NASASpaceFlight.com Forum
General Discussion => General Discussion => Topic started by: Brian P on 05/20/2009 03:27 am
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How does the Hubble change it's orientation in orbit if it doesn't have any thrusters? I didn't see a hubble part of the q&a section.
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The gyroscopes that were just replaced. It also has magnetic torquers, which are magnetic coils that react to the Earth's magnetic field, and are used for de-saturating the gyros when needed.
Hubble doesn't need rapid pointing change ability. What it does need is very fine pointing stability, which the gyros are specially designed to accomplish.
So it's slow to slew to a new target (and most targets are pre-scheduled well in advance to allow for its orbit orientation to do a lot of the work), but has almost rock-solid stability once it's aligned there.
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The orientation is primarily controlled by four reaction wheel assemblies (RWA). Speeding or slowing a wheel causes a rotation about the center of mass in the opposite direction.
Orientation can also be controlled by four magnetic torquer bars (MTB) which are electromagnets that interact with the earth's magnetic field.
When an RWA is spinning too fast and needs to be slowed down, an MTB is used to hold the position. This is called angular momentum management.
There are also various complications that arise with failures of RWAs combined with limitations of the MTBs.
This paper discussion some of that: http://www.emergentspace.com/pubs/AAS%2008-279.pdf
Gyros are part of the sensor system. There are several different kinds of sensors that determine where the telescope is pointing.
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Anybody know if the gyros have fixed axis's. They say they can keep it pointed with two working gyros and I can't think of any way to do that unless they can exert a force to change gyro orientation.
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Anybody know if the gyros have fixed axis's. They say they can keep it pointed with two working gyros and I can't think of any way to do that unless they can exert a force to change gyro orientation.
The gyros they usually talk about are used for sensing orientation when the telescope is not locked onto guide stars. Orientation control uses reaction wheels, of which there are 4 (I think three mutually perpendicular and one skew to be able to replace any of those, but can't find that detail right away). The two-gyro pointing mode uses input from the star trackers to give the 3rd axis location (essentially roll about the telescope axis). This was engaged almost 3 years ago as a measure to stretch the telescopes useful life when SM4 was in doubt, turning one gyro off. Its major impact has been restricting sky visibility at a given time dramatically; the star trackers point about 135 degrees from the telescope, and both the telescope and trackers have to not be pointed at the Earth.
Lose two reaction wheels and you're in real trouble. (FUSE was able to operate with less using magnetotorquing on the geomagnetic field. This was an amazing piece of re-engineering and gave a couple of years' additional data, but it was pretty inefficient and didn't work over much of the sky).
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Thanks ngc. I was wondering about using the magnetorquers for control, but SM4_engineer said Hubble only used them for desaturation.
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Why was Hubble intended for servicing missions but not Compton? They were similar is size and scope plus developed around the same time, yet their servicing requirements were different.
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Why was Hubble intended for servicing missions but not Compton? They were similar is size and scope plus developed around the same time, yet their servicing requirements were different.
GRO was built for servicing. The subsystems were from the MMS program and the prop system could be refueled.
The instruments weren't made for servicing because they were large and there was no need to change them out. There aren't that many types of ways to observe gamma rays.