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Changing orbits

Posted by: Ekkehard Augustin - Fri Dec 10, 2004 9:53 am
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Changing orbits 
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Post    Posted on: Wed Apr 13, 2005 1:47 pm
That's a misunderstanding - I am not talking about releasing a vehicle or an object at a point of the space elevator below geostationary altitude.

What I am talking about is an artificial object with its own axis orbiting Earth. The plane of rotation is crossing the plane the cable of the space elevator is placed in - the plane of rotation is nearly a sphere high above Earth's surface. It's a plane all circular orbits of equal altitude are placed in.

After a vehicle has docked to that object, moved to a special point of its rotating part and stopped there that vehicle is rotating around the object's axis - and it is rotating parallel or nearly parallel to Earth's surface. In this situation it is possible to release the vehicle when it's moving NOT parallel to the object's orbit but parallel to Earth's surface.

That's what I have in mind.



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Post    Posted on: Wed Apr 13, 2005 1:54 pm
Do you mean a second tether, or beam, at right angles to the elevator and rotating around the elevator cable, with the object at the end of the beam, like this?

X <--- counter weight above geosynchronous orbit.
|
|
|---------------O <-- object to be released.
|
|
| <-- elevator cable.
|
|
|
|
EARTH


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Post    Posted on: Wed Apr 13, 2005 2:02 pm
Second tether at right angles to the elevator is right but currently I don't think of rotation around the elevator cable - it's separated from the elevator. It could be compared to a rotating space station that is orbiting Earth. But this space station isn't designed to be habitated by astronauts tourists or others - it's designed to catapult vehicles into new orbits. By far similar to a transit-spaceport...

To let it rotate around the elevator cable would be one or more steps further. In the past sometimes I was thinking about enabling to change orbits in three dimensions but currently it seems to be to early to consider that.



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Post    Posted on: Wed Apr 13, 2005 2:12 pm
OK, it is not rotating. If it is a flexible tether it would hang down parallel to the elevator, so it is a stiff beam. I'll have to think about that, but it seems the beam would at least have to be very long (hundreds of km) and very strong. Also it would probably need to extend on both sides of the elevator, to balance.


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Post    Posted on: Wed Apr 13, 2005 2:23 pm
I have been misunderstanding obviously. It is rotating - but not around the elevator cable.

It consists of an axis orbiting Earth. Around the axis a beam or a tether is rotating - if a tether is rotating then a counterweight would be required. But the axis is not the cable of the space elevator.

That's the current state of my thought(s).



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Post    Posted on: Wed Apr 13, 2005 2:35 pm
I guess I don't understand what you mean by "an axis orbiting Earth".


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Post    Posted on: Wed Apr 13, 2005 2:46 pm
The object is like a satellite or a space station. It consists of a rotating part and that part it is rotating around. The non-rotating part is the axis. One pole of the axis is at the lower side of the satellite or station pointing down to Earth's surface and the opposite pole is at the upper side pointing to the stars.

But this axis is not connected to the surface like the cable of the space elevator is - that cable is anchored at the surface.

This thing can be installed at any orbit - at a Low Earth Orbit at requiring higher velocity, a High Earth Orbit requiring less velocity than LEO, at Geostationary Orbit requiring geostationary velocity or a higher orbit. It can be installed at a polar, equatorial orbit or at an orbit between them. There could be more than only one such orbit-changer.

Currently I don't want to consider the special case of rotating around the elevator cable because that situation would be more complex.



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Last edited by Ekkehard Augustin on Wed Apr 13, 2005 2:53 pm, edited 1 time in total.



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Post    Posted on: Wed Apr 13, 2005 2:51 pm
OK, This sounds like a regular tether. Two masses connected by a cable and rotating around their common center of mass. And the whole assembly is in orbit.


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Post    Posted on: Wed Apr 13, 2005 3:03 pm
To me that looks as if their were two alternative versions of the thought:

1. A beam or a tether with a counterweight at one end and an axis to rotate around at the other end. This would be similar to the space elevator which has the counterweight at the space-side end and the Earth as axis to rotate around at the other end.

2. A beam or tether with the counterweight at one end, the axis to rotate around in the middle and the other end beyond that axis.

May be that in the case I have in mind the second alternative is working only.

If there only is the tether then the relation of the masses would determine where the axis is - what I have in mind is to let the engineers determine the point of the axis once for all...



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Post    Posted on: Wed Apr 13, 2005 3:10 pm
You may be confusing the words "axis" and "axle". An axis of rotation for an object that is not attached to anything but is freely falling, which is what an orbiting object is really doing, is just the center of mass of that object. It is not the same situation as a wheel attached to an axle and rotating in a bearing.


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Post    Posted on: Thu Apr 14, 2005 7:04 am
That may be - in German we say "Achse" in both of these examples: Earth has a "Rotationsachse" and the wheels of a car are mounted on a "Achse" which can break ("Achsenbruch"). For both of these I used the term axis here. The "Achse" the wheels of a car are mounted to sometimes is called a "Welle" in German - but this is considered to be a wrong use of the word "Welle". I never use it in such a context. May be that "axle" means "Welle" in German - I didn't know the word "axle".

Thank You Very Much for the hint.

In my last posts I imagined an axle rotating with a tether mounted to it. Then the same effect should result as in the case of the cable of the space elevator mounted to Earth's surface - the axle would be a substitute of Earth.



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Post    Posted on: Thu Apr 14, 2005 1:05 pm
Ekkehard Augustin wrote:
In my last posts I imagined an axle rotating with a tether mounted to it. Then the same effect should result as in the case of the cable of the space elevator mounted to Earth's surface - the axle would be a substitute of Earth.
It wouldn't be quite the same. The elevator attached to the Earth would be more stable, although the flexible cable would wobble and twist. An orbiting station not anchored to anything and would tend to spin in the opposite direction. By that I mean that as you spin an object about an axle in a clockwise direction, the rest of the station will want to spin counterclockwise.


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Post    Posted on: Thu Apr 14, 2005 1:22 pm
Depends on the relation between the weight of the axle and the counterweight.

The counterweight is required only to keep the tense of the cable.

A vehicle would contribute to the counterweight and I could imagine that the cable is enrolled around the axle as long as there is no vehicle to be docked and catapulted into another orbit. The cable then would be rolled down if a vehicle is to be docked. An alternative would be to readjust the location of the counterweight or the counterweight itself.

To me it seems that for keeping oversight there should be a heavy weight of the axle and a not that heavy weight of the counterweight.

A litlle bit this is a derivative of the concept of a vehicle to Mars that has artifical gravity: a crew module would be connected to an engine or drive module by a cable, a part of the engine/drive module would be set to rotation and this way the crew module would be rotating around the engine module. This would be done even if the engine would fire permanently (plasma drive, pulsed fusion drive and the like).



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