NASA is loaning a rocket engine to Rocketplane Ltd. , the Oklahoma City-based firm that intends to carry paying passengers into suborbital space starting in 2007.

Under a Space Act Agreement negotiated with NASA’s Johnson Space Center in Houston, Rocketplane is borrowing a single RS-88 engine that the company says it plans to use to get a head start on ground- and flight-testing the four-seat Rocketplane XP vehicle.

The engine was designed and built by the Boeing Co. ‘s former Rocketdyne Power & Propulsion unit, for use on Lockheed Martin’s Pad Abort Demonstration vehicle, a project that was canceled when NASA dropped the proposed Orbital Space Plane in favor of the Crew Exploration Vehicle (CEV) it plans to use both for missions to the international space station and trips to the Moon. (Boeing sold the Rocketdyne Unit in 2005 to United Technologies Pratt & Whitney. The company is now known as Pratt & Whitney Rocketdyne.)

The engine was never flown but was test fired 14 times for a total of 55 seconds.

In exchange for the three-year loan of the engine, Rocketplane agreed to provide NASA with data from its design, development and test efforts, as well as about $30,000 to cover NASA’s expenses associated with the equipment loan, according to NASA spokesman J.D. Harrington.

In addition to the engine being loaned to Rocketplane, Harrington said NASA has three RS-88s in storage at its White Sands Test Facility near Las Cruces, N. M . He said those engines have been set aside for NASA’s Exploration Systems Mission Directorate, which is expected to decide by March whether it wants to use them in the CEV program.

Rocketplane Executive Vice President Dave Urie said in a Jan. 25 interview that the company has a custom kerosene-fueled engine in development for the Rocketplane XP at Polaris Propulsion in Simi Valley, Calif., but borrowing the slightly overpowered RS-88 engine from NASA will enable the company to initiate test flights in early 2007.

The Rocketplane XP consists of a Lear executive jet fuselage that has been outfitted with a delta wing and v-shaped tail meant to provide good subsonic and supersonic flight characteristics. Powered by two CJ610 turbo jet engines and a liquid oxygen-kerosene rocket engine, the Rocketplane XP is being designed to carry four people to an altitude of nearly 100,000 meters providing several minutes of weightlessness before making a runway landing.

Urie said the RS-88 has the potential to carry the Rocketplane XP all the way through a test flight program meant to culminate around mid-2007 with a series of suborbital spaceflights. But the company ultimately plans to power the Rocketplane XP with the regenerative-cooled AR-36 engine that Polaris is working on. Regenerative-cooled engines circulate propellant or oxidizer through the combustion chamber wall to keep the engine from overheating. They tend to be lighter and more efficient than other engines.

Urie said the AR-36 should be undergoing ground testing by the time Rocketplane is flying with the help of the RS-88.

Before Rocketplane can mount the RS-88 to the aircraft and start flying, Urie said the company must first demonstrate on the ground that the engine is a good match for the vehicle.

Although the RS-88 was designed to run on kerosene, Urie said the engine has only been tested using an alcohol-based fuel. Additionally, the engine’s demonstrated ability to kick out 60,000 pounds of thrust is well in excess of the 36,000 pounds needed to power the Rocketplane XP. Urie said a big part of the ground-test effort planned will entail showing that the RS-88 can run on kerosene and safely operate below its intended thrust level and at a lower chamber pressure.

Rocketplane’s interest in incorporating the RS-88 into its test program is fairly recent. “I first got onto the RS-88 last April when I was visiting my old friends at Rocketdyne and they suggested that might be the engine to use,” Urie said. “I offered to buy it from Rocketdyne and they said it belonged to NASA.”

Urie said Rocketplane initially pursued a fully reimbursable Space Act Agreement with NASA under which the company would pay NASA for the engine and take title to it. “NASA suggested they loan it to us and that appealed to us because as a start-up we try to conserve cash,” he said.