Thursday, April 28, 2011

Our generation will go to space...

Don't tell me that the concept of a space elevator doesn't fill you with thrills...

Some years ago (checking my archives, I find it was actually way back in 2006), I championed Liftport's Space Elevator programme, believing that—amongst other things—the whole concept was incredibly cool. Although they ran into difficulties at one point, Liftport is still going and still keeping the space elevator dream alive.

There are times when the world conspires to remind you of certain things, are there not? I have just started re-reading Kim Stanley Robinson's Red Mars—which features a space elevator—and now I have just stumbled across this fantastically enthusiastic post over at Counting Cats.
In 1995 I started my MSc in astrophysics (yeah, I have a dog in this fight but mine really is worthwhile—do I need to say why? If so I have lost you and you can grab your coat on the way out and basically I hope the door doesn’t bang your arse) at Queen Mary, London. I met a Spaniard there and you know what? She’d only written her undergrad dissertation on the space elevator! I was like wow! I really was. The idea, like so many others, like the Silbervogel or whatever had just been in the aether (which Einstein demonstrated doesn’t so much not exist as just not matter—ouch!).

Whether or not great minds think alike is irrelevant. Competent ones can do and we were far from alone. Both Agnetha and I had dreamed independently of something grand and this was not the meeting in a pub in Stepney of two geniuses. It was better than that for it was written on a beer mat. It was simply the realisation that it could be done and that we were not alone in conceiving this scheme. Yeah, I know it was not original but it honestly had been to me and her. That is my point. If the idea can occur without separate cause to the likes of me or Agnetha then…

… Maybe it’s a good one. Not an Earth-shaker. Not a Quantum Mechanics or whatever but basically, physically, (the engineering is as ever something else—I have have the greatest respect for engineers—they make dreams real—and that is way cool) absolutely obvious.

NickM finishes his post with this extraordinary video: it seems that the designs have not much changed since those that I saw in 2006, but the technology has finally arrived. It's time...

So, so cool...


Frank Davis said...

They could have at least explained what a space elevator is. The idea is that once you have a satellite in geostationary orbit above the earth's equator, you can join it to the ground below with a cable (or ribbon), and you can haul stuff up to the top with a mechanical lift. No need for rockets.

The difficulty, last time I looked, was in in finding materials strong enough to resist the stresses in the cable.

Personally, I think that the related idea of an 'orbital siphon' is an even cooler idea. A siphon is essentially a VERY long cable extending out from the equator, far longer than a space elevator cable. A cable longer than about 170,000 km will pull upwards, and haul up anything on it. No need for rockets, and no need for mechanical lifts either. It's powered by the rotational energy of the spinning earth. If they ever build a space elevator, the next thing they'll make is a siphon.

Mark Allen said...

S10 billion? Is that all? When can we start?

Michael Fowke said...

You must be mad. I don't even like rollercoasters.

James Higham said...

A Russian mate showed me pics recently that they're going hell for leather for the SE over there. Seem to think it's possible.

Dioclese said...

I hope you are proven right. The end of the space program was the greatest missed opportiunity in human history.

If the human race is to survive, we must get of the single rock and disperse - especialy as we are forecast to have a very close encouter with an asteroid on Firday 13th 2029!

If we just sit tight, we are probably buggered!

Weekend Yachtsman said...

Why will we go to space?

It will cost unbelievable amounts that would be better and more wisely spent here. Especially if the State gets involved.

There's nothing out there worth having unless you go such distances that nobody would live long enough to get there, to say nothing of radiation hazards.

When you do get there, every single thing you need, right down to the air you breathe, has to be brought from Earth, at unbelievable expense and risk, see above.

Comparisons with the early American settlers are moot - at least when they got there, there was fresh water and they could grow stuff.

Every boy wants to be Dan Dare, or Captain Kirk, or Doctor Who, or something - that's about it really, isn't it?

bloke in spain said...

Sorry Frank, but your 'orbital Syphon 'project isn't actually a goer.
In a space elevator the cable doesn't just go to geostationary orbit. It goes a lot further. Theoretically you could extend the cable to twice the distance & use the mass of the centripetal (?)(for purists)pull on the extra to balance the mass of the cable dangling into the gravity well. Actually you'd do what Kim Stanley Robinson did in the Mars trilogy & hang a large mass on the end & a much shorter distance. Works just like a lever.
Trouble with your idea is twofold. One you don't want to be putting any more tension on the cable than absolutely necessary because if the tension is increased the strength & therefore the mass of the cable increases. And the increase isn't linear it's logarithmic because you're having to increase the strength of the cable to hold up its own mass as well.
Two. The 'syphon' only works a few times. Conservation of angular momentum. As the payload goes up it moves back along the orbit pulling the cable with it. Eventually you have to use energy to push the cable back into the correct position & that takes exactly the same energy as you've gained from the earth's rotation.
There is however a neat cheat. You start with a shorter cable in a much lower orbit. The cable is made to rotate end over end in the plane of the earth's equator. If I remember correctly the cable rotates 6 times per orbit. Set the whole thing up right & when the bottom tip of the cable touches the atmosphere it's travelling at almost the same velocity, relative, as the rotating earth below it. If it touched the ground the two would be stationary so, to an observer, it would just seem to descend vertically through the clouds & then rise again.Attach a spaceship to the cable & it gets yanked up into space.
But it gets better. When the tip of the cable gets to the point opposite to the earth's surface it's travelling at well over escape velocity. Let go & if you get the geometry right your spaceship coasts all the way to Mars.
Of course the energy has to come from somewhere & that's from the cable itself. But now for the neat trick. If you capture a returning ship at the endpoint of the cable & return it down to earth you put all the energy back
OK, so now you put a rotating cable orbiting Mars & you can send ships backwards & forwards. As long as the mass going both ways balances the system works. You could ship colonist farmers to Mars & their farm produce back to earth to feed the 'starving millions'. At no energy cost whatsoever although you would get Caroline Lucas on your back for altering the length of the days ( about a microsecond every million years).

bloke in spain said...

Weekend Yachtsman - stick to boating.
If we get out of the prison of the gravity well we've access to infinite energy & infinite resources.
Who said it's raining soup out there & all you need's a spoon? Heinlein? Pournell?

Frank Davis said...

In a space elevator the cable doesn't just go to geostationary orbit. It goes a lot further.

Depends on the relative mass of the station at the top and the cable below. If the cable mass relative to the orbiting space station is negligible, the space station will be at geostationary altitude.

Two. The 'syphon' only works a few times. Conservation of angular momentum. As the payload goes up it moves back along the orbit pulling the cable with it. Eventually you have to use energy to push the cable back into the correct position & that takes exactly the same energy as you've gained from the earth's rotation.

No. You're wrong. The siphon works indefinitely (at least while the earth keeps spinning). At operating length, mass is continually released at the top, and pulled up from the bottom. The siphon remains at the same angle.

I agree though that the stresses along the cable are much higher than a space elevator.

The great thing about the siphon, once you've got it working, it that it will just keep pumping stuff out into space. Over time that will be thousands and thousands of tons. Which is the sort of thing you need if you're going to colonise space.

RantinRab said...

Will it have a faint whiff of farts, like most elevators today?

bloke in spain said...

Sorry. With ref the long post above: Just rethought my orbital dynamics & I reckon the cable gets pulled forward in it's orbit not backwards,again due to conservation of angular momentum. The geostationary point moves up the cable so the cable below has to move faster. Anyone know enough about this stuff to confirm? To me it's a tad counter-intuitive.

Frank Davis said...

In addition, a siphon doesn't consist of a single payload, but a long string of them, stretching 170,000+ km into space. When the siphon is stationary, it extends radially into space, with maximum cable stress as geostationary radius. When it starts hauling the whole chain up, the siphon train tilts back at an angle (which is greater the higher the speed at which the train rises). Payload masses are released at the top as they are added at the bottom, so the net mass of the siphon chain remains constant. The siphon requires a braking system at the base to stop it from accelerating. This braking system would be used to generate electrical power for the siphon chain. The speed at which the siphon chain rose would be relatively slow, and it could take a couple of days for a payload added at the bottom to rise to the top and be released. Depending on the release radius, payloads could have sufficient velocity to escape from the solar system.

Try modelling it some time.

bloke in spain said...

Sorry Frank, but I still don't think that works. To lift something up the cable the energy has to come from somewhere. The only available energy you've got is the total angular momentum of the system. Raising the mass up the cable causes the cable to orbit slightly lower. Lower orbits are faster so the centre of mass moves east relative to a point on the surface. Once the payload passes the geostationary point its travelling too fast for an orbit if it's still attached to the cable so it pulls the cable into a curve.
Yes, the energy comes from the rotation of the planet, because the planet's part of the system, but you can't buck conservation of angular momentum.

Frank Davis said...

I originally thought siphons would have to be rigid.[1].2.07.pdf

But they don't have to be.

(click on the applet to start the simulation)

Frank Davis said...

you can't buck conservation of angular momentum.

Indeed you can't. The earth's angular velocity (and momentum) would fall fractionally as mass was lost.

bloke in spain said...

Interestingly, you can really model this with a hose & a bucket of water. Tried it as a kid when the idea of whether this would work as a pump occurred.It's a good way of cooling down on a hot day & learning about conservation of angular momentum. You get very wet when the end of the hose hits the back of your head.. Then I realised it's really no different from a centrifugal pump & gave up trying to reinvent the wheel.

Yes, given effectively infinite tether strength & a massive counterweight relative to payloads it's theoretically possible, but unattainable with materials that would conform to any possible strength/weight ratios. There's a limit to the strength of molecular bonds. Surely we need to considering things that could actually be engineered?
The rotating tether is probably doable in some versions with something the strength of current aramids. Maybe carbon nanotubes, if they could be produced by the hundreds (thousands?) of tons, would be better.
You can also considerably shorten the tether length by having the closest to surface point in the upper atmosphere & there's less problems with atmospheric friction heating as the dip starts. Just fly up & hook on.Precisely steering the whole thing isn't such an issue either as you can chase the endpoint.

Frank Davis said...

given effectively infinite tether strength & a massive counterweight relative to payloads it's theoretically possible

I see you're retreating from your earlier position! ;-)

But there's no need for a massive counterweight. The upper siphon chain is itself the 'counterweight'.

Nor is there need for 'infinite' tether strength. It has to be an order of magnitude or two stronger than the tether on a space elevator.

Surely we need to considering things that could actually be engineered?

When Jerome Pearson first calculated the requirements for a space elevator, I think he estimated that the tether would need to have the tensile strength of diamond. There was no way that could have been engineered 50 years ago. But things move along, and if some people think that a space elevator is now a feasible engineering possibility, then it must be because there are new materials which have sufficient strength.

I agree that a siphon remains a practical engineering impossibility right now, but the space elevator was when it was first conceived.

bloke in spain said...

Incidentally, it surprises me that whoever wrote the paper didn't realise that the tower isn't actually necessary. Once you've got your string of payloads past the geostationary point you just need the right degree of braking to keep the line paying out. Start with a spider's web thickness of unobtainium fibre tied to the tail of a rocket & let it pull up an increasing taper of cable once it's past the geostationary point till you're hauling freight. Look!No tower!

Frank Davis said...

You have to go a lot further than geostationary (35,000 km) to get the payload chain to pull itself upwards. It's more like 175,000 km (it's in the paper somewhere) before the net centrifugal force (or centripetal force, if you prefer) exceeds net gravitational force. That's half way to the moon.

Yes, the rocket idea might be possible. A slightly more plausible way would be to start building the chain upwards and downwards from geostationary until you've got the required length, and then gradually release the brakes at the bottom.

Incidentally, the tower wasn't there to build the siphon. It was to give it rigidity. If you can build a rigid radial siphon, it can raise far greater masses at higher speeds than a laterally-unconstrained siphon. In the example of the latter shown in the simulation model referred to above, the siphon is only lifting 43 kg/day, or about 16,000 kg/annum.

bloke in spain said...

I see you're retreating from your earlier position! ;-)"

Not really. I build shit so I tend to work with what's conceivably doable. All the problems I mentioned would arise using materials that can be envisaged. Nuclear forces really do enforce a cap on what tensile strengths could be obtainable. Last I looked an orbital tether tapered to around 10m at max load point for realistic payloads. Maybe a way of reducing that's been thought of since.
The siphon (although pump's a better analogy)is another world. For a start there's no taper. Those further packages are pulling the mass of everything below & that includes the links that have to be strong enough to take the load when they get to the top. For a quarter of a million kliks?
Maybe this is the way you dismantle planets.

bloke in spain said...

Incidentally, the tower wasn't there to build the siphon. It was to give it rigidity."

Typically, the load bearing capacity of materials is less than half their tensile strength. A free-standing tower to geostationary? I think you'd be talking more in terms of an elliptical planet.

Frank Davis said...

No, it's not a free-standing tower. It's not in compression or anything. It's in tension along its whole length, just like the siphon. But it has lateral rigidity, unlike a long piece of string.

bloke in spain said...

For the tower to be laterally rigid half the tower, as shown , will be in compression. Lateral rigidity is achieved by a balance of compressive & tensile forces. I suppose you could manage to get a measure of rigidity from pure tensile loading by attaching guy lines up the cable & anchoring them to the planet's meridians ie build the thing @ 90degW 0degN @ the attachment points along the Greenich & the IDL, but that only gives a measure of rigidity out to a couple of diameters.And you're building 4 towers not one.
Incidentally, are you proposing to build in sit, or fabricate elsewhere & tow it into place? There's lengths of cable will have harmonic sympathy with the regular tidal effect of the moon. Whole thing will twang like a guitar string.
With reference to the cargo carrying capacity, I wouldn't imagine that's a factor. We're talking gigatons of structure here.The engineering problems are so great it'd probably be easier to spin up the rotation of the planet so things just lift off naturally at the equator.

Frank Davis said...

The engineering problems are so great it'd probably be easier to spin up the rotation of the planet so things just lift off naturally at the equator.

Maybe you're right! :-)

bloke in spain said...

Enjoyable mental exercise though. Thanks for the fun.

The rotating tether is probably almost doable now provided that incoming & outgoing loads balance. Or you put energy in using an ion drive at the midpoint.
I think I may have thought of a variant of it that would work in one direction energy free though:
The rotating tether has an elastic cable attached at it's mid-point connects with a mass in a constrained orbit further out from the planet. The sum of mass/centripetal force of the whole issue is slightly negative allowing the tether to keep hauling stuff up the gravity well with the elasticity damping the system. I hope the whole lot just precesses round the orbit getting its energy from the planet's rotation but my math isn't good enough to cope with proving it.
It's sort of your syphon without actually having to build the tower or the full length of cable.

Anonymous said...

Being pulled slow upwards, that will be fun.

Sliding quickly downwards for the return trip - not so much.

neil craig said...

You may be interested in this, which uses an expert's opinion that a space elevator could be built, over the next 12 years for $20 bn (£12bn) - only twice the cost of one government warming alarmist quango (NERC) out of dozens.

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