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Keeping rotating tethers in balance
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One of the problems with the rotating tether idea (see link) is that when the payload is captured, the balance is thrown off by it's weight. This could cause a decay in the teher's orbit. My solution is to have the couterweight move closer to the COG when the payload is attched. the torque will ajust to keep it spinning right.
 — the great unknown, Jun 27 2007

rotating tether http://www.tethers.com/MXTethers2.html
a way to fling stuff into orbit. note the tether's orbit decays after the release. [the great unknown, Jun 27 2007]

the lever http://en.wikipedia.org/wiki/Lever
note excample 6: wheel [the great unknown, Jun 27 2007]

 How can the counterweight NOT move FARTHER from the COG when adding a weight at the far end.

 Picture it this way. You have a ten kilogram counterweight on the end of a ten kilometer cable that weighs one kilogram per kilometer. Your center of gravity, and therefore center of spin is right at the point where the counterweight attaches to the cable. Ten kilograms of counterweight on one end, ten kilograms of cable on the other. Now lets say you attach a one kilogram payload to the other end of the cable, your total system now weighs 21 kilograms and the COG is a half kilometer from the counterweight. You have a ten kilogram counterweight and a half kilometer of cable, weighing a half kilogram on one side of the COG, totaling 10.5 kilograms and a one kilogram payload and 9.5 kilometers of cable on the other, also weighing 10.5 kilograms.

Your Center of Gravity (actually a center of mass, but it is really the same thing). is always going to be at the balance point between the two ends of the cable. If you have some sort of hub, it is going to have to move back and forth on the cable to find the dynamic center of mass.
 — Galbinus_Caeli, Jun 27 2007

 //it is going to have to move back and forth on the cable to find the dynamic center of mass.//

 That is what I'm saying. Have the counterweight move on the cable when it picks up the payload so the COM stays the same. since the cable is spooled in/out, it won't add/remove weight from the system.

Another thought experiment: say you have a lever/fulcum, 1 m long. if you put 1 kg weights on either end it will balance when the fulcum is 1/2 m away (in the middle) now add 1 kg to one side. you will have to either move the fulcum (as you suggest) or add length to the side that still weighs 1 kg.(my idea) in order to keep balance. a wheel/axis, as well as the tether, works on the same principles as the lever.
 — the great unknown, Jun 27 2007

 Two things. First, why do you have anything at the center of spin of this tether?

 Second, why do you care how far the center of mass is from the counterweight?

 I suppose you could spool cable onto a winch on the counterweight to keep it a constant distance from the center of mass, but why?

 In my example above the 10kg counterweight could just be a 10km spool of cable and when you add a payload it could just spool up enough cable so that the mass of spooled cable equalled the mass of unspooled cable plus the payload mass. But I don't see what you are gaining here.

 The orbiting tether is still going to be losing momentum to the payload it accelerates. (Or gaining it in the case of a deorbiting payload.)

Adding complexity without adding function only makes sense when you are selling software or cars.
 — Galbinus_Caeli, Jun 27 2007

//...the balance is thrown off by it's weight. This could cause a decay in the teher's orbit.//

You don't understand the basics. The orbit decays because of loss of momentum (which goes to the boosted payload), not because of imbalance.
 — ldischler, Jun 27 2007

 What would you rather have: a rotating tether system that can knock itself off it's axis each time it picks up a payload or a system that has the ability to correct this?

If you see the diagram on the 1st link, if the rocket was coming from the opposite direction, the payload would add momentum to the tether.
 — the great unknown, Jun 27 2007

Why do you have an axis? It is a pointless complication.
 — Galbinus_Caeli, Jun 27 2007

//If you see the diagram on the 1st link, if the rocket was coming from the opposite direction, the payload would add momentum to the tether.//

No, according to the diagram, at the point of capture, they--the end of the tether and the payload--are traveling at the same speed, which makes the capture possible. But the CG of the tether system is travelling faster, so it is slowed by the capture of the payload.

Using a car tire as an analogy, the axle travels at a constant velocity, but any point on the circumference comes to a stop once each revolution when it comes in contact with the road surface. The tread can then pick up a rock at zero speed and release it at the uppermost point, where it's traveling forward at twice the axle speed. This slows the car.

To add momentum to the tether system, the payload would have to travel faster than the system CG, be caught by the tether as it was traveling forward, then released on the back-swing.
 — ldischler, Jun 27 2007

 Exactly, ID, why can't the rocket carrying the payload move faster than the tether? The momentum would transfer. It would be just like how the old railroad used to catch mailbags without stopping.

 I know I'm playing the "what if" game:

Let's say that there is some equipment that the tether needs, like most satellites, to stay in orbit. And let's say this stuff needs to be in a stable position. The only place on the tether is the COM. But wait, when it picks up the payload, the COM shifts, so all that junk is thrown out of whack. My solution keeps this area stable.
 — the great unknown, Jun 28 2007

The way this works is, the tether grabs the projectile when it's travelling at the same speed as the end of the tether--otherwise, you're trying to catch a bullet with your teeth. If you do this right, you can transfer momentum from the tether system to the payload, which decreases the cost of putting the payload into orbit. The momentum loss by the system is then made up by other means, described in the article. Changing the COM will not cause the orbit of the tether system to decay (that's caused by the exchange of momentum). And moving your counterweight closer will not keep the COM "stable." Quite the opposite, in fact.
 — ldischler, Jun 28 2007

Then how about moving it away? I know keeping the COM will not effect the orbit, but I'm suggesting there may be other reasons to keep it stable.
 — the great unknown, Jun 28 2007

 Ok, so you have something at the hub of your tether system.

 You had not mentioned that before. Tethers are usually considered in isolation purely as tools to transfer momentum to payloads. If you are including one as part of a larger system you are going to have do something like you suggest.

Most tether/catapult systems I have seen analyzed are also designed to orbit very low to minimize the need for high energy lift vehicles. Then they just transfer the payload to a higher orbit where it can rendezvous with its true destination.
 — Galbinus_Caeli, Jun 28 2007

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