Discovery's fuel leak stopped during overnight work
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: October 19, 2010
A small fuel leak in the plumbing used by the shuttle Discovery's maneuvering rockets apparently stopped during work overnight to double-check the torque on six bolts in a suspect flange. There were no obvious problems with the bolts and it's possible increased pressure in the line may have helped seat internal seals.
"Just because the leak went away doesn't mean it will stay away," a NASA official said.
The trouble involves seepage of toxic monomethyl hydrazine, or MMH, fuel at a crossfeed flange in the propellant plumbing of the shuttle's right-side orbital maneuvering system rocket pod. The shuttle is equipped with two such rocket pods, one on either side of the ship's vertical tail fin, that burn monomethyl hydrazine with an oxidizer, nitrogen tetroxide, to carry out maneuvers in orbit. Crossfeed lines in the shuttle's aft engine compartment allow propellants to be moved from one pod to the other.
After testing over the weekend, engineers were able to trace the leak to a specific area of a large flange in the crossfeed system where two sections of propellant line come together. But it is not yet clear whether the seepage was the result of trouble with seals in the flange or the flange itself.
The right-side OMS pod was removed after Discovery's most recent flight in April to repair a helium isolation valve in another part of the system. After the pod was re-installed in July, leak checks were normal and no obvious loss of pressure has been noted. But insulation around the flange was found to be damp with MMH over the weekend.
Overnight Monday, engineers checked the torque, or tightness, of the six bolts holding the flange together and none of the bolts moved, indicating they were properly torqued. At the same time, the temperature of the system was changed, resulting in increased pressure in the fuel line. Subsequent measurements found no more traces of leakage.
"That's not the scenario they anticipated," the official said.
The flange features two internal seals and it is possible the increased pressure helped seat one or both seat properly. In any case, engineers planned to pump helium into a test port Tuesday to pressurize the volume between the primary and secondary seals to look for any signs of additional leakage. Troubleshooters also may pressurize the line to flight levels of around 250 pounds per square inch to make sure the seals are properly seated and secure.
The propellants are extremely toxic and it is unlikely NASA would launch a shuttle with a known leak, however small. If problems persist, engineers could be forced to drain the propellants and replace one or both seals in the flange, work that would interrupt normal launch processing for two to three days. Replacing the flange itself at the launch pad would be much more difficult.
Discovery is scheduled for launch on a space station resupply mission at 4:40 p.m. EDT (20:40 GMT) on Nov. 1. If the shuttle is not off the pad by Nov. 5 or 6, the flight will be delayed at least to early December because of conflicts with other launches, already planned space station spacewalks and temperature restrictions due to the station's orbit.
Senior NASA managers plan to attend an executive-level flight readiness review at the Kennedy Space Center next Monday to review ground processing and to set an official launch date.