SCI/TECH: Loss of a Second Gyro on ISS

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]

This thread was moved to ATSNN from the space forum. Gosh, I guess that means you liked it. Thanks!

Amet,

I changed the title, because the gryo hasn't been determined to have "failed", they just can't use it. And the EVA sounds like it might be replacing the RPC instead of the gyro, right?

[Edited on 4-23-2004 by Valhall]

Originally posted by Valhall
Amet,

I changed the title, because the gryo hasn't "failed", they just can't use it. And the EVA sounds like it could be work on the RPC instead of the gyro, right?

They don't need to do an EVA on the gyros. They are all internal to the ISS.

[Sorry, I accidentally clicked edit...nothing edited though...I'm doinked.]



[Edited on 4-23-2004 by Valhall]

[Edited on 23/4/04 by COOL HAND]

Uuuuhh...negatory good buddy. They are EXTERNAL to the lab.



[Edited on 4-23-2004 by Valhall]

Originally posted by Valhall
Uuuuhh...negatory good buddy. They are EXTERNAL to the lab.

They don't need to do an EVA for the Gyro's.

Really? Where at? I was told that they were placed at opposite axis inside the modules to allow the crew access to them for checkups.

They are on the Z1 Truss. They were taken up on STS-92. The FGB and Lab were without CMG's until this assembly mission.

P.S. This is on the zenith hatch of the Unity module which is between the FGB and Lab.


[Edited on 4-23-2004 by Valhall]

Here ya go:



Sorry that the pic has the station "on its side". The "top" (zenith) is to the right. See that square thingy attached with the antenna? That's the Z1 Truss. The CMG's are housed in it.

The CMG went out due to RPC failure. The bad RPC is on the S0 truss, and it will require an EVA to replace it.

Cool Hand: the CMGs are external on Z1.

Okay, I have gotten confirmation that this is, in fact, a RPC. And it is an external one. It looks as though the plan is to fix this on Expedition 9. Mike Fincke will be the American onboard at that time.