Constant's pations

If it's more than 30 minutes old, it's not news. It's a blog.

Tuesday, November 16, 2004

Space elevator: Cautionary thoughts

The theories are in outer orbit. Ref With all the political excitement ebbing, a few days ago I found myself wondering, "What would be more interesting to think about."

I recalled the space elevator, but thought "who's going to want to think about that."

I made a couple of notes and tucked them away thinking, "Maybe one day I'll mention this." Well, with the above link .... I thought I'd share them now.

Mechanism to move the elevator

There are a couple of things that I hope people have thought about. When creating a space elevator need to make sure really consider the weight of the cables.

I'm not suggesting that "the right answer" involves what I'm concerned about. Rather, I'm suggesting there's perhaps an obvious problem that has yet to be explained.

Cables Let's be specific. Elevators have cables. Those cables, as they pull something up, have to go somewhere. Sometimes the cables get wrapped around things and stay at the top; other times, the cables using a counter-balance simply fall down the other side.

Brilliant. Small problem. With a space elevator going up to 60,000 miles [or whatever number you want to use], that's alot of cable.

Design considerations

A library architect neglected to include the weight of the books in the building design.

It remains to be understood how much the entire cable assembly will weigh. Recall that breaking is not simply a function of speed [which may be low to accommodate passengers], but the force required to counter-act momentum of 60,000 miles of cable moving at 1G. That's alot of energy.

Some might suggest that there's no need for cables as there will be "anti-gravity technology." Brilliant. Then if that exists, there's no reason to have a space elevator then is there.

In other words, the space elevator "no reason for cables argument" could be premised on the assumption of technology, if it existed, that would negate the reason for having the space elevator.

Housing Space The "right answer" is to explain: Where is the cable going to be stored; how will it physically move up the space elevator, and where does the "leftover cable" after dragging "whatever they're pulling up" actually end up.

Breaking The other problem is decent. Think back to your days of bike-riding. Remember those annoying little reverse pedals on tricycles? They worked.

But the advancing technology meant that you could use rubberized brakes for far more effective breaking at high speeds. Breaking means control.

Think about a space elevator that is "on the way back down." It's all well and good to talk about "getting up there", but the return trip is going to be a challenge.

As the platform returns to earth, something has to control the speed, otherwise you're talking something that is going to reach terminal velocity of something far higher than I'm willing to calculate.

Regardless the actual speed, the only way to slow it down is to dissipate that heat using something. But it's not a one-time break. Each time you "release the breaks" to let the object continue its descent, you have to start breaking again.

Heat The key question becomes: What happens to the heat, how is it dissipated, and what physical mechanism is going to be used to actually slow the returning elevator.

You can't move something for free. Can't simply "use a counterbalance" to lift something as that "whatever counterbalance you use" has to be controlled, cannot be left to simply fall without control, and again the heat-dissipation has to be handled.

Transfer points Let's consider the length of the cable for the moment. Suppose it is physically possible to create a single 60,000 mile cable. Do you propose making it in one piece; and what of the repairs?

However, if the option is going to be, "We'll use multiple transfer points" and "multiple channels" and "the cable will not have to be that large," great: Tell me how many intermediate levels are needed; and how wide the elevator shafts are going to be to allow this.

Recall the World Trade Center. There were many shafts for elevators. This space elevator, if it has multiple shafts, is going to physically have to be wide enough to hold that many shafts. I'm not clear that there's been a clear explanation or understanding of what is physically required [space -wise] to allow this many transfers.

Sure, we may have the technology to do it, but the time to think about the physical space is now not assume its going to get solved. We can only go with what we have, not with "what will be in the future."

Acceleration Let's suppose there is an alternative "acceleration system" related to anti-gravity; or something using magnets. How big does the "anti-gravity-apparatus" have to be to adjust gravity; where do you physically put this object; how do you get it there.

My thought is that "suppose there is a way to move the elevator inside the shaft" using something other than cables, that "whatever it is" is going to have to be maintained, housed, and in a position to be repaired. That takes up space. At worst, that "machine that creates anti-gravity" has to be physically moved up into space, and plopped on top of the space elevator.

Do we propose using "something that doesn't exist at the top" to create the object and pillar below? That's analogous to saying in the 1700s that we're going to have a technical interchange meeting to discuss railroads; and we'll meet at the airport."

Summary Before we spend alot of money seriously developing this space elevator, I'd like some quick estimates from credible independent engineers on the heat that will be dissipated; the physical size of the breaking system to slow that elevator down; and the feasibility of moving a cable 60,000 miles long; or what alternative transfer-mechanisms are required to ensure that the cable is shorter.

I'm all for the idea of "modern technology" but let's not get so caught up in the idea of a "cheap and easy way to space" without thinking about the basics: Heat, physical spaces to store the cables, and the physical mechanism to transfer between various levels.

It might be possible, but explain it to me in terms that address the basics. Until then, it remains simply science fiction. It remains to be seen how many consultants are on the gravy train on this one.

Update: 22 Aug 2005

Google has announced plans to issue stock to raise funding for a space elevator.

One solution to the "massive heat build-up" in the space elevator is to use a series of airlocks.

By removing air from the upper regions, airpressure could be used to push the space elevator up into space.

The problem is going to be to figure out how to get the elevator back down to earth.