# Will there be torques on spacecraft due to interaction of magnetic torquer with the residual magnetic field of the spacecraft?

Assuming the magnetorquer and the field generating electronics are both rigidly fixed to the spacecraft cube. Reference to any supporting papers/articles would be great. Thanks.

Would the residual field affect the performance of the magnetorquer due to generation of back emf. To cancel which, more current must be passed to get the same torque (in case polarity of emf is opposing) and less current in the other direction where emf polarity is favorable. Constant external field assumed.

• Are you looking for the theoretical existence of some torques, however small, or are you looking for some practically significant value? If the latter, you should probably clarify a bit. Commented Sep 14, 2016 at 5:21
• Any torque on the magnetorquers will cause an equal and opposite torque on the spacecraft components they're interacting with; but since both are part of the spacecraft, the net torque is zero. (Otherwise you'd be able to turn "for free," violating conservation of angular momentum.) Commented Sep 14, 2016 at 8:09
• I agree with the previous comments. I can see two other directions for your question: a) just to calibrate the unintended effects what are the effects of other current loops - either from a changing current or a steady current on a spacecraft moving through a changing magnetic field, it might be easiest to evaluate these yourself for the situation of interest as its otherwise open-ended Commented Sep 14, 2016 at 9:16
• b) in a more complex system where there an active control loop with some attitude sensor (possibly even a magnetometer) being used to control the magnetorquer then how would you isolate the attitude disturbances from the unintended satellite current loops so that the control loop wasn't constantly firing the magnetorquer circuit to make corrections? I don''t know what normal practice is, just putting it out there. Commented Sep 14, 2016 at 9:18
• @Puffin I am not using magnetometer for the closed loop attitude control. However, would the residual field affect the performance of the magnetorquer due to generation of back emf. To cancel which, more current has to be passed to get the same torque (in case polarity of emf is opposing) and less current in the other direction where emf polarity is favouring. Constant external field assumed. Commented Sep 16, 2016 at 14:11

Yes, there is absolutely torques generated on the spacecraft from a combination of residual spacecraft magnetic field (static, produced by magnetic or paramagnetic materials in the construction) and dynamic resulting from the flow of electrical current around the spacecraft. There will be some interaction of these magnetic fields with the magnetic field of the magnetic torquers, however, that is typically minimal since the torquers generate much stronger fields. The individual torque sources, however, all add together in a vector sum.

On spacecraft where there is a desire to minimize these net disturbances on the control system, a magnetic balancing exercise may be conducted. During the design phase, use of magnetic materials are minimized. If they cannot be eliminated, subassemblies can be degaussed prior to integration. After integration, the residual spacecraft dipole can be measured then adjusted by adding trim magnets. This involves selecting a location which has minimal magnetic sources, measuring the location, then bringing the spacecraft to this area and spinning the spacecraft around to measure the effect on a stationary sensor to characterize the spacecraft dipole. This exercise can be done with the spacecraft in various power states to characterize the magnetic torque in the various basic modes of spacecraft operation. Note that solar arrays can be large contributors to the dipole as the loops the current flows through can be relatively large if careful attention is not paid to precisely how the backwiring is designed, so it may be critical to illuminate the panels and draw power from them for this test. Each principle axis of the spacecraft is measured by spinning the spacecraft around that axis. A vector sum can then be calculated and if desired, small permanent magnets may be epoxied to the spacecraft and the test rerun until the net magnetic dipole measurement is below the desired limit. A good description of a magnetic cleanliness program is found here: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710029225.pdf

• I don’t think a spacecraft can exert a torque on itself, via any mechanism. That would violate conservation of angular momentum. Magnetic cleanliness is about reducing the torque due to the craft’s magnetic field interesting with external fields. Commented Dec 30, 2019 at 3:12
• I've down voted (at least temporarily) because I can't understand how a spacecraft or any isolated body can exert a net torque on itself. Certainly there can be local torques, but I assume the question asks only about a torque about the entire spacecraft's center of mass, one that would induce rigid-body rotation of the spacecraft about it, and I don't think that that's what you are describing in this answer.
– uhoh
Commented Dec 30, 2019 at 6:05
• Maybe if you can just caveat your answer at the beginning explaining that there's no net torque, that's enough. Right now we have a yes and a no answer to the same question, but I don't think either answer intends to contradict conservation of angular momentum.
– uhoh
Commented Dec 30, 2019 at 6:07
• The magnetorquer provides an external torque while what you are talking about is an internal torque. If you think about it in a practical sense of any engineering system, when we say there is no effect, it means that the effect is infinitesimally small that it becomes negligible. I think the question you should ask is whether the magnetometer readings will be corrupted if a magnetic field is being produced internally.
– John
Commented Dec 30, 2019 at 19:01

No. No object can exert a torque on itself via only internal interactions. That would violate conservation of angular momentum.

Spacecraft are constructed to have controlled, usually zero, magnetic fields to reduce torque from external fields they encounter.