The motors -- two are used per flight -- are the primary component of the Shuttle Solid Rocket Boosters, providing 6.6 million pounds of thrust or 71.4 percent of what the Shuttle needs for liftoff.

"This purchase will support Shuttle launches for several more years," said Ben Goldberg, manager of the motor project at NASA's Marshall Space Flight Center in Huntsville, Ala.

"This contract includes performance, as well as cost incentives for our industry partner. We're seeking ways to reduce cost while maintaining the important level of safety. Our overriding requirement in this program continues to be safety," added Goldberg.

The contract calls for manufacture and delivery of the new motor components to NASA's Kennedy Space Center, Fla., to begin this fall and continue through September 2004.

Thiokol also will conduct post-flight review of the last motors flown, carrying the contract through 2005. In addition to 35 sets of flight motors, the contract also includes three motors that will be used in ground testing to ensure quality and prove new materials, manufacturing techniques and hardware suppliers.

The original solid fuel motor was redesigned in 1986. Each motor is about 126 feet long and 12 feet in diameter and contains 1.1 million pounds of propellant. The propellant is mixed and then cast in four hollow, cylindrical metal casings. When it solidifies, it has the color and consistency of a pencil eraser.

A forward dome containing the igniter and an aft dome with a steerable exhaust nozzle are attached. The motors burn for about 123 seconds before they burn out and are jettisoned to descend by parachute into the Atlantic Ocean.

During flight, each motor puts out the equivalent of about 15.4 million horsepower. The solid fuel motor's combustion gas temperature approaches 6,000 degrees Fahrenheit -- nearly two-thirds the temperature of the Sun's surface.

At that temperature, steel doesn't melt; it boils. After each flight, the segmented motors are recovered and disassembled. The cylindrical motor cases are cleaned, reinsulated and refilled with propellant. The exhaust nozzles are refurbished, and other components are replaced as needed. Nose cone and aft skirt assemblies are added to turn the motor into a completed booster.

With this new contract, NASA will have purchased 230 flight motors and 11 flight support motors to date. The Marshall Center manages the motor program. Thiokol, a division of Cordant Technologies, is the prime contractor.