Tether Varient: Just playing around with new idea

[barrykirk]

What if we take a satellite in a higher orbit that has an orbital velocity of 1Km/Sec less than that of LEO.<br /><br />If a tether is dropped straight down from that satellite to an altitude of 200Km without rotating the tether. Tidal forces should keep the tether and satellite in a fairly vertical line.<br /><br />If the vast majority of the mass is in the higher orbit, the whole assembly, ie Satellite and tether will have an orbital velocity of 1Km/Sec less than LEO.<br /><br />A slightly suborbital rocket would catch the end of the tether at 200Km altitude and hang on. It doesn't have to use rockets to acheive the full orbital velocity.<br /><br />Can an electric motor be used to winch up the tether straight up until the payload is in orbit.<br /><br />In this example we have in effect a space elevator that isn't connnected to the ground and doesn't extend all the way out to GEO.<br /><br />This should have the following advantages over a standard space elevator.<br /><br />1) Lower Strength to Weight requirements.<br /><br />2) The cable is kept entirely outside of the earth's atmosphere.<br /><br />3) Instead of trying to power the climber, the power is provided by the larger satellite, which should have ample power provided by large solar cells.<br /><br />4) Sending payloads directly to the satellite, such as reaction mass for the reboost ION engines would be possible.<br /><br />5) Climb time to orbit would be substantially less than a space elevator because the distance travelled is considerably less.<br /><br />Advantages over a standard rotating tether would be the following.<br /><br />1) It is possible to send cargo such as reaction mass for an ION engine to the satellite and use the tether itself to assist in boosting that reaction mass.<br /><br />2) Using a set of counterweights, it should be possible to reduce energy required to hoist cargo. It won't be as effective as counter weights on a ground based elevator. But it would reduce the energy requirements some. Especiall

 

[thinice]

There are no miracles. Where do you think the energy needed to speed up a suborbital rocket will come from? A classic space elevator slightly slows down the Earth's rotation. In your scheme, catching slow moving rocket on a tether will be like a deorbit burst for the satellite.

 

[no_way]

You need to compensate for lost velocity<br />like this for example:<br />http://spacetethers.com/<br />

 

[barrykirk]

Yes, I know that I need to compensate for lost velocity with either an ion engine or running a current through the tether to push against the earth's magnetic field.<br /><br />The problem with a "conventional" rotating tether that I'm trying to solve are the following.<br /><br />1) After boosting from a rotating tether at the release point, the boosted objected can't have a circular orbit unless and I don't know about this. Is it possible to put a rotating tether in a elliptical orbit such that their is a release point that has a circular orbit?<br /><br />The vertical non-rotating tether can provide the object with a good approximation of a circular orbit.<br /><br />2) How do you supply a rotating tether system with spare parts, ie solar cells, or reaction mass if using ion engines for re-boost? The vertical non-rotating tether makes that possible, maybe not easy but less difficult.<br /><br />3) When using a rotating tether to provide a de-orbit assist, it's hard to catch it unless your in an elliptical orbit matching the tip. The vertical tether can catch an object in a circular orbit.<br /><br />4) When a rotating tether boosts an object doesn't it transfer some of it's angular momentum to that object? How is that angular momentum replaced? How is the initial angular momentum imparted to the tether?<br /><br /><br />I suspect that that angular momentum will be more difficult to introduce/maintain/restore than a simple orbital reboost.<br /><br />As a slight variation on this them. Make a vertical tether with a massive satellite as a climber on that tether. Rather than winding and unwinding the tether. I don't know what the relative merits of the two systems would be.<br /><br />I'm just throwing this out to see what people say.<br /><br />

 

[josh_simonson]

A 1km/sec boost would improve the STS mass fraction by about 5%. That isn't very revolutionary. Also leaving the tether hanging in LEO would endanger LEO satelites more than a rotating tether would. The rotating tether can adjust it's length to change it's period of rotation so that it steps over satelites and avoids hitting them.<br /><br />Here's a neat video showcasing a rotating tether concept: http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=1295&start=1

 

[barrykirk]

Actually, this tether that I'm describing can be reeled in out of LEO.

 

[mikeemmert]

<p>Hi, BarryKirk.In reply to:1) After boosting from a rotating tether at the release point, the boosted objected can't have a circular orbit unless and I don't know about this. Is it possible to put a rotating tether in a elliptical orbit such that their is a release point that has a circular orbit?I'm pretty sure that you can indeed do this. Most people, I think, visualize a rotovator as being in a circular orbit, but I think elliptical orbits are the way to go. And release doesnt' neccessarity have to be at the top of the tether's arc, it can be at any point. I'm sure you can get at least a good approximation of a circular orbit this way.<br /><br />I like rotating tethers, myself. They're not quite as simple, but they are much cheaper. What you are doing is trading higher acceleration for shorter length. Length is crucial, since flying tethers have a problem of the induction of electric currents by cutting the Earth's magnetic field. I looked in Wikipedia to try to find out more about tethers, but the article's kind of short. Google the TSS-1R mission for more. (This is Your Project, so You have to Google it. I have to Google Coriolis effect, force gradients, and vectors). <br /><br />I remember following that mission. The tether was made out of Kevlar with a copper wire embedded in it. The copper wire had so much current induced in it that it arced at the base. The tether broke and got away. Lesson learned? Do not put copper wires in your tether. That's why they want to beam energy to climbers and crawlers with a laser, a huge technological challenge ($xillions...) in it's own right. Rotovators don't have this problem. They simply introduce a little bit more thinking about the Coriolis effect and elliptical orbits, and I think it's well worth the effort to have a simple, useable system.</p>

 

[mikeemmert]

Dear josh_simonson;<br /><br />I clicked on your link and got:<br /><br />This section of the BBS is restricted to specific user accounts. Please return to the BBS.<br /><br />So I came back here.<blockquote><font class="small">In reply to:</font><hr /><p>A 1km/sec boost would improve the STS mass fraction by about 5%. That isn't very revolutionary.<p><hr /></p></p></blockquote>I don't know where you got your figures. Rotovators work by reducing the delta v needed to get into orbit. If you reduce the needed delta v from 8 km/sec to 7 km/sec and your booster has the 410 second ISP of the RD-68 engine, then according to my calculations, you can boost 28 % more payload. It should be more than that, since every kilogram except the latching device goes directly into payload. The RD-68 engine already exists (Boeing Delta IV) and does not need to be reinvented.<br /><br />Does anybody from the Mission Control Team want to check my math? We have a resolvable problem here. I have tried to define it fairly exactly so that anybody who does the math should get the same results. You will, of course, get different results if you use the SSME's 453 seconds ISP or if you make a pickup at one kilometer per second less than normal and deposit it in an elliptical orbit rather than LEO, which will result in a 2 km/sec delta v improvement.<br /><br />I would think that the first job of rotovators will be to pick up geosychronous satellites from less than LEO and put them on a LEO/GEO transfer orbit. This would indeed result in a revolutionary cost reduction with a completely reusable system.

 

[mikeemmert]

<blockquote><font class="small">In reply to:</font><hr /><p>...remember that carbon is a great conductor too...<p><hr /></p></p></blockquote>Yeah, I know... <img src="/images/icons/tongue.gif" /> ...<br /><br />Copper is, of course, a much better conductor than carbon, but I worry that ions will be implanted on surfaces of anything speeding through the ionosphere. This might make materials that are normally insulators into good enough conductors to cause a problem. <br /><br />I think the solution might be to make the tether from shorter pieces, a few kilometers long, with connectors that isolate the currents and maybe do something useful with them, like drive ion engines or something. Such stations are in a position to better control harmonic vibrations so you don't have a flying guitar string. Stations along the length would also make it much easier to do things like change the plane of the rotation of the tether. There would be less harmonic vibration produced in the first place.<br /><br />Places where the tether is connected together would form speed bumps (literally!) for crawlers. The connectors also might cause problems reeling the tether in and out. That's one reason I like rotovators.

 

[josh_simonson]

http://www.nasaspaceflight.com/content/?id=4202<br /><br />Try that link. Hopefully it'll work since it's an article not a thread.

 

[mikeemmert]

Thanks, josh_simonson. But there are no figures about how much or little rotating tethers can reduce launch costs. I would think quite a bit more than 5%.<br /><br />Anyway, I like the name, trebuchet which is an ancient device that the rotovator closely resembles.<br /><br />Your link give a useful and simpler formula, r = v ^2/a, where r is the radius of the tether, v is the tip velocity, and a is the acceleration. It's interesting that the mass of a tether depends only on tip velocity. A shorter tether is fatter, and those factors exactly cancel out assuming you make the taper right.