NASA aiming for APU full resolution

by Chris Bergin

NASA is wrapping up troubleshooting on a pair of issues that affected Shuttle orbiter Discovery’s Auxiliary Power Units (APUs) during last month’s mission, STS-121.

Shuttle managers are hopeful the issue won’t arise with Atlantis, as she prepares to launch Sunday on STS-115, as engineers continue to gather more understanding related the APU double trouble.

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The first issue came up mid-way through the flight of STS-121, when one of two heater thermostats on APU #3 experience random power fluctuations.

The crew was commanded to switch to a backup unit, which experienced no problems for the rest of the mission. However, Space Shuttle Program Manager Wayne Hale explained that mission managers don’t want a repeat occurrence on Atlantis’ upcoming flight.

‘There are a number of potential causes we would like to rule out, things like are thermostats just getting old and failing. That would not be a good thing. We would also like to make sure that if it was a collateral damage or workmanship issue on Discovery, that we don’t have that same kind of problem on Atlantis. So those are the kinds of areas where we are continuing to do work.

‘Atlantis is okay, and there may have been some electrical fault due to workmanship or aging parts of something on Discovery. So whatever problem there is on Discovery is likely not there on Atlantis, but you would like to have a full resolution rather than going in with troubleshooting still in works. So we are going to talk about that at the L-Minus-2 (meeting).’

The more serious concern and one that was discussed in greater detail in the STS-115 MMT meetings was the appearance of a small fuel leak from APU #1 that came to light during the later part of STS-121. The leak was so minor that it ultimately also had no impact on the mission. Managers again don’t want to see a similar issue during next week’s mission, but now believe they have a better understanding of the issue.

‘What we are doing, the first anomaly that we really tried to resolve with the APU was this potential of a leak on APU-1 because that has somewhat more serious consequences.
‘So we spent quite a bit of time troubleshooting this leak on APU-1, and because that involves hazardous or toxic chemicals, hydrazine, and there it gets right next to the other APU, that means it is a serious operation to the troubleshooting. So we actually came to troubleshoot the thermostat problem later in this sequential nature of that troubleshooting.

‘We now believe that there was what they call a quick disconnect cover on the gaseous nitrogen side that had a very small Teflon seal that was deformed,’ added Hale, ‘and that probably was the source of that very, very slow leak, although folks are off doing the final tests to wrap that up.’

With those two issues being close to being resolved, Atlantis and her crew of six appear ready to resume space station with a launch on Sunday afternoon local time. They will complete a nine day mission to add the 17.5 ton P3/P4 Truss to the burgeoning International Space Station.

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