NASA and its industry partner in the X-33 Advanced Technology Demonstrator program, Lockheed Martin, plan to meet these challenges by taking a 30-year-old idea - the aerospike engine - and making it a reality for the 21st century by incorporating a host of new technologies and materials.

The aerospike engine will be developed from groundwork laid in the 1960s and 1970s by Rocketdyne. Unlike conventional rocket engines which feature a bell nozzle that constricts expanding gasses, the basic aerospike shape is that of a bell turned inside out and upside down.

The aerospike features a series of small combustion chambers along the spike, also called the ramp, that shoot hot gases along the ramp's outside surface to produce thrust in a spike-shaped plume, hence the name "aerospike."

With the aerospike, the ramp serves as the inner wall of the bell nozzle, while atmospheric pressure serves as the "invisible" outer wall. The combustion gasses race along the inner wall (the ramp) and the outer wall (atmospheric pressure) to produce the thrust force.

Efficient at all altitudes

The key to a conventional bell nozzle's level of performance is its width. At high pressure - i.e. sea level - the gasses are more tightly focused, so a bell nozzle with a narrow interior surface works best. At low pressure - i.e. higher altitudes - a wider interior works best as the gasses will expand farther.

For example, the initial stage of the Saturn rocket which carried the Apollo astronauts to the moon featured a narrow nozzle to produce an ideal straight-edged column of exhaust at sea level. However, the command module which orbited the moon featured a much wider bell nozzle better suited for controlling the combustion gasses in the vacuum of space.

Since the width of the bell nozzles can't change to match the atmospheric pressure as the rocket climbs, bell nozzles are normally designed to provide optimum performance at one certain altitude or pressure. This is called a "point design," and designers accept the performance loss the nozzle will encounter at any altitude other than the one it was designed for.

The aerospike eliminates this loss of performance. Since the combustion gasses only are constrained on one side by a fixed surface - the ramp - and constrained on the other side by atmospheric pressure, the aerospike's plume can widen with the decreasing atmospheric pressure as the vehicle climbs, thus maintaining more efficient thrust throughout the vehicle's flight.

However, the aerospike engine's ramp is cut short to save weight, resulting in a less-than-ideal plume. Designers compensate for this by pumping secondary exhaust from the aerospike's gas generator into the base region to add to the atmospheric pressure and elongate the wake.

The linear aerospike - which is more compact than a conventional bell nozzle engine - also distributes the thrust along the X-33's long base. This helps distribute the structural loads and makes for a lighter vehicle since heavy structural reinforcement in a confined area isn't required.

The X-33's direction of flight will be controlled by varying the thrust side to side and engine to engine, rather than by adjusting the direction of a bell nozzle. The lack of systems for thrust vectoring - such as gimbals, hydraulics and flex lines - also will make the aerospike easier to maintain than conventional engines and help keep the X-33's weight down.

Lockheed Martin's industry partner for the engine, Boeing North American's Rocketdyne Division, plans to build four liquid hydrogen and liquid oxygen-fueled, fuel-cooled linear aerospike engines with help from NASA's Marshall Space Flight Center in Huntsville, Ala. Two engines will be used for ground testing, and two for flight. One ground engine also will be refurbished after testing for use as a spare.

Marshall Space Flight Center also will work with Rocketdyne to develop the engine technologies required for a full-scale reusable launch vehicle (RLV), if industry decides to develop an RLV after the X-33 program is completed.

The X-33 will feature two aerospike engines, each independently supplying 20 of the 40 combustion chambers running the length of the X-33's long base to produce a combined 412,000 pounds of thrust at sea level. The specific impulse is projected at 339.9 seconds at sea level, 429.8 seconds within a vacuum. Lockheed Martin's planned full-scale RLV, dubbed the "VentureStarTM," will feature seven engines, each supplying eight chambers, to produce a total thrust of 3,010,000 pounds at sea level.

Testing

NASA engineers at Marshall Space Flight Center have conducted a number of tests for the linear aerospike engine. In the spring of 1997, Marshall Center conducted tests of three hydrogen-cooled thrusters, or thrust cells, mounted side by side and attached to a 4-foot-long copper alloy ramp. The test series at Marshall collected data on cell-to-cell plume interaction, cell-to-cell feed system interaction and heating.

Marshall Space Flight Center followed the thrust cell tests with tests on aerospike ignition and gas generator systems.

Testing of two completed, 20-cell X-33 aerospike engines is scheduled to take place in 1998 at NASA's Stennis Space Center in Mississippi.

NASA and Lockheed Martin also have tested a 5-percent scale model of the lifting body/aerospike configuration in the supersonic wind tunnels at the Air Force's Arnold Engineering Development Center in Tennessee. And, NASA and Lockheed Martin are conducting the Linear-Aerospike SR-71 Experiment, better know by its acronym LASRE. LASRE consists of a small linear aerospike rocket engine mounted within a half-span, 10-percent scale model of the X-33 flown on the back of an SR-71 Blackbird.

Both the wind-tunnel and SR-71 tests are aimed at characterizing the interaction between the aerospike engine exhaust and the lifting body's aerodynamic behavior.

Background

The X-33 is a subscale technology demonstrator prototype of a RLV which Lockheed Martin has named "VentureStarTM," and which the company hopes to develop early in the next century. Through demonstration flights and ground research, the X-33 will provide information needed for industry to decide by the year 2000 whether to proceed to the development of a full-scale, commercial single-stage-to-orbit RLV.

The X-33 is being developed under a cooperative agreement between NASA and Lockheed Martin Skunk Works, Palmdale, Calif., which began July 2, 1996.

NASA has budgeted $941 million for the X-33 program through 1999. Lockheed Martin will invest at least $212 million in the X-33 program.

Lockheed Martin plans to conduct 15 flight tests of the X-33 beginning in July 1999. All 15 tests will originate from Edwards Air Force Base, Calif., and fly to Michael Army Air Field, Dugway Proving Ground, Utah, and later to Malmstrom Air Force Base, Mont.

For more information on the aerospike engine and the X-33, visit NASA's Space Transportation Programs Web site.