Is it possible to have some sort of a spaceship that basically runs on electricity and "butt kicks" itself forward? Or would it stand still because every force has an equal force in the opposite direction?
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2$\begingroup$ The only form of space propulsion we currently know requires the ejection of a reaction mass. This spacecraft "butt kicks" against what? grc.nasa.gov/www/k-12/airplane/conmo.html $\endgroup$– Jacob KrallCommented Jun 7, 2016 at 13:46
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1$\begingroup$ @JacobKrall currently it's no longer fashionable to say photons have mass. One can also deflect solar wind, and push against magnetic fields. In those cases there is a reaction mass, but not really "ejection" thereof. $\endgroup$– uhohCommented Jun 7, 2016 at 13:51
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3$\begingroup$ @uhoh Yes, the answer is in the question, and it has to do with regular physics--nothing special about space. On the other hand, if the asker wishes to know if it is possible to somehow "push against the vacuum", he should check out our posts on the EM drive keeping in mind that it seems to defy our current understanding of physics and may not be a valid form of propulsion (it is not yet proven). $\endgroup$– called2voyage ♦Commented Jun 7, 2016 at 14:20
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1$\begingroup$ In case the ship "ejects" the "hand" afterwards... A railgun as rocket engine? If the Yugoslavian EDO-1 indeed reached 4500m/s of muzzle velocity, that would make a respectable 450s of Specific Impulse. The others (like the US Army's 3km/s) aren't that respectable - a very sub-average rocket can do 300s in void, and with vastly less necessary dry mass. $\endgroup$– SF.Commented Jun 7, 2016 at 14:37
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1$\begingroup$ Reactionless drives violate the laws of physics, but reactionmassless drives don't. Would you consider a gravity assist (flyby) to be mechanically propelled? $\endgroup$– gerritCommented Jun 8, 2016 at 11:19
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3 Answers
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It would stand still, because of the equal and opposite force rule (aka conservation of momentum).
On earth's surface it's possible to have a self-propelled butt-kicking machine by having a slow "wind-up" phase alternating with an abrupt "kick". During the wind-up, static friction with a ground surface holds the vehicle in place, but the kick phase can break the frictional threshold. However, the momentum of the system of (ground + vehicle) remains conserved, ultimately using the Earth's vast mass as a "momentum sink".
In space, with no ground to brace against, the butt-kicking machine goes nowhere.
answered Jun 7, 2016 at 15:15
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3$\begingroup$ It's been tried: en.wikipedia.org/wiki/Dean_drive but those darn laws of physics just get in the way. $\endgroup$ Commented Jun 7, 2016 at 15:38
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2$\begingroup$ So Queen's documentary on what makes the world go 'round was close, but slightly inaccurate? $\endgroup$– corsiKaCommented Jun 7, 2016 at 17:28
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$\begingroup$ I'm trying to imagine this machine - one mass (the butt) is "standing" and the other mass (the kicker) sticks out to the side - when they come together, the whole thing actually moves in the direction of the kicker and not in the direction of the kick? This sounds somehow familiar - is there already something thad does this - a toy, or an animal or something? $\endgroup$– uhohCommented Jun 8, 2016 at 1:46
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1$\begingroup$ I wasn't thinking of any particular real world implementation. If you slowly hoisted a rigid pendulum "boot" then released it to swing against the "butt" resting on the ground, I think net movement would be in the kicking direction? $\endgroup$ Commented Jun 8, 2016 at 1:50
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$\begingroup$ This is the Newtonian explanation, but he is wrong at more exotic scales. I think it boils down to: Is gravity weaker that electricity? $\endgroup$ Commented Jun 8, 2016 at 5:49
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If you used 1.000.000 cubic meters of steel to create a 1 mm² wire spanning a large distance of 1000 million km you might be able to tug spacecraft along it.
Not very practical though and the wire might be unstable or snap. IF it worked and you could get something like 70 000 N out of it (~70 kg on Earth) that would be quite good though.
You would always need to have spacecraft going both ways to keep it stable and each craft would have to share the max load with all the others.
answered Jun 7, 2016 at 20:00
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$\begingroup$ Doesn't look anywhere near practical: 70.000 N isn't going to accelerate that billion kilometer of wire at any measurable rate, let alone the space ship attached at the end. $\endgroup$– MSaltersCommented Jun 8, 2016 at 11:00
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1$\begingroup$ The cable would orbit the sun. The ship would pull on the cable using say a pair of wheels. I'm not saying it would work, just my best guess for mechanical propulsion. Very similar to/same as space tethers I think. Btw 70000 N is a lot in space when applied over many days. $\endgroup$ Commented Jun 8, 2016 at 15:08
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Depending on what you count as a 'mechanical' you could consider Project Orion. (Technically a nuke uses nuclear energy, not mechanical but it would be 'pushed' by the explosion)
Basically, detonate a nuke and use it to propel you. So a very hot kick in the butt.
http://99percentinvisible.org/article/retro-rockets-nuclear-explosion-powered-spaceships-atomic-age/
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1$\begingroup$ It doesn't use electricity for propulsion, and it kicks out a very hot, very radioactive, very fast-moving plume of exhaust behind it in pulses. Essentially, it's just an unusual type of rocket, rather than the unique mode the OP describes. $\endgroup$ Commented Jun 8, 2016 at 4:18
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$\begingroup$ What are you referring to with the "It doesn't use electricity for propulsion"? And that is a fair assessment of this answer. $\endgroup$ Commented Jun 8, 2016 at 5:29
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$\begingroup$ The fact that, at most, it uses electricity for a very minor thing (positioning the nuclear charges), not the primary motive force. The propulsion proper is entirely nuclear-thermal. $\endgroup$ Commented Jun 8, 2016 at 5:31