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Using the Humanity Star (see this answer for example for background/context), consider two possibilities for spinning a small spherical shell at a low rate, say perhaps in the neighborhood of 2 to 10 RMP.

To spin a spacectraft in LEO, one could use tangential thrusters as discussed in How did the tangential thrusters for the 2014 LDSD test spin-up then spin-down so nicely? and also What ever happened to SpinSat - did it work? for example, or one could use a battery, motor, and flywheel. In either case a timer or signal would activate the spin-up once it is safely in orbit.

What would be the tradeoffs for each? Perhaps mass, reliability, safety? Are there others?

Let's assume it's a 1.3 meters in diameter shell (most mass at the surface) perhaps a mass of 10 kg.

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    $\begingroup$ The kick stage has attitude thrusters that could spin the stage before releasing the payload. That way you don't need a dedicated mechanism. $\endgroup$ – Hobbes Feb 1 '18 at 17:14
  • $\begingroup$ @Hobbes that's a good point! The background here is the idea that the "mystery triangular panels" are for tangential thrusters and so your comment might be instructive there as well. $\endgroup$ – uhoh Feb 1 '18 at 22:56
  • $\begingroup$ @Hobbes I'm now wondering if they are instead photovoltaic panels which might be useful to maintain the sphere's spin over its ~8 month lifetime, considering Rocket Labs' emphasis on electric power. $\endgroup$ – uhoh Feb 1 '18 at 23:00
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In general, mechanisms such as reaction wheels are the most "undesired" parts of a spacecraft as they are statistically the ones most prone to failure. This is why they are typically used for station-keeping requirements like instrument pointing and attitude corrections.

On the other hand, small solid boosters are relatively inexpensive (compared to mechanisms), much more reliable and flight proven. They also do not require batteries, a bus and perhaps solar arrays. Thus, if the objective is to only spin-up and/or spin down a spherical spacecraft, this would be the obvious choice.

Finally, as a side note, for spin-stabilised or other satellites that require spinning to fulfil their mission, the spin is usually provided by the upper stage of the launch vehicle before it releases the payload.

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    $\begingroup$ Some electric power will be necessary for ignition of the solid boosters. $\endgroup$ – Uwe Feb 1 '18 at 15:30
  • $\begingroup$ You might want to rephrase "mechanism" to emphasize the high rotational speed that is so characteristic of these. $\endgroup$ – Nathan Tuggy Feb 1 '18 at 15:59
  • $\begingroup$ @NathanTuggy this disposable satellite is an empty shell, and possibly quite light, so in this particular case it might not need very high speed. Because it is spherically symmetric, it also is not likely to pick up much angular momentum, so it won't be accumulating angular momentum until a "maxed-out" wheel can be unloaded. It just needs a nudge, so that it looks 'pretty" when it slowly rotates. $\endgroup$ – uhoh Feb 1 '18 at 16:02
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    $\begingroup$ @uhoh: True, but in general the failure rates for these come because they have to frequently rotate at tens of thousands of RPM (or even higher). "Mechanisms" in general do not have such a failure rate; it's specifically flywheel-type devices that are being referred to. $\endgroup$ – Nathan Tuggy Feb 1 '18 at 16:09
  • $\begingroup$ @NathanTuggy Ah, I understand what you mean now. $\endgroup$ – uhoh Feb 1 '18 at 22:52
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Any acceleration of the spacecraft requires a reaction mass. With a rocket, that reaction mass is immediately expelled. With a flywheel, that reaction mass is retained. If one wishes to repeatedly increase and decrease the rotational velocity, then retaining the reaction mass means that it can be reused. If we're simply accelerating the spacecraft once, then there's no advantage to retaining the reaction mass. Since the change in momentum in the reaction mass must match the change in momentum of the spacecraft, increasing the velocity of the reaction mass decreases the amount of reaction mass required. Rocket exhaust can be accelerated to greater velocity than a flywheel , and so is more efficient.

Furthermore, if the purpose of the spinning is to create gyroscopic stability, then retaining the flywheel would mean that the spacecraft as a whole has zero angular momentum, and thus would defeat the purpose.

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  • $\begingroup$ A good point! Since the purpose here is to establish a constant rotation, the throw-away reaction mass of thrusters seems more suitable than the rotating wheel, which would require nearly-lossless rotation (no sources of mechanical friction or eddy-current losses), or replacing some of the triangular reflective panels with photovoltaics to maintain a small source of battery power for occasional make-up torques by the motor. Of course, there may be sources of mechanical friction or eddy-current losses that slow the spacecraft rotation, in which case the PV segments and motor would be helpful. $\endgroup$ – uhoh Feb 1 '18 at 22:50

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