Vern Wedeven calls it the most perplexing – and most critical – tribological problem hes ever faced. In 1993, he was working as a consultant for Pratt and Whitney on a huge contract to stretch the life of bearings in the turbopumps of Americas space shuttles. Seven years into the project, Pratt and Whitney remained stymied in their search for a way to reduce the tremendous wear to which those bearings were exposed.
With spending on the project hundreds of millions of dollars over budget, Congress decided enough was enough. The project was given a deadline of three months. If a solution wasnt found by then, the plug would be pulled.
A tribology problem had put that whole program in jeopardy, said Wedeven, president of Wedeven Associates,whowill recount his experience this month in a keynote address at the annual meeting of the Society of Tribologists and Lubrication Engineers. If the problem was not resolved within weeks, they were going to cancel the program.
The bearings that were causing such a big problem provided the rolling surface for the shaft of the turbopumps that moved liquid oxygen and liquid hydrogen to the shuttles main engine. These pumps operated for just a brief period during each flight: the eight and a half minutes needed for the shuttle to reach orbit.
Those eight and a half minutes, however, subject the bearings to horrendous conditions. They start outat cryogenic temperatures, courtesy of the liquid oxygen. When the pumps start, hot gases used to power a turbine raise temperatures at one end of the shaft to the neighborhood of 1,400 degrees F. In addition, the pumps – one pair for the oxygen, another pair for the hydrogen – have to work at tremendous speeds to expend 500,000 gallons of fuel in eight and a half minutes. How fast? The pump shafts turn at 30,000 rpm.
As if that wasn’t tough enough, these bearings are straddled with a big handicap. They cannot be lubricated with hydrocarbons, which explode when in contact with liquid oxygen. Nor could NASA use any other type of liquid lubricant, since it would freeze at the cryogenic temperatures.
From the first shuttle launch, in 1981, NASA used a solid film lubricant. But the pump bearings still wore excessively so that in early years, the space agency had to replace them after every flight. Later,NASA stretched the interval to two flights. Still, each pump replacement cost millions of dollars. The expense, not to mention the risk associated with such high wear rates, was deemed unacceptable.
The Pratt and Whitney contract was not the first attempt to find a better solution. Rocketdyne, the space contractor that designed the pump, sought an answer for several years in the late 1970s and early 1980s. It could not find a way to lengthen the life of the bearings.
NASAs Marshall Space Flight Center, in Huntsville, Ala., took up the challenge next, but did not find a solution. Finally, NASA decided to try another contractor, awarding an Alternate Turbopump Development contract to Pratt and Whitney in 1986. Seven years later, whenthe three-month deadline was imposed, it looked as if Pratt and Whitney would fail, too.
Out of desperation, and with one or two weeks before the deadline, researchers tried a combination of design changes, which included making the rolling elements of bearings out of silicon nitride, instead of stainless steel. The new design also included pre-coating the bearings with a different dry film lubricant, but Wedeven says he cannot discuss details because they are proprietary to Pratt and Whitney. None of those involved knew if the silicon nitride would hold up, Wedeven said, but it did.
No one inhis right mind at that time would dare to enter a ceramic like that into a man-rated flight machine, he said. With steel bearings, we were worried about excessive wear, but now youre worried about them breaking apart and causing a catastrophic failure of the mission.
We really just tried it out of desperation based on limited testing at NASA and Wedeven Associates. But it worked so well it was like a miracle, and people couldnt figure out why it did work. Later on, we came to understand why. It brought into play different types of tribology mechanisms.
The silicon nitrite solution was quickly incorporated into a new turbopump design, and the new pumps were first installed in a shuttle two years later. Last month, NASA removed the last of the old pumps still in service. Whereas the old pumps had to be overhauled every two missions, the new ones now run 10 missions before they are scheduled for overhaul.
Part of the benefit is the tremendous cost savings of not having to do those expensive overhauls so frequently, Wedeven said. The bigger benefit is the reduced risk that comes from not having so much wear on the bearings.
The shuttle turbopumps marked the first major use of ceramic rolling elements in a man-rated system for flight. Silicon nitride bearings have since been used in a variety of otherapplications, such as machine tool bearings.
But Wedeven says he was struck most by the nearly show-stopping effect that a tribology problem had on such a huge engineering program.
This was the most dramatic example that Ive been associated with where tribology was such a key issue, he said. It demonstrates what an important role tribology can play.
Wedeven plans to discuss his experience on Pratt and Whitney’s Alternative Turbopump Development projectduring a keynote session on May 20 at the Society of Tribologists and Lubrication Engineers’ Annual Meeting in Houston. The session is titled “Improving Equipment Reliability Through the Science of Technology.” Other speakers include Astronaut Bonnie J. Dunbar; Judith Harrison, of the U.S. Naval Academy; and Ward O. Winer, of the Georgia Institute of Technology.