I'm mildly skeptical about the idea of using carbon nanotubes to make strong compressive structures- particularly using tensegrity ideas.

In my experience tensegrity structures aren't used much (ok, this is a fair overstatement, space frames are used in a number of structures), but the problem with tensile cables is that they often tend to be quite flexible- particularly stretchy. Usually this flexibility often means that the structure flexes to failure, often with a buckling failure.

Besides, even for tensegrity structures you still need to provide a compressive member, and this member presumably isn't going to be nanotubes under compression. Even if you apply external nanotubes to prevent buckling, the compression of solids is typically the same or worse than their tensile force, and we already use compression plenty, so I don't see that this is going to be a big win. In fact tensegrity structures have internal stresses that mean that you need *more* compressive material than a similar purely compressive structure.

On the other hand; inflatable structures sound potentially interesting, and seem to be tensile-only structures. I can't off-hand see a limit for how big they can get...

You can't launch straight up because the coriolis effect due to the earths rotation pulls you westwards away from the elevator (in the earth/elevator frame of reference). You actually have to point eastwards quite noticeably to keep up with the elevator.

Under 1 kWh (if I haven't messed up my maths). Well under 20c per kg, even assuming expensive electricity.


That works too. It's not clear that that would be cheaper though, and it takes 9 months or so to ship.


Eventually...

I'm sorry, perhaps I'm being slow, but why do you have to use an asteroid for the counterweight?

Couldn't you just lift some rock up the elevator ahead of time from the Earth and then do the slingshot trick with that?

It's actually fairly difficult to move stuff into earth orbit from elsewhere- you need rockets for that in nearly all cases.

I suppose you could also use lunar material raised up using a lunar beanstalk, that would work too, and is probably somewhat easier; although attaching it to the earth beanstalk might be "interesting".

Yes, either works and works well. You need a fair amount of shielding- a meter or two. But because it is in orbit, you only have to carry it up the elevator once, and once there it doesn't take away from the capacity of a beanstalk.

It's just at the upper edge of the Van Allen belt. In fact, quite a bit of the time it's outside it- IRC the sun throws out solar wind in all directions and this hits the earths magnetic field, and this makes an asymmetrical radiation field held somewhat 'downwind' of the earth. GEO swings you in and out of this radiation zone, but you are past the worst of it. But in a solar storm you can get plenty toasty, both direct from the sun and from the edges of the earths Van Allen belt- which gets pumped up at those times.

And there's continual bondbardment from cosmic radiations- this cosmic radiation would cause cancer about every 15 years or so if you don't shield it. So 1-2m of shielding deals with that.


Yes, I don't think it works as stated.

I am wondering if a modified form might work though. If you hang a rotating bolus off of the elevator, then a rocket might be able to jump vertically off the ground and grab the tip, and then climb along the cable to the beanstalk. That wouldn't endanger the beanstalk too much, and provided it wasn't at too low an altitude you might be able to fully shield it without adding too much weight to the beanstalk. But I haven't done the maths, and it's probably a bit iffy.


I'm thinking taxis from LEO to anywhere using fuel raised up the beanstalk and aerobraked down to LEO. Access to LEO by rocket. Refuelling like that is *great* if the beanstalk is cheap- rocket fuel is cheap enough. But right now we have to launch it on rockets, and that's not so good.

Shielded man-rated containers work too. But it's slow; and probably going to be cramped initially. But eventually, with a really massive cable it's maybe not a problem.

p.s. incidentally if you lift a LEO launcher up a beanstalk, say 300km before lighting the rocket, it helps a lot- the vehicle avoids having to hack through the atmosphere, it doesn't have to gain any further altitude, and the rocket nozzles see only vacuum which makes them about 20% more efficient, and at 300km you're below nearly all the Van Allen belts, so the radiation isn't a problem. Also you're only using the beanstalk for a couple of hours, so you can use the beanstalk more frequently.

Nitpick: LEO is Low Earth Orbit. So LEO Orbit is redundant.


Yes, I know, but it doesn't make any sense. Don't forget that the cable *is* moving *fast* at GEO, and even at GEO you aren't completely out of the belts even there- and you need a GTO capable rocket to get that high; never mind about actually connecting to the cable.

So, if you use a LEO capable rocket going straight up, can you get past the belts? No; you'll latch onto the cable *way* below GEO, still well within the belts, and if you miss the cable you are doomed (reentry angle being way off); and if you hit the cable, the cable is doomed as well.

I'm not saying it wouldn't help, I'm just saying it wouldn't help enough; and there seem to be large practical issues with doing it anyway.

Unless I'm missing something, which is entirely practical, it doesn't seem terribly practical. The closer you connect to the beanstalk at GEO the nearer to being a GEO-capable launcher you need. GEO-launchers are expensive, roughly 3x more expensive than LEO launchers.

And the Van Allen belts extend most of the way to GEO. So it doesn't look like you would be saving much. Then there's the problem of standing on a column of flame whilst you try to attach to the cable; and without cutting the cable.

It's probably better to just launch to LEO, and refuel with fuel dropped off of the elevator and aerobraked into a suitable orbit for docking. Or something.

The problem with this thread is that it is predicated on the idea that going a few thousand miles suborbitally is substantially cheaper than going to orbit.

Well, it is cheaper.

Consider that more than one orbital launcher is just a modified ICBM.

The payload difference between orbital and intercontinental suborbital for the same launcher is only about ~30%; so the price is 1/(1-0.3), about 1.4x higher for orbital than a few thousand miles.

So a few thousand miles is only going to be 70% the orbital price. That's currently running at $10 million or so. So the suborbital price is about $7 million. You'd have to be in a *tearing* hurry.

Still, if you stick wings on it, you can do better than a purely ballistic approach, but it still won't be cheap.

Then there's the safety issue. Currently passengers are lost about 1-2% of launches. That would probably preclude it being used for anything but reaching orbit. It's only acceptable for orbit because there's no other way to reach orbit.

So I think it's a no go, unless someone does something *really* clever.

So make it 500 or 1000 km apart. It doesn't matter much.

Tensile strength of 6063 -T6 aluminium alloy is 240MPa. So, to hold up 25,000kg you require 250,000N of force. That's an area of 0.001m^2, so a hook length of about 30cm would be about a kilogram (~2.7g/cc).

But that's in the middle, at GEO. Near the bottom it would be a teeny, tiny fraction of this weight.


Nanotubes? You just need to spread the pressure over a wider area. You use nanotubes to do that. :-)

You lose. Aluminum atoms are even smaller! :-)

Seriously, I don't think nanotube cables are made of single nanotubes. They're good, but not *that* good.

I don't see that there should be just 2 segments. There's no reason that I can see why you can't have 22 segments or 222 segments for that matter. The only downside is that the joints weigh more than the nanotubes, so the average weight of the cable go up as you add more segments, but the strength is the same (the joints would be atleast as strong as the nanotubes either side, for obvious reasons).

But a joint might only weigh a kilogram or less; and the nanotubes weigh around a kilogram or so per kilometer, so a joint every hundred miles is only adding 1% to the weight of the cable or so- I don't think that this is terribly significant.

There are questions about how to make a joint at all though. Nanotubes are pretty slippery. In principle nanotubes can be formed into a long loop I guess, but how easy that is in practice I have no idea. If a loop can be formed then wrapping the loop around a rod at each end would be a reasonable architecture I guess.

Space elevators do have one thing in their favour however- they're new. That means that there's a half-decent chance that the seed elevator may be launched for pure research purposes- as in, stick it up, and see how long it lasts.

Research has a pot of money that can be plundered for this. If the seed elevator survives then funding for the full thing probably becomes a lot more obtainable- but beyond the seed there's still going to be that market risk angle, and nobody knows how that will turn out.

OTOH, the SE is likely to have a safety factor of perhaps 2, so losing even 1/10 of it's strength isn't likely to be terribly significant.

My gut feel is that whatever the answer is, the answer is way, wayy wayyyyy too much shielding is needed.

I had an email exchange a couple of years ago, Brad indicated he was hoping a foot of metallic shielding might be enough. But that's a very high weight; and even for the most optimistic projections it's gonna *cost*- and the cost gets added on to the price of the ticket, since you have to lift it each time, and it takes payload away from the elevator.

The best way is probably to use freight to protect the passengers, for example water, or rocket fuel, or building materials. That way somebody *else*pays for their payload and the passengers only pay for themself and their consumables.

Still, it's going to require a big elevator to be able to carry all the shielding that people need. Maybe 100 tonne payload or more.

Incidentally, the bigger the elevator, the better. Shielding weight is proportional to surface area (diameter squared); whereas shielded volume is proportion to diameter cubed. So the shielding weight per person is inversely proportional to the number of people carried.

As I understand it, electromagnetic shielding can be employed, but needs a very large amount of power, and doesn't end up saving much weight; or atleast it didn't with designs for protecting space habitats. I expect that this would be similar.

Not quite. I'm saying that by far the most important point is to address the tourism market, since that is the only way that launch volume can grow, since it seems for all launch systems, and all future launch systems that prices will be brought down. It is *not* simply a matter of improving rockets, quite the contrary, cheap, inefficient and nasty rockets might be perfectly good if they were reliable; it is a question of developing more appropriate launch systems to address that market; whether they are nanotube based or fuel based.

I think somewhat higher would create a cycle; it's not even clear that we aren't already in the cycle. I've seen potential costs a lot lower than $500/lb for rockets.
 
I found a reference to 9000 ft, so well over a mile anyway.


No, that isn't my point either. My point is that the cost models for a laser-powered space elevator is not massively if at all lower than rockets, until you are launching *massive* quantities.

That's *really* bad. Nearly all big systems are small systems with the capacity to grow. Look at the Channel Tunnel. That's a big system. It took 200 years to build...

The 'fuel'- electricity is more expensive. The development and construction of the infrastructure is more expensive and the risks are enormously higher. It seems to me that few people here have really got this point. If I were to bet, even if nanotubes popped out tomorrow, I might well still bet on rockets. Indeed, because rockets seem to support tourism, and elevators don't (directly), it seems that rockets need to flourish for elevators to grow.

It's not that the emperor has no clothes, he's certainly decent, but he's certainly wearing a skimpy swimming costume, and it's rather chilly. The clothes designers need to put some more clothes on him.

No doubt you're jesting, but a fully fueled Atlas very much is designed to fall atleast a few inches. An early rocket did that, due to engines shutting down and exploded. It's now part of the spec that it be able to fall maybe a foot intact.

Now you're just being facetious.

Wrong!

Wrong again. So far as I can tell British Airways ran Concorde at a profit. The real problem was that the airlines wouldn't buy Concorde, because they were happy with their subsonic transports- they didn't have to take the risk.