Author Topic: Space elevator  (Read 46294 times)

Offline joshuabaker

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Space elevator
« on: 03/21/2010 03:26 am »
I have developed a picture which shows you how a space elevator could be possible one day.

Offline Rabidpanda

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Re: Space elevator
« Reply #1 on: 03/21/2010 03:40 am »
I am not an expert on space elevators but I am fairly sure that if we did build one it would not work anything like your illustration.  For an overview of how a space elevator would actually work I suggest you read the wikipedia article.

http://en.wikipedia.org/wiki/Space_elevator

Offline Mark S

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Re: Space elevator
« Reply #2 on: 03/21/2010 03:42 am »
Joshua, thanks for the graphic illustration of a space elevator.  Here are a few things to think about on the subject:

1) The elevator will not require engines unless we plan to enable avoidance maneuver capability.  The centripetal force generated by having the upper counterweight anchored above geosynchronous orbit will keep the tether taut.

2) The elevator structure will not generate an appreciable amount of gravitational force.  It will of course be subjected to the Earth's gravitational pull.  Centripetal acceleration, as stated above, will counter the force of gravity.

3) The elevator will not be located beyond the Earth's gravitational field, since even the Moon is subject to Earth's gravity and it is much further away than the upper end of a space elevator would be.

4) You show the lower end of the elevator anchored somewhere in the Rocky Mountain range of North America.  Although it may be desirable to place the anchor site on a mountain top, the lower end of any space elevator will probably always be located directly on the equator.

Keep up the good work!
Mark S.

Offline joshuabaker

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Re: Space elevator
« Reply #3 on: 03/21/2010 06:37 am »
Thank you for your informative reply. I am confused though, if there was a counterweight anchored above orbit like the diagram @ http://en.wikipedia.org/wiki/File:Space_elevator_structural_diagram.svg
Then wouldn't that counterweight have to travel in correlation to the earth below rotating? In that case the space elevator with its enormous mass would break unless made out of extremely flexible material.

Offline spacetraveler

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Re: Space elevator
« Reply #4 on: 03/21/2010 07:23 am »
Then wouldn't that counterweight have to travel in correlation to the earth below rotating?

yes, that's what "geosynchronous orbit" means.
« Last Edit: 03/21/2010 07:23 am by spacetraveler »

Offline Mark S

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Re: Space elevator
« Reply #5 on: 03/21/2010 01:09 pm »
Then wouldn't that counterweight have to travel in correlation to the earth below rotating?

yes, that's what "geosynchronous orbit" means.

If I could expand on that a little bit...

At geosynchronous orbit, the orbital speed of any object is exactly 24 hours, or 1 day.  Therefore any object in a circular equatorial geosynchronous orbit will always remain in the same location in the sky, and appear to be stationary to anyone on Earth.  This is also called a geostationary orbit.  Obviously it is very useful for communications satellites.

At altitudes above geosynchronous orbit, the orbital period is longer than 24 hours.  Below, the period is shorter.  Thus, for a space elevator where every point on the tether is rotating with a 24 hour period, all points below geostationary altitude will be pulling downward, because they are traveling at less than orbital speed.  And all points above geostationary altitude will be pulling upwards, because they will be traveling faster than orbital speed. 

So, if you place a large counterweight at the top of the tether, and position it above geostationary orbit, and do all your calculations just right, then the upper part will pull upwards with the same amount of force as the lower part pulls downward.  The entire structure will be balanced around the station located at geosynchronous altitude, which is why you don't need any engines to keep the structure in orbit!  Cool, huh?

I hope this helps you understand the problem a little better.  At the rate technology is advancing, I'm sure we'll have space elevators much sooner than a few thousand years from now.

Now here's a question for you: How long would it take an elevator car to climb all the way to geostationary orbit from the Earth at any reasonable speed, say 500 km per hour?  (Hint: geostationary altitude is about 35,800 km high!)

Mark S.

Offline joshuabaker

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Re: Space elevator
« Reply #6 on: 03/21/2010 07:39 pm »
About 71.6 hours, very interesting thank you for your responses.

Offline aero

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Re: Space elevator
« Reply #7 on: 03/21/2010 08:44 pm »
What would the trip time be with a reasonable constant acceleration/deceleration, say 0.2 g, earth relative?
Retired, working interesting problems

Offline Bill White

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Re: Space elevator
« Reply #8 on: 03/21/2010 09:03 pm »
About 71.6 hours, very interesting thank you for your responses.

An interesting consequence of this is that to reverse directions, the cable must be "emptied" over a 72 hour period.

In other words the first "down" car cannot begin its journey until the last "up" car arrives at its destination in space and at 500 kph that is ~72 hours. At less than 500 kph the cable is idle for a longer period of time.

Empty cables will significantly increase your cost per kilogram for delivered payload. Personally, I believe space elevators only become economical if significant mass is lifted upwards each and every day of the year.

= = =

Quick and dirty back of envelope calculations:

Suppose $10 billion dollars is needed to build the elevator.

Use 8% interest for cost of capital, amortized over 8 years.

Aim for a target price of $100 per kg to LEO.

Ignoring operational costs, you need to launch 50,000 kg of payload per day to cover the mortgage payment.

50,000 kg x $100/kg = $5 million per day in revenue

$5 million x 30 days = $150,000 million per month in revenue

$10 billion at 8% amortized over 8 years (96 months) is $141,270,000 per month unless I slipped a decimal somewhere.

http://www.bretwhissel.net/cgi-bin/amortize

At $1,000 per kilogram to LEO target cost, you only need 5,000 kg of payload per day, again, ignoring operational costs.

But you need 5,000 kg per payload per day ever day of the year and at $1,000 per kilogram to LEO other systems become economically competitive.

= = =

Technology cannot solve the "cheap access to space" challenge.

Only demand for very significant amounts of mass lofted to LEO can solve that and the US taxpayers cannot possibly buy enough Earth-to-LEO lift to change the game, even if commercial crew to ISS is a useful symbolic gesture.
EML architectures should be seen as ratchet opportunities

Offline Bill White

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Re: Space elevator
« Reply #9 on: 03/21/2010 09:08 pm »
xkcd space elevator cartoon:

http://xkcd.com/697/
EML architectures should be seen as ratchet opportunities

Offline hop

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Re: Space elevator
« Reply #10 on: 03/21/2010 10:07 pm »
An interesting consequence of this is that to reverse directions, the cable must be "emptied" over a 72 hour period.
Compared to the other technical challenges involved, putting some kind of sidings at intervals along the cable shouldn't be a big deal.

Offline Mark S

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Re: Space elevator
« Reply #11 on: 03/22/2010 12:53 am »
What would the trip time be with a reasonable constant acceleration/deceleration, say 0.2 g, earth relative?

Remember that the climber is physically attached to the tether with some kind of rolling gear, so there will be a practical upper limit to the speed of any elevator.  I don't see how such a cable could support any kind of maglev / linear induction motor mechanism, that would be way too heavy.

Also, remember that the elevator car will be accelerating at >1 g as it climbs up out of Earth's gravity well.  Of course the g force will drop off towards zero as the car climbs, but it will take a lot of energy just to get off the ground.

And for the 144+ hour round trip time, maybe it would be better to not make round trips.  Just use the elevator to ship the personnel and materials up where they can be used for in-space construction of additional infrastructure.  The elevator running gear can be collected and returned in a large disposable return capsule, which would remove the need for round trips at all.

Or, go all out and build two tethers that would work in parallel.  One for outbound traffic and the other for inbound.  Think big!

Mark S.

Offline hop

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Re: Space elevator
« Reply #12 on: 03/22/2010 01:22 am »
Remember that the climber is physically attached to the tether with some kind of rolling gear, so there will be a practical upper limit to the speed of any elevator. 
An important point. At speeds over a few hundred MPH, just running wheels over a surface starts to get challenging.
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The elevator running gear can be collected and returned in a large disposable return capsule, which would remove the need for round trips at all.
Hard to imagine deorbit propellant and TPS being cost effective. Just shoving off the tether will not get you back to earth unless you are quite far below GSO, you'll end up in an elliptical orbit that intersects the tether. This would also have big impacts on your climber design. Since they will probably use beamed power, they are going to have some large, relatively fragile collectors. The only other alternative is nuclear, in which case they'll have large radiators.

Here's another way to deal with the round trip problem. Have a bunch of climbers that just go back and forth over shorter stretches, and transfer standard containers in each direction. This may allow you to design more optimal climbers anyway: The ones that do the first few hundred km will need to deal with atmosphere and near one G, but will be close to a power source. They will also be on the thinnest part of the tether. The ones a bit higher will be in the harshest radiation environment. As you get closer to GSO, the apparent gravity goes down and the tether gets wider.

Offline Steve D

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Re: Space elevator
« Reply #13 on: 03/22/2010 01:25 am »
I have always wondered about how this space elevator would work. Any mass going up the elevator is also accelerated at right angles to the cable until it reaches orbital velocity when it gets to the top. Where does this energy come from? would it not eventually slow down the station at the top of the elevator? Wouldnt it eventually pull down the counterweight? That energy has to come from somewhere.......

Offline A_M_Swallow

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Re: Space elevator
« Reply #14 on: 03/22/2010 01:27 am »
Or, go all out and build two tethers that would work in parallel.  One for outbound traffic and the other for inbound.  Think big!

Until we mine asteroids/moon the return space elevator does not need to be as big.

The climber's solar panels can be used to power SEP spacecraft.  Frequently the same one the climber just lifted as cargo.

Offline hop

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Re: Space elevator
« Reply #15 on: 03/22/2010 01:54 am »
I have always wondered about how this space elevator would work. Any mass going up the elevator is also accelerated at right angles to the cable until it reaches orbital velocity when it gets to the top.
They will also exert force in the opposite direction if they come back down.
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Where does this energy come from?
The energy comes from whatever powers your climbers.
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would it not eventually slow down the station at the top of the elevator? Wouldnt it eventually pull down the counterweight?
No, the top of the tether is under tension, not just floating neutrally. But the dynamics will be interesting!

If you don't bring your climbers back (edit: or actually, if you let them fall off the end!), you will steal some very small amount of angular momentum from the earth, just like the yo-yo weights used to de-spin rocket stages.

« Last Edit: 03/22/2010 01:54 am by hop »

Offline Steve D

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Re: Space elevator
« Reply #16 on: 03/22/2010 02:02 am »


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     Where does this energy come from?

The energy comes from whatever powers your climbers.


That energy is only pulling up. Where is the energy coming from to accelerate the mass to orbital velocity when it reaches orbit at geo? Wouldnt the tether be pulled west also pulling the counterweight down?

Offline Rhyshaelkan

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Re: Space elevator
« Reply #17 on: 03/22/2010 02:15 am »
This is why you need a counter-weight with substantial mass.

Whether your SE is designed to lift 100, 1000, or 10000 tonnes at a time your counterweight will need to be proportionately larger. As well as re-boosted(or boosted as the climber comes up) to GSO speeds if need be. But you can have a solar-powered VASIMR drive to do that efficiently.
I am not a professional. Just a rational amateur dreaming of mankind exploiting the universe.

Offline joshuabaker

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Re: Space elevator
« Reply #18 on: 03/22/2010 04:18 am »
Personally, I do not believe the production of a space elevator would be possible/beneficial until new technologies and materials become available. This may take a few ten-thousand years. It would simply cost too much money to build and maintain and additionally have a too-higher chance of catastrophe. Unless we got our calculations EXACTLY correct, the powerful force of the earths spin on its axis would snap the building like a twig. Also, we don't have the technology to land on a asteroid (counterweight) and then transport the huge mass over to exactly the correct spot.

Offline hop

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Re: Space elevator
« Reply #19 on: 03/22/2010 08:27 am »
That energy is only pulling up. Where is the energy coming from to accelerate the mass to orbital velocity when it reaches orbit at geo? Wouldnt the tether be pulled west also pulling the counterweight down?
Consider where it would come from if the elevator were a solid tower. Since the tether is under tension, the effect is the same.

 

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