exiphm@eutrc3.urc.tue.nl (h.munk) (07/19/89)
In article <28782@ames.arc.nasa.gov>, yee@trident.arc.nasa.gov (Peter E. Yee) writes: > continues to perform twice daily momentum wheel desaturations ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Forive me my ignorance, but what is "momentum wheel desaturation", and why is it done ? Harm Munk
henry@utzoo.uucp (Henry Spencer) (07/20/89)
In article <805@eutrc3.urc.tue.nl> exiphm@eutrc3.urc.tue.nl (h.munk) writes: >Forive me my ignorance, but what is "momentum wheel desaturation", >and why is it done ? There are a number of methods of attitude control for spacecraft. Just using control jets works, but if the spacecraft is going to have to do a lot of turning, it uses a lot of fuel. Magellan is going to be pivoting around two or three times per orbit for months on end (it alternates mapping Venus and transmitting back to Earth, through the same antenna), so one would prefer an alternative. If you simply put a heavy little wheel on a motor inside, when you spin the wheel in one direction the spacecraft turns slowly in the other. When you stop the wheel, the turn stops too. In theory. The problem is that you can get torques on the spacecraft from outside forces, like gravity gradients and light pressure. Not large ones, but not insignificant. If the momentum wheel is used to counter those, it can end up spinning faster and faster when the spacecraft is stationary. So once in a while you need to put on the brakes for the wheel to keep its speed within bounds. (This generally means firing control jets at the same time, to keep the spacecraft from just picking up the wheel's rotation.) This is momentum-wheel desaturation. -- $10 million equals 18 PM | Henry Spencer at U of Toronto Zoology (Pentagon-Minutes). -Tom Neff | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
pgf@athena.mit.edu (Peter G. Ford) (07/21/89)
In article <805@eutrc3.urc.tue.nl> exiphm@eutrc3.urc.tue.nl (h.munk) writes: >In article <28782@ames.arc.nasa.gov>, yee@trident.arc.nasa.gov (Peter E. Yee) writes: >> continues to perform twice daily momentum wheel desaturations > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ >Forgive me my ignorance, but what is "momentum wheel desaturation", >and why is it done ? When mapping Venus by radar, accurate antenna pointing is very important. Unlike an optical imager, a synthetic aperture radar is "pointed" electronically, by choosing a precise time and frequency "window" within which to listen for echoes. If the antenna isn't pointing at the correct part of the planet, you don't pick up any useful data at all! During each orbit of Venus, the Magellan antenna, along with the rest of the spacecraft to which it is rigidly attached, must slowly rotate to trace a line on the planet's surface that follows the pre-chosen "mapping swath" for that orbit. Unlike the earlier generation of spacecraft which used small rocket motors for orientation, Magellan uses a set of three small gyroscopes, called "momentum wheels", each of which spins along one of three mutually perpendicular axes. To rotate the spacecraft about one of these axes, it is necessary to change the rotation rate of that particular momentum wheel IN THE OPPOSITE SENSE. Magellan, as an isolated body, must conserve angular momentumm, and the change in the angular momentum of the wheel is offset by that of the rest of the body. A rotation about an arbitrary axis can be put together out of a series of rotation changes of any two wheels. Wheels are better than rockets because they use only electric power (from the solar panels), not precious liquid fuel. But what, you ask, is a "momentum wheel desaturation?" Well, the wheels are much less massive than the rest of the spacecraft (technically, their moments of inertia are much smaller), so they must spin up to very high rates to turn Magellan in a reasonable time. To avoid their spinning so fast that they fly apart, there are auxiliary rockets on Magellan that are fired from time to time to give the spacecraft a strong twist in the opposite direction, thereby allowing the wheels to be spun down and "desaturated". The desaturation burns are bad news to those of us interested in the precise location of the spacecraft--e.g. for the altimetry experiment, or to determine the shape of the Venus gravity field--because, in addition to a rotation, they tend to push Magellan sideways by a random amount which must be allowed for in subsequent data processing. By a careful combination of wheel turns, the need for desaturation burns is minimized. Once in orbit, we hope to keep them down to less than one per day. Peter Ford MIT and Magellan Project
tneff@bfmny0.UUCP (Tom Neff) (07/21/89)
In article <12864@bloom-beacon.MIT.EDU> pgf@space.mit.edu (Peter G. Ford) writes: >But what, you ask, is a "momentum wheel desaturation?" Well, the wheels >are much less massive than the rest of the spacecraft (technically, >their moments of inertia are much smaller), so they must spin up to >very high rates to turn Magellan in a reasonable time. To avoid their >spinning so fast that they fly apart, there are auxiliary rockets on >Magellan that are fired from time to time to give the spacecraft a >strong twist in the opposite direction, thereby allowing the wheels to ^^^^^^^^ >be spun down and "desaturated". Jeez I hate to disagree with someone on the Project, but wouldn't the thruster burns be twisting the spacecraft in the SAME direction as the momentum wheels' accumulated spin, rather than in the OPPOSITE direction? If you add more opposite torque you'd have to spin the wheels even faster to compensate. By tweaking in the same direction, you allow[require] spindown of the wheels to compensate. That's what I said in my mailed explanation anyway. If it's wrong I'd like to know it. -- "My God, Thiokol, when do you \\ Tom Neff want me to launch -- next April?" \\ uunet!bfmny0!tneff
bobmon@iuvax.cs.indiana.edu (RAMontante) (07/25/89)
<12864@bloom-beacon.MIT.EDU> pgf@space.mit.edu (Peter G. Ford) : ->spinning so fast that they fly apart, there are auxiliary rockets on ->Magellan that are fired from time to time to give the spacecraft a ->strong twist in the opposite direction, thereby allowing the wheels to ->be spun down and "desaturated". tneff@bfmny0.UUCP (Tom Neff) <14480@bfmny0.UUCP> : - -Jeez I hate to disagree with someone on the Project, but wouldn't the -thruster burns be twisting the spacecraft in the SAME direction as the -momentum wheels' accumulated spin, rather than in the OPPOSITE -direction? If you add more opposite torque you'd have to spin the -wheels even faster to compensate. By tweaking in the same direction, -you allow[require] spindown of the wheels to compensate. No, think of it this way --- You can stop the spacecraft's spin by transferring it's angular momentum to the momentum wheel(s). If the craft is spinning in the same direction as the wheel, then its angular momentum has the same sign, so transferring the ang. momentum to the wheel increases the wheel's spin. So use the thrusters to give the craft a spin OPPOSITE to the wheel's spin. Then transfer this spin (ang. momentum) to the wheels, incidentally causing the craft to stop spinning again. The momentum has the opposite sign to that of the wheel, so the transfer decreases the net momentum, and hence the spin, of the momentum wheel. The "opposite torque <=> spin wheels even faster to compensate" argument would be true IF the torque came from the wheels, but it's coming from an outside source. The outside source, thrusters, apply a counter torque, which just happens to be transmitted through the spacecraft itself as an intermediary. Yet another view: imagine "hitting the brakes" on the momentum wheels. This will completely stop the wheels, *relative to the spacecraft*, and the craft will begin spinning in the same directions that the wheels were (albeit slower). Now fire up the thrusters to kill this spin.