NASA is continuing to look at two problems that arose on STS-121 with Shuttle orbiter Discovery last month, of which neither proved to be a problem for the Return to Flight mission.
The source of a suspected leak on Auxiliary Power Unit (APU) 1 continues to undergo testing to find the source of the leak, while an overflowing Water Spray Boiler (WSB) is also being tested by the vendor.
The APU issue came to light on Flight Day 7, when mission controllers noted that one of Discovery’s three APUs – which produces pressure for the orbiter’s hydraulic system – was slowly leaking fuel.
These APUs are vital to the orbiter because they control the vehicle’s wing flaps, rudder, body flap and landing gear. Three APUs were built into the orbiter to provide some redundancy in case of a total failure. Nevertheless, APU 1 was the only unit built to hydraulically lower an orbiter’s landing gear.
After further on-orbit tests cleared the suspect APU for landing, it was used during the re-entry phase and performed flawlessly.
After landing in Florida, KSC personal performed leak point detection tests on APU 1 and were unable to find a source for the leakage seen during the mission, although the process is on-going.
‘Investigations have been delayed by ongoing main propulsion systems work in the aft compartment,’ noted documented information acquired from sources. ‘Work to date includes soap bubble checks at Quick Disconnect (QD) panel and hydrazine ‘sniff’ checks of the fuel tank. No hydrazine leakage has been detected. The tank insulation was intact and per spec, thus eliminating it as a cause of the signature seen during the flight.
‘In addition, the tank insulation was found to be completely intact, thus eliminating it as a cause of the signature seen during the flight.
‘Hydrazine leak checks will continue and if no source is found, the tank will be pressurized to around 400 psia with Nitrogen. If there is no success in detecting a leak during that test, the tank will be purged and filled with Helium. This will aid in leak path detection.’
Another issue came to light after landing, when it was discovered that one of Discovery’s Water Spray Boilers (WSB), which cools the lube oil system and hydraulic system by spraying water onto their lines, overflowed causing several blankets on the starboard side at the base of the vertical stabilizer to get wet.
‘WSB 3 was flown with a mixture of water and Propylene Glycol Monomethyl Ether (PGME) in its tank to test out the mixtureâ€™s ability to prevent the freeze up problem that has affected the WSB’s since the beginning of the shuttle program,’ noted the information.
‘WSB 3 overfilled its heat exchanger core with the water/PGME mixture. The mixture flowed out of its steam duct and several blankets were wet on the starboard side at the base of the vertical stabilizer.’
The issue was spotted by a technician, who noticed a stain of rust coloured appearance in the area. Upon further evaluations it was noted the blankets were actually wet from the PGME/Water overflow – which is a potential ignition source.
‘Prior to the mission, the problem resolution team (PRT) had not fully evaluated the hazard of PGME/water boiler overflow. The PGME water mixture is flammable and there are potential ignition sources in the vicinity of the WSB steam vent orifices,’ the information continued.
WSB 3’s tank on Atlantis has already been filled with a PGME water mixture in preparation for the STS-115 mission, but it is yet unknown how much of a hazard the water boiler overflow is for Atlantis. This could lead to WSB 3 being removed from Atlantis prior to flight.
For now, Discovery’s WSB is undergoing testing to find a reason for the problem.
‘The hydraulic PRT is also working on failure analysis for WSB 3 from Discovery. The plan is to remove WSB 3 from Discovery and send it to the vendor, Hamilton Sundstrand, for testing.’