NASA Relies on Thrusters to Steer Station

NASA Relies on Thrusters to Steer Station

NASA Relies on Russian-Made Thrusters to Steer International Space
Station Following Malfunction

The Associated Press


CAPE CANAVERAL, Fla. Dec. 5 — NASA is relying on Russian-made
thrusters to steer the international space station following a new
malfunction with the U.S. motion-control system, officials said
Friday.

Flight controllers detected spikes in current and vibration in one of
the station's three operating gyroscopes on Nov. 8. Last week, when
the gyroscopes were used again to shift the position of the orbiting
outpost, all three worked fine.

To prevent further trouble and give engineers time to evaluate
everything, the gyroscopes will not be used for at least the next
month and the Russian thrusters will assume control, said flight
director Joel Montalbano. The station must be periodically moved into
a new position to prevent the exterior from getting too hot from the
sun.

The main drawback is the use of thruster fuel. For now, the two-man
station has more than enough fuel to spare, said program manager Bill
Gerstenmaier.

A fourth gyroscope broke in 2002. Only two good gyroscopes are needed
at any given time to control the space station.

"It's not where we want to be and we definitely don't want to get
there, but we have much backup capability ... and we're not in any
kind of real crisis," Gerstenmaier said.

Gyroscopes are too big to fit into a Russian supply ship, so NASA
cannot send up a spare until the shuttles are flying again. The
shuttle fleet has been grounded since Columbia broke apart during
re-entry in February.

http://abcnews.go.com/wire/US/ap20031205_1639.html

Rusty B wrote:
NASA Relies on Thrusters to Steer Station

Last time I saw the numbers for required upmass it was pretty tight
conserning especialy water. Without gyros and back to RCS the station would
spend a lot more fuel (IIRC it was a new feature of Mir). Is there someone
here that has an idea of the problems this might lead to?

Sincerely
Bjørn Ove

Bjørn Ove Isaksen wrote:

Rusty B wrote:
NASA Relies on Thrusters to Steer Station

Last time I saw the numbers for required upmass it was pretty tight
conserning especialy water. Without gyros and back to RCS the station
would spend a lot more fuel (IIRC it was a new feature of Mir). Is there
someone here that has an idea of the problems this might lead to?

Sincerely
Bjørn Ove

If they only have two CMGs, then they can use only two and still maintain
attitude control without using any fuel. They might have to slightly modify
the software, but it should be doable.

Treat the sum of the torques on the Station as a controller in place of the
broken CMG. Leave the remaining two CMGs perpendicular to each other, but
not perpendicular (45 degrees might be best) to the sum of the torques.
Allow the two remaining CMGs to fight the sum of the torques. When a CMG
become saturated, flip over changing the sign on the sum of the torques to
desaturate the CMG. The two remaining CMGs and sum of the torques would be
able to perform the flip maneuver. I would think that it would give an
almost full range (only slightly degraded) of attitude control away from
the stable attitude. Essentially, allow the sum of the torques to act as a
controller in place of the third CMG and pick an attitude where the sum of
the torques is never close to being perpendicular to the other two CMGs.

Of course, maybe they already have this downmode capability built into the
software and have downmoded to thrusters because they aren't concerned
about fuel usage.

Things get a little tougher with only one CMG, but it could still be used
to minimize fuel usage.

Craig Fink

Craig Fink wrote in
ink.net:

If they only have two CMGs, then they can use only two and still
maintain attitude control without using any fuel. They might have to
slightly modify the software, but it should be doable.

Correct. ISS uses two-axis CMGs, so two CMGs gives you three-axis control
with one redundant axis. No software changes needed. And in fact, that is
what they are doing right now.

Several of the stories in the media appear to be incomplete (surprise!) ISS
is currently using CMGs for attitude hold, thrusters only for maneuvering
from one attitude to another. Or at least that was the case last I checked.

--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.

Jorge R. Frank wrote:

Several of the stories in the media appear to be incomplete (surprise!)
ISS is currently using CMGs for attitude hold, thrusters only for
maneuvering from one attitude to another. Or at least that was the case
last I checked.

Thanx for clearing that up. The only diffrence is as I understand it that
only attitude manuvering is now done by thrusters, instead of CMG's.
Attitude hold is still done by CMG's. This should'nt have a big impact on
fuel usage as it is'nt preformed all the time.

Sincerely
Bjørn Ove

Craig Fink wrote in message link.net...

If they only have two CMGs, then they can use only two and still maintain
attitude control without using any fuel. They might have to slightly modify
the software, but it should be doable.

It's already in the software. Two can be used (and have been used).

Of course, maybe they already have this downmode capability built into the
software and have downmoded to thrusters because they aren't concerned
about fuel usage.

There were some inaccuracies in the reports that came out. The use of
thrusters only was for attitude *maneuvers*, not attitude control
during normal bore-hole-in-the-sky operations. Maneuvers are
relatively uncommon events, maybe once every few weeks. During
steady-state ops, control is still going to be maintained on CMGs and
that's where all the propellant savings comes from. The reason there
isn't much concern on prop usage for maneuvers on thrusters is
because, frankly, they really aren't much less efficient than using
CMGs with thruster assist. Maneuvers were utilized with CMGs in
control more for operational convenience than anything else (no
control handover to the Russian Segment required).

Things get a little tougher with only one CMG, but it could still be used
to minimize fuel usage.

Interesting point. Someone who worked on Skylab told me IBM was
developing one-CMG control software when they started to have problems
with a second CMG after the first one failed (Skylab only had three).
I've not seen any evidence of how far that got.

I would imagine that the Hubble Space Telescope staff is working the
same issue right now.


(cyclone96) wrote in message . com...
Craig Fink wrote in message link.net...

If they only have two CMGs, then they can use only two and still maintain
attitude control without using any fuel. They might have to slightly modify
the software, but it should be doable.

It's already in the software. Two can be used (and have been used).

Of course, maybe they already have this downmode capability built into the
software and have downmoded to thrusters because they aren't concerned
about fuel usage.

There were some inaccuracies in the reports that came out. The use of
thrusters only was for attitude *maneuvers*, not attitude control
during normal bore-hole-in-the-sky operations. Maneuvers are
relatively uncommon events, maybe once every few weeks. During
steady-state ops, control is still going to be maintained on CMGs and
that's where all the propellant savings comes from. The reason there
isn't much concern on prop usage for maneuvers on thrusters is
because, frankly, they really aren't much less efficient than using
CMGs with thruster assist. Maneuvers were utilized with CMGs in
control more for operational convenience than anything else (no
control handover to the Russian Segment required).

Things get a little tougher with only one CMG, but it could still be used
to minimize fuel usage.

Interesting point. Someone who worked on Skylab told me IBM was
developing one-CMG control software when they started to have problems
with a second CMG after the first one failed (Skylab only had three).
I've not seen any evidence of how far that got.

(Explorer8939) wrote in
om:

I would imagine that the Hubble Space Telescope staff is working the
same issue right now.

Not quite the same issue. HST's gyros are rate sensors, while ISS's CMGs
are effectors.

--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.

Flight controllers detected spikes in current and vibration in one of
the station's three operating gyroscopes on Nov. 8. Last week, when
the gyroscopes were used again to shift the position of the orbiting
outpost, all three worked fine.

They are talking about the CMG's.

I'm not thrilled about the use of the word "gyroscope" as a gyroscope
is generally a sensor rather than an actuator (hence the "scope" part
of "gyroscope").

On different web sites I saw CMG expanded as "Control Moment
Gyroscopes" or "Control Moment Gyros".

The Russian term "gyrodynes" does seem like a logical one when
considered in that light.

Anyway, enough discussion of terminology. Let's hope that they can
keep the CMG's limping until they can replace some of them. The
replacement of at least one of them is on the return to flight shuttle
mission according to
http://spaceflight.nasa.gov/shuttle/future/index.html

Re below, it shows the prudence of having two different techniques for
attitude control though, in my view. Good bit of design sense there in the
original design.

anyway, exactly how big are these gyros? Could one be sent via some kind of
expendable launcher and a cobbled together manoeuvring system?

Brian

--
Brian Gaff....
graphics are great, but the blind can't hear them
Email:
__________________________________________________ __________________________
__________________________________


"Jim Kingdon" wrote in message
news | Flight controllers detected spikes in current and vibration in one of
| the station's three operating gyroscopes on Nov. 8. Last week, when
| the gyroscopes were used again to shift the position of the orbiting
| outpost, all three worked fine.
|
| They are talking about the CMG's.
|
| I'm not thrilled about the use of the word "gyroscope" as a gyroscope
| is generally a sensor rather than an actuator (hence the "scope" part
| of "gyroscope").
|
| On different web sites I saw CMG expanded as "Control Moment
| Gyroscopes" or "Control Moment Gyros".
|
| The Russian term "gyrodynes" does seem like a logical one when
| considered in that light.
|
| Anyway, enough discussion of terminology. Let's hope that they can
| keep the CMG's limping until they can replace some of them. The
| replacement of at least one of them is on the return to flight shuttle
| mission according to
| http://spaceflight.nasa.gov/shuttle/future/index.html


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