WASHINGTON -- Two novel linear aerospike engines are ready to roar, perhaps putting more life into NASAs trouble-plagued X-33 rocketplane program.

The aerospike motors are mounted side by side in a test stand at NASAs Stennis Space Center, in Mississippi. This first tandem firing test is set for early next year, to be followed by at least nine more tests of the twin rocket power plants.

The X-33 relies on two aerospike engines. The upcoming firing is considered a program milestone in propelling the rocketplane skyward in 2003.

Eventually, longer engine-burn tests will mimic durations required to launch the pilotless X-33 from an Edwards Air Force Base, California, takeoff point to landings in either Utah or Montana.

Once the engines are fully certified and ready for flight, they are to be shipped to Palmdale, California, where the X-33 is being built.

"The vehicle is in excellent shape, sitting in Palmdale," said Gene Austin, NASA's X-33 program manager. "The engine tests are another significant mark in our program."

Austin said the importance of the X-33 to lower the cost of access to space is at the heart of the project. "We're trying to change the nature of how we get into space," he said.

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Don Chenevert, NASA's X-33 program manager for aerospike engine testing at Stennis Space Center, told SPACE.com that the first ignition test will be very brief, 1.2-second test. That burp of the engines is planned for sometime in January. This set of aerospike engines are the actual units that will carry the X-33 into space.

"Based on that, we would go on out and continue to do the remaining 10 tests throughout the rest of 2001. At the later part of next year we will ship those engines to California," he said.

A main hurdle for the engines is to perform for 210 seconds. That is the length of time the engines must fire for X-33 to begin a series of suborbital test hops in 2003.

Mike McKeon, X-33 aerospike engine program manager at Rocketdyne Propulsion and Power, The Boeing Company, in Canoga Park, California, said that the aerospike engine tests are important to the future of space transportation. If the engines don't move forward and are not used on X-33 due to a program shutdown, "we would try to sell aerospike technology to whoever wanted to use them. It's a significant technology and it will be around for a long time," he said.

Previously, more than a dozen single-engine tests of an earlier version of the unique aerospike engine have been completed.

The linear aerospike XRS-2200 engines are developed by the Rocketdyne Propulsion and Power Unit of the Boeing Company.

The difference between the linear aerospike engine and a conventional rocket engine is the shape of the nozzle. For the aerospike motor, the nozzle is V-shaped and called a ramp.

Hot gases are shot from chambers along the outside of the ramps surface. This movement of hot gases along the ramp produces thrust along the length of the ramp -- hence the name linear aerospike.

Aerospike engines are considered more efficient and effective than todays rocket engines.

During launch, the plume spewed from aerospike engines can widen with decreasing atmospheric pressure as the X-33 hurtles through the sky. Doing so means the aerospike engines can maintain more efficient thrust throughout the vehicles flight.

The X-33 is being built under cooperative agreement between NASA and the Lockheed Martin Aeronautics Co. As a shrunk-down version of a reusable single-stage-to-orbit (SSTO) vehicle, the X-33 is evaluating key technologies and concepts for a commercially developed and operated spaceliner -- the VentureStar.

X-33's critical composite liquid-hydrogen tank failed in November 1999, a problem that spurred a major overhaul of the program.

In late September, after much debate, NASA and Lockheed Martin agreed to move forward on the X-33. The vehicle will now carry yet-to-be-built hydrogen tanks made of aluminum, replacing the composite version.

In restructuring the X-33 program, NASA and Lockheed Martin will carry on work through March 31 of next year. At that time, Lockheed Martin must compete for additional funds for X-33 work through NASAs newly funded Space Launch Initiative.

Without NASA monies -- or more funds poured into the project by Lockheed Martin -- it is not clear whether the craft becomes "ex-33" -- more a museum piece than a tried-and-true rocketplane.