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# Gravity Swing

We don't need no steenkin' rockets
 (+7) [vote for, against]

A space station parked between two gravity sources - for example the "zero g" point beween Earth and Moon - is going to need a method of station-keeping. Chemical or ion thrusters are obvious, but are also a bit wasteful.

In stead, this station - powered by a nuclear generator andor solar panels - throws out a weight on a cable to a precise distance and angle to be pulled in that direction before being reeled back in.

Gravity strength is cube law [edit: okay, inverse-square, similar result] so it actually works : despite the rest of the station being temporarily moved in the other direction, the mass on a string is being pulled more since it's closer to a gravitational source.

This method could be used to propel(impel ?) a spacecraft from such a resting point either into orbit around one or the other bodies, or perhaps outside the system altogether, building up velocity while "swinging" back and forth, the last pass a slingshot maneuver. That would probably work best with two cable+weights, one for each gravity well.

 — FlyingToaster, Oct 06 2017

When the cable goes taut, it's going to jerk the whole system into a weird little dance unless you have very carefully aligned the center of inertia of the rest of the craft.
 — RayfordSteele, Oct 06 2017

 //a weird little dance//

 So science: spacecraft: performance art ?

[+]
 — pertinax, Oct 06 2017

 //Gravity strength is cube law//

 Or "square law" as we say in this dimension.

[+]
 — MaxwellBuchanan, Oct 06 2017

[RS] If more than one were used simultaneously...
 — FlyingToaster, Oct 06 2017

Using bungee cord ought to damp out the "jerk" a bit.
 — Wrongfellow, Oct 06 2017

 //it's going to jerk the whole system//

Dampening is as easy as playing the line out and reeling it in as slowly as necessary. If that's not fast enough your line is insufficiently long and the weight isn't heavy enough. But wouldn't internal magnetism, material tension, and gravity keep this from working?
 — Voice, Oct 06 2017

 //But wouldn't internal magnetism, material tension, and gravity keep this from working?// ?? explain, pls.

 As far as "gravity" is concerned, that's the thing(s) that actually moves the station/spacecraft around.

 For a spacecraft (which is the fun one), imagine two cables with weights, shooting out the front of the craft at opposing 45deg angles. To start, the (much heavier) station moves straight backwards until the cables are taut, so that bit's useless.

 However, now each weight is closer to one gravitational source than the other, so will start to fall. As they fall (in opposite directions) they will pull the spacecraft forwards.

 Before the cables are at 90deg to the spacecraft, the weights are reeled in and snugged. At which time the craft will have a net forwards velocity, along the zero-G line between the two gravitational sources.

 At this point it's a swing that's been started from scratch: the spacecraft coasts along that line until it slows down, stops and starts to fall backwards, at which time the cables are sent out again (in the other direction this time). Etc.

As far as "internal magnetism" and "material tension" are concerned... ?
 — FlyingToaster, Oct 06 2017

 //throws out a weight on a cable to a precise distance and angle to be pulled in that direction before being reeled back in.//

hmm, wouldn't there be an opposite and equal reaction throwing the same amount of your stations' mass backwards?
The only way I can see to eliminate that is if the station is spinning.
 — 2 fries shy of a happy meal, Oct 06 2017

 Yes, Newton isn't on holiday and the rest of the station will move in the opposite direction. But the umm cannonball on a string goes quite a distance before stopping, whereas the much heavier station barely moves.

 This means the cannonball is closer to the gravity source it's been launched at than the station is to the other one.

 If gravity dropped off with distance linearly then that would be that, there'd be no net force one way or the other. But gravity drops off with distance exponentially, so the station is not only under less gravity but much less gravity, than the cannonball which is much closer to its source.

So the ball pulls the station in its direction.
 — FlyingToaster, Oct 06 2017

 I don't know that you can use gravity to effectively stay in orbit against gravity. Something seems amiss. You can decay the orbit I guess if that's the goal.

 I would go with 3 strings in 3 directions in order to keep the center of mass from whipping around in out of plane.

And unless you are able to stop them at exactly the same time, that weird little rotational dance is still going to occur.
 — RayfordSteele, Oct 06 2017

[RayfordSteele] I think [FlyingToaster] is dancing a reel at a Lagrange point. Playing the moon's gravity against the Earth's.
 — wjt, Oct 07 2017

 [RS] If it was under human control then you'd probably want to have a gyroscope assembly handy to sop up the spin. But, like most orbital mechanics it'd have to be done with a computer anyways. Nothing says the weight has to shoot out directly in line with the station CM.

One line is showing off, true : like one foot on a skateboard instead of two in rollerblades. Maybe four: one for each caltrop point.
 — FlyingToaster, Oct 07 2017

 I'd be interested in the masses, cable distances that give good numbers to the impulsion to verify feasibility.

Also it would be nice to have extremely dangerous sections of the station space gapped.
 — wjt, Oct 07 2017

 You mean like real numbers for say Terra<>Luna L1 ? dude, I forgot (or didn't know) gravity was inverse-square with distance, not cube. I'm not doing a 4 or 5 body problem.

 But I can do a simple one for illustration.

 Say I want to move a station at an L1 LaGrange point closer to one or the other gravitational sources.

 Specs: - Station masses twice as much as the cannonball. - Station is situated halfway between two identical point sources of gravity, 4 distance units away from each. - Tether is conveniently 3 units long.

 So we start by tossing the cannonball at one of the grav sources. It runs away really fast then stops short. At this time, thanks to Newton's meddling, the station will be 3 units away from one source; the cannonball is 2 units away from the other.

 Before we separated them, at 4u away from each grav source, net gravity (influence of each grav source) on the station was 1/16g - 1/16g = 0g, ie: balanced: the station's not going anywhere. But now, in their new positions...

 Station: 1/9g - 1/25g Cannonball: 1/4g - 1/36g

 The pull of the cannonball on the tether - even though it's half the mass of the station - is still more than that of the station.

So the system will move towards the cannonball side.
 — FlyingToaster, Oct 07 2017

I do think you'll want to keep the cm in line. Not doing so induces a torque. Simple. Bungee or no bungee.
 — RayfordSteele, Oct 07 2017

But - with the exception of the really simple manoeuver from my previous anno - the cannonball will be coming back at a different angle than it was sent out, pretty much by definition of the mechanism.
 — FlyingToaster, Oct 07 2017

 [+] But why keep throwing the cannonball out and pulling it back?

 Just leave the tether extended all the time. Gravity will put tension on the cable and keep it lined up. The station will be positioned slightly off from the L1 point. To adjust position, slowly wind up the cable a little or play it out a little depending on which way you need to move. As long as it is never pulled in more than some amount, gravity will help extend it. Initial deployment would still be tricky, but since that's a one-time thing, it could be done using rockets or something.

Unfortunately I suspect that you'd need an unreasonably long cable to be useful.
 — scad mientist, Oct 08 2017

 [FlyingToaster] Like [scad mientist] indicated, those calcs flesh out the idea's basis but it needs scaling/modelling to put on the stage of reality.

Nice idea though, two different sized Deathstars with a balanced highway system between them, running through the Lagrange point. That's extreme watching of your population's weight.
 — wjt, Oct 08 2017

 // leave the tether extended all the time // That would solve the problem of having to reposition the cannonball one side or the other.

 // unreasonably long cable //

"unreasonably" is such a harsh word. There's almost no tension on the cable.
 — FlyingToaster, Oct 08 2017

Well I'm all for it. Let's crowd source this puppy. Put me down for fifty.
 — 2 fries shy of a happy meal, Oct 08 2017

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