Here’s a report from the conference. There are still a lot of issues to work out. The smart money is on (reusable) rockets, in the near term, I think.
13 thoughts on “Space Elevator Update”
Comments are closed.
Here’s a report from the conference. There are still a lot of issues to work out. The smart money is on (reusable) rockets, in the near term, I think.
Comments are closed.
I’ve always thought a LEO equatorial ring (which can be tensioned by a small increase in its rotational velocity) makes more engineering sense, with downcables dropped from maglev carriers riding the ring.
Another method I came up with is to put lots of small particles in highly elliptical polar orbits, then put structures over both poles that magnetically reflect the particles, forcing them into a football shaped half orbits. The particles (which could even be millions of small smart satellites) would have a perigee over the equator and be crossing the pole (heading toward a distant apogee) when the structure would reflect them (the positive vertical velocity becomes negative), using the particle’s momentum change to stay aloft. As long as the reflection was done with 100% efficiency, the orbits wouldn’t degrade. The two structures hovering over the poles then drop downcables to complete the lift system.
It sounds wierd but the physics work out.
I could make a dozen analogies, but really there’s no need. Space Elevator advocates need to get with the freakin’ program. Learn how new technology is developed and start actively working on flight hardware. Fundamental part of SE technology: space tethers. Got any plans to fly one? Nope. Have you even managed to get them on the NASA technology roadmap? Nope. Instead they have a materials competition every year with no serious contenders and use that as an excuse to ignore all the myriad other problems that have yet to be solved by *actually flying*.
“Got any plans to fly one? Nope. Have you even managed to get them on the NASA technology roadmap? Nope.”
There are proposals being considered. You may see it sooner than you expect.
“I’ve always thought a LEO equatorial ring (which can be tensioned by a small increase in its rotational velocity) makes more engineering sense…”
Would this work? In a spinning, rigid, circular ring, centrifugal forces on each mass element are canceled by the tug from neighboring elements. Spin it in a gravity well, and there is still no net centrifugal force on an element. So, without the centrifugal force to offset the gravitational attraction, how would a component mass element stay in effective orbit?
I would expect a ring with center displaced from the center of the Earth would continue falling in the direction of the displacement until it collided.
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IMHO, the main problem with the space elevator concept I see is, how to get the material strength needed, and yet be able to dissipate enough energy to keep the thing from going Tacoma Narrows on us? It appears likely a cable to GEO would have resonances near the frequency of once per day, and that would also be the fundamental harmonic of thermal pumping from eclipse entry and exit. Driving a low energy dissipation system at its resonant frequency is generally bad.
“I would expect a ring with center displaced from the center of the Earth would continue falling in the direction of the displacement until it collided.”
Or, would there be any net gravity at all? I really don’t fell like doing the calculations, but I seem to recall the gravity within a solid sphere is zero. Would not the same be true in 2-d?
A LEO loop based elevator system would need at least 3 anchor cables to prevent drift away from nominal orbit.
The related Langstrom loop is another long term option. Put the loop high enough that orbital velocity for a payload is viable, but low enough that atmospheric drag cleans out debree. The loop doesn’t need to circle the planet if it runs inside a vacuum casing.
“Would not the same be true in 2-d?”
Some back of the envelope calcs suggest to me it is not the same in 2-d, and a freely floating ring displaced from the center would continue “falling” until the closest segment hit the Earth. I think you would need anchor cables, as Peterh suggests. How to construct such an animal, I have no idea.
Bart, there are several ways to look at an orbital ring. Given its diameter versus practicable thickness (small and lightweight or it would cost too much) I wouldn’t regard it as rigid. It’s going to act like a big loop of spaghetti under very slight tension. Then you apply force at the downcables, which makes the behavior get complicated.
If the cable has enough freedom of movement (stretching and compressing) then it could be regarded as a stream of particles, each obeying the normal laws of orbital mechanics. Since the force from a downcable is applied to the ring stream at a fixed point over the Earth, the effect would be like having each particle in the cable firing a reaction jet (an OMS burn) directed downward as they pass that particular point in their orbit. They follow the adjusted path till they hit another downcable, etc.
The ring shouldn’t really drift if off center, any more than a satellite in an elliptical orbit does, especially since the downcables can apply balanced forces to correct any orbital issues. More importantly, each bit of the cable is spinning very slightly faster than orbital velocity, perhaps 10 to 100 mph, so if there’s a problem you just relax the tension (or even sever the connections to create thousands of ring segments) and the systems will drift to a slightly higher orbit.
I haven’t crunched hard numbers on the system, but I think the behavior of a stretchy, wiggly cable will have to be applied to modeling a stiff ring because no ring is going to be truly stiff.
The football-shaped orbit idea I mentioned is just an extension of the thought, using seperate orbiting masses in a stream as opposed to a physically connected cable. The downcables at the pointy ends of the football orbit kick each mass heading toward apogee back toward perigee, and the change in vertical momentum supports the down cable. Any energy losses in the reflecting system would have to be compensated by power delivered up the down cable, so that the orbits of the masses don’t decay.
Think millions of big ball bearings supporting two slightly bent mass drivers. Of course, if an errant ball bearing (traveling at orbital velocity) impacts the mass driver structure or downcable, it’s going to be a very bad day, so maybe connecting the bearings with a cable to keep them from wandering out-of-plane is a good thing, which marries the idea back to the LEO cable.
@Bart,
That was fast! 🙂
I recently read Larry Niven’s Ringworld series, and in a preface to one of the later ones he talks about how the story evolved. After the first book came out he was doing a book signing in Boston and a bunch of MIT students showed up chanting “The ring is unstable! The ring is unstable!” So in the second book he introduced Bussard ramjets on the ring’s edge that were used to stabilize the orbit.
But if the ring was unconnected segments then the problem couldn’t exist, as each segment would just follow the laws of orbital mechanics like any planet. Of course their mutual gravitation might eventually make them clump and bump, just like planetoids in any early solar system evolution, but it seems safer than crashing into the sun.
So I guess a good design rule is that the components of a ring need to be free to change their orbital eccentricity in response to applied forces.
Well, George, I would not dream of claiming I am absolutely right without spending a lot more time looking at the problem. My prima facie impression, however, is that it wouldn’t be stable and would require active control of some type. I thought we might get others who had studied the problem chiming in here but, not so far at least.
Here’s an idea, though: suppose the ring were equatorial so that the magnetic field lines are more or less vertical everywhere on the ring. You run a current around the ring, so you get a radial Lorentz force pushing out instead of in like the gravity. Now, I believe you might have a stable configuration (again, caution is advise – I’m more or less just thinking out loud here).
It might only be stable in the 2d plane, though. I think you’d have a net instability along the Z-axis since, as you move up or down, radial field lines crossed with the tangential current would give a force along plus or minus Z. So, maybe you could have like 3-rings directing current in different directions to stabilize the configuration… or something. Maybe the guys who make those little magnetic levitation novelties would have some good ideas.
But, then the J2 bulge might help stabilize translationally in the X-Y plane… It’d be an interesting dynamical problem!
…translationally to the X-Y…
Bart, others who have studied the problem reach the same conclusion, that a ring is gravitationally unstable. A sphere would be stable, but a ring is not.
If you can provide an outward pressure that matches gravity’s inward pressure then stability can be maintained, but this is difficult, and monkeying with the Van Allen belts might not be wise. The magnetic field surely affects the Earth’s electric field, and the electric field is tied to charge seperation in clouds and possibly thunderstorm formation, and thus cloud cover and albedo. So before such a system were built scientists would have to figure out the possible consequences to the climate. Given the track record of climate scientists, by the time they reach a conclusion free of political bias and self-interest that everybody trusts, we’ll have developed anti-gravity lifters and faster-than-light travel, making the ring moot.
Well, yeah, I guess I forgot about birds flying North for the winter and all. Usually, I don’t have to design for such details!