Special space shuttle fueling test expected next week
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: June 23, 2009
Shuttle managers plan to meet Wednesday to review procedures for a fueling test next week to assess the performance of an alternative internal seal and shim-like washers intended to eliminate a leak in a gaseous hydrogen vent line that has twice grounded the shuttle Endeavour.
If higher-than-allowable leakage is again detected, program managers could be forced to move Endeavour to a different tank and boosters, triggering more significant downstream delays.
Engineers believe they understand the problem - a slightly misaligned vent port housing built into the side of Endeavour's external tank - but it remains to be seen whether the proposed fix will, in fact, eliminate the leaks that have grounded Endeavour.
The 7-inch-wide hydrogen vent line in question is needed to carry away potentially dangerous hydrogen vapor that builds up in the tank as it is loaded with fuel. By cycling a valve in the vent line, the proper internal pressure is maintained and excess hydrogen gas is routed to a "flare stack" near the pad where it is harmlessly burned away.
The launch pad vent line is attached to the side of the tank with a ground umbilical carrier plate and a quick-disconnect fitting. At liftoff, an explosive bolt is detonated and the vent line drops away from the tank.
Endeavour was grounded June 13 and 17 when sensors detected hydrogen concentrations greater than 40,000 parts per million at the umbilical plate interface.
Engineers initially believed the problem involved a slight misalignment of the vent line umbilical plate and/or trouble with a Teflon seal in the quick-disconnect fitting. A similar problem grounded the shuttle Discovery for four days last March.
The seal was replaced, but the system leaked again during fueling for the June 17 launch try. After additional analysis, engineers concluded the rectangular vent port housing itself, which was riveted into the side of the tank about six-tenths of a degree out of vertical, was preventing the umbilical plate/quick-disconnect fitting from maintaining a tight seal under cryogenic conditions.
The proposed fix is to install a two-piece flexible seal in the quick-disconnect fitting in place of the more rigid single-piece Teflon seal used earlier. In addition, the umbilical plate that connects the vent line to the tank will be mounted using different shim-like washers to help counteract the effects of the vent port misalignment.
In so doing, engineers hope to maintain a tight fit at the vent line interface despite the temperature-induced shrinkage and flexing that occurs when the tank is filled with propellants. The leak only shows up when the equipment is exposed to cryogenic conditions.
To save time and minimize the chance of introducing any variables that might affect the test, Program Manager John Shannon told the engineering team to forego adding any additional instrumentation for the upcoming fueling test. Instead, Endeavour's tank will be in normal flight configuration when it is loaded with a half-million gallons of liquid oxygen and hydrogen rocket fuel.
Two fueling test timelines are under consideration but managers favor a plan that calls for installing the new seal late this week and re-rigging the umbilical plate attachments Saturday. The fueling test itself would be expected between 7 a.m. and 10 a.m. on either June 30 or July 1.