The maiden voyage of the space shuttle Enterprise will not take place before Thanksgiving of 1979, a delay of at least five months in the space agency's plan for the spacecraft.
The second postponement in a schedule that originally called for the first orbital flight next March, the new delay is the result of continuing development troubles in the testing of the space shuttle's main engine. The engine tests are more than six months behind schedule, and are no more than one third of the way to being complete.
"Our progress in being able to bang off the tests and accumulate the time we need is slower than we'd hoped," said J. R. Thompson, space shuttle engine manager at the Marshall Space Flight Center in Huntsville, Ala. "We're running into development problems."
What Thompson has set as the space agency's goal is an accumulation of 60,000 seconds of engine running time, which means starting up the space shuttle's engine and running it at full power for several minutes several hundred times. So far, the accumulated engine test time is 20,000 seconds.
The shuttle engine was tested three times last week, the last test having been terminated after 65 seconds of a scheduled 300-second test run. A fuel turbine overheated just after the test began, forcing the shortened test run.
"It was nothing serious," Thompson said. "A typical nagging developmental problem."
The space shuttle engine is the most advanced rocket engine ever built. Made to burn supercold liquid hydrogen and oxygen, the engine has a rated thrust of 350,000 pounds. Its high-speed turbine blades spin at 36,000 revolutions per minute. The amount of energy released during combustion in a space shuttle engine is the equivalent of 60,000 horsepower.
When the space shuttle lifts off the ground for the first time, it will have three of these engines burning at full bore to help life the shuttle into space. Solid rocket motors strapped onto the sides of the shuttle also will be burning at liftoff, providing the rest of the thrust needed to propel the DC9-sized shuttle into orbit.
The toughest developmental trouble in the space shuttle's engine has been in the turbopumps that feed hydrogen and oxygen into the combustion chamber. Twice, turbine blades in these pumps cracked during tests, once at the 3,000-second mark and a second time at the 4,000-second mark.
At the time the turbine blades cracked, there was some question about the suitability of the steel used to make them. Thompson said a review cleared the blade material, blaming improper damping devices between the blades for the two cracks.
"We improved the damping," Thompson said. "These are the fastest turbines we've ever built, so there was bound to be something about them we didn't know ahead of time."
The pump that feeds liquid oxygen into the firing chamber has also been troublesome. The injector tubes cracked from fatigue on several occasions, causing the oxidizer to burn through the tubes instead of going into the firing chamber. The tubes have since been strengthened.
Tests also showed that the turbine's drive shaft bent too easily when the engine went to full power. Thompson said this problem has been corrected.
"We stiffened the seals along the shaft and went to heavier-duty bearings," he said. "We've had 800 or 900 seconds of engine time with this stronger shaft right now, and it seems to be holding up quite well."
The last problem that could be described as serious occurred July 18, when a fire in an instrument installed in one of the pumps forced the engine to shut down prematurely. Thompson said the fire had nothing to do with the engine.
"We're not using that instrument anymore," he said. "It was a one-of-a-kind test anyway, and we think we got all the information we needed."
Upcoming is a test planned with the engine running at full power for 520 seconds, which is what all three engines will be asked to do when the shuttle makes its first orbital flight.
When that test is finished, tests will begin in which three of the shuttle's engines will be run at full power simultaneously. This is the same way the engines will be firing when they actually lift the space shuttle off the ground. At least four of these tests are planned.
The tests are being run at two engine test stands Marshall operates at the National Space Technology Laboratory at Bay St. Louis, Miss. Two months from now, a third test stand will be opened at Santa Susana, where the Rocketdyne Co., which builds the shuttle engine, is located.
Thompson said that everybody involved in developing the shuttle engine is convinced that it's a good engine. Said Thompson: "The basic design is a good one, but in something like this you always go a little slower on the front end. We'll get this thing moving at the right speed before long."