Author Topic: 12 Feb 2014 - Commercial Mining with a Lunar Elevator  (Read 20230 times)

Offline cfrjlr

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12 Feb 2014 - Commercial Mining with a Lunar Elevator

Oregon L5 Society and PNW AIAA Joint Event

Commercial Mining with a Lunar Elevator

By Charles F. Radley

Wednesday, February 12, 2014 | 6:00 pm to 7.45 pm  (Pacific USA)

Midland Library , 805 SE 122nd Ave, Portland, Oregon 97233 USA


** This event will be streamed online as a Google hangout at this web link:

https://plus.google.com/events/c7g5u2ngrlq1qpnhl8lbmt720ho

The Earth's Moon is a treasure trove of mineral resources, such as precious metals, rare earth elements, Helium-3 and Oxygen for propellants. However, the cost of soft landing on the Moon is currently very high. Using modern fibers we can build a lunar elevator which reduces the cost of lunar landing sixfold. Furthermore, it makes the cost of collecting material from the Moon and sending it to Earth essentially free.   The cost of lunar sample return is reduced by about one thousand times versus chemical rockets.  For soft landing payloads, the LSE pays for itself in 20 payload cycles;   for sample return it can pay for itself in as little as a single payload cycle, depending on the sample site.

The lunar elevator concept is a long tether which is loaded under tension by terrestrial and lunar gravity. One end is anchored on the Moon and the other end free, hanging towards Earth.  The orbital center of mass of the system is located at an Earth-Moon Lagrange location, either L1 or L2, approximately 50,000 kilometres from the lunar surface.  Such a tether can now be built inexpensively from commercially available materials, e.g. Zylon, Dyneema, M5.

The near-side L1 tether is attached to the lunar equator at Sinus Medii. 

For a one time capital cost of US$800Million [2012], a lunar elevator can be built today using existing available materials. This first generation lunar elevator will softly deliver an infinite number of payloads to the lunar surface, each weighing 100 kg, and retrieve the same amount of material from the lunar surface. The alternative of using chemical rockets to soft land on the Moon [or return material] is prohibitively expensive.

The first generation lunar elevator kit weighs 11,000 kg and can be delivered today to the Lunar L1 lagrange libration location, using a single Delta-IV (or Ariane-V) launch. From there the tether is unreeled upwards and downwards. The lower end anchors itself into the lunar soil using robotic penetrators.

The first market will probably be Helium-3 which currently sells on the terrestrial market for one million dollars per ounce.  There is a critical shortage of Helium-3 which is in great demand for various industrial applications.  Terrestrial supplies of Helium-3 will be exhausted by 2030.   Helium-3 is abundant on the lunar surface.

The lunar elevator can also transport oxygen from the Moon to Low Earth Orbit where it can refuel tugs to take satellites from LEO to GEO, a significant revenue source. This reduces the cost of launches to GEO by a factor of Eight times.

 The lunar elevator represents a game changing technology which will open up the Moon to commercial mining and long term human exploration.
 

Charles Radley is President of the Oregon L5 Society, and an Associate Fellow of the AIAA. He has worked extensively in the space industry since 1981 as a product assurance and systems engineer. He is an adviser to Liftport Group.

** This event will be streamed online as a Google hangout at this web link:

https://plus.google.com/events/c7g5u2ngrlq1qpnhl8lbmt720ho

Background:

A very nice lunar elevator study report from Israel. Student Project at The Technion, Israel, 2008. A full year under the supervision of Dr Alexander Kogan, now retired to Canada. The team is now disbanded, some work at the Israeli Aerospace Industries.

Conclusions
• Cargo delivery from the Moon to the Earth can
be done within 6 days using solar power and no
propellant.
• The cargo system uses a cable car moving
along a stretched ribbon.
• The ribbon is kept stretched by terrestrial and
lunar gravity. One end is anchored on the Moon
and the other one free.
• The cargo released from the cable car performs
a passive flight to the Earth. At landing, no
parachute is needed.

Here is the link to the details:

http://lunarjacobsladder.webs.com/Jacobs%20Ladder%20IACAS%202010.pdf

more details here too ... http://asri.technion.ac.il/jacobs-ladder/

JACOB’S LADDER | Asher Space Research Institute

Year 2008 “Jacob’s Ladder” Lunar Elevator Student Team: Ran Qedar, Natan Grinfeld, Georgy Bezrodny, Ortal Reuven, Alex Tatievsky Faculty of Aerospace Engineering, Technion – Israel Institute of Technology Supervisor: Dr. Alex Kogan, Asher Space Research Institute, Technion , Israel.

There is a detailed 94 page report written in Hebrew, I expect to have English translation available in a couple of weeks.
« Last Edit: 01/31/2014 02:39 pm by cfrjlr »

Offline mheney

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #1 on: 01/31/2014 04:40 pm »

Did the team look at stability issues with a lunar space elevator?  There are several significant problems I see:

(1)  Because the moon's orbit is elliptical, L1 and L2 are not at fixed locations.  They vary as the Earth-Moon distance varies.

(2) L1 and L2 are "negative stability" points, in that if you drift away from them,  you keep drifting away.  Given that L1 and L2 are moving, you have a where, rather than the tether's CM drifting away from the L1/L2 point, the point drifts away from t he CM - but the effect is the same.

(3) The sub-Lagrangian point is not fixed, due to libration in longitude (an effect of the Moon's orbital eccentricity.  You'd get a "tug and relax" effect as the ground station moved away from, the back towards the sub-Lagrangian point, causing the CM for move away from he Lagrangian point.

The difference between a terrestrial and a lunar space elevator is what they're in orbit about.  A terrestrial space elevator is in orbit around the parent body - the Earth.  Assuming a "floating" (detached) end point at the ground, if the tether drifts a bit, it stays in Earth orbit, and doesn't stray to far (n orbital terms).  A lunar space elevator, however, is NOT in orbit around the parent body - it's not in a selenosynchronous orbit.  It's in Earth orbit; and if it drifts a bit, it stays in earth orbit - and departs the vicinity of the moon.   Or comes crashing down onto the lunar surface.

I'd go so far as to say that I don't think a lunar space elevator would survive for one lunar day - a single orbit around the Earth - given the instabilities involved.


Offline cfrjlr

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #2 on: 01/31/2014 05:05 pm »
the lunar elevator will maintain orbital position using  active station-keeping, involving varying the tether length, electric propulsion and/or solar sailing.

This is similar to communications satellites which also have to do active station-keeping to remain on station, otherwise they would also drift away.

Most comm-sats use bi-propellant chemical thrusters with storable propellants, some use Hall thrusters.
« Last Edit: 01/31/2014 05:09 pm by cfrjlr »

Offline mheney

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #3 on: 01/31/2014 06:19 pm »
Most comm-sats are in orbit around their parent body.  Small station-keeping impulses work in those cases.
And those impulses are short, and infrequent.  Add to that the fact that the mass of a comsat is not even remotely comperable to the mass of a space elevator.

A lunar space elevator is in orbit around the earth, not the moon.  It has *zero* relative velocity to the moon.
Things like that have a tendency to drop straight down.

The difference between lunar apogee and perigee is 43,000 km.  Looking at earth-moon L1, it's about 6 times farther from the moon than it is from the earth.   That suggests that the moon-L1 point distance is going to vary by thousands of kilometers over the course of a month.   I'm going to write off varying tether length as a viable option here - reeling hundreds to thousands of kilometres of cable in and out in lunar conditions (temperature extremes, dust/grit...) (plus, how do you PUSH a cable up???) is beyond impractical.  Plus, it's a solution that you can't take off-line for maintenance.   If you stop adjusting, the elevator fails.

So now we're looking at a fixed length cable.  If the center of mass (CM) is at the "average" L1 location, then for half the month, the CM is going to be BELOW the L1 point.  That means that there is going to be a continuous net force straight down on the cable.  and if you let the CM drop - at all - the downward force is going to increase.

Just to stay put, you're going to have to apply a continuous upward force - starting at zero but climbing to several thousandths of a G - to the ENTIRE mass of the elevator.  How long can you apply even a .0001g constant force to the entire mass of the elevator?  If you can't manage two weeks, uninterrupted, then your elevator will Drop.   Straight.  Down.

OK, well, let's put the CM beyond the L1 point.  Then the structure will be in tension.  We don't need to worry about the elevator falling, then!   Except - the CM is in EARTH orbit, not lunar orbit.  Since it's in a lower orbit (earth-wise) than the L1 point, it's going to try and pull ahead of the moon.  And, from the moon's point of view, also "up" away from the moon.  At some point, the surface anchor point fails (lunar bedrock doesn't have the tensile strength of carbon nanotubes..) and the elevator sails away, returning to the Moon's environment every couple of years, perhaps.

Now, lets' say that you actually have the active control in place to maintain your lunar elevator in place, burning through reaction mass month in, month out, year after year, simply to keep the thing from falling down or wandering far, far away (and in either case, rendering it useless.)   How does that compare to what it would take to simply launch the payloads from  the surface via conventional rocketry?  Remember, the purpose of the thing is to provide an efficient, sustainable method of transport from the surface to lunar orbit (and beyond). 

TL;DR:  It's easy to say "Oh, we'll use active control" - bu have you looked at how much active control you'll actually NEED?  Cheaper, simpler, more efficient, and safer to simply launch payloads from the lunar surface via conventional rocketry.

Offline A_M_Swallow

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #4 on: 01/31/2014 07:36 pm »
The operating surface anchor does not have to be designed for minimum weight, so we can use steel or even stone.  The initial anchor needs to last until it can be replaced.  There are plenty of boulders on the Moon that can weigh the anchor down.  The maximum lifting force on the anchor is known.

Offline cfrjlr

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #5 on: 01/31/2014 08:12 pm »

The active control algorithms have been analysed by Pearson (under NASA contract), Kogan (at Technion)  and others, they are adequately understood and manageable.   

The initial lunar elevator weighs 11 tons, not much more than large communication satellite.

Using a lunar elevator, sample return is over one thousand times cheaper than using chemical rockets.  And soft landing is about six times cheaper than chemical rockets.

I have attached a copy of the "Jacob's Ladder" report to this email thread, take a look at that PDF file and you will see control algorithms are presented.

Offline mheney

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #6 on: 01/31/2014 10:16 pm »
Initial mass is irrelevant - they're listing a total mass (including end mass) of 261 tons.

And FWIW, it appears that their control algorithms are focused on oscillations and  disturbances caused by the payloads traversing the elevator under idealized conditions (moon in a circular orbit).  What I don't see is any estimates of what it's going to take in terms of reactant mass, or total force needed per 24 hours, or other analysis of just what it's going to take to keep the thing standing under real (vs idealized) conditions.

Offline KelvinZero

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #7 on: 01/31/2014 11:37 pm »
Under a billion dollars to get over 200 tons of any technologically non-trivial object (including station keeping technology etc) we haven't built yet to a BEO position. Surely that misses some costs? Wasn't it about a billion just to build that 40kw SEP tug?

Im not competent to evaluate something like this and I feel the US government taken as a herd isnt either. Individuals, sure, but not the entire decision making machine.

This is why I like to sidestep all the technological aspects by simply saying it would be great if the US government would create a market for lunar payloads by buying say $500m of lunar cargo every year from it's HSF budget, and letting commercial competition sort out the winner. (the same goes for all those wacky and wonderful earth launch schemes: NASA should just create a market for propellant in orbit and let the lunar ISRU, laser launch schemes or F9R just fight it out)

edit: For the 100kg lunar cargo in particular, something like this should be a winner eventually, but it's not clear to me anyone would step up right now and exploit lunar ISRU even if we could teleport lunar regolith to any point beyond earth orbit for free. Prove the market and you won't need to argue its worth on an internet forum.
« Last Edit: 01/31/2014 11:50 pm by KelvinZero »

Offline cordwainer

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #8 on: 02/01/2014 01:56 am »
Well if cheap, pragmatic Americans don't build it. I have no doubt the Chinese or somebody else with more cash than brains will. People criticized Apollo and the Space Shuttle as being impossible and a waste of money.(which they probably were) At least if this works you could no doubt expect a return. Active control costs would probably be a fraction of the profit so only the engineering and initial cost of set-up would be the issue.

Offline cordwainer

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #9 on: 02/01/2014 02:00 am »
Nice thing about this is that delivery of mined resources would be relatively quick so unlike asteroid mining there would be no long wait between deliveries and therefore no revolving "boom/bust" cycle.

Offline high road

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #10 on: 02/05/2014 12:07 pm »
Well if cheap, pragmatic Americans don't build it. I have no doubt the Chinese or somebody else with more cash than brains will. People criticized Apollo and the Space Shuttle as being impossible and a waste of money.(which they probably were) At least if this works you could no doubt expect a return. Active control costs would probably be a fraction of the profit so only the engineering and initial cost of set-up would be the issue.

If it's for mining, China's going to be the last country to invest in it. The only material 'worth' mining in the short run are rare earths, and they are only 'worth' it to reduce the dependency from China, which produces more than 50% of the global supply, and has already blocked exports to 'unfavorable nations'. If China ever stops exporting, global prices would skyrocket. That would probably be good news. It would be the second time restricted exports from China would lead to a wave of exploration, colonisation, and the industrial revolution.

If it's built to make further exploration and other missions cheaper, that's something else. Although you probably want the payload to be bigger. 300 kg doesn't feel like it's enough to carry big rovers or big equipment for human exploration. Not everything can be built in situ, not even with 3D printers.

To avoid discussion: 'worth it' means: market price * volume of sales > development cost + production cost. Water, titanium, etc. can't be brought back cheaper than it can be produced here, and there's very little demand for it in space unless space budgets increase (not going to happen in the short run) or the price is lowered considerably. In an inelastic market like space, lower prices means lower revenue.

Offline clongton

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #11 on: 02/05/2014 01:42 pm »
People criticized Apollo and the Space Shuttle as being ... a waste of money.(which they probably were)

What is apparent is that folks who think this way don’t understand the difference between public and private financing. And until 2008 I was among them. Then I went to the Senate and got taken to school.

In private financing, if a project makes a profit it was a success. If it does not then it was a waste of money.

In public financing, if a project executes the policy that created it then it was a success. If it did not achieve its stated goals then it simply didn’t achieve its goals, but it was never a waste of money because public policy was carried out. Public financing is not intended to provide a monetary ROI. It is designed to achieve a goal and often to create employment or opportunity along the way regardless of the amount of capital spent. The concept of a positive ROI does not apply in any way, shape of form to public financing. It is anathema to public financing.

For example:

From a private financing pov both Apollo and Shuttle were a waste of money because neither one ever created a positive return on investment. Both programs cost significantly more than they ever returned in cold cash returns in spite of all the wonderful things they both accomplished.

From a public financing pov both Apollo and Shuttle successfully executed public policy and achieved their stated goals. They were both a total success because of that and nobody on the public side cared how much they cost so long as capital was available to fund the policy.

That’s the main difference. Private financing must create a profit to be considered a success. All public financing has to do to be considered a success to to successfully execute public policy.

Government is not business and business is not government. When one attempts to run a government like a business the government becomes completely screwed up because it’s not designed to function that way; all governments - all over the earth.

Public policy sets goals and as part of accomplishing them they create a budget. What has to be understood is that from a private pov that budget needs to be adhered to and create a profit but from a public pov that budget is just “operating capital” and if it runs low Congress just appropriates more. I sat in a Senate staffer’s office in DC and he explained all this to me in great detail. The single statement he made that I will always remember was “it’s only money and if we need more we just appropriate it. That’s the difference between them and us. One way or the other we will continue to execute the policy until somebody changes it. Then we will execute the next policy”. From this I understood that business is about profit while government is about policy. Money is only a tool they both use, but designed to achieve two completely different objectives.

Both Apollo and Shuttle were public policy. Both policies were executed to a conclusion. Therefore neither one was a waste of money.

Bottom line: If The US Government creates a policy that it will build and operate a lunar elevator, then it won't matter how much it costs. The government will just finance it. But if the elevator is financed by private funding then it become a much deicer situation because it must create a positive ROI or it will be scrapped as a waste of money.
« Last Edit: 02/05/2014 01:51 pm by clongton »
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Offline cordwainer

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #12 on: 02/07/2014 12:04 am »
While government should be concerned with public policy more than private policy, to have no concern for how a program impacts public policy or how efficient or successful a public policy is carried out is foolish. The space raced was a popular policy project that rushed the development of "miniaturization" and propulsion technologies from a private policy perspective and generated enthusiasm for science and engineering within a public policy perspective. Problem was it did not achieve the "patriotic" and nationalistic objectives of beating the Russians in many avenues when it came to space exploration and military weapons development. It was also a costly affair for which money would have been better spent on welfare or the military industrial complex depending on whatever political leaning you might have. While the Shuttle was not nearly as expensive as people make it out to be it's overall design was inefficient for the mission it was given in both a public and private policy sense. Also there is a point of diminishing returns when you start thinking you can just appropriate more money for a project, the government budget is not unlimited and you can't just keep printing more money. Currency devaluation, inflation, increased taxes, a rob "Peter to pay Paul" mentality and misappropriation of funds can result from such thinking.

Offline high road

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #13 on: 02/07/2014 08:01 am »
Problem was it did not achieve the "patriotic" and nationalistic objectives of beating the Russians in many avenues when it came to space exploration and military weapons development.

The problem isn't that there were no victories, the problem was that there WERE victories. If the Russians would have continued to try to go to the moon after the US had landed, the moon program would never have stopped. If the Russians had had a space plane able to compete with the Space Shuttle, it would have been 'upgraded' much faster. If the USSR hadn't collapsed, space station Freedom would have been assembled far quicker than the ISS. Competition, it's rarely a bad thing. (far healthier than war, hot or cold). Let's see what happens when China starts to plan things that haven't been done before.

Quote
It was also a costly affair for which money would have been better spent on welfare or the military industrial complex depending on whatever political leaning you might have. While the Shuttle was not nearly as expensive as people make it out to be it's overall design was inefficient for the mission it was given in both a public and private policy sense. Also there is a point of diminishing returns when you start thinking you can just appropriate more money for a project, the government budget is not unlimited and you can't just keep printing more money. Currency devaluation, inflation, increased taxes, a rob "Peter to pay Paul" mentality and misappropriation of funds can result from such thinking.

NASA gets what? 0.50% of the federal budget? Even during the final years of the race for the moon, when they were speeding closer to the deadline, it was only about 6% of the budget... How exactly does that require you to keep printing money?

Offline cordwainer

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #14 on: 02/07/2014 08:54 am »
I am not debating whether a mere 6 percent of the budget would have prevented us from "printing money or crippled the economy" obviously it wouldn't have. Putting that money into more focused goals for enriching the public in a more direct sense than space travel might very well have had positive consequences, though. That is not to say that NASA has not enriched the public in many ways, either. Competition is a good thing but spending your money wisely can be just as rewarding. Many other nations on the planet have a better quality of life for less government dollars spent than we do. Admittedly, we did have to fight the Cold War with one hand tied behind our backs which in turn gave rise to the need for a large and expensive military industrial complex. Also the Soviets did develop a better Shuttle called the Buran but there government and economy failed before they could put it into service. The moon landings were part PR stunt and part voyage of discovery. Even if the Russians had mounted a successful moon landing it is very likely that after a few landings the magic would have worn off for them as it did for us and we all would have moved onto something else less costly and more rewarding. Mining resources from the Moon during the Apollo era would have been even more technically and economically unfeasible than it is now. I think in time mining resources from space and more advance manned exploration of space will become feasible but it will take time and some sort of economic rationale not just "patriotic showmanship".

Offline cordwainer

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #15 on: 02/07/2014 09:00 am »
Also, governments are lot like the average person. When times are good we spend money like crazy and when times are bad we tighten our belts. Rich people know that you should actually do the exact opposite.

Offline high road

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #16 on: 02/07/2014 12:39 pm »
I am not debating whether a mere 6 percent of the budget would have prevented us from "printing money or crippled the economy" obviously it wouldn't have. Putting that money into more focused goals for enriching the public in a more direct sense than space travel might very well have had positive consequences, though. That is not to say that NASA has not enriched the public in many ways, either. Competition is a good thing but spending your money wisely can be just as rewarding. Many other nations on the planet have a better quality of life for less government dollars spent than we do. Admittedly, we did have to fight the Cold War with one hand tied behind our backs which in turn gave rise to the need for a large and expensive military industrial complex. Also the Soviets did develop a better Shuttle called the Buran but there government and economy failed before they could put it into service. The moon landings were part PR stunt and part voyage of discovery. Even if the Russians had mounted a successful moon landing it is very likely that after a few landings the magic would have worn off for them as it did for us and we all would have moved onto something else less costly and more rewarding. Mining resources from the Moon during the Apollo era would have been even more technically and economically unfeasible than it is now. I think in time mining resources from space and more advance manned exploration of space will become feasible but it will take time and some sort of economic rationale not just "patriotic showmanship".

I agree on the general idea, but a few remarks though: a Moon base in the 60's (or seventies) would have been a military research facility, not a mine. The point would not be to make money, but to keep an eye on the Russians, and keep toe to toe with everything they would have developed. NASA's budget has been dwindling ever since, but it hasn't exactly been replaced at some point by something less costly.

Calling the Buran better than the Space Shuttle is like calling the Skylon better than the SpaceX rockets. It has to fly (regularly) before the competition really feels the pinch. But IMO the Buran would have prompted the 'upgrade' to the Space Shuttle I talked about.

Offline JasonAW3

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #17 on: 02/07/2014 01:44 pm »

Did the team look at stability issues with a lunar space elevator?  There are several significant problems I see:

(1)  Because the moon's orbit is elliptical, L1 and L2 are not at fixed locations.  They vary as the Earth-Moon distance varies.

(2) L1 and L2 are "negative stability" points, in that if you drift away from them,  you keep drifting away.  Given that L1 and L2 are moving, you have a where, rather than the tether's CM drifting away from the L1/L2 point, the point drifts away from t he CM - but the effect is the same.

(3) The sub-Lagrangian point is not fixed, due to libration in longitude (an effect of the Moon's orbital eccentricity.  You'd get a "tug and relax" effect as the ground station moved away from, the back towards the sub-Lagrangian point, causing the CM for move away from he Lagrangian point.

The difference between a terrestrial and a lunar space elevator is what they're in orbit about.  A terrestrial space elevator is in orbit around the parent body - the Earth.  Assuming a "floating" (detached) end point at the ground, if the tether drifts a bit, it stays in Earth orbit, and doesn't stray to far (n orbital terms).  A lunar space elevator, however, is NOT in orbit around the parent body - it's not in a selenosynchronous orbit.  It's in Earth orbit; and if it drifts a bit, it stays in earth orbit - and departs the vicinity of the moon.   Or comes crashing down onto the lunar surface.

I'd go so far as to say that I don't think a lunar space elevator would survive for one lunar day - a single orbit around the Earth - given the instabilities involved.

I'm not 100% certain that your point is valid in this case.

It appears that instead of depending solely of the stability of the L-1 or L-2 points, the L-1 point would be held stable by dipping the counterweight fairly deeply into teh Earth's Gravity Well.  As the orbis IS elliptical, I forsee having to real the counterweight in and out a bit to maintain the proper tension on the ribbon, but that should be mostly a math and mechanical issue that should be fairly simple to solve.  (Allowing for other gravitic influences would, of course, require a very active monitoring of ribbon tension and counterweight position).

The L-1 point, in this case, is simply where craft could launch or dock, with nearly zero gravitic influence for either Earth or the moon.

A space elevator from L-2 would require a fairly high mass counterweight on a long lead past the L-2 point.

The basic point I'm making here is that the L-1 and L-2 point drift shouldn't really be a factor in this calculation as the ribbon tensioning system extends well beyond either Lagrange points, keeping both points stable via both Gravity tensioning on the L-1 side and centripedal force on teh L-2 side.
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Offline A_M_Swallow

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #18 on: 02/07/2014 03:43 pm »
A practical Earth facing lunar space elevator could be designed to take the tension of maximum gravity without breaking.  Its payload could be limited to the mass that minimum gravity can support.  This avoids having to change the length every month.

A spacestation at EML-1 would have to be a gigantic climber that goes up and down the cable in time with the Moon's orbit.

Offline mheney

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Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator
« Reply #19 on: 02/07/2014 08:10 pm »
Good grief - I hadn't even thought about the problems renedezvous-ing with the EML-1 station as EML-1
moves about!

My biggest concern with structures that require significant active control is "what happens when the active control fails??"   Because it will - maybe today, maybe next week, maybe in 20 or 75 years - but it eventually will. 

If your center of mass is closer to the moon than EML-1, the tether drops to the lunar surface.

If your center of mass (Cm) is farther from the moon than the EML-1 point, the situation is a bit more complex.  People talk as if this puts "tension" on the cable and helps stabilize it, staying along the Earth-moon centerline.   In fact, what actually happens is that, since the Cm is in a lower Earth orbit than the EML-1 point, it's going to pull ahead of the moon.  From the surface, the elevator would be seen to start tipping over to the East.  The Cm, which was not at the L1 point in the first place,  is also moving East, away from the EML-1 point.  Now, since the cable is attached to the lunar surface, the Cm is going to pull "up" away from the Earth as it moves out ahead of the moon (the angle from the ground becomes increasingly acute), until the limiting case where the CM is  directly ahead of the moon in it's orbit.  Of course, that'd be MORE than a 90 degree angle from the ground - and long before that, the elevator would suffer from the fate of any object above the surface but not in orbit (it's velocity relative to the surface is essentially -zero-) - it drops to the surface.  The "tipping tower" scenario would actually have the elevator wrapping itself around the globe a couple times ..

Losing sufficient active control results in your elevator crashing to the ground - possibly in a pile of cable, possibly wrapping around the moon a couple times.  Given that you can't allow the Cm to diverge from the (moving) EML-1 point - either in altitude (moon's elliptical orbit) or to the East or West (libration), you have a need for a significant amount of constant active control.  Add to this the fact that any control you apply to a multi-thousand kilometer cable is going to set up oscillations - and for a fixed path lentgh (the amount of cable you have), an oscillating (sine wave) cable doesn't reach as far as a straight cable.  Which pulls your Cm toward the moon, which ....

Yes, you can blow the anchor point at the surface if Cm is farther from the moon than EML-1 to preserve the elevator (it'll wander away in VHEO, but it'll be findable...) - but Cm further than EML-1  has the problems desrcibed above.  At some point fairly early on, the failure mode converts to the Cm below EML-1 situation - dropping to the ground.

All that to say - if you lose sufficient active control, you're SOL.  And the amount of active control that's "sufficient" is significant - and constant.

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