Tethered Space Stations

John Creighton · 2004-08-28 19:37:52

One Reason for a tethered space station is artificial gravity. Another reason may be an elevator between orbits. Such an elevator could spin but it might not be necessary. In a non spinning arrangement the bulk of the mass would be at a high stable altitude well the docking port would extend down lower where there is more drag. Although such a station wouldn’t be stable maybe it would still require less fuel to keep it stable then is used to boost the ISS. Probably some engineering could be done by balancing the drag and gravity to get as stable an arrangement as possible.

GCNRevenger · 2004-08-28 21:26:47

Time for a little conservation of energy concern... as the saying goes, you can't get somthing for nothing, so you are going to have to supply the energy to move a payload from a lower altitude to a higher one, it just depends on how.

dicktice · 2004-08-29 12:00:28

Also "up" don't forget, to help acheve escape velocity from LEO. Lot's of ways to replace the loss of orbital height, too numerous to mention.

Dook · 2004-08-29 17:46:21

How come they've never built a giant turning wheel in space? One just like you see in the old science fiction movies. It seems to me that wouldn't be too difficult and would provide many advantages. Gravity on the outer edges where the living spaces would be and no gravity at the center where you could have a large pressurized hangar for servicing satellites or space shuttles.

I was disappointed when I learned the ISS was going to be just a bunch of modules. Is it that hard to build a wheel?

Grypd · 2004-08-29 18:06:16

It is dificult to build a wheel type station without the ability to create orbital construction. Modules sent from Earth are easier to assemble as they fit inside the shuttle or in the rocket sent up. To create a wheel you must actually make the tresses in space and then fill them up. A hard bit would be if you wish to make the central part of your station not spin as this causes design problems.

Some of the dificulty in creating a wheel station has now been solved if the use of inflatable modules is used

GCNRevenger · 2004-08-29 19:56:05

The biggest hurdle to cross to make a spinning space station is size... The Coriolis Force, the thing that powers whirlpools and hurricanes and gyros and such, will make a force tangent to the wheel or direction of spin. That is, if you are walking around the wheel the force will pull you horizontaly and you will have a rough time standing up.

The effect becomes smaller as the wheel gets bigger or spins more slowly, so the thing is, there is a minimum size for a given amount of artifical gravity where the Coriolis Force becomes small enough to ignore... and unfortunatly, its pretty big.

comstar03 · 2004-08-30 02:13:58

Tethered lines are going to play havoc with navigation for droids, docking for transfer vehicles and other vessels. I think spinning sections would be a better use of design or a magnetic floor systems would also provide artifical gravity against the body.

In developing rotating system a flywheel in the core would generate energy that could power turbines that would then power ion drive systems to maintain artifical gravity. Outside the rotating sections you could use magnetic floor panel systems to create force for applying to systems, suits and other with metal threading, or metal cores, or other metallic attachments.

GCNRevenger · 2004-08-30 06:03:59

Magnetic systems in your shoes aren't a long-term solution, because the bones in your body will still aptrophy, your food and everything else still floats away, and it would make getting around pretty hard.

The idea of using the stations' rotation as a means of generating electricity won't work, because as the conservation of energy requires, the whole station will stop spinning as electricity is made.

dicktice · 2004-08-30 08:53:23

GCNR: You're going to hate this, but I have to mention this for others who are less opinionated regarding the Russian space program. I just had a gander at the IMAX video called "Mission to Mir" and, except for the accumulated trash from a decade of occupation(s), Mir in every way reminded me of the ISS as it is right now. Just goes to show: the Russians seem to get it right from the beginning, every time. I felt sick, as I watched the video, to recall that Mir had already been ditched. Made me want to bawl--except that spaceguys don't cry, right?

John Creighton · 2004-08-30 09:25:51

Time for a little conservation of energy concern... as the saying goes, you can't get somthing for nothing, so you are going to have to supply the energy to move a payload from a lower altitude to a higher one, it just depends on how.

Maybe when some talk about tethers and momentum exchange they miss that point. Anyway elevators could move cargo much quicker then an ion engine. If the cargo flow was continuous this may not matter as much but currently space stations don’t receive cargo each day. In the mean time while the station is weighting for new cargo ion engines in the upper part of the station can slowly boost the altitude of the station.

The biggest advantage of having one part of the station in a higher altitude is the fuel tanks and the ion engines used to maintain its altitude will experience less drag. Consequently they will be more effective at maintaining or increasing the altitude of the space station. Another interesting point is the part of the station at the lower altitude will be at a lower orbital velocity then a station would normally be at that altitude. This will mean that a space ship will need less orbital velocity to reach it and the lower part of the space station would experience less drag then a space station normally would at that altitude.

John Creighton · 2004-08-30 09:33:33

Tethered lines are going to play havoc with navigation for droids, docking for transfer vehicles and other vessels.

Is see three issues with tethers and orbital transfers. The first would be don’t run into the tether. The second is a vehicle docking with the tether will need some kind of holding thrust to counter act gravity when the connection is made. The third is that the tether is not a ridged connection. This would have advantages and disadvantages in a docking procedure. I think the biggest challenges are material science. The longer the tether is the more effective it will be. However a longer tether will weigh more and have to deal with a greater amount of load. Thus the effectiveness of the technology depends on having light weight materials with high tensile strength. Clearly nano tubs will have the required strength. I don’t know how useful present day materials would be for such a tether.

mboeller · 2004-08-30 10:19:16

One Reason for a tethered space station is artificial gravity. Another reason may be an elevator between orbits.

Do You mean something like this:

http://www.affordablespaceflight.com/]h … light.com/
and this:
http://members.aol.com/Nathan2go/]http: … Nathan2go/
?

John Creighton · 2004-08-30 10:38:53

Do You mean something like this:
http://www.affordablespaceflight.com/
http://www.affordablespaceflight.com/

and this:

http://members.aol.com/Nathan2go/
http://members.aol.com/Nathan2go/

Yeah. For instance here is one disign. I don't know if it is a good design. Someday I will have to work out the numbers. I don't think the paper says anything about how much fuel is required to maintain and stabilize its orbit. Also the tether seems to have a failly big cross section. I think this would produce alot of drag.

The system is based on a "space elevator" and space hotel in low Earth orbit. The hotel would orbit 775 miles above the Earth, and would suspend a space dock 160 miles above the Earth, via a hanging tether. Passengers and cargo would be brought to the dock by a new suborbital reusable launch vehicle, and would travel up the tether via a space elevator. The launch vehicle latches onto the dock, and is carried back to the launch site. The dock moves at only 79% of orbital velocity, which quadruples the payload capacity of the launch vehicle.

from: http://members.aol.com/Nathan2go/SPELEV.HTM
Cost Space Elevators, Space Hotels, and Space Tourism

GCNRevenger · 2004-08-30 12:53:47

"The dock moves at only 79% of orbital velocity, which quadruples the payload capacity of the launch vehicle.

But, this is why it just won't work. If the dock is traveling at below orbital velocity, then the station will have to exert a huge constant thrust to keep tugging it along. There would be such a big fuel expenditure, that the whole idea is pointless.

"Mir in every way reminded me of the ISS as it is right now. Just goes to show: the Russians seem to get it right from the beginning, every time. I felt sick, as I watched the video, to recall that Mir had already been ditched."

Well, I pretty much doubt that you are a biiig expert on the condition of space stations that are more than a decade beyond their design life. Mir had power failures. Coolant leaks (resulting in the poisoning of a Cosmonaut). Air leaks. Communications failures. Solar pannels damaged. Collision with a Progress-B that breached the hull. Oxygen generator fires (that nearly killed the Mir AND Shuttle crews). Smoke/gas contamination. Jammed airlock hatches (that nearly killed two Cosmonauts). Attitude control issues. Microbiological contamination station-wide... but more then all that, more then all that... Mir was worthless. There was simply nothing up there to do worthwhile other than to prop up the Russian space agency a little longer... The Russians were MORONS for leaving the thing up there manned as long as they did. "Get it right" my foot.

The ISS will have several serious laboratory modules, very large power arrays... you don't think they'd put up those big arrays to look pretty do you?... gyro stabilization, and tending by real cargo vehicles and not that silly toy the Progress-B... And frankly, the ISS is in much better shape than Mir, having exhausted only some of its design life. Once Shuttle flights resume and real maintenance can be done, then it will be in pretty good shape for several more years.

John Creighton · 2004-08-30 13:33:40

"The dock moves at only 79% of orbital velocity, which quadruples the payload capacity of the launch vehicle.
But, this is why it just won't work. If the dock is traveling at below orbital velocity, then the station will have to exert a huge constant thrust to keep tugging it along. There would be such a big fuel expenditure, that the whole idea is pointless.

I do wonder if the fuel expenditures in that particular plan would be too great. However the lower velocity (75% of orbital) means less drag friction since drag is proportional to the velocity squared. And obviously the doc would be smaller then the station which would also mean less drag since drag is proportional to surface area. Additionally there are always are planes in the air and the drag they experience per surface area would be more then the space doc. Finally since it will be easer to get the fuel to that altitude refueling a space station with the lowered doc system would be less costly per pound of fuel then refueling the ISS.

Maybe the altitude proposed for the doc in the paper mentioned above is to low. However if the doc was at the altitude of the ISS then the bulk part of the station would experience less drag per surface area and consequently require less fuel to maintain its altitude. The real Key technology is the tether. How thin must tether be so that the drag introduced by the tether is less then the reduced drag on the station components achieved by using the tether. Clearly this depends on how big the upper part of the station is. The bigger the upper part of the station the greater the fuel savings will be by having it in a higher altitude. But even if the upper part of the station is real big it is still necessary to look at the overall fuel savings between direct transport vs transport to the doc then taking an elevator to the bulk part of the station.

John Creighton · 2004-08-30 13:48:55

Momentum Engine:
The momentum engine is required somewhere along the length of the tether (a few dozen miles below the zero gravity point would allow light-weight construction, and still allow liquids to settle) in order to offset drag and the momentum transferred to payloads as they move up the elevator, to the extent that is not offset by downward moving payloads (i.e. during construction of the hotel, or for cargo released into orbit). In this way, the elevator with engine acts as an upper stage rocket, except that more fuel efficient engines can be used.
The thrust required is relatively low. The upper atmosphere will exert about 13 lbs of drag on the the main structure (this varies a lot with design and upper atmospheric temperature)10. When the docking grapple is extended down to 90 miles, it will contribute another 30 lbs (perhaps 10 lbs average with 4 dockings per day). If cargo is carried to the zero Gee point at the rate of 100,000 lbs per day, another 84 lbs of average thrust is required11. The thrust must be applied symmetrically about the orbit, so if thrust is not available during the 31.6% of the orbit that is not in direct sunlight (775 miles up12), then the engine must be shut down another 31.6% of the time on the sunlight side of the orbit to balance it out. The engine is then only usable for 37% of the orbit and must therefore have a thrust of 289 lbs when on to average 107 lbs overall.

Okay, assuming these numbers are right 13 pounds of drag on the main structure and 84 pounds on the docking modual. But how much drag does the tether produce. I don't think it will be negligable given:

For the case of a space hotel, the space elevator (say 860 miles long in this case) links a zero gravity station orbiting 775 miles above the Earth, with a space dock hanging only 160 miles from the surface, and a low gravity (low "Gee") station 1,020 miles above the Earth. The low Gee station feels about 1/10 of Earth normal gravity except it's directed upwards, and serves to counterbalance the force of the hanging dock (the Moon-like gravity makes it an interesting place for hotels rooms). The dock is much easier to reach by rocket than a zero-Gee station would be, since it travels at Mach 19.5, only 79% the full Mach 25 required for orbit (more precisely: 13,800 m.p.h. vs. 17,450 m.p.h.).

The tether is hundreds of miles long and thick enoguh to support a gondala track.

SpaceNut · 2004-08-30 13:50:13

So basically the counter balance part of the stationis a higher orbital speed but matching the orbital speed of the Earth to syncronize it over one spot. The delta in speeds being from end to end a relationship to how far in orbit about the Earth with regards to each others end of the Tether. Would not centrifical forces come into play also on such a design.

Nothing say that the tether has to be kept as a small diameter cable. This could also be of other functions. Depending on how low in orbit it would get or if it could into the atmoshpere it could then lessen the amount of oxygen needed to be transported to the station by creating a syphon system to pull in the needed O2.

GCNRevenger · 2004-08-30 13:55:06

No no, that is not the issue at all...

First of all, the drag effect on the ISS is not a major issue. It only requires a delta-V of under a hundred meters per second a year to maintain orbit on or abouts that figure, and Progress + ESA ATV are quite sufficent to bring up the few tons of fuel needed now and then.

Second, the problem of dragging a docking station well below orbital velocity has nothing at all to do with drag, the problem is that if the dock is not moving at orbital velocity, then its momentum will not be enough to counteract gravity. But instead of falling out of the sky immediatly, the space station above pulls on the docking station to cancel gravity out. But, since you can't get somthing for nothing, the docking station will drag the space station down out of orbit and the whole thing will fall without constant rocket thrust to keep it up there.

John Creighton · 2004-08-30 14:06:59

Second, the problem of dragging a docking station well below orbital velocity has nothing at all to do with drag, the problem is that if the dock is not moving at orbital velocity, then its momentum will not be enough to counteract gravity. But instead of falling out of the sky immediatly, the space station above pulls on the docking station to cancel gravity out. But, since you can't get somthing for nothing, the docking station will drag the space station down out of orbit and the whole thing will fall without constant rocket thrust to keep it up there.

With regard to drag perhaps the ISS doesn’t experience much drag but it experiences more drag then an ion engine can support. For long lived space stations the cost of maintaining the orbit should be kept as low as possible. The doc isn’t at orbital velocity but the center of mass of the system is. The effect of the earth’s gravity pulling the doc down is counterbalanced by the pull needed to keep the part of the station that is above its center of mass from flying off into orbit. It is mainly drag that needs to be fought and not earths gravity. There is a little question of whether the station might start to spin. To keep the station vertical the station must spin with the exact period that it rotates around the earth. Drag will cause it to want to spin and gravity will try to counter act the spin. Hopefully the system will be stable.

GCNRevenger · 2004-08-30 14:52:44

Its more than a reasonably sized ion engine with a reasonably sized power source can support.

The trouble with simply using a counterweight isn't obvious, but its an orbital mechanics concern: that the rotation that the difference in orbit of the dock would induce and that the counterweight would induce in your schematic are different. That is, one of the pieces would be spinning around the center of gravity faster than the other, and the whole thing would never be stable.

Euler · 2004-08-30 16:57:25

Its more than a reasonably sized ion engine with a reasonably sized power source can support.

The tether is at a pretty high altitude, so drag should not be too bad. I think an ion engine could support the tether. However, if it is receiving a lot of heavy payloads, this might no longer be true.

The trouble with simply using a counterweight isn't obvious, but its an orbital mechanics concern: that the rotation that the difference in orbit of the dock would induce and that the counterweight would induce in your schematic are different. That is, one of the pieces would be spinning around the center of gravity faster than the other, and the whole thing would never be stable.

I disagree. Gravity would work to stabilize the tether, so it should not be that hard to keep it from spinning.

My concerns are that it would be expensive to build and operate, it would be limited in payload capacity, and docking may be difficult.

GCNRevenger · 2004-08-30 17:16:21

An ion drive would have to be pretty big to counteract the drag on such a big object or the ISS I would think... perhaps you could dial up the thrust, but it would come at the cost of specific impulse most likly.

If the counterweight is on a shorter teather than the docking station as in the drawing, then it would be traveling at a signifigantly different orbital velocity faster than the space station than the teather flying slower, which is going to make for asymmetric forces even if the counterweight is heavier to make up for its shorter cable to counter the orbital velocity difference.

Even if the counterweight is the same mass as the docking station, which it cannot be if you take on payloads, and has the same length then there is another question if the spinning induced by the different orbital velocities of the componets can be made to match the whole stations' rotation around the Earth... if the length required is too long and hits the atmosphere or too short and is of useless length, then the whole contraption won't be much good.

Euler · 2004-08-30 18:33:28

An ion drive would have to be pretty big to counteract the drag on such a big object or the ISS I would think... perhaps you could dial up the thrust, but it would come at the cost of specific impulse most likly.

The tether system is in a pretty high orbit, and the lower parts of it are moving slower than they normally would. That should keep the drag down to a reasonable amount.

there is another question if the spinning induced by the different orbital velocities of the componets can be made to match the whole stations' rotation around the Earth...

If the tether system was spinning at a rate that did not match the whole station's rotation around Earth, then gravity would alter the rate at which it was spinning until it did match the whole system's rotation around Earth(ignoring atmospheric drag). The effect is the same one that causes the moon to always have the same side facing Earth, but due to the shape of the tether system and it's proximity to Earth, the effect would be greatly exaggerated.

SpaceNut · 2004-08-30 20:39:20

So what if the end of the tether is an anti gravity or magnetic repulser system being constantly recharged by solar panels or from a nuclear reactor. Does that change the equation of drag versus the need for rocket boosting fuel use.

ya maybe to star trek ish with the tracter beam idea but just a thought.

mboeller · 2004-08-31 05:35:15

Second, the problem of dragging a docking station well below orbital velocity has nothing at all to do with drag, the problem is that if the dock is not moving at orbital velocity, then its momentum will not be enough to counteract gravity. But instead of falling out of the sky immediatly, the space station above pulls on the docking station to cancel gravity out. But, since you can't get somthing for nothing, the docking station will drag the space station down out of orbit and the whole thing will fall without constant rocket thrust to keep it up there.

There is no problem at all.

The dock is moving slower than orbital speed, the counter-balance is moving faster than orbital speed and the space station in the middle is moving at orbital speed.

Together the complex of all three parts is moving at orbital speed. So unless you cut the tether the system will remain in orbit just like an normal space station like the ISS.

The added benefit of such an system is, that the docking station is moving (in this example) only at 79% of the orbital speed so you need only around 62% of the energy to reach the docking station. This would make SSTO's very simple and practical (see also the website "affordable Space Flight"). And with the counter balance moving faster than orbital speed you can save a lot of fuel going to the moon, mars or only GEO too when you launch space crafts or just satellites from the fast moving counterbalance.

Yes, the docking station will drag down the system when an Shuttle docks and nothing is done. But it does not happen fast cause the weight of the shuttle is far lower than the weight of the complete station and you don't need normal fuel to raise the orbit of the system with the docked shuttle. You can use an ion engine instead or even an electrodynamic system to raise the orbit. Just let electricity flow in the tether and the interaction with the magnetic field of the earth will raise the orbit of the station without any fuel.

All in all you don't need a lot of energy for this cause when the shuttle departs from the dock then the orbital system will raise it's orbit from itself and you have only to counter the effect of the payload in the long run unless the payload from and to earth is the same, then (theoretically) you don't need any fuel at all.

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#1 2004-08-28 19:37:52

Re: Tethered Space Stations

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Re: Tethered Space Stations

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Re: Tethered Space Stations

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Re: Tethered Space Stations

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Re: Tethered Space Stations

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#7 2004-08-30 02:13:58

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#8 2004-08-30 06:03:59

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#9 2004-08-30 08:53:23

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#11 2004-08-30 09:33:33

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#12 2004-08-30 10:19:16

Re: Tethered Space Stations

#13 2004-08-30 10:38:53

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#14 2004-08-30 12:53:47

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#15 2004-08-30 13:33:40

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#16 2004-08-30 13:48:55

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#17 2004-08-30 13:50:13

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#18 2004-08-30 13:55:06

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#19 2004-08-30 14:06:59

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#20 2004-08-30 14:52:44

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#21 2004-08-30 16:57:25

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