Space elevator. Dream, hoax or reality?

I am not trying to be silly here but why do we need the mass transporter system? The most efficient payload we can send into space is people and their tools right. In space, energy is easier, cheaper, and more readily available. In space resources are converted into products. Products can be more easily returned to earth. At first demand for those products would be so low that energy wont matter too much. How can we make space so attractive to business that they will pay to colonize/pioneer it. Remember the new world would have stayed the new world if there was no profit in it. Now I am not some money hungry capitalist, however the taxpayers dont seem interested in space because they dont see the $$$. When the demand is there, I think we will be surprised how fast, cheap and easy a mass transporter will be designed and implemented. MV

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>I know a lot of serious work has been done and I am not saying it is impossible. Skyhook only has efficiency if the load coming in roughly equals the load going out. If you can show me compelling evidence that that will be the case then I will be more interested. Once the hook is built we will want to build big space stations and start taking out material to make moon bases, moon based radio telescopes etc. What mass are you going to bring back to balance this tremendous amount of material? It will be some time before moon or space based products will be created to ship down and most of the material will itself come from the Earth for a long time.
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>Even if you deliberately waste energy by NEVER using a balancing mass when raising or lowering something with a beanstalk, it will be more efficient than a rocket.

The payload fraction for a typical rocket is about 5%. As a rough approximation, you have to raise about half of the launch mass of the rocket to the same altitude as the payload (or, all of it to half the altitude of the payload). Bottom line is about 10 to 1 in favor of the elevator, without energy storage. With energy storage that advantage becomes 100 to 1 or better.

A lot depends on exact design details, of course. One could propose specific circumstances where this advantage might drop to 1 to 1, or rise to 10,000 to 1.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>The initial platform has to go up to manufacture the first cable. That platform is not light. Orion could do it in one launch. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>???
Do you understand what Orion is? An atomic bomb "rocket"?

I'm not aware of any serious proposals to use such a machine for planet-to-orbit shuttle work. They would be huge and massive. The reaction chamber has to be able to physically contain a rapid series of nuclear explosions. The kick from each blast would be pretty strong. If you don't mount all of the rest of the structure on a Really Big Shock Absorber instantaneous accelerations would disable or kill the crew and damage or destroy many types of cargo.

IIRC an Orion type rocket would need to process 2 or 3 bombs per minute to produce a reasonable average acceleration on the other side of the shock absorber. But this was for a purely space-based mission.

I'm guessing here, but in order to get one of these big boys to take off from the surface of the Earth you might have to process several dozen bombs per minute.

And if you think getting one of these things UP is a challenge, just wait till you try to LAND one.

The exhaust, of course, would be radioactive. There would be a lot of exhaust. Each round trip would be the equivalent of 40 or 50 (or more?) atmospheric bomb tests. NFW!!.

I'd like to build one some day. It would be fun and you could probably learn a lot from it. But I can't imagine it ever having much in the way of practical application for transport.

BTW, your expectations for the initial mass requirement for a beanstalk seem to be a little high. The proposals I've seen call for three or four shuttle loads, max. ALL of the rest will be lifted on the initial cable or be brought in from off-planet.

Regards,
LB

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>This means that to start the 100 Kg mass at the bottom going up your counterbalance must be almost 28,000 Kg! ... How do you control the velocity of the huge mass so it does not cause a huge crater at the bottom. A counterforce must be applied because the mass itself going down and the mass going up will be contributing to increasing velocity.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>???
You do not want to balance forces at the begining of a trip.

Break the problem down into several parts to make the analysis less confusing. (A massles rope is fine for this type of analysis. BTW, in the real world you can approximate a massless rope. Use a continuous rope that goes from bottom to top and back. This doesn't eliminate inertial mass, but it does eliminate some aspects of gravitational mass. For the rope only. It is very unlikely that a rope system would be used in practice.)

Start with your 100 Kg mass to be lifted from the surface. Assume that there is NO counterbalance. Consider the following formulas.
f = m*a (force, mass, acceleration)
E = f*h (potential energy, height)
P = E/t (power, time)

Acceleration and deceleration of the mass at the beginning and end of a trip are ignored.

1) The 100 kg mass is sitting on the floor. How much force must you apply to lift it two meters?

Approximately 1010 newtons (220 pounds). Some people can do this with their bare hands.

The beanstalk is 100,000 km tall but assume for the moment that that the lifiing station is attached at an altitude of 2 meters.

2) How much force has to be applied to the (massles) rope to lift the mass 2 meters?

About 1010 newtons (220 pounds).
The energy needed to do this is about 2,000 netwon*meters.
The motor/generator needed to do this in one second must be able to operate at a power output of 2,000 n*m/sec. A little more than 3 kW. About 2 hp.

Now assume the the lifting station is attached at geosync altitude - approx 36,000 km).

3) How much force has to be applied to the (massles) rope to lift the mass 2 meters?

(same answers, trick question)

4) How much force has to be applied to lower this mass back to the ground (from an altitude of 2 meters)? Again, no counter balance is being used.

About 1010 newtons (220 pounds).
The energy recovered in doing this is about 2,000 netwon*meters.
The motor/generator needed to do this in one second must be able to absorb power at a rate of 2,000 n*m/sec.

In practice it becomes difficult to use electromagnetic breaking when velocities drop below a certain level, so some extra power has to be used as the mass approaches either end of the elevator. This is one of the many reasons why energy recovery is never 100% efficient. (No big deal - anything above zero is a benefit.)

Clever design can minimize a loss like this, sometimes to a surprizing extent. But not eliminate it.

(continued...)

( ... continued)

5) With no counter balance, the force needed to lift the mass against gravity and maintain a fixed velocity drops contunuosly as the mass gains altitude. You must supply energy to create this force.

On the return trip the force needed to maintain a fixed velocity continuously increases as the mass looses altitude. You must dissipate (or, preferably, collect and store) energy to create this force.

The motor/generator must be sized to handle the largest of these forces. Generally not a problem, unless you want to lower a small asteroid all at once.

It is easy to imagine circumstances where an unbalanced load would need to be raised or lowered. A medical emergency? An accident or sabotage? You can't wait for a balancing load to be put in place. Its got to go NOW.

6) With a counter balance in place not much changes, except that you get your energy back on the same trip instead of on the next trip.

Regards,
LB

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>
Do you understand what Orion is? An atomic bomb "rocket"?
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

Sure do. So what? Your skyhook is pretty dangerous too if the cable breaks and flops down over thousands of miles of territory rather than being localized like Orion.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>
I'm not aware of any serious proposals to use such a machine for planet-to-orbit shuttle work. They would be huge and massive. The reaction chamber has to be able to physically contain a rapid series of nuclear explosions. The kick from each blast would be pretty strong. If you don't mount all of the rest of the structure on a Really Big Shock Absorber instantaneous accelerations would disable or kill the crew and damage or destroy many types of cargo.
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

Most of the work on Orion is classified but Freeman Dyson has always maintained that it was quite practical. They did indeed design hydraulic dampers to smooth the detonations. Orion on a payload/weight basis is far less massive than your typical Saturn V, the rig is big because we're also lifting a hell of a lot. The crew would not be turned into jelly and a mission to Mars from Earth surface was studied. Landing could indeed be a problem, I don't know if studies were done for reentry by nuke or a combination of nuke and parachute.

I guess my main point here Larry is that should we spend a titanic sum of money developing something that doesn't even have the materials it needs to work in existence yet or should we maybe implement something that has already been pretty well worked out, doesn't require unobtanium and could be implemented sooner? You want to go from crawling to running without walking first.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>I guess my main point here Larry is that should we spend a titanic sum of money developing something that doesn't even have the materials it needs to work in existence yet or should we maybe implement something that has already been pretty well worked out, doesn't require unobtanium and could be implemented sooner? You want to go from crawling to running without walking first.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>
Ah. I didn't realize someone had proposed that we begin spending Big Bucks on this thing. Until carbon nanotubes have been successfully turned into a cable (ribbon, actually) with the required properties there is no point in doing any of the expensive stuff. Feasibility studies and preliminary design work are the only parts of this project that can be justified at this stage.

Cable breakage is definitely something that has to be considered. A beanstalk built from our best steel would have to have a maximum diameter of thousands of kilometers, and even so the safety margin would be only a few percent at best. Kevlar would shrink that to hundreds of kilometers, still no safety margin. If nanotubes live up to their promise, that maximum diameter shrinks to a few meters and the safety margin goes to well over 100%. The bottom few thousand kilometers (the only part that is likely to fall DOWN) is literally only centimeters across. In the event of a break it would fall mostly on and around the anchor point. Parts of it would burn up during re-entry. The remainder COULD cause some damage if any pieces hit something.

The ground track for this debris would be well known. Proper siteing is an important part of the design for something like this.


As for Orion. If Dr. Dyson truely thinks that Orion is a practical way to move stuff from Earth's surface to outer space, well, I guess I'm not as impressed with him as I was. I hope he has been misunderstood.

Orion also has an unobtainable component - permission to operate in some country's air space. Magic pixie dust would be easier to find. NO ONE is going to let you set off a couple hundred atom bombs in the sky above their least favorite national park, or even their least favorite city. If you try to take off from the middle of the Pacific Ocean the countries situated downwind would probably launch a pre-emptive strike to stop you. And be treated as heros by the world community.

Even nuclear thermal rockets, which have been test fired extensively with essentially zero radiation release, are unlikely to ever operate legally within the Earth's atmosphere.

Chemical rockets obviously work, but are VERY EXPENSIVE and not really practical outside of cislunar space. Nuclear rockets are great for interplanetary work. They are much less expensive, but have some real issues if operated near inhabited planets.

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So I guess my point is - why waste time thinking about a technology that can/should never be used (in the way you have advocated here)? Orion is technically feasible, *IF* you ignore that landing thing (which means it really isn't). It is also economically feasible *IF* you ignore that fallout thing (which means it really isn't). But it's not socially and politically feasible. People will shoot you if you try to fly an Orion from the surface.

We need something other than a rocket to get into orbit. Beanstalks, *IF* the nanotube or something similar becomes real, are a VERY cheap alternative. If the nanotube turns out to be another almost technology, like hot fusion, then we'll have to do something else. We should know if this is the case in another year or two.

Regards,
LB