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  1. Would an electric centrifugal-force fan (like that found in a blower motor) work in space to create force, by putting a substance (like silica sand or something slippery) through the centrifugal fan and directing the output like an engine?

  2. Furthermore, could you then recapture the substance with a similar centrifugal-force fan spaced 1 or 2 meters apart with the input of the #2 fan directed toward the output of the first fan, and the #2's fan's output fed back to #1's fan as a closed looped design? Would this still create any type of force in space or a vacuum without gravity??

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Answer to question 1: I think Newton 3rd law would say that there will be a reaction force. So yes, I think it would work.

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    $\begingroup$ Please consider conservation of momentum in a closed system. $\endgroup$ – PM 2Ring Sep 1 at 12:01
  • $\begingroup$ @PM2Ring It looks like this answer is just focusing on point 1, which would be fine I believe. Point 2 is where we get into trouble. $\endgroup$ – Aaron Stevens Sep 1 at 12:05
  • $\begingroup$ @Aaron Ok. Point 1 is fine. Point 2 is not. $\endgroup$ – PM 2Ring Sep 1 at 12:07
  • $\begingroup$ Sorry if it was not clear, will edit the answer. $\endgroup$ – drandran12 Sep 1 at 12:16
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Point #1 is indeed how any "reaction motor" operates, including the small rocket thrusters normally used to do this.

Point #2 is ruled out by conservation of momentum: there is no way to make a "reactionless thruster" like this except in science fiction literature and movies. Here is an example which illustrates why:

Imagine that on our ship in space we have a big box of loose bicycle chain. we extend the end of that chain over a cog wheel connected to an electric motor. we turn on the motor so the cog wheel spits the chain out of the box and notice that this action produces a thrust force on the cog wheel, and our ship is propelled in the direction opposite the direction of the departing chain. That thrust lasts until we run out of chain.

Now imagine that we hook the two ends of the chain together so we never run out of chain, and turn on the cog motor. It spits chain out as before, but to recover the chain we must pull on it to reverse its direction and bring it back to the ship so we can spit it out again, right? The forces required to halt the chain's motion and reel it back in cancel the thrust generated by spitting the chain in the first place, and so this scheme cannot propel the ship.

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  • $\begingroup$ To point 2, then. Say instead of a fan, we just put a big collection vessel behind the first motor a few meters after its output. There's no way we could use the material in that collection vessel again, as long as it's tethered to the first motor? $\endgroup$ – bostongio Sep 2 at 11:48
  • $\begingroup$ as long as that collection vessel is tied to the body of the ship in any way, it will apply a force to the ship which exactly cancels the thrust generated by the cog wheel throwing the chain. $\endgroup$ – niels nielsen Sep 2 at 16:42

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