Today the ISS lost a second control moment gyro (CMG). One suffered mechanical failure 2 years ago, leaving 3 functional CMGs for attitude control.
Now, we're down to only 2 - the bare minimum needed for attitude control.
"We're not dealing with a safety issue," says ISS Ops Mgr Mike Suffredini. While he is technically right, we ARE dealing with increased risk.
With the CMG attitude control system now zero fault tolerant, there is great risk of wasting Russian Segment propellant for attitude control. You
see, the Russian Segment thrusters can handle attitude control if the CMGs fail completely.
There is enough prop to maintain attitude by RS thrusters for 6 months or more, but this is the same prop
used to reboost the ISS. The reboost
schedule depends on regular help from visiting vehicles (Progress, Soyuz, Shuttle). Visiting vehicles burn their own
prop while docked to
conserve station resources, and Russian visiting vehicles may even transfer propellant.
So, if we fall back to RS attitude control the reboost strategy gets very sporty. There may not be enough prop to both maintain attitude and reboost
if they stick to current 6-month crew rotation.
There is good news though. The newly failed CMG is believed to be in good shape, and it's only shut down due to a failed Remote Power Controller
(RPC) on the S-0 truss. There is an onboard spare available which, although sporty, can be replaced during during an increment ISS-based EVA after
the crew rotation is complete.
That means Mike and Gennady will have to EVA on their own, with no IVA crewmember to mind the shop inside. The good news is they are fresh and have
logged tank time doing similar tasks. The bad news is, increment EVAs are risky business, and they have not trained this particular procedure.
It will be interesting to see what happens.
Houston Chronicle (AP) report on gyro failure
[Edited on 23-4-2004 by Zion Mainframe]
[Edited on 4-23-2004 by Valhall]