dietz@SLB-DOLL.CSNET (Paul Dietz) (01/10/86)
Even if they aren't used for launching things, tethers may be very useful for generating angular momentum in spinning space structures. For example, a space station could be spun up by extending two very long cables with small reaction engines on the ends. The cables would be spun up and, because of the long moment arms, would acquire large amounts of angular momentum. An electric motor anchored to the cables at the hub could then spin up the station, gradually slowing the cables. When the cables are stationary they would be retracted. This is much more mass efficient than using reaction engines on the space station itself.
andrew@cadomin.UUCP (Andrew Folkins) (01/13/86)
In article <8601101806.AA00639@s1-b.arpa> dietz@SLB-DOLL.CSNET (Paul Dietz) writes: >Even if they aren't used for launching things, tethers may be very >useful for generating angular momentum in spinning space structures. >For example, a space station could be spun up by extending two very >long cables with small reaction engines on the ends. The cables would >be spun up and, because of the long moment arms, would acquire large >amounts of angular momentum. An electric motor anchored to the cables >at the hub could then spin up the station, gradually slowing the cables. >When the cables are stationary they would be retracted. This is much >more mass efficient than using reaction engines on the space station itself. Umm, what stops the cables from just wrapping around the station? This scheme would work, but you would need rigid 'towers' instead of cables, and the mass needed for these towers might be large enough to offset any fuel savings. One last point, the cables will only be stationary with respect to the station, and they will still have a considerable amount of angular momentum. Retracting them would spin up the station even more, just like a spinning figure skater. -- Andrew Folkins ...ihnp4!alberta!andrew All ideas in this message are fictional. Any resemblance, to any idea, living or dead, is purely coincidental.
dietz@SLB-DOLL.CSNET (Paul Dietz) (01/16/86)
>Umm, what stops the cables from just wrapping around the station? This >scheme would work, but you would need rigid 'towers' instead of cables, and >the mass needed for these towers might be large enough to offset any fuel >savings. One last point, the cables will only be stationary with respect >to the station, and they will still have a considerable amount of angular >momentum. Retracting them would spin up the station even more, just like >a spinning figure skater. By stationary I meant with respect to an inertial reference frame, not the space station (you torque the cables until they stop). The point about winding the cables up is a good one, but can be overcome by not torquing them even that much, so leaving them with some residual rotation. This rotation can be eliminated by firing the rockets at the ends again, but in the opposite direction. One might acquire angular momentum by catching masses at the ends of the cables. This would transfer angular momentum from the orbits of the masses to the rotation of the structure; because the orbits have such large radii this could be extremely mass efficient.
throopw@dg_rtp.UUCP (01/20/86)
Seems to me there is a much better, and far more efficent method of using tethers to spin up (or down) a space station or satellite, using tidal forces. Might take a while, but so it goes. The notion is to extend two tether ends (with some small mass or other on each) away from the station, one primaryward, the other anti-primaryward. Tidal forces will produce a tension on the string. Then merely use a solar powered eletcric motor (with the tension on the tether as an "anchor") to spin up the station, and an electric generator to spin it down. No fuss, no muss, no reaction mass, and you can even get some or most of your energy back by spinning the station back down. In fact, isn't this method used (at least partly) to stabilize the space telescope? I had read where at least some satellites were stabilized using this method, but I may be remembering some fictional scenario... -- Wayne Throop at Data General, RTP, NC <the-known-world>!mcnc!rti-sel!dg_rtp!throopw
henry@utzoo.UUCP (Henry Spencer) (01/21/86)
> The notion is to extend two tether ends (with some small mass or other > on each) away from the station, one primaryward, the other > anti-primaryward. Tidal forces will produce a tension on the string... > Then merely use a solar powered eletcric motor (with the tension on the > tether as an "anchor") ... > > In fact, isn't this method used (at least partly) to stabilize the space > telescope? I had read where at least some satellites were stabilized > using this method, but I may be remembering some fictional scenario... Not the space telescope, but a number of satellites are stabilized this way. It's called "gravity gradient" stabilization, because the reason for the tidal forces is that the primaryward and antiprimaryward masses are at different distances from the primary and hence feel slightly different gravitational accelerations from it. Tethers probably would not do; you need rigid or semi-rigid structures to transmit torque. The distances don't need to be all that long, either: LDEF (the Long Duration Exposure Facility satellite), which is maybe 3 times as long as it is wide, is stabilized this way. Remember that the tidal forces on your "anchor" mass must be stronger than those on the mass you are rotating. Remember also that these forces are fairly small, so don't expect rapid rotation rates. The Space Station probably will use gravity-gradient stabilization, if for no other reason because gravity-gradient effects are unavoidable in something that big, and it's easier to use them than fight them! -- Henry Spencer @ U of Toronto Zoology {allegra,ihnp4,linus,decvax}!utzoo!henry
hogg@utcsri.UUCP (John Hogg) (01/22/86)
In article <6307@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes: >> The notion is to extend two tether ends (with some small mass or other >> on each) away from the station, one primaryward, the other >> anti-primaryward. Tidal forces will produce a tension on the string... >> Then merely use a solar powered electric motor (with the tension on the >> tether as an "anchor") ... > >Not the space telescope, but a number of satellites are stabilized this >way. It's called "gravity gradient" stabilization, because the reason >for the tidal forces is that the primaryward and antiprimaryward masses >are at different distances from the primary and hence feel slightly >different gravitational accelerations from it. Tethers probably would >not do; you need rigid or semi-rigid structures to transmit torque... Actually, tethers might provide an advantage here. You need a solid lever arm to change a force into a torque. This should ideally be as large as possible. However, if it must be small, a tradeoff can be made made by increasing the force, which can be done by creating a large gravity gradient via a long tether. In other words, a case can be made for a small (=> light) satellite, a small (=> also light) plumb bob and a LONG string. Kevlar cable will weigh less than a rigid boom providing the same stabilizing torque. I have no idea what vibrational properties a design of this type would have, and that could be the most serious problem. Note, by the way, that two balancing tethers are not required. Furthermore, for a given amount of cable, it will be far more efficient to have one long tail (up or down, dealer's choice) than two small ones; in fact, for half the plumb bob mass (one instead of two) you'll get twice the gradient force. There's an r^2 in there. -- John Hogg Computer Systems Research Institute, UofT ...utzoo!utcsri!hogg Standard disclaimer: the above may or may not contain sarcasm, satire, irony or facetiousness. It does not contain smiley-faces.
hogg@utcsri.UUCP (John Hogg) (01/22/86)
In article <1976@utcsri.UUCP> hogg@utcsri.UUCP (John Hogg) writes: >Note, by the way, that two balancing tethers are not required. >Furthermore, for a given amount of cable, it will be far more efficient to >have one long tail (up or down, dealer's choice) than two small ones; in >fact, for half the plumb bob mass (one instead of two) you'll get twice the >gradient force. There's an r^2 in there. If you do the high school physics on the envelope BEFORE posting, you look like far less of an idiot. Tether force is a linear, not square, function of tether length. You still gain (in simplicity apart from force strength) by using one long tether, but not by quite as much. -- John Hogg Computer Systems Research Institute, UofT ...utzoo!utcsri!hogg Standard disclaimer: the above may or may not contain sarcasm, satire, irony or facetiousness. It does not contain smiley-faces.
dietz@SLB-DOLL.CSNET (Paul Dietz) (01/28/86)
Tidal forces... but of course! Seasat used this scheme, I think, as does the long duration exposure facility.