HOUSTON The deafening roar of rocket engine firings should fill the air at Stennis Space Center near Bay St. Louis, Mississippi in October as the innovative Aerospike engine nears the final phase of testing.

The Aerospikes, built by Boeing Rocketdyne, are in the home stretch for certification for use in the oft-delayed X-33 spacecraft, a half-scale prototype of the VentureStar craft proposed by NASA and Lockheed Martin. The VentureStar is supposed to be a completely reusable, single-stage-to-orbit craft that lowers the cost of spaceflight.

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Dr. Donald Chenevert, the Aerospike project manager at Stennis, said he expects few problems with this round of dual-engine tests, following a successful run of 14 single-engine tests ending in May.

The current round of tests will certify how the pair of engines work together in preparation for installation on the X-33, now scheduled for flight tests no earlier than 2002. Engine tests at Stennis are expected to last until early summer 2001.

"The first few tests are designed to see how the two engines start together," said Mike McKeon, the Aerospike program manager for Boeing Rocketdyne. "The last five tests let us do a mission profiling and then we can say were ready to fly."

All rocket engines do the same thing they turn propellants into thrust. Liquid-powered engines use pumps to force the propellant into combustion chambers for ignition and then to a nozzle to produce thrust.

McKeon explained that the biggest difference between the Aerospike and other engines is the nozzle shape.

"In the bell shape, gas expands at the end of the nozzle," he said. "A bell nozzle is most efficient at one altitude."

Unlike space shuttle engines that use an engine bell, the Aerospike is shaped like a "V," called a ramp. An engine using a bell-shaped nozzle expands the hot gases on the inside, but the Aerospikes series of small combustion chambers shoots them down the ramp on the outside using the atmosphere on the other side to act as the engine bell.

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Building engine nozzles is a study in tradeoffs an engine must operate at a wide range of altitudes, but can operate at peak efficiency in only a narrow range. At higher altitudes, gasses could expand farther and produce better performance if a bell-shaped nozzle was longer, but conventional nozzles cant grow longer, thus the compromise in performance.

The Aerospikes plume is open to the atmosphere on one side, allowing it to expand, while it pushes against the ramp allowing it to operate efficiently at different altitudes.

Another feature of the Aerospike is the ability to vector or adjust the direction of thrust without actually moving the engine or "gimbaling." The space shuttle and other craft use hydraulic systems to physically shift the direction of the engine bell to direct the thrust. A pair of the boxy Aerospikes in parallel can adjust thrust in their four chambers to change the direction of flight.

McKeon said not needing the equipment to gimbal the engines, results in a major weight savings.

The box-shaped Aerospike also offers an aerodynamic edge for craft such as the wedge-shaped X-33 because the engines fit inside the profile of the X-33 while conventional engine bells might extend beyond the crafts outline.

Aerospike engines were initially developed during the Apollo era and a ring-shaped engine was considered for use on the space shuttle in the 1970s, but the thrust was somewhat less than conventional engines at the time.

One throwback to the moon landings is the current crop of Aerospikes uses the turbopump hardware and designs from the Saturn 5s J-2 engines.

McKeon said the Aerospikes turbopumps are a mix of modified and refurbished ones from a Saturn 5 and those built from scratch.

"Its really helped us a lot in development of engine, we knew the turbopump machinery and could design the engine around it," he said.

Following tests of the current Aerospikes or XRS-2200, engineers will begin work on the full-size engine, the RS-2200.

Even though the Aerospike is a mix of old and new, McKeon said he thinks the four years of development and testing this time around have been well worth it.

"As far as a major engine development goes, theres probably no better place in the industry to be," he said. "Our team is extremely excited about what weve done and what we have to do."