Mechanical Space Propulsion. Why isn't this possible?

  • Thread starter kerbtrek
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So I've been told this is impossible before, and while I understand some of the reasoning as to why, I still can't wrap my head around how this wouldn't work, or rather, what it would do instead of working.

So here's an image of what it looks like:

Its function is this:

Step 1: 'Forward' sphere rotates, flipping the whole panel and other sphere over.
Step 2: Other sphere performs the same action.
Result: Object changes position in space along a specific path.

DOdZWWe.png


Now there may be other things needed, such as mass shifting from one sphere to the other (i.e. a pumped liquid) so that one has more mass than the other and the sphere doesn't spin like a tire stuck in the mud. But is it really completely impossible?

While a mechanism like this would need to move very rapidly to be practical for space travel, it does seem like it could be much more practical in say, re positioning itself in orbit, without fuel, run solely on electricity without any need for gases to exhaust.
 

BvU

Science Advisor
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What it would do instead of accelerating (which is what you want from propulsion) is: move along with the center of mass following a straight trajectory at a constant velocity. There are no external forces, so Newton rules !
 

Mech_Engineer

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I think you may be thinking in terms of the "spacecraft" sitting on a table rather than free-floating. If you start one of the spheres rotating, you will be adding angular momentum and as such the whole system will rotate in the opposite direction around its center of mass (which is in the middle of the part). This method is loosely similar to how satellite gyroscopes control orientation of the craft in orbit, but a gyroscope cannot impart net linear acceleration of the craft.

https://en.wikipedia.org/wiki/Control_moment_gyroscope
 
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But if the center of mass is changed to only one sphere at a time (not the center of length) wouldn't that essentially make the rest of the apparatus rotate around the sphere? This is why I spoke of pumping a fluid between the spheres.
 

berkeman

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But if the center of mass is changed to only one sphere at a time (not the center of length) wouldn't that essentially make the rest of the apparatus rotate around the sphere? This is why I spoke of pumping a fluid between the spheres.
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