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I need to determine the torque that should be provided by the RCS thrusters for a slew maneuver of a 3axis-stabilized spacecraft (say 50x50x60cm and 50kg). Too much torque can cause unwanted structural vibrations and too little results in a very high thruster activity. Is there a method or systematic/analytical approach to determine the torque output required by the Reaction Control Subsystem?

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closed as unclear what you're asking by kim holder, Fred, Deer Hunter, TildalWave Dec 27 '15 at 10:10

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Please clarify what you're after. The inertia tensor is given by the mass distribution, slew rates are mission-driven. Torque is simply the one necessary to achieve mission maneuvers and counteract external and internal forces. $\endgroup$ – Deer Hunter Dec 26 '15 at 17:43
  • $\begingroup$ Sorry for the confusion. I need to determine the torque that is required for a slew maneuver. Too much torque can cause unwanted structural vibrations and too little results in a very high thruster activity. $\endgroup$ – Mr.Bones Dec 28 '15 at 9:56
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    $\begingroup$ Please edit the conditions into the question. It kind of gets unwieldy as soon as you do away with the rigid approximation, all the frequency response spectrum has to be taken into account, and "very high thruster activity" is an imprecise term. The usual way of doing that is having an objective function to maximize, constraints to respect, and a structural model to spit out numbers. $\endgroup$ – Deer Hunter Dec 28 '15 at 10:01
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If you are asking about how to determine the torque output from a given system with reaction wheel and/or thrusters, then you need to start with the specifications of those devices. Reactions wheels will generate torque only along their specific axis of rotation, otherwise you can simply look up what the rated and maximum torques are for that wheel. Thrusters generate torque based on their offset distance away from the vehicle centre of mass. To compute the torque you need to look up the rated thrust for that thruster then use the cross-product of the thrust vector and the position vector from the centre of mass to the thruster position. You can then combine all of the torques from each device to determine the next torque in all three directions.

On the other hand, if you are asking about how to determine how much torque you need, then you will have to follow Deer Hunter's comment and break down your problem some more so that you can determine what motions are needed and then convert that to torque requirements based on angular acceleration. For example, you could assume you'll be slewing from one orientation to another using a bang-bang maneouvre (apply some constant torque to start moving, then do nothing while moving, then apply some constant torque to stop moving). Then you can define how quickly you want to accelerate/decelerate and determine how much torque is required.

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  • $\begingroup$ There's one more possible option of where this question might be going: writing specs for prototype devices. You're projecting a reaction wheel and you want to determine what torque it generates. $\endgroup$ – SF. Dec 27 '15 at 2:15

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