As you know, last week Terry, Anton and I passed our final exams. The really big one, of course, is the full day Soyuz exam, where we simulate everything from launch to reentry. I’ve talked about how it all works when we took the exam as backup crew:
Back then, when it was time to pick one of the five envelopes with the exam scenarios, we happened to pick the most difficult (and physically uncomfortable) scenario, the one with the fire. Since we picked that one, it was not available for the prime crew to pick on the next day.
This time, our backup crew did us the same favor: they had to face the fire scenario in their exam on Thursday, so when we showed up on Friday to pick from the four remaining envelopes, at least we knew it wouldn’t be fire for us again!
Our first failure was after insertion: a thermal control system valve failed, so for the rest of the sim we had to control temperature manually by turning on and off the pump that circulates water to the radiators.
Our CO2 scrubbing system in the orbital module also had a minor failure: the primary fan engine failed and the automatic switchover to the backup engine didn’t happen, so we had to take care of that manually as well.
Then we had a computer failure before docking, at a couple of km from Station, and Anton had to fly the approach manually from there. As you might remember, we practice that quite a bit and there’s even a separate exam for that:
After the lunch break it was time to simulate undocking and descent. We could assume that the main computer was back online at this point and did a nominal undocking, after which we realized that one of the oxygen tanks, the one located in the descent module, was leaking oxygen into the cabin. That’s a dangerous situation, because we don’t want the oxygen percentage to go over 40%, which is considered a flammability hazard. So we closed a valve to isolate that tank. Until separation, we anyway have four more oxygen tanks in the service module and after separation, we had enough oxygen in the cabin to breath until landing, so no need to open the valve. (If you don’t remember what separation is, I explained it here: https://plus.google.com/+SamanthaCristoforetti/posts/dm5pW651AkL).
But we did have to reopen the valve after the thermal shield was blown away: that happens at about 5 km altitude, well after parachute opening. Once the thermal shield is gone, a command is sent to open two redundant valves (over which we have no manual control) that allow remaining oxygen in the descent module tank to be vented. It would be quite dangerous to have the tank full of oxygen at landing, so we had to remember to open the manual valve as well, to allow the venting to occur.
In between we had a couple more failure, of course. The main engine failed on us halfway through the deorbit burn and a signal converter in the reentry control system didn’t work, leaving us without gyroscope and rate sensor: only solution, switch to ballistic reentry. Not even the “prime” ballistic reentry system, but a backup one, that makes use of a its own backup rate sensor.
The Soyuz does have a lot of options to down-mode reentry following all kinds of failures: one way or another, it brings you home!
Photo credit: GCTC