West Palm Beach, FL -- Consider a rotating mechanism, turning at about 24,000 rpm, with an end-to-end temperature differential of 1,300° to 1,500°F. Consider further that it carries a shaft horsepower rating exceeding 25,000 and pumps some 7,400 gallons of liquid oxygen per minute. These are the operating conditions for the oxidizer turbopumps that serve the main engines of the Space Shuttle-not a trivial design.
The original pumps for both fuel and oxidizer have served admirably with no failures. After each flight, personnel remove them from the Shuttle and inspect them. After two flights, the pumps must return to the factory for analysis, at a cost of between $300,000 and $1 million.
In 1986, NASA awarded Pratt & Whitney a contract to develop alternate high-pressure fuel and oxidizer turbopumps for the Space Shuttle Main Engine (SSME). The goal: Further improve reliability and reduce the costs of removal and inspection. While the liquid hydrogen (fuel) pump project was suspended for a time, the oxidizer pump is now in flight-qualification tests.
High-quality, fine-grained, investment castings in the new pump replace approximately 200 welds in the previous design. The seven welds in the new pump are all easy to inspect-many in the old design were not reachable for inspection. The new design includes no sheet-metal components.
Equipped with silicon nitride balls running in steel races, the specially designed pump bearing further enhances reliability. At the pumping end of the turbopump, liquid oxygen starts at its boiling point, -297°F, and warms to about -250°F.
At these temperatures, and particularly in the presence of liquid oxygen, conventional lubricants will not work. In this bearing, bronze-filled Teflon™ inserts on the bearing-ball separator provide lubrication through a mechanism called solid- film transfer. As the balls rub against the inserts, they wipe off a thin film of Teflon made by DuPont, which then transfers to the ball-race contact point.
"We have had tremendous success, beyond our expectations, with the silicon nitride balls in this bearing," says Program Manager John Price. "The silicon nitride is harder, lighter, and has higher thermal conductivity. This design reduces all the wear-producing mechanisms and also generates less heat."
Other advantages of the silicon nitride balls include lower centrifugal loads and lower thermal expansion. Their reduced sliding characteristic leads to improved fatigue and wear life.
A typical Shuttle mission requires main-engine firing for about eight minutes. The new oxidizer pump provides a design life of 55 missions or about 7.5 hours of operation.
Silicon nitride balls are also candidates for the cryogenic end of the liquid-hydrogen turbopump. Operating conditions resemble those found in the oxidizer pump, but liquid hydrogen is some 100°F colder than liquid oxygen. Development has resumed on the fuel pump, and the schedule calls for first flight in September 1997.
Additional details...Contact Patrick W. Louden, United Technologies, Pratt & Whitney, Box 109600, West Palm Beach, FL 33410, (407) 796-6793.