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A concept known as a space elevator has been "doing the rounds" as a cheap way into orbital space for several decades now.

Basically, a cable of some description is anchored to an orbiting mass, and vehicles climb up and down the cable to transport materials and passengers to and from orbital space cheaply and easily (in any space journey, the most expensive and dangerous parts of the journey are the first and last few kilometres).

There are, technologically speaking, two main hurdles - the cable, and the vehicles.

Carbon nanotube composites are a strong contender for the cable, but travelling up and down the cable takes power. Rockets are out of the question (what would be the point of the cable?). The hours, possibly days, of the journey make batteries unfeasible, and the turbulence of the atmosphere precludes the use of PV panels large enough to power the vehicles from daylight.

That leaves transmitted power.

So far, the transmission ideas have been electromagnetic - lasers aimed at smaller PV panels on the underside of the vehicles, or microwaves aimed at receiver dishes.

Now, a new proposal suggests transmitting the energy mechanically.

European Space Agency ground station engineer Dr Riise ... proposed sending power mechanically - effectively by providing a carefully timed jerk of the cable at its base.

To demonstrate, he employed a broomstick to represent the cable held in tension, and an electric sander to provide a rhythmic vibration to the bottom of the stick.

Around the broomstick's circumference he tied three brushes representing the climber with their bristles pointing downwards - meaning it took slightly more force to lower the brush assembly than to raise it.

The vibration from the sander allowed the assembly to slide upward along the broomstick as it moved slightly downward, but grip it as it moved slightly upward. The net effect: the assembly rose against gravity straight to the top of the stick.

That's a bristlebot (or vibrobot)! Dr Riise, though, calls it a longitudinal wave climber or LWC.

It sounds easy enough to replicate the demonstration model, although I can't imagine how you would efficiently vibrate a 36,000km cable (I have images of damaging harmonics getting set up as the moving carriages change the effective length of the vibrating part of the cable, and shaking important bits off the vehicles).

Apparently, though, the idea is being looked at by lift manufacturers for installation in the next generation of super skyscrapers.

Demonstration video
BBC story

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kelseymh (author)2009-01-05

There's been substatial discussion of cable oscillations in space elevators, recently. Any oscillation turns out to be a very bad positive feedback loop (if you calculate the gravitational gradient this becomes obvious). One potential consequence (and a showstopper for actual use) may be that you have to travel the elevator adiabatically -- that is, so slowly that any induced oscillation has an amplitude of zero. It seems to me that using a vibrating system for the job would only make the oscillations worse. However, I am neither a rocket scientist nor a space-systems engineer :-)

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NachoMahma (author)kelseymh2009-01-05

> adiabatic . This is only the second time I've run into that word. The first when maintaining a calorimeter.

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kelseymh (author)NachoMahma2009-01-05

Hah! At least the first was in the correct context, rather than in a mixed metaphor like mine :-) What sort of calorimeter? An actual "chemical energy" device (burn something and measure the temperature rise)? In particle physics, the detectors we use like CsI crystals or stacked lead plates with a noble gas, where the particles stop and deposit all of their energy in a measurable way, are also called calorimeters.

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NachoMahma (author)kelseymh2009-01-05

. It's not such a big leap to apply it to energy in general. It didn't sound right, but it got the point across. . . Yep. We measured the Btu content of hazmat so the operators would be prepared for "hot" or "cold" feed when fed into the incinerator. Our's were ancient (green CRT display) and the bomb/bath units had to be serviced quite often. I learned a lot more about calorimeters than I really wanted to know. . . heehee I tried to Google CsI and all I got was hits for a TV show. "cesium iodide" works much better.

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Kiteman (author)NachoMahma2009-01-06

"Ancient" means "green CRT display"?? So what does that make the one we used in high school, with a mercury thermometer sticking out the top?

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kelseymh (author)NachoMahma2009-01-05

Technically, the BaBar crystals are CsI(Tl) (tellurium-doped cesium iodide). Since we were doing e+-e- collisions at 10 GeV, the incident photons top out around 5-6 GeV, so CsI(Tl) does fine to fully capture the shower. At LEP back in the 1990's, they were doing e+-e- at 100-200 GeV, so they used BGO (bismuth germanate) crystals in their calorimeter. Picture fully transparent "glass" blocks, heavier than the same sized piece of lead! The ATLAS electromagnetic calorimeter is a lead-liquid argon system, where the shower energy is only sampled, and calibrations are applied to derive the incident photon (or electron) energy.

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Kiteman (author)kelseymh2009-01-05

Have they been discussing just transverse waves, or the also the longitudinal waves needed for this idea?

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kelseymh (author)Kiteman2009-01-05

I want to go back and find some of the article citations before I respond in detail. For an incompressible (i.e. solid) cable, I think you'll find that the longitudinal and transverse perturbations couple -- a longitudintal driving force induces a buckling, which leads to a transverse displacement.

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Goodhart (author)kelseymh2009-01-05

I was about to comment, but thankfully I read down the posts first.....I haven't much to add :-) Curious though, would said oscillations create any problematic sound waves? What about the wind at higher altitudes?

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Kiteman (author)kelseymh2009-01-05

Probably, but some of the recent "versions" of the cable I have seen have been "woven tape" structures. I know they wouldn't normally carry a longitudinal wave, but under tension...?

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whatsisface (author)2009-01-05

That's an interesting demo. I wonder how long before something like that gets shown to us in physics (our physics teacher is always showing us cool things he finds on the web, like videos of thermal runaway in glass).

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Kiteman (author)whatsisface2009-01-05

Outside the BBC article and the website that hosts the video, this doesn't seem to have had much publicity - maybe you ought to be the one introducing a cool video to your teacher?

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whatsisface (author)Kiteman2009-01-05

Maybe so, i might see if there's a longer video first, otherwise it doesn't really seem worth it.

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Kiteman (author)whatsisface2009-01-05

So you'll have to build your own demo model instead...

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whatsisface (author)Kiteman2009-01-05

Maybe at some point. before then, there's an A level Physics exam, A level Chemistry exam and general studies exam. All in the space of a week.

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Kiteman (author)whatsisface2009-01-05

A-levels, pshaw, they're no harder than the old O-levels...

Runs like b*****y into the distance...

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whatsisface (author)Kiteman2009-01-05

I'd chase you if I didn't have a mound of revision to do :-P

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KentsOkay (author)2009-01-05

Sounds incredibly Douglasadamsian...

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