Control Moment Gyroscope Notes
Below is a picture of a full-scale mockup the Control Moment
Gyroscopes (CMG's) as they'll be mounted on the International Space
Station. Some notes:
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- The two large black circles you see on each CMG are not part of
the flight configuration. Those are holes cut in the WETF mockups to let
- Note the box on the CMG at the lower right. That is the
Electronics Assembly, a removable avionics box containing all the
electronics associated with control and data collection for that CMG. Each
CMG has its own EA box. Note that they do not appear to be in the same
position on all four. That's because the CMG's are actually mounted in
four different orientations.
- The CMG's are mounted diagonally, but only because Rocketdyne had
to tilt them over to fit them all onto the Z1 truss. These CMG's were
originally designed to go on the S2 truss, where they mounted in a similar
cluster but oriented in alignment with the space station's Z axis. The
mounting angle doesn't really matter since the actual gyroscopes are free
to rotate every which way inside the CMG housing.
- Scale references: The CMG's are 48 inches in diameter. The
parabolic antenna at the top of the Z1 truss is 72 inches in diameter.
That is the space station's main Ku-band antenna. (Actually, the mockup is
a Radio Shack TVRO antenna, from the local Radio Shack at Baybrook Mall in
Houston.) These four CMG's have sufficient control authority to control
the attitude of a half-million-pound space station, so something this big
would be overkill on the Reference Mission
spacecraft or assembly node.
- The silvery box to the left of the CMG's is an EVA tool box.
- The gray thing the CMG's are bolted to is mockup of the Z1 truss.
This unit is a kludge, replacing the functions of the old S2 segment. The
Z1 truss exsits because of a commumnication problem between NASA and the
Russian Space Agency. NASA thought the Russians' FGB module would be able
to provide control for the whole space station, but the Russians meant that
it could control the Russian elements. The FGB also cannot provide the Nav
Base functions or power requirements of the whole space station. When NASA
and RSA finally got that communication across, it was clear that the U.S.
elements would have to provide those functions. Rather than bring back the
S2 segment, NASA decided to add the Z1.
- Everything you see is a WETF mockup. The Ku-band antenna and the
CMG's were built for one of my tests, the S2 Phase 1 WETF test, conducted
in Feb 92 at the JSC WETF. Rocketdyne built the Z1 mockup structure. The
mockup was tested as shown in this picture at the Neutral Buoyancy
Simulator at the Marshall Space Flight Center in Huntsville, Alabama.
- Each CMG has a mounting ring, sort of a belly band, that bolts in
six places to space station primary structure. You can see the little tabs
on the ends of the mounting rings. The two tabs next to the axis of each
CMG might not be so noticeable. The mounting rings on the real CMG's are
more distinct; the mockups' mouting rings were just molded as part of a
single unit along with the rest of the housing. The mockup housings are
made from vacuum-formed plastic. The real CMG's will be covered with
multi-layer insulation, so their texture will look quite a bit like the
mockups in the photo.
- There are handholds on the mounting rings, to aid the crew in
handling the CMG's.
- The black holes in the Electronics Assembly are mockup-specific.
- Trunnion pins and scuff plates are visible on the right side of the
- All those yellow things are EVA handrails.
- Photo credit goes to NASA, I think; it might be a Rocketdayne photo.
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