Space Tether Questions

[barrykirk]

Couple of questions I've got about space tethers that I haven't seen posted anywhere else, or maybe I just missed it.<br /><br />Assume a rotating tether in LEO or maybe a little higher with 25 Km radius and a tip speed of 1 Km/sec. This should provide a boost of 2Km/sec to it's cargo.<br /><br />1) When picking up a slightly suborbital cargo that is 1 Km/sec short of full orbit and boosting it, does the tether transfer any of it's angular momentum of rotation around it's center of gravity to the boosted cargo?<br /><br />If so, then what systems are being considered to restore that lost angular momentum.<br /><br />2) Can the orbit of the center of gravity of the tether be circular? Or will it wobble due to it's rotation? <br /><br />3) Assuming that the best place to pick up a cargo is when a tip is at the lowest point of the rotation, what is the best place to release the cargo.<br /><br />Is it at the top of the rotation or somewhere else?<br /><br />What would be the best place to release a cargo if it's final destination is LEO?<br />

 

[nexium]

Hi barryKirk: I think the conditions you stated are realistic. Circumfrence = 6.28 times 25 = 157 kilometers, so it will take 157 seconds for one rotation and attachment opportunities will occur at 79 second intervals. I think nearly all of the speed boost is transfered from the the momentum of rotation and nearly all the perhaps 40 kilometer altitude gain comes from the orbital energy of the tether.<br />2 I think, minor variations from circular are not serious, wobble may need to be reduced, occasionally to avoid excessive stress on the ribbon just after attachment. The orbit will need to be boosted to a higher altitude, and the 1 kilometer per second restored, after each group of three atachments, perhaps oftener.<br />3 Approximently the top is best. The direction the payload is thrown can be adjusted by a second sooner or later release. The release can also be timed to take advantage of transients which travel on the ribbon = a crack the whip effect.<br />40 kilometers above the suborbital attachment is just barely LEO = low Earth orbit, so we will likely release a second early so the the pay load is boosted to perhaps 400 kilometers altitude. The pay load will then need a small rocket burn to circularize the orbit of the payload unless a eliptical orbit is desired. Please embellish,refute and/or comment. Neil

 

[mikeemmert]

<blockquote><font class="small">In reply to:</font><hr /><p>If so, then what systems are being considered to restore that lost angular momentum.(?)<p><hr /></p></p></blockquote>Yes, you would have to restore both angular momentum and orbital altitude. No free rides.<br /><br />Consider this in Missions and Launches, "Europeans and Australians Make Space Propulsion Breakthrough". It's a new ion engine with <b><i>215 km/sec !</i></b> exhaust velocity. This is an interesting thread, but I haven't seen anybody yet apply this to the rotovator. One load of reaction mass would probably last for years even under heavy usage.<br /><br />I seriously doubt that you would be making a pickup every 157 minutes. You would probably have weeks to power up the rotovator for a fresh pickup. Where the point of closest approach to Earth is during those weeks doesn't really matter, as long as it makes the rendezvous. You should reel in the cable to reduce the hazzard of orbital debris/meteoroids and reduce wind resistance.<br /><br />Orbits don't necessarily need to be circular, and the plane of rotation of the rotovator doesn't need to be vertical. It could be up to horizontal. This is important for making pickups from KSC or Tyuratam and putting them into equitorial or polar orbits. Might be some confusing math there but it would be worth it. <br /><br />The engines, powerplant (probably solar cells) and what ever else is needed like a rotating bearing between the powerplant and the tether and a spool to reel in the tether, would just be ballast.<br /><br />There are four-stage rockets, why not four-stage rotovators?<br /><br />On a somewhat more pessimistic note, when the Space Shuttle reeled out a 12 km long tether, the voltage induced by the tether cutting through the Earth's magnetic field generated a voltage potential, causing arcing which destr

 

[barrykirk]

If your going to GEO, than a tether doesn't waste much. My example with 1 Km/Sec tip speed would add 2Km/Sec total velocity. This could increase the payload capacity of any given rocket headed to GEO.<br /><br />Yes you would need rockets to continue to boost to GEO after the tether but the total Delta V requirement to reach it would be reduced by 2 Km/Sec. GEO just got a lot cheaper.

 

[barrykirk]

When the shuttle made it's tether tests. What altitude orbit was it in?<br /><br />Would increasing the orbital altitude of the tether reduce the induced voltage?<br /><br />If your going to GEO anyway, your going to have to reach a higher altitude anyway and you can still reach it on a suborbital trajectory with the tether boost changing direction as well as adding velocity and energy.<br /><br />The other advantage of tethers as I see it is to improve the downmass capabilities and lower the TPS requirements for sending stuff downhill.

 

[barrykirk]

Reeling in the cable?<br /><br />I've seen several places in some of the tether websites that mention reeling in/out cables, but I've got to ask some questions about that.<br /><br />Reeling in a cable would take some serious energy, remember that you reeling in against a huge backforce and a considerable distance. That's going to take a lot of power and a huge motor/gearbox to generate that kind of force. More gearbox than motor because it doesn't have to reel it in rapidly and the power source, if solar, is not going to generate a huge amount of power.<br /><br />Also, since angular momentum is conserved, rotational speed is going to skyrocket as the cable is reeled in. Will the total force on the cable increase?<br /><br />Yes the amount of cable at a distance has decreased and the distance has decreased, but the velocity goes way up as you reel in the cable.