exit when operating at low altitude. This can lead to loss of performance and possible structural failure. Therefore, a compromise altitude must be used as the design point for a bell nozzle.
In an aerospike nozzle, the opposite happens - the exhaust is directed inward from several smaller thrusters at some distance away from the centerline. This flow is directly exposed to the airstream and coupled to the external atmospheric conditions by continuous altitude compensation, with no moving parts.
The linear aerospike engine incorporates linear thrusters on both sides of a central truncated cone nozzle. Hot gas from each nozzle expands along the spike on one side and into the free air on the other. One-axis vectoring is accomplished by running the top and bottom nozzles at different thrusts, providing pitch and roll control during the ascent, while throttling between the engines provides yaw control. Thrust is distributed evenly along the upper and lower expansion ramps into the vehicle aft-body structure. The thrust is spread uniformally across the entire base of the rocket.
The success of the X-33 programme will depend largely on its array of these engines, and a small prototype X-33 version of the engine is due to undergo a programme of 15-20 tests, known as LARSE, aboard a NASA Lockheed SR-71 research aircraft at the Dryden Flight Research Center, at Edwards AFB, California.
The tests will involve a 1/10th scale, half-span model of the X-33, with a 6.4kN (1,440lb)-thrust engine , riding on top and towards the rear of the SR-71's fuselage. The idea is to gather data on slipstream interaction and thrust-vector-control effectiveness on exact X-33 ascent profile conditions up to Mach 3. Helium and nitrogen will initially be used as "propellants" for slipstream interaction data. Understanding the interaction between the aerodynamics of the X-33 demonstrator and its planned two aerospike engines is vital because the engines are at the rear, making center-of-mass control difficult.
Much will also depend on the configuration and light weight of the engine nozzle. A fully fledged orbital RLV would fly with seven aerospike engines. Ground testing the flight-configuration engines is due to begin in 1998 at NASA's Stennis Space Center, Mississippi, but many doubt that the aerospike engines will be ready on time. The LASRE tests, already delayed since April, should help allay fears, expressed in some quarters, that the technical challenge to be surmounted presents too great a risk. It is also hoped that the tests will also show that the engine will produce the hoped-for benefits. Further delays in preparing the tests, however, may yet force their cancellation, placing fresh doubts on the X-33 project.
Source: Flight International
