From Henry Spencer, over at sci.space.policy:

...as Jordin Kare noted a while back, the elevator people say they could give us launch cost of a few hundred a kilogram for a ten-billion investment... but there are plenty of rocket people who think they could match or beat that launch-cost number with a lot less up-front money. "They aren't Boeing, but neither are you."

And the nanotube materials that the elevator people need will do wonders for rocket structure, well before they're good enough for elevators.

My thoughts exactly.

Long before CNT tech becomes mature enough to invest in a space elevator, it will have transformed life on Earth.

Imagine CNT insert plates and fabric in ballistic armor. It will make Kevlar and Spectra look like tissue paper.

Imagine an auto with CNT replacing most steel. The weight will be a fraction and fuel mileage will soar. I am sure it will do wonders for the mass fraction of space transports as well.

If lasers get better in the way Jordin expects, his up-front beam facility construction costs for the laser HX launcher could easily be an order of magnitude lower.

And, at least according to my calculations, that beam facility adds to each payload cost something on the order of the energy cost of launch.

Still, nobody believes such calculations and probably rightly so. I doubt I'll start writing formal stuff on costs until I've had a chance to do a full-blown probablistic cost analysis. Does anyone know if the space elevator folks have done that?

If lasers get better in the way Jordin expects, it might also make laser beaming from orbital powersats practical. This could revolutionize the powersat concept, since the minimum scale would be drastically reduced.

I've often had the same thoughts as Henry on the potential applications of CNT-based composites to spacecraft structures. Not sure however if the fibre/matrix combination that would be useful for a space elevator ribbon would be transferrable to something like a monocoque spaceplane structure. I would think that there would be plenty of similarities, however, and spaceplanes (or whatever) would probably not require the same magnitude of strength as a ribbon (but possibly need to have a greater toughness or thermal resistance). Anybody from a materials engineering background here?

I think the best near term option for very cheap access to space is to combine a reusable suborbital rocket and a rotating space tether.

For a really big mass transportation system, the suborbital rocket should probably start and land directly from the ocean like sea dragon or the interorbital neptune vehicle.

Of course such a system would benefit tremendously from CNT advances, but it could be built today using commercially available materials.

I know that I sound like a broken record by now, but...

CNT will outdo the revolution of plastics in the 50s. Folks who get ahead of the game and start figuring out how to use them as they drop in price should for the most part make some nice change.

Rockets aren't going away. CNT, efficient heat shields, and above all else heavy launch schedules will change orbital lift before the elevators get there. They also provide more than 20ish launches/year per system.

Skyhooks in Earth orbit are messy. Leave them at the L-points. The elevators will have enough fun by themselves dodging stuff up there.

Elevators have some really *nice* benefits. Cost is a really big one, but not the only one. For starters, they barely get over 1G at all. Excellent for hauling people to space. The downside, of course, is that you have to have ample shielding (water tanks work).

Using an elevator to get a rocket the first couple hundred miles might (somebody wanna do the numbers?) increase the payload fraction significantly--most of the atmospheric considerations go away entirely, and slingshot trajectories to orbit become possible. Likewise, using the full length of the elevator provides a major advantage in interplanetary trips.

Question on laser space power beaming... I thought one of the advantages of microwave beaming (in addition to the formerly huge gap in transmission efficiency) was that it spread the beam out over a large area, reducing the risk of the powersat turning into a Death Star. What do the numbers look like with a near-future fiber laser? Can we reach a compromise between too big a receiver and too power-dense a beam?

Any one laser can be made to have a sufficiently low intensity at the surface, just by making the aperture small. If you stack many laser beams, you can achieve high power at a target -- but that's true of microwave beams as well. The main difference is that there will be many more laser beams (assuming a large number of mutually incoherent emitters), so more can be stacked on smaller targets.

CNT will outdo the revolution of plastics in the 50s. Folks who get ahead of the game and start figuring out how to use them as they drop in price should for the most part make some nice change.

For example, a private company that builds a lunar elevator to the vicinity of EML-1 and keeps the acquired knowledge and expertise in house would appear well positioned to make money as a prime contractor for building Terran elevators. Learning to handle tether oscillations and stuff like that.

Also, owning a station at EML-1 before the elevator gets built could be profitable as well, like buying land near the interstate highway BEFORE the new on/off interchange is built.

Paul: How small, and what power density?

I haven't followed the math closely, but my understanding with microwaves was that a multi-GW receiver would be spread out over hundreds of acres, yielding a power density low enough to give someone in the beam a slow tan.

How small can you make the receiver before your power density becomes potentially lethal? A few acres? A small building? And what kind or receiver would be used? CNT-based photovoltaic? Mirrors onto a steam/Stirling engine?

s Jordin Kare noted a while back, the elevator people say they could give us launch cost of a few hundred a kilogram for a ten-billion investment...

Maybe I'm not an elevator people. I don't seem to be as strident about the thing as people seem to assume one of those must be. And my membership card never arrived. Damn catch all labels that don't fit.

And the nanotube materials that the elevator people need will do wonders for rocket structure, well before they're good enough for elevators.

Sure will. But structure cost isn't the only thing that keeps costs up.

I come not to praise rocketery but to ... no. I want a cost effective method to get from here (stamps on the ground) to there (points up).

A space elevator seems a reasonable method to investigate for this end. You may note that most of the bleats about how damned _good_ space elevators must be don't come from anyone who is actually working to that end.

Paul: How small, and what power density?

You can figure this out from r_emitter * r_receiver ~ wavelength * distance. For wavelength = 1e-6 m, distance = 4e7 m, the product is 40 m^2, so a 1 meter diameter aperture at the emitter would illuminate an 160 meter circle at the earth (I've omitted a factor of O(1) that would make the circle a bit larger, and it would be tilted at an angle for sites off the equator.)

Call it 4e4 m^2. If you want a power density equal to sunlight, you need about 40 megawatts of power. Kare's lasers would be maybe 10kw, so about 4000 emitters. Larger emitters in space would let you get by with a smaller spot at Earth and fewer sources at lower overall power. Note how small this all is compared to microwave powersats (but advances in lasers and efficient receivers are needed.)

In any power beaming scheme, you can incoherently add beams from multiple sources to get as high a power density as you want, even if the power from a single satellite is designed to be incapable of exceeding some safety threshold.

Actually, come to think of it, you'd use the same lasers and collectors to beam power down that the elevator would to beam power up. For pretty much the same reasons (although receiver mass isn't as much of an issue dirtside).

Doh!

I have said it many times before but a space elevator has a couple of orders of magnitude times the drymass of a high flight rate rocket for a given payload rate. Hence it will cost a lot more than a comparable rocket. The energy savings are in the noise and it is disingenuous to argue the effectiveness of a space elevator on the basis of energy savings. Well, not until costs are down to the $10/kg range.

Pete.

Sure, CNT will be a great material for lots of things, and it may make some improvements in rockets. But rockets are inherently inefficient because the first 1% of your fuel has to lift not only your payload and the rocket casing but also the other 99% of your fuel, and the 2nd 1% of your fuel has to list the other 98%, and so forth. Saving on the weight of the casing and the payload is not going to make that go away. The cost of just the energy -- never mind insurance, ground support, research, the rocket itself, etc. -- just the price of the energy needed to get a kg to LEO, given that you're using a system as inefficient as a rocket, is enormous. That's why I'm more convinced by those experts who argue that it's critical to find a way to get off Earth without using rockets, if space travel is ever to become economically viable.

As Rand has pointed out, rocket fuel is cheap.

It's everything else--but particularly human physical and mental labor--that costs the big bucks.

"But rockets are inherently inefficient because the first 1% of your fuel has to lift not only your payload and the rocket casing but also the other 99% of your fuel"...

But that's only for a low specific impulse (Isp) scheme. On the engineering side, the real costs come in trying to make the rest of the structure like an eggshell so you can have a payload at all. And that's only an acute problem at low Isp too.

But rockets are inherently inefficient because the first 1% of your fuel has to lift not only your payload and the rocket casing but also the other 99% of your fuel, and the 2nd 1% of your fuel has to list the other 98%, and so forth.

In actuality, if your Isp is not too bad, rockets are surprisingly efficient. If you have high thrust and a variable Isp, and ignore drag, the efficiency at which a rocket converts jet kinetic energy to vehicle kinetic energy can be made arbitrarily close to 1 (idea: make the exhaust velocity at each point equal to the total delta-V so far, except very near the beginning, and also use high thrust to make gravity losses small.) Even with more practical assumptions the vehicle can be quite efficient.

You guys are thinking too small. The problem is we've got all this Earth gravity to overcome. The solution is to start lobbing mass quantities into orbit! Ok, I've provided the big idea... you guys can fiddle with the details. ;-)

(Think parallel railguns - what they call a couple in physics.)

The big problem the space elevator has is not just that the materials it requires makes rockets far more competitive. It's that it lags those competitive rockets by a generation, because nobody is going to sink billions into a full-scale beanstalk without building a test model first. And the best place for the test model would be on the moon, where there is nothing to screw up if it crashes, and where the scale and tensile strength requirements would be far less.

Nah, the best place for a test model is across some stretch of water, with traffic driving over it.

Believe me, when space elevators are about to happen you'll see it on the ground first.

CNT composites could make your rocket structure arbitrarily light. But you still have to fight the rocket equation, which means your payload fraction is always going to be small (unless you can start burning much hotter fuel). So I don't see how rockets can gain all that much from advanced materials (does increasing your payload fraction from, say, 10% to 15% really have a huge impact?). If you spent $10B wisely you could maybe come up with a rocket system that was more cost effective. But how likely is that? Much of the SE money will be spent on research, and then on ribbon material and other infrastructure. So avenues for waste and system mis-design will be different.

The fact that CNT composites don't exist yet is the biggest argument against the space elevator. But once they come along, as others have said, you'll see life on Earth transformed before they get used in a space elevator. (In fact, the cost of the SE will probably be an afterthought in the money to be made from high-strength CNT materials.)

Pete says a space elevator is much heavier than a rocket. But the space elevator can be used for decades, and can over time transport many times its weight to orbit (much better than a disposable rocket, or even any reusable rockets I've seen). So weight is a bad way to compare the two.

Remember, the SE will not replace rockets. Until you can afford to hang a ton of shielding on your vehicle, SEs will not be transporting people to orbit. And once you get to orbit, you'll still need rockets to get to different orbits.