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# Space Elevator Light

Tethers or railguns at low earth orbit
 (+12, -3) [vote for, against]

While learning for the space systems test, I was reading the part about tether systems (connecting masses with strong cables) in the script and had some ideas about it. What if one would place a big mass in LEO, let us say 10,000 tons in the form of a rotating cable or as a long magnetic accelerator, probably like a railgun.

Let us assume first that it is a rotating cable with a 300 kilometer radius and a relative speed at the edge of 3 km/sec (you would have a centripetal acceleration of 3g at the ends with this configuration). This means that in the vertical position one end is moving at 5 km/sec relative to earth surface and the other end at 11km/sec.

Now if a spaceship with an actual mass of 100 tons at 5km/sec speed (after burning the fuel to get to it) would dock at the slower end, then travel in to the mid, which is moving at 8km/sec (LEO speed). The whole configuration would loose 30 metres/sec of orbital velocity (not so much that would be enough to bring the thing down), but the rotational impulse would be increased by the mass of the ship. I know there is nothing unusual in the proposal yet, so now it comes.

When the same spaceship departs back to earth it does so by travelling back out to the slower end and leaving the device with the same 5km/sec it came with. By going back to the low speed end it gives back orbital velocity, while decreasing back the rotational impulse. In the end you have zero energy losses for the rotating device (assuming your spaceship has the same departure mass as arrival mass). For a spaceship with an exhaust velocity of 3.5km/sec (space shuttle has some more) could bring this mass to 5km/sec with a starting mass of 417 tons (space shuttle has over 2000 tons, so a spacecraft of that scale could bring about 500 tons to this speed). An SSTO doesn't seem impossible with these masses.

In the end you have created an "energy bank" in space from which you can take or give energy by docking spaceships to it. The same device at LEO would of course be good to launch ships from LEO to moon and catch arriving ones. It could have highly effective ion engines to compensate eventual arriving-departing mass differences, since you wouldn't have to worry about a big thrust with such low speed changes.

The same idea also works with a 600km long magnetic rail of the same mass (also deccelerating at 3g), that should be able to catch some anker from the arriving space ship flying with a 3km/sec relative speed to the rail (somewhat difficult but surely not impossible).

For the rotating version the equation for the maximal perimeter velocity for an evenly thick cable is v_max=square(2*tensile strength/density) for carbon fibres you have tensile strengths of 3600MPa and a density of about 1.5 ton/meter^3. This allows a max velocity of 2.19 kilometres/sec, not far from the example (that is for an evenly thick cable, for one that is getting thicker to the middle the 3km/sec should be doable with carbon fibers). With carbon nanotubes tensile streghts of 22000MPa to 60000Mpa have been measured so far with comparable densities so this material is easily cabable of achieving the speed I calculated with (although no nanotubes of that length have been manufactured to date).

Ok so much for now, I am looking forward to your comments and have some other crazy ideas if anyone is interested.

 — Fledi, Jan 18 2005

Similar idea? [cloudface, Jan 26 2005]

http://bz.pair.com/outwith/Fledi/LEOCable.gif http://bz.pair.com/.../Fledi/LEOCable.gif
Here's a diagram of the idea. [Fledi, Jan 31 2005]

http://www.islandone.org/LEOBiblio/SPBI126.HTM http://www.islandon...OBiblio/SPBI126.HTM
I just feel like someone who re-invented the wheel [Fledi, Jan 31 2005]

http://www.spaceelevator.com/docs/355Bogar.pdf http://www.spaceele...m/docs/355Bogar.pdf
Ok, this finally explains it in detail. At least I don't have to design it myself now *sigh* [Fledi, Jan 31 2005]

Anti Space Elevator Group http://groups.yahoo...up/stoptheelevator/
Brooklyn residents: just say no to space elevators! [jaksplat, Feb 01 2005]

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Annotation:

 What about all the drag of the tether moving through the lower (upper?) atmosphere at near 5 km/s?

Or is the lower end closer to 400 km above the ground?
 — my-nep, Jan 19 2005

[+] interesting. The actual disruption to the orbit of the energy bank is where I see it getting very tricky. You may work best with a near constant stream of traffic going up & down.
 — sophocles, Jan 19 2005

Docking really sounds like hell. Ok, maybe not hellish, but certainly challenging. Of course that doesn't apply to a railgun.
 — tiromancer, Jan 19 2005

World class first effort, [Fledi]. Even with the math showing and all.
 — bungston, Jan 19 2005

 I am very much looking forward to your next idea, though I'm not sure I've fully grasped this one yet. Can't see how the energy is transferred from the ship to the cable and back. Will re-read this tomorrow.

Welcome to the Halfbakery. You seem to suit it.
 — wagster, Jan 19 2005

 //other crazy ideas if anyone is interested//

 ...of course we're interested, darling. That's why we're here.

I share [wagsters] wariness about the whole conservation of momentum thing. It's okay to lose some speed, I suppose, but you're still connected to the ground, right? So you'd be losing/gaining ground on your anchor point all the time. Orbit variations might be okay, but while tethered to the ground? Maybe the whole station can move up and down the cable, how 'bout that?
 — moomintroll, Jan 19 2005

 // What about all the drag of the tether moving through the lower (upper?) atmosphere at near 5 km/s? //

 The center of mass is let us say moving above 400km, so with a radius of 300km the lowest part would always be above 100km. Besides there is another configuration possible, where the plane of the rotation is parallel to earth surface. In this case you have no big altitude differences at all.

 However it is more effective to do it vertically because of less average atmospheric drag (there is a small rest atmospheric drag above 100km (this is why the ISS altitude has to be raised regularly using the shuttle or progress) that decreases the farther you go out) if most of it is higher up.

 The other advantage is that your suborbital spaceship doesn't have to come up that high to catch it.

 // Docking really sounds like hell. Ok, maybe not hellish, but certainly challenging. Of course that doesn't apply to a railgun //

 I have thought about that this morning and one possible solution is to get the spaceship a few kilometres higher than the lower end of the cable, to which anchors are attached. It could also fly sideways to the cable to some degree, then fire another cable, maybe a kilometer long (nothing too heavy), also with an anchor at its end, sidevays and wait for the rotating cable to cross the fired one's path. Still tricky but it would give us some tolerance to orbit shape and time.

 // World class first effort, [Fledi]. Even with the math showing and al //

 Thank you very much. Though I have to admit that I have an advantage because I'm studying aerospace engineering and was just in the process of learning for a test about space systems.

 // Can't see how the energy is transferred from the ship to the cable and back //

 The main idea about it is that you are using the spaceship as fuel to accelerate the rotator back to it's former speed & rot. impulse, because it is docking at 5km/sec rel. to earth and leaving at the same speed while the center of mass of the rotator is almost constantly flying at LEO speed ( somewhat below 8km/sec ) So the energy you use is the same that is wasted in atmospheric drag during reentry when you don't use such a system.

 Of course this also means that this way spaceships require much less heat shielding (the first part of reentry is the critical one in terms of heating up because of the big 8km/sec relative speed to air molecules in present configurations) So space shuttles would be much safer and easier to build this way, too.

 // but you're still connected to the ground, right? //

 Not at all, the whole thing is flying at orbital velocity out in low earth orbit.

 For sophocles concern I will calculate the orbital change by docking a 100 ton ship to it, but I guess it is not that critical.

 I also thought about way to put this large mass into orbit. It could be done in a similar fashion as the space elevator is planned by first putting up a thin and light cable, let us say 20tons, to which you can dock 200kg pieces and thus slowly increase capacity. It would have an ion engine to compensate for the speed loss in the construction phase.

 I have made some very rough calculations about the energy requirement and it seems it would be possible to build a 10,000 ton device in a few years time with a 500kW ion engine (it would be a lot smaller at the beginning, then grow bigger with the other parts) powered by solar cells (you need about a 20 meters x 100 meters surface for that) or preferably a small nuclear battery or reactor.

You would also need to develop suborbital spacecraft of different sizes. I think Burt Rutan is the right man to ask about this.
 — Fledi, Jan 20 2005

 Just calculated the orbit drop for a docking. It is for a 30m/sec deccelaration (caused by a spacecraft docking with 1/100 of the rotator mass and a rotating speed of 3km/s. I assumed the center of mass would move at 400km altitude in a circular orbit.

The decceleration would result in an elliptical orbit with a perigee (lowest point) of 300km above earth surface. This means that you have to be 200km high with the lowest part of the rotator in the starting orbit. So with a cable radius of 300km this would result in a 500km starting orbit for the center of mass.
 — Fledi, Jan 20 2005

 This is a potentialy usefull cross betwean a full ship-thrust to orbit and a (elegant, IMO) 0 ship-thrust cable "wheel" that dosn't have any movment at the pickup position.

It's cheaper to build but probably more expensive to operate.
 — my-nep, Jan 20 2005

Of course it is more expensive to operate than a space elevator, but this can be done with existing material (nanotubes are existing too but I don't know how difficult it will be to manufacture them in the desired length)
 — Fledi, Jan 20 2005

 No, a space elevator is different than one of these on a larger scale.

 The cable wheel I discribed... Think of it as a tire rolling on the atmosphere. There is NO relitive movment at the point of contact, but it's not ancored like a space elevator.

This device is "burning rubber" or "spinning out"
 — my-nep, Jan 21 2005

 Oh that way, that's a great idea, too. The space station guy from the institute told me about something like that has been proposed for the moon.

He also told me that one question is how the cable would react to a sudden change of the center of mass by a docking, so I will look at that next. But this could take a while since there are also some tests I have to learn for...
 — Fledi, Jan 21 2005

Don't take this halfbakery that seriously. You go prepare for finals. When you're done, and come back here, there'll be no lack of discussion.
 — sophocles, Jan 21 2005

 Wow.

[Fledi], does the phrase 'get out more' mean anything to you? I mean that in the nicest possible way, of course ;-)
 — moomintroll, Jan 21 2005

Hehehe, yes actually it does
 — Fledi, Jan 21 2005

Its a very cool idea. But why have it roll along the top of the atmosphere? Why not put it into an eccentric or elliptical orbit? That way it could dip briefly lower into the atmosphere to pick up transit then perhaps lift up and fling out at the same time (or would this require too strong a structure?). I suspect there's a sweet spot in the trade-off between atmospheric friction/structural stress and to-orbit capacity... - I kept considering the problem of anchoring one of these rings to a space elevator, but couldn't come up with a reason to do so; it just seemed more ornate. I kept thinking of the various carnival ride analogies... Does it have to be a ring? Would it make sense to build it as a cloverleaf with two or more rings whirling at once, tethered in the center? Most recent space elevator concepts involve (laser) beamed power from a ground station to the crawler; is this concept finally a rationale for the old L -5 concept of orbiting solar power stations? What would a superheated ring like this do to the ozone layer?
 — cloudface, Jan 21 2005

 — athome, Jan 22 2005

Forward into the past?
 — Aegir, Jan 23 2005

 // Why not put it into an eccentric or elliptical orbit? That way it could dip briefly lower into the atmosphere to pick up transit then perhaps lift up and fling out at the same time (or would this require too strong a structure?) //

 The problem with an elliptical orbit is that your orbital speed is highest at the lowest point of the ellipse, so this would be contraproductive, for the main problem of spaceflight is not reaching that 100 or 200 km altitude but the high orbital speed.

 For sweeps into the atmosphere I really wouldn't want this below 100km. Any lower and it will start to heat up/loose speed/tear apart.

 // Does it have to be a ring? //

 Why should it? If you mean placing ther cable in a ring form the tensile stress is much higher in that configuration making the mentioned perimeter rotating speeds impossible even for carbon fibres.

 // is this concept finally a rationale for the old L -5 concept of orbiting solar power stations? //

 Well I don't know what you mean with L5, is it Lagrange Point 5? Anyways the main purpose I thought about for this is for example to bring a man to the moon for below some 100,000\$ or place cheap space stations into orbit, although it could maybe even effectively be used as a power station.

 // What would a superheated ring like this do to the ozone layer? //

 Superheated? It shouldn't be even warm and stay out of the atmosphere (and the ozone layer) all together.

Sorry, but I have no homepage right now so I could scan in some drawings but not place them on the net.
 — Fledi, Jan 24 2005

I'm for editing out the references to railgun altogether and fashioning that piece into a separate idea.  Interesting idea, and I might add thanks for the elaborate scenarios you've put up as clarification.
 — reensure, Jan 24 2005

I just did some shopping for nanotubes, did you know you have to buy them by the gram? And that a gram of tubes costs more than the most amazing street drug you can dream of?
 — cuckoointherye, Jan 24 2005

 Yes, nanotubes are very expensive right now, but that might change. They're only manufactured in laboratories right now as far as I know.

 On the other side, carbon fibres seem to be quite cheap, I already found a place where they can be bought from the net in over 1000m cables.

 But I have another idea especially about nanotubes in case they become cheap enough to use them as fuel. Everybody could build his own spaceship in a garage in that case.

For the railgun, it's basically the same principle as the rotator, only you use electrical means to store and release energy. The railgun has the problem that it would have to be a very long structure and difficult to stabilize, while the rotator is self stabilizing so to say (the cable is always under lots of tension)
 — Fledi, Jan 24 2005

Re velocity: I guess it would have to be a very large ring to be slow enough to hook up to at 100 km or lower... Could a permanent aerostat launching platform of some kind (i.e. balloon platform) reach that high? The other thing I like about this idea is that it is also an enabling technology for a ground-to-orbit space elevator.
 — cloudface, Jan 24 2005

 Sheer genius. Took me a while to get my head around it, but yup, it seems to work. The only caveat I have is that there's a chicken and egg problem, where I think you need fairly regular traffic in both directions to make it feasible, yet you have to have the thing up to make the traffic possible.

Have a rapidly rotating carbon fibre bun from me.
 — BunsenHoneydew, Jan 25 2005

My only problem with this is its structural integrity. A 600 km diameter cable ring won’t be able to transfer loads. Unless it’s built like a huge hula hoop or bicycle wheel it’ll be as flimsy as a loop of thread in the wind.
 — FarmerJohn, Jan 25 2005

 Well you only need to place the initial 20ton cable into orbit with a single conventional launch (shuttle, ariane 5, proton or delta 4 heavy) That is already enough to place 200kg satellites into orbit using small suborbital spaceships (maybe some kind of UAV). You can take your time to build it out to larger capacities then.

For the ring question I never thought it to be a ring, just a single straight cable line going right through the rotational axis. Loads can easily be transferred by climbing in to the middle along the cable.
 — Fledi, Jan 25 2005

Sorry, so this is a stiff cable twirling in space like a drum major(ette)’s baton? It would somehow be held straight (to lift/accelerate the spaceship) like the mast of a ship or a long irrigation pipe on wheels?
 — FarmerJohn, Jan 25 2005

Exactly. Although the outward centripetal acceleration should auto-straighten it out even without a supporting structure.
 — Fledi, Jan 25 2005

 [Fledi] I can host images for you on my site if you want. Email them to me (try to make them reasonably small, like under 500Kb each). My e-mail info is on my halfbakery profile page.

I'd try to illustrate this myself but I am not at all sure I am seeing this idea with the needed fidelity.
 — bristolz, Jan 25 2005

I get it now. (?) Spokes without the wheel. Dang. I liked the idea of a big space wheel... OK, what about taking six tethers, and making of them a tetrahedron, then dipping one end after the other into the atmosphere? You would only have four points where you could pick something up, I guess, but it would look really cool going by from the ground and could be stronger/safer than a single tether as well as simpler to build than a ring?
 — cloudface, Jan 26 2005

Yep, that is exactly the same idea as proposed by Robert Zubrin in cloudface's link. Seems like it's already baked after all.
 — Fledi, Jan 31 2005

 I'd put NASA brainstorming sessions in the same category as the Simpsons, in that they've done everything. You just have to pretend they don't exist, and get on with your life.

PS Isn't your spaceship facing the wrong way?
 — tiromancer, Jan 31 2005

Uhmm yes, that's different in this redraw from bristolz, she was kind enough to create a new drawing based on the poor quality one I sent her (she's not the only one complaining about my unreadable writings)
 — Fledi, Jan 31 2005

I was thinking about docking again... what about an vehicle with wings in a pronounced V shape, with a cable stretched between the wingtips? The vehicle could "park in front of the teather, catch the a hook on the end and ride it almost 1/4 rotation. A bit of thrust would kick it off of the hook. A clamp on the bottom of the vehicle could be used on return.
 — tiromancer, Jan 31 2005

 Then the maximal tolerance of the vehicle's path would be as big as the distance from one wing-tip to the other. Possibly a useful solution, although you'd still have less tolerance than with a kilometer long cable floating along sideways.

 But certainly your space plane would be more maneuverable that way, which is probably more important.

 By the way, I'm quite happy with having found this as a serious paper from Boeing, certainly better than finding out it doesn't work after all :)

I hope it will be placed into orbit soon, that would be a big step towards colonies on Moon and Mars.
 — Fledi, Jan 31 2005

I know it ain't so, but I keep opening this idea expecting it to be a low-carb version of a space elevator.
 — normzone, Jan 31 2005

 Carbon nanotubes have fewer fats and oils than plastic string.

 What about a big, flying cannon that fires it's cargo into a basket?

Actually, I've been badly overestimating the difficulty of docking. I keep forgetting how high up this thing is.
 — tiromancer, Jan 31 2005

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