Mission managers update crew on shuttle APU issues
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 13, 2006
The Discovery astronauts took the day off today, relaxing and enjoying the view from space after a hectic week in orbit highlighted by three spacewalks and work to transfer supplies and equipment to the international space station. Texas Gov. Rick Perry plans to call spacewalker and fellow Texas A&M graduate Mike Fossum later today and flight engineer Lisa Nowak will join Fossum for two media interviews. Engineers, meanwhile, continue analysis of two seemingly minor issues with Discovery's hydraulic system.
In the morning "execute package" of instructions and timeline changes uplinked to the astronauts, mission control passed along a few tongue-in-cheek ideas for spending an off-duty day in space:
Good morning, Discovery. Since today is an off-duty day for you, the planning shift team has come up with a few ideas* to help you pass the time:
The morning execute package also included the latest thinking from NASA's Mission Management Team about the status of two of Discovery's auxiliary power units, or APUs, which provide the hydraulic power necessary to operate the shuttle's aerosurfaces, landing gear brakes and nose wheel steering during entry and landing.
The shuttle is equipped with three APUs and can safely land with just one. But given the critical nature of the system, engineers pay close attention when anything out of the ordinary occurs.
In a brief call to the crew late Wednesday, mission control told shuttle commander Steve Lindsey that engineers were monitoring two unrelated issues with APUs 1 and 3. The pressure in APU 1's fuel tank appears to be decaying faster than expected, indicating a possible hydrazine fuel leak or a leak in the nitrogen (N2) gas system used to provide pressurization. And APU 3 is experiencing heater problems.
Both issues may be related to work done before Discovery's launch, but that remains to be seen. Here is the MMT explanation of what has been observed to date:
"APU 1 Tank Pressure Decay - The APU 1 fuel pressure is decaying differently than the other two tanks which is indicative of a small N2 or hydrazine leak. The data is very subtle so it has taken eight days to detect this change in slope between the three APU fuel tank plots. APU 2 and APU 3 shows a normal pressure decay of 6 psi over the eight days of the mission. This is due to the temperature change in the tanks which is caused by a 9-10 degrees Fahrenheit decrease in the tank temperatures as the aft structure cools. It takes some time to see this change because ascent does a good job of warming up this part of the vehicle.
"APU 1 Tank pressure has dropped a total of 22 psi over these eight days which is indicative of a leak which is most likely N2. The Entry Flight Director, CB (astronaut office) Reps, and the entire APU community are meeting several times daily to understand the data and determine a forward plan of action. There was an N2 leak at the QD (quick-disconnect) panel during a portion of the STS-121 OPF (orbiter processing facility) flow that was later fixed; that has many in the community thinking that the leak is N2. Analysis is in work to determine if it is feasible to leak hydrazine based on the very slow leak rate and the propensity of hydrazine to freeze and slow an actual fuel leak. We hope to have more details on this subject (Thursday) but as of today there is no change to any mission plans based on the data collected. The APU is not considered failed and the N2 pressure will be more than 100 psi above the lower operating limit at EOM (end of mission).
"APU 3 GG/Fuel Line/Pump/Valve Heater - Both the A and B heaters appear to be controlling at their upper limit on the overtemp thermostat. Initially the B heater appeared to fail ON, as indicated by the Supply Line Temp, increasing from ~82 deg F to 190 deg F. The heater then operated nominally for two cycles, and then failed ON again. MMACS (mission control mechanical systems officer) asked to reconfigure the heater to the A system, where it is also operating on the overtemp thermostat.
"There are a few theories on the failure mode that could affect both the A and B heaters. This area was reworked before STS-121 with a new sealant such that the heaters were removed and rewrapped. If the wrap came loose on orbit, it would bias both thermostats cold and would appear as a failed on heater. If the heater is debonded, a similar signature would be expected. The teams continue to meet multiple times each day on this topic and additional data will be provided as the analyses mature. APU 3 is not considered failed for entry at this time and there are no significant impacts to entry operations."
At present, these do not appear to be serious issues, but given the critical nature of the APU system they bear watching. More information will be posted here as it becomes available.
The Mission Management Team congratulated the Discovery astronauts on the successful completion of their third spacewalk Wednesday and told the crew a lost spatula posed no threat of returning to the shuttle-station complex.
Shuttle and station managers "were extremely happy with the results of EVA 3 including the get-ahead task of transferring the grapple bar to ISS," the MMT report said. "FYI - The spatula that was released inadvertently during the EVA is well clear of the orbiter and ISS. It was 4 miles in front of the ISS/Shuttle at crew sleep on FD9 (flight day 9) and continues to move in front of and below the ISS with the distance increasing about 2 miles each rev."
Here is an updated timeline of today's activities (in EDT and mission elapsed time):
TIME...........DD...HH...MM...EVENT 07/13/06 01:08 AM...08...10...30...STS crew wakeup (flight day 10) 01:38 AM...08...11...00...ISS crew wakeup 03:43 AM...08...13...05...Crew off duty time begins 04:13 AM...08...13...35...ISS European Space Agency PAO event 09:03 AM...08...18...25...Crew meal 09:08 AM...08...18...30...ISS Russian PAO event 10:00 AM...08...19...22...ESA, Russian briefings replay with translation 10:03 AM...08...19...25...Crew off duty time resumes 11:00 AM...08...20...22...Mission status briefing on NASA TV 12:53 PM...08...22...15...MPLM cargo module cleanup 12:58 PM...08...22...20...Texas Gov. Rick Perry calls Mike Fossum 01:23 PM...08...22...45...MPLM cargo module racks configured 01:23 PM...08...22...45...MSNBC, FOX News interview Fossum and Nowak 03:30 PM...09...00...52...ISS-14 crew news conference 05:08 PM...09...02...30...STS/ISS crew sleep begins 06:00 PM...09...03...22...Daily video highlights reel on NASA TV