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Space Forum / Space Flight / December 2003Lunar Sample Return via Tether
My father (Henry Cate) and I have come up with an initial tether project that might be fun, affordable, and profitable. The idea is to use a rotating tether to pickup some Lunar samples, bring them back to Earth, and sell them. The Apollo Lunar Orbit Rendezvous was a big win because they did not need to land their return vehicle or fuel on the moon, just the lander. With a rotating tether we could win even more by only having a small scoop at the end of the tether touch the moon. Since there is no landing vehicle, we can also use a high ISP ion drive the whole time. By lifting a small scoop of regolith (probably under 10 Kg, maybe under 1 Kg) many times we could lift a reasonable total mass of lunar regolith using a small tether. For the 1.6 km/sec tip speed of a tether for lunar pickup, the tether only needs to be like 3 to 10 times the payload mass. The ion drive has to replace the momentum before the next pickup. The Dnepr at $10 to $13 mil for 9920 lbs (4500 Kg) to LEO seems like a good deal. http://www.spaceandtech.com/spacedata/elvs/dnepr_sum.shtml Starting with a 4500 Kg vehicle in LEO we use an ion-drive to go to the moon, spin up a tether (probably winching in and out 2 tethers), and start picking up samples when the end of the rotating tether touches the moon. After picking up enough that it is running low on fuel, it could head back to earth and have a capsule reenter with the samples. The question of how many Kg of lunar sample you could bring back depends on a lot of things, like ISP of thruster, watts/Kg for solar, ratio of capsule mass to payload mass, how long the mission can be, etc. To really get a detailed answer would take some real work. You can also trade off time and mass returned to some extent (higher ISP can bring back more but takes longer). Our initial guesstimate is that you could return between 1,000 Kg and 10,000 Kg in something like 1.5 to 4 years. All of the Apollo missions combined returned 381.7 Kg. The Apollo costs have been estimated at $100 billion in 1994 dollars (next URL). If this project can be done for $50 million (not by NASA for sure) then this would be like 2000 times cheaper and return 2.5 to 25 times as much. :-) http://www.asi.org/adb/m/02/07/apollo-cost.html In a more fair comparison, the Artemis people were going to start with 44,000 lbs in LEO (so 4.4 times as much as us). They would return between 200 lbs (95 Kg) and (227 Kg) of lunar material (we return ~4 to ~100 times as much). http://www.asi.org/adb/04/03/10/05/returned-mass.html If you think of the investment cost as scaling with the lbs to LEO (a reasonable first approximation) and the financial return value as the Kg of Lunar material returned (not totally fair as the price goes down with a bigger supply), then the tether method is 16 to 400 times better. Artemis was sort of marginal as an investment, but this could be a reasonable investment. It is of course hard to estimate what people would pay for lunar regolith once there was a real supply. If it was $1,000/gram and you had 2,000 Kg, that would be $2 bil. Probably be hard to get that much, but it could really be a good return on investment. Additional missions would cost much less than the first, since you would not have the development costs again. You could even design the vehicle to be resupplied for a new mission (more xenon, new reentry capsule, etc). This would be able to pick up samples from many parts of the Moon, any part that passed under the orbit. If it was in a polar orbit it would even be possible to get a sample from a dark crater at the North or South pole to see if there was water ice. Most of the regolith is very fine dust. You might get more money selling lunar rocks. It might be possible to have a computer guided harpoon on the end that could target small rocks. Another interesting option is to use the tether to toss small reentry capsules from the Moon in such a way that they fall back to Earth. One nice thing about this is that you could start selling your product much sooner. The other nice thing is that and if at some point their was a catastrophic failure you would at least have what had been returned so far. People bidding on what you had so far would not know how much more would be coming. So concern that your vehicle might fail at any time might keep the price of Lunar material high longer. We have not seen this idea of using a rotating tether to pick up lunar samples anyplace else and think it looks very promising. What do you think? -- Vince Vincent, Nice idea, but not original. Hoyt, Forward and Moravec have each proposed using tethers for lunar landing and sample return some years ago. It can be done even more cheaply than you propose. You do not even need any ion drive or propulsion at all actually. You can do it entirely with zero momentum exchange, you simply deposit payloads on to the lunar surface whose mass equals that of the samples you remove. For the details, take a look at these web links: http://www.tethers.com/MXTethers3.html http://www.tethers.com/MXTethers.html Best regards, Charles F. Radley AFAIAA > My father (Henry Cate) and I have come up with an initial tether project > that might be fun, affordable, and profitable. The idea is to use a > rotating tether to pickup some Lunar samples, bring them back to Earth, > and sell them. [snip] > We have not seen this idea of using a rotating tether to pick up lunar > samples anyplace else and think it looks very promising. What do you > think? > > -- Vince > Vincent, > [quoted text clipped - 6 lines] > do it entirely with zero momentum exchange, you simply deposit payloads on > to the lunar surface whose mass equals that of the samples you remove. Assuming a spherical moon. For non-spherical, non-ideal gravity moons, things get more interesting. > > Vincent, > > [quoted text clipped - 9 lines] > Assuming a spherical moon. > For non-spherical, non-ideal gravity moons, things get more interesting. Maybe. A lot of control can be gained by reeling the tethers in and out. A small propulsion system might help, but it is too early to say whether it is an essential component. Some analysis is needed. >> Assuming a spherical moon. >> For non-spherical, non-ideal gravity moons, things get more interesting. [quoted text clipped - 4 lines] >small propulsion system might help, but it is too early to say whether >it is an essential component. Reeling in the tether between loads (just enough to miss terrain) seemed like the obvious idea to me, too. Therefore, there must be something wrong with it. :D Stupid question time: assuming that the cargo schedule isn't very heavy, why would we necessarily need incoming cargo or propulsion to keep the tether's momentum up? If we had a momentum wheel powered by a solar-electric motor mounted in the hub of the tether, couldn't we win back rotation that way with minimum effort? It *would* take quite a while, but if we have days or weeks between loads, it should be within reach. The first problem that comes to mind is the longevity of a mechanical mechanism in hard vacuum for long periods without maintenance, but with magnetic bearings that should be surmountable, surely? Actually, come to think of it, the momentum wheel would probably be less of a mechanical problem than reeling the tether in/out. > Stupid question time: assuming that the cargo schedule isn't > very heavy, why would we necessarily need incoming cargo or propulsion > to keep the tether's momentum up? If we had a momentum wheel powered > by a solar-electric motor mounted in the hub of the tether, couldn't > we win back rotation that way with minimum effort? There are 2 types of momentum you need to keep under control. One is the rotational momentum around your own center of mass and the other is orbital momentum around the moon. The rotational momentum of even 1 Kg at the end of a 100,000 meter long tether is so huge that no momentum wheel will have any impact on it. However, you can easily control a tether's rotational momentum, when near a gravitational body, by winching the tether in and out as it is going up or down relative to the body. If you want to rotate faster you let it out on the down side and winch in on the up side so it spends more time going down (and pulled faster) than going up (and pulled slower). So rotational momentum is an easily solved problem. The orbital momentum needs to be controlled by either leaving something on the surface of the moon of equal mass to what you are picking up, or using some kind of thruster. The thruster could be a conventional chemical rocket, an electric thruster, or a solar sail. All of these, including leaving something on the surface, can be looked at in terms of ISP or exhaust velocity. ISP Exhaust velocity Leaving mass on surface 163 1.6 km/sec Chemical Rocket 400 3.9 km/sec Hall Thruster 2,000 19.6 km/sec Ion Drive 10,000 98 km/sec Solar Sail infinite speed of light - solar photons The Hall Thrusters and Ion Drives come in different ISPs, these are just some sample values. Note that ISP times 9.8 equals the exhaust velocity in meters/sec. The ratio of lunar-pickup-mass/reaction-mass is the same as the reaction-mass-exhaust-velocity/lunar-orbital-speed so that momentum is conserved. The orbital speed you will be giving the regolith is about 1.6 km/sec. The higher the exhaust velocity the less reaction mass you need. In the solar sail case the reaction mass keeps coming to you from the sun, so it is sort of an infinite ISP. We are used to needing lots of rocket fuel to lift a small payload, since launching from Earth you might use 30 to 100 times as much reaction mass as you get payload to orbit. With a 10,000 second ISP ion drive and a tether, we could lift 61 Kg of lunar regolith for every 1 Kg of reaction mass (98/1.6=61). This is so amazingly good that it takes awhile to sink in. With a solar sail the only thing limiting how much you can lift is how long your system keeps working. -- Vince >The rotational momentum of even 1 Kg at the end of a 100,000 meter long >tether is so huge that no momentum wheel will have any impact on it. Another option to consider: it could be better to use a smaller rotating tether centered at the lower end of a non-rotating tether. Downsides are a more complex system and higher G-forces on the rotating tether (because at constant tip velocity, centrifugal force scales inversely with radius). Upsides include independent control of rotation rate and altitude (by independent length control of the two tethers). >With a solar sail the only thing limiting how much you can lift is how >long your system keeps working. Assuming you can make a solar-sail-based system work at all. Don't forget that solar sails impose some troublesome constraints, like the need for quite large surface areas and some sharp limitations on what direction you can thrust in.  SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net > >With a solar sail the only thing limiting how much you can lift is how > >long your system keeps working. [quoted text clipped - 3 lines] > need for quite large surface areas and some sharp limitations on what > direction you can thrust in. My simulator can do solar sails, so I have played with them some. Tethers and solar sails can fit together well in terms of the time and direction of thrust. When you do a tether pickup it drops the far side of your orbit. To reboost you need to thrust on the same side that you picked up. A solar sail does its best thrusting on the side of its orbit where it is going away from the Sun. If you plan your pickup so that you are in the part of your orbit where you are going away from the Sun, then all works out amazingly well. The large area is not an easy fit with any real G loading. So having it be built into a rotating tether may not be practical (I have not totally given up, but it does not seem easy). One way around this is to have 2 solar sails out in front pulling (one a bit to the left and one a bit to the right) and attached to the center of mass for your rotating tether. A hanging tether could be a bit easier. So for a big tether in GEO, it could work well. It is less clear for a small probe picking up regolith from the moon. I now prefer tossing regolith for thrust to reboost the Lunar tether. Anyway, solar sails have their troubles and constraints, but I think they have a bright future with tethers. -- Vince Here is picture of the craft proposed by Henry Spencer: http://www.islandone.org/LEOBiblio/SPBI1325.JPG and http://www.medianet.pl/~andrew/SPBI1325.JPG I improved it a little; the graspers can move in direction perpendicular to the orbit plane. > Assuming you can make a solar-sail-based system work at all. Don't > forget that solar sails impose some troublesome constraints, like the > need for quite large surface areas and some sharp limitations on what > direction you can thrust in. On the other hand, they don't need any fuel at all. And I'm not sure the thrust direction problems cannot be handled - you "just" need more complex sail configurations. SignatureSander +++ Out of cheese error +++ >> Assuming you can make a solar-sail-based system work at all. Don't >> forget that solar sails impose some troublesome constraints, like the [quoted text clipped - 4 lines] >sure the thrust direction problems cannot be handled - you "just" >need more complex sail configurations. There are concepts like Forward's "solar photon thruster" which are essentially solar sails that can thrust in any direction. However, you pay for that flexibility with much more demanding requirements: you still need huge surface areas, but now their shape has to be controlled quite precisely. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net >>> Assuming you can make a solar-sail-based system work at all. Don't >>> forget that solar sails impose some troublesome constraints, like the [quoted text clipped - 10 lines] > need huge surface areas, but now their shape has to be controlled quite > precisely. Yes. Thats more or less what I mean by "complex sail configurations" - sorry, i'm horribly bad at remembering names and attributions, so it usualy easier to omit them. Ultimately, I think solar sail based systems and very fuel efficent hybrids (that is less than 1/2000 of fuel/payload) are workable. SignatureSander +++ Out of cheese error +++ > There are concepts like Forward's "solar photon thruster" which are > essentially solar sails that can thrust in any direction. However, you > pay for that flexibility with much more demanding requirements: you still > need huge surface areas, but now their shape has to be controlled quite > precisely. I have had a google for the solar photon thruster and have come up blank. Also i can't find any refs in the solar sail litrature. I can't see how it can thrust in the direction of the light (ie toward the sun). You wouldn't have some refs handy would you? Thanks Greg > I have had a google for the solar photon thruster and have come up > blank. Also i can't find any refs in the solar sail litrature. I can't > see how it can thrust in the direction of the light (ie toward the > sun). You wouldn't have some refs handy would you? Pages 91 to 95 of "Solar Sailing - Technology, Dynamics and Mission Applications" by Colin R. McInnes. It is not that it can thrust toward the Sun. It is just that "Since the main collector always faces the Sun, the solar photon thruster is not subjected to the reduction in projected area that a flat solar sail suffers when directing the solar radiation force away from the Sun-line." (page 92,93) -- Vince >> Assuming you can make a solar-sail-based system work at all. Don't >> forget that solar sails impose some troublesome constraints, like the [quoted text clipped - 4 lines] > sure the thrust direction problems cannot be handled - you "just" > need more complex sail configurations. Nope. Solar sails can be adequately modeled by a fixed paddle reflecting a stream of balls. You can make the balls bounce in any direction, but the resultant thrust can never have any component in the direction of the source. You are strictly limited to thrusting to a direction at least 90 degrees away from the light source. And near 90 degrees, the thrust is vanishingly small. To get thrust towards the sun, you need either a reflector outside your position (impractical due to fundamental optics for more than a hundred times the width of the reflector) or a solar powered laser of some sort pushing on the sail. >>> Assuming you can make a solar-sail-based system work at all. Don't >>> forget that solar sails impose some troublesome constraints, like the [quoted text clipped - 6 lines] > > Nope. Only for some values of nope. > Solar sails can be adequately modeled by a fixed paddle reflecting a stream > of balls. But only of you have a single, fixed geometry sail. > You can make the balls bounce in any direction, but the resultant thrust > can never have any component in the direction of the source. [quoted text clipped - 6 lines] > than a hundred times the width of the reflector) or a solar > powered laser of some sort pushing on the sail. Not at all. If you make the sail be slightly bowl shaped instead of flat, then you should be able to make both "newtonian" and "cassegrain" sails which will allow you to both thrust normaly, at odd angles to the sail and even directly towards sun by moving of and changsing the angle of the "secondary sail". Sure, its not free. SignatureSander +++ Out of cheese error +++ >>>> Assuming you can make a solar-sail-based system work at all. Don't >>>> forget that solar sails impose some troublesome constraints, like the >>>> need for quite large surface areas and some sharp limitations on what >>>> direction you can thrust in. <snip> >> Nope. > [quoted text clipped - 15 lines] >> than a hundred times the width of the reflector) or a solar >> powered laser of some sort pushing on the sail. > Not at all. If you make the sail be slightly bowl shaped instead of flat, > then you should be able to make both "newtonian" and "cassegrain" sails [quoted text clipped - 3 lines] > > Sure, its not free. But the overall thrust on the sail system can never be towards the sun. And the seperation between them can never be larger than some hundred times the diameter of the primary sail, sharply limiting the use. >> Not at all. If you make the sail be slightly bowl shaped instead of flat, >> then you should be able to make both "newtonian" and "cassegrain" sails [quoted text clipped - 8 lines] > And the seperation between them can never be larger than some hundred times > the diameter of the primary sail, sharply limiting the use. Why is that limiting? This is not, after all a telescope, so you don't really care about optical quality and mass-center problems should be handle-able (or probably even handled better) by placement of the payload. Well, if the solar sail has a cenrtal pole that connects primary and secondary sails, the mass-center could also be movable (by moving the payload), so as to wary where the sail goes. but may well not be worth it. Oh, a question - why would a "cassegrain" sail not have its thrust towards Sun? | ... | | /|\ | | / | \ | |/ | \| ..... | ...... (allowing that ... are curved surfaces, etc). SignatureSander +++ Out of cheese error +++ vince@offshore.ai (Vincent Cate) wrote in message > If you want to rotate faster > you let it out on the down side and winch in on the up side so it spends > more time going down (and pulled faster) than going up (and pulled slower). > So rotational momentum is an easily solved problem. >From page 18 (or 20 by PDF count) in: http://www.tetherapplications.com/papers/guidebook.pdf (which is the space tether guidebook / bible) It says: "As discussed under Tether Control Strategies, changing a tether's length in resonance with variations in tether tension allows pumping or damping of libration or even spin. Due to Coriolis forces, in-plane libration and spin cause far larger tension variations than out-of-plane libration or spin, so in-plane behavior is far easier to adjust than out-of-plane behavior. Neglecting any parasitic losses in tether hysteresis & the reel motor, the net energy needed to induce a given libration or spin is simply the system's spin kinetic energy relative to the local vertical, when the system passes through the vertical." So not only can we use a winch to pump the spin up, it is very efficient to do so. -- Vince vince@offshore.ai (Vincent Cate) > However, you can easily control a tether's rotational momentum, when > near a gravitational body, by winching the tether in and out as it is > going up or down relative to the body. If you want to rotate faster > you let it out on the down side and winch in on the up side so it spends > more time going down (and pulled faster) than going up (and pulled slower). > So rotational momentum is an easily solved problem. The real algorithm is to pull in when the tether is vertical and let out when it is horizontal relative to the body. The method in the paragraph above does not work. The working algorithm comes from the Tether Guidebook and I have confirmed it by simulation. But it is true that you can control your rotational momentum without using any reaction mass when near a gravitational body. -- Vince > Nice idea, but not original. Hoyt, Forward and Moravec have each > proposed using tethers for lunar landing and sample return some > years ago. I realize that they proposed tethers for the Moon; however, I have not seen them point out that you could pick up samples without any infrastructure in place on the Moon. The thing that might be original is that a single probe, with no advance landing on the moon, could use a tether to scoop up samples. > It can be done even more cheaply than you propose. > > You do not even need any ion drive or propulsion at all actually. > You can do it entirely with zero momentum exchange, you simply > deposit payloads on to the lunar surface whose mass equals that of > the samples you remove. If we use an ion drive of 10,000 seconds ISP, it is throwing xenon out the back at about 98 km/sec. To pick up something from the Moon we need to give it 1.6 km/sec of momentum. So with the momentum from 1 Kg of xenon we can pick up 98 kps / 1.6 kps or about 61 Kg of regolith. If you simply deposited equal mass on the lunar surface you would only pick up 1 Kg for every 1 Kg you put down. So this way is cheaper for a probe on a sample return type mission. > For the details, take a look at these web links: > > http://www.tethers.com/MXTethers3.html > > http://www.tethers.com/MXTethers.html When I say "lunar sample return" I mean that there is nothing already in place on the Moon. I have not seen anything in these or any others papers I have read that indicates they were thinking of a sample return type mission. Sure people have looked at Lunar tethers. And I do think that two way lunar tether traffic would be *really cool*. In particular once we have lots of tourists going to the moon and coming back. -- Vince > > Nice idea, but not original. Hoyt, Forward and Moravec have each > > proposed using tethers for lunar landing and sample return some [quoted text clipped - 5 lines] > original is that a single probe, with no advance landing on the moon, > could use a tether to scoop up samples. The Hoyt/Forward proposals do not need infrastructure on the Moon. A bit of infrastructure might be nice to collect the specimens ahead of time and make them ready for pick-up, but that is not essential. > If we use an ion drive of 10,000 seconds ISP, it is throwing xenon > out the back at about 98 km/sec. To pick up something from the [quoted text clipped - 3 lines] > you would only pick up 1 Kg for every 1 Kg you put down. So this way > is cheaper for a probe on a sample return type mission. OK. Whether that is better than zero momentum exchange depends on whether you are interested in putting payloads on the lunar surface. Since soft landing on the Moon using rockets is extremely expensive, the tether method of depositing payloads is much cheaper. It would open up new markets for lunar development. Adding an ion drive has the advantage of lowering the amount of mass which must be launched from Earth, but it loses the opportunity benefit of soft-landing lots of payloads on to the Moon. > > For the details, take a look at these web links: > > [quoted text clipped - 6 lines] > others papers I have read that indicates they were thinking of a > sample return type mission. Sure people have looked at Lunar True, their paradigm is rather the inverse, soft landing high-value payloads on to the Moon. They did not recognize collecting lunar samples as a significant market. You have taken the precise opposite viewpoint, going for the lunar samples, and ignoring the oppportunity to soft-land payloads. Perhaps reality will be somewhere in between. > tethers. And I do think that two way lunar tether traffic would be > *really cool*. In particular once we have lots of tourists going to > the moon and coming back. In the short term we can soft land lots of infrastructure on to the Moon basically for free (except for the cost of launch from Earth), and build hotels ready for the tourists when they come. > The Hoyt/Forward proposals do not need infrastructure on the Moon. > A bit of infrastructure might be nice to collect the specimens ahead > of time and make them ready for pick-up, but that is not essential. It could be nice to have some tinny rovers collect some rocks and put them in a basket for pickup. I really suspect customers would pay more for lunar rocks than for lunar dust. But to pickup the basket you need to be accurate about where the end of your tether goes, and there is no GPS system on the moon so far. So there is added complexity and cost. >> OK. Whether that is better than zero momentum exchange depends on > whether you are interested in putting payloads on the lunar surface. > Since soft landing on the Moon using rockets is extremely expensive, > the tether method of depositing payloads is much cheaper. It would > open up new markets for lunar development. If there were people willing to pay big bucks to land small payloads on the moon then it would be something to do. But if you give up 60 Kg of regolith that you can sell for $250+/gram in order to lower 1 Kg onto the moon you would have to charge at least 60*250*1000 or $15 mil/Kg. But if their are customers, sure. > In the short term we can soft land lots of infrastructure on to the > Moon basically for free (except for the cost of launch from Earth), > and build hotels ready for the tourists when they come. Also the cost of what we give up by having that payload. We only have a limited total mass to work with. If we put on 1 Kg of something going to the moon and we have to give up 1 Kg of xenon, it would costs us a lot of valuable regolith. -- Vince >> You can do it entirely with zero momentum exchange, you simply >> deposit payloads on to the lunar surface whose mass equals that of [quoted text clipped - 5 lines] >you would only pick up 1 Kg for every 1 Kg you put down. So this way >is cheaper for a probe on a sample return type mission. Yes and no and maybe. It uses less fuel, but that does not necessarily equate to "cheaper". The issue is not fuel cost -- xenon is one of the few fuels whose cost actually *is* comparable to LEO launch costs, but even so, that's a relatively minor issue here -- but hardware costs and complexity and reliability. SignatureMOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer pointing, 10 Sept; first science, early Oct; all well. | henry@spsystems.net > >...with the momentum > >from 1 Kg of xenon we can pick up 98 kps / 1.6 kps or about 61 Kg [quoted text clipped - 7 lines] > even so, that's a relatively minor issue here -- but hardware costs and > complexity and reliability. I should not have said "cheaper", I really meant "a better business case". If you did not use a high ISP thruster, xenon, or lots of solar cells, then you could clearly make something that was cheaper. However, without a high ISP thruster the deltaV from LEO to LLO (low lunar orbit) and the deltaV from LLO back to Earth each reduce your mass by some factor, maybe a factor of 10 together. So on top of the factor of 61 above we have reduced our return by another factor of 10 for a total reduction factor of 610. So if we had 6,100 Kg of returned regolith in the high ISP method, now we have 10 Kg of regolith to sell for profit. It may not be exactly this bad because you save something on solar Kg too. But whatever the exact number, but I don't think it would be better business. If we have a high ISP thruster for getting to the moon and back, then we might as well use it to reboost momentum after we do a tether pickup. It may well be that a Hall Thruster makes more sense than an Ion Drive because it is cheaper, needs less solar power, (and lasts longer?). I can can not say what the ideal ISP or type of thruster is just yet. But I am convinced that some kind of high ISP thruster makes sense for this type of mission. -- Vince <>> You can do it entirely with zero momentum exchange, you simply <>> deposit payloads on to the lunar surface whose mass equals that of <>> the samples you remove. >>...with the momentum >>from 1 Kg of xenon we can pick up 98 kps / 1.6 kps or about 61 Kg >>of regolith. If you simply deposited equal mass on the lunar surface >>you would only pick up 1 Kg for every 1 Kg you put down. So this way >>is cheaper for a probe on a sample return type mission. >Yes and no and maybe. It uses less fuel, but that does not necessarily >equate to "cheaper". The issue is not fuel cost -- xenon is one of the >few fuels whose cost actually *is* comparable to LEO launch costs, but >even so, that's a relatively minor issue here -- but hardware costs and >complexity and reliability. How much worse would argon be? Argon is easily obtained in large quantities, being on the order of 1% of the atmosphere. It probably does not need to be highly purified for this purpose. SignatureThis address is for information only. I do not claim that these views are those of the Statistics Department or of Purdue University. Herman Rubin, Department of Statistics, Purdue University hrubin@stat.purdue.edu Phone: (765)494-6054 FAX: (765)494-0558 >>...The issue is not fuel cost -- xenon is one of the >>few fuels whose cost actually *is* comparable to LEO launch costs, but >>even so, that's a relatively minor issue here... > >How much worse would argon be? With current thrusters, you do take a significant performance hit, but it may be acceptable, depending on what you're doing. (Also worthy of note is that ion thrusters are not necessarily the optimal solution, when delta-V requirements are modest. Ion rockets need a lot of power, which means rather heavy solar arrays and related equipment. Something like an arcjet, with lower Isp but also lower power demands, may well be preferable.) >...It probably does not need to be highly purified for this purpose. You want ion-thruster propellants fairly pure, to keep various annoying secondary effects under control. In any case, purifying gases is not particularly hard. SignatureMOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | henry@spsystems.net >We have not seen this idea of using a rotating tether to pick up lunar >samples anyplace else... I *think* I have seen rotating tethers for orbital samplers mentioned before, but I'm unable to locate a specific reference. >...and think it looks very promising. There are some obvious issues with precision navigation over an uneven surface, but they don't seem insuperable. It certainly does have potential. SignatureMOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer pointing, 10 Sept; first science, early Oct; all well. | henry@spsystems.net > I *think* I have seen rotating tethers for orbital samplers mentioned > before, but I'm unable to locate a specific reference. Maybe we are the first to propose an Artemis style business of "selling lunar samples for profit" using rotating tethers. Since it seems like it could be a profitable business, this could be a useful idea. -- Vince > I *think* I have seen rotating tethers for orbital samplers mentioned > before, but I'm unable to locate a specific reference. From: http://www.uah.edu/library/archives/forward/BusinessBio.html >... "Tether-Assisted Planetoid Sampler" that would use a sampling >penetrator on the end of rotating tether to obtain a sample from the >surface of an airless planetoid (from comets and asteroids to Luna >and Mercury) during a flyby trajectory and return it to the Earth, ... Our idea of orbiting for months and winching in sample after sample using a high ISP thruster is a little bit different from this single flyby idea. Though going from that to ours is not much of a leap. :-) Anyway, if you or anyone finds any references I am interested. -- Vince I believe there is more money to make on hands-on space experience than on the Moon rocks. It would be fun for the overweight Internet generation to control a low Earth orbit satellite and take pictures of a home town. It would be even more fun to do some real work. For example, the lunavator can deposit toy size, rock hunting excavators on the visible half of the Moon so that relatively cheap terrestrial microwave dishes can communicate directly with the excavators. The Internet users would control the excavators with a mouse. They would hunt for rocks and scratch graffiti on the Moon dust. A few years ago a businessman considered launching a remotely controlled lunar buggy and renting it for "virtual" joy riders. Whatever you do, keep in mind that novelty of such space hardware will wear off quickly unless the hardware can do new things that have not been done before. I would start with finding a massive piece of junk in low Earth orbit and attaching a 100 kg Zylon bolo to the junk. The bolo has internal wires which can be used as electrodynamic tethers and power supply for a remote manipulator riding on the bolo. The wires can make lots of electric power at night, and they can control the orbit of the junk/bolo satellite. Add a 200 kg sounding rocket, a few Hall thrusters, and you can use the bolo to launch your lunavator in 20 kg pieces assembled in space with another remote manipulator. When the lunavator is assembled, you have your own, dirt cheap, lunavator bolo relay, which is Earth-to-orbit, Earth-to-Moon, and Moon-to-Earth space transportation system. I am not sure how far you could scale this project down. I don't think it would be too hard to get by on the 1,300 lbs to LEO of Space-X. I expect you could even get down to 500 lbs. At some smallness it gets hard to make fault tolerant tethers, but I am not sure where that is exactly. I don't think space junk is nearly the issue around Luna as around Earth. There has not been the human stuff smashing into each other and making lots of orbiting junk. You can get fault tolerance by having spare tethers, as the tethers you need for the moon are not really too heavy. You can make your tether shorter than 100 km, say 5 km, since the non-human payload can tolerate high Gs. This also reduces the chance of collision. Very small diameter Spectra lines are available. And if your last tether did happen to break, you just head back to Earth with whatever regolith you have so far. So a very small tether is probably doable. Scaling down solar power is easy. You can get small Hall Thrusters. At busek.com they have one that is just 900 grams. It might really be possible to do this project in under 100 lbs, though that would be impressive. -- Vince Charles F. Radley wrote: CFR> A lot of control can be gained by reeling the tethers in CFR> and out. A small propulsion system might help, but it CFR> is too early to say whether it is an essential component. If your lunavator is a sophisticated craft, rocket or ion propulsion is needed for emergency only. You can increase your orbital energy by picking up Moon dust, reeling it in and dropping it backwards. With the help of an anchor you can even change your orbital plane; just drop your anchor on the Moon off your orbital plane and pull it gently. ============================================================== VC> I am not sure how far you could scale this project down. VC> I don't think it would be too hard to get by on the VC> 1,300 lbs to LEO of Space-X. I expect you could even get VC> down to 500 lbs. I believe the size of the transponder that can send live video to the Earth determines the minimum mass of the lunavator craft. VC> At some smallness it gets hard to make fault tolerant VC> tethers, but I am not sure where that is exactly. I VC> don't think space junk is nearly the issue around Luna VC> as around Earth. True. Ribbon-shaped tethers are better than rope-shaped tethers. I do not have lunar meteoroid data, but I have terrestrial data at: http://www.islandone.org/LEOBiblio/SPBI1RE1.GIF VC> You can make your tether shorter than 100 km, say 5 km, VC> since the non-human payload can tolerate high Gs. True. VC> This also reduces the chance of collision. False. The longer the lunavator, the safer it is. Make sure the centrifugal acceleration (v*v/r) of the lunavator tip is at least several times the gravity on the lunar surface. If the centrifugal acceleration is too low, your lunavator will vibrate (librations). VC> Very small diameter Spectra lines are available. Plastic fibers are cheap. You can weave your own ribbon shaped tether from the fibers. A much more serious problem is space radiation. Inorganic (glass, carbon) fibers are immune to space radiation. VC> It might really be possible to do this project in VC> under 100 lbs, though that would be impressive. You are on the right track. VC> ...there is no GPS system on the moon so far. If your mini excavators are conspicuous, you do not need GPS on the Moon. There are many ways to make them conspicuous: radio beacons, laser retro-reflectors, microwave retro-reflectors... VC> The rotational momentum of even 1 Kg at the end of a VC> 100,000 meter long tether is so huge that no momentum VC> wheel will have any impact on it. Having a small momentum wheel is useful for fine adjustment of the angular velocity of the lunavator. If you do the adjustments with the winch only, the lunavator shakes too much, and you have to use mini rocket thrusters. VC> The orbital momentum needs to be controlled by either VC> leaving something on the surface of the moon of equal VC> mass to what you are picking up, or using some kind of VC> thruster. False! Pick up Moon dust, reel it in and drop it backwards. You will gain both orbital energy, and angular momentum about the center of lunavator's mass. There are many ways to loose the extra angular momentum. Probably the cheapest one is dragging an anchor on the dusty Moon surface. PS. You do not need rocket or ion thruster to fly your lunavator back to the Earth. You can use the Moon dust as reaction mass. ============================================================== Henry Spencer wrote: HS> The issue is not fuel cost -- xenon is one of the few fuels HS> whose cost actually *is* comparable to LEO launch costs, but HS> even so, that's a relatively minor issue here -- but hardware HS> costs and complexity and reliability. The complexity and reliability must be vied in the context of fail safe design. We are not talking about rocket launchers, so we are not talking about catastrophic failure. > VC> You can make your tether shorter than 100 km, say 5 km, > VC> since the non-human payload can tolerate high Gs. > AN> True. > VC> This also reduces the chance of collision. > AN> False. The longer the lunavator, the safer it is. If you are going to say someone is wrong, it is nice to say why. So, why do you think that a longer tether is safer from collision? The odds of hitting a large object are not increased much by the thickness of the tether and they increase linearly with the length. Even on a simple swept area basis, a shorter cable is going to be safer. If you make a cable twice as strong and half as long, it is not twice as wide, only sqrt(2) as wide. > VC> The rotational momentum of even 1 Kg at the end of a > VC> 100,000 meter long tether is so huge that no momentum [quoted text clipped - 4 lines] > adjustments with the winch only, the lunavator shakes > too much, and you have to use mini rocket thrusters. Do you have anything to backup either of these claims? > VC> The orbital momentum needs to be controlled by either > VC> leaving something on the surface of the moon of equal [quoted text clipped - 4 lines] > > Pick up Moon dust, reel it in and drop it backwards. Ok, you can use the tether as a thruster with the regolith as your reaction mass. And this is fun, for sure. There are a couple issues, though it could be a reasonable thing to do. When you picked up the regolith it was going backwards relative to your center of mass about 1.6 km/sec. So you have to throw it backwards faster than this to get any net thrust. If you want to throw it back twice as fast then you are going to winch in like halfway first. So now you have twice the tip speed and half the length. Given that centrifugal force is v^2/r your tether now needs to be 8 times as strong (2^2/0.5). This is an issue, but it would not be as bad if you tossed slower. The second issue is that you can easily make all kinds of space junk that hits you or someone else on some future orbit. By picking your velocity to be something less than 3.2 km/sec backwards relative to the center of mass of the tether, then it will be less than 1.6 km/sec relative to the moon and fall to the moon. You might want to avoid hitting Apollo sites, or any man made objects on the moon. But this does not seem like killer problem either. Ya, I think it could work. Both issues get easier if you don't toss very fast. Since there is no shortage of mass, that is ok. Since you would not be limited by a fixed supply of xenon reaction mass, you could bring back more lunar regolith this way. It should take less in the way of solar power since the energy put into the reaction mass goes up with the velocity squared (1/2 m v^2). So you save mass on solar and on the xenon. So you can put more mass into making a bigger reentry capsule and bring back more regolith. I can imaging starting with 4,200 Kg in LEO from a Falcon-5 and bringing back a few times this mass in regolith. > There are many ways to loose > the extra angular momentum. Probably the cheapest one is > dragging an anchor on the dusty Moon surface. If you are rotating too fast, then you are going to be hitting the moon as you rotate (dragging sounds too gentle). It seems like it would be very hard on your anchor and tether. Winching in and out to use the gravity of the moon to increase or decrease you rotational momentum just has to make for a more reliable system. -- Vince VC> So, why do you think that a longer tether is safer from VC> collision? You mean collisions with the meteoroids. I mean collision with the Moon. If the lunavator (tether) is short, it must be in an elliptic orbit to reduce probability of colliding with the Moon. The elliptic orbit limits efficiency and flexibility of the lunavator. AN> Having a small momentum wheel is useful for fine adjustment AN> of the angular velocity of the lunavator. If you do the AN> adjustments with the winch only, the lunavator shakes AN> too much, and you have to use mini rocket thrusters. VC> Do you have anything to backup either of these claims? The real rotating tether is not a rigid straight line. Gravity and reeling make it flex and shake. Finite element analysis is the only way to model the rotating tether. The small momentum wheel has the biggest influence on the inner end of the tether, while gravity and reeling influence mostly the outer end of the tether. This means that the wheel can make corrections that cannot be easily done by other means. By the way, having a small winch near the outer end of the rotating tether is also useful, because it can be used as a shock absorber and as a means to swing the outer tip in a plane perpendicular to the orbital plane. Needless to say, the winch does not waste rocket propellant. VC> When you picked up the regolith it was going backwards VC> relative to your center of mass about 1.6 km/sec. VC> So you have to throw it backwards faster than this to get VC> any net thrust. If you want to throw it back twice as fast VC> then you are going to winch in like halfway first. So now VC> you have twice the tip speed and half the length. Given VC> that centrifugal force is v^2/r your tether now needs to VC> be 8 times as strong (2^2/0.5). This is an issue, but VC> it would not be as bad if you tossed slower. I agree as long as we ignore the tether mass. VC> The second issue is that you can easily make all kinds of VC> space junk that hits you or someone else on some future VC> orbit. By picking your velocity to be something less VC> than 3.2 km/sec backwards relative to the center of mass VC> of the tether, then it will be less than 1.6 km/sec relative VC> to the moon and fall to the moon. You might want to avoid VC> hitting Apollo sites, or any man made objects on the moon. VC> But this does not seem like killer problem either. Being an environmentalist I do not like space junk, but it seems to me that you exaggerate the danger. I would go as far as launching sacks filled with Moon dust into Moon orbit and using the sacks as the reaction mass. The lunavator grabs the sacks one by one to climb out of the gravity well. It flies by the Earth, drops its Moon rocks on the Earth and returns to the Moon. The sacks are used again to climb down to low Moon orbit. This scheme has a flaw: the Moon rocks enter the atmosphere at about 9.6 km/s. Will they survive? A simple terrestrial bolo can reduce the velocity to 3.4 km/s. A complex system of bolo and sling can reduce the velocity to zero. I am talking about the lunavator bolo relay. It does much more than reducing the Moon rock velocity: it is a two-way space transportation system. AN> There are many ways to loose the extra angular momentum. AN> Probably the cheapest one is dragging an anchor on the AN> dusty Moon surface. VC> If you are rotating too fast, then you are going to be hitting VC> the moon as you rotate (dragging sounds too gentle). It seems VC> like it would be very hard on your anchor and tether. Winching VC> in and out to use the gravity of the moon to increase or VC> decrease you rotational momentum just has to make for a more VC> reliable system. I like your method because of its novelty, but I have no idea which method is better. The gravity method is useful only when gravity is significant in comparison with the centrifugal force. Unfortunately, this condition shakes the tether. The anchor method has the advantage of increasing the orbital energy of the tether, but it shakes the tether as well and the anchor may hit a rock buried in the Moon dust. Maybe a chain would be better than the anchor? PS. I have been frequenting the sci.space.tech and a few other newsgroups before the World Wide Web was invented. Until now I have never seen any novel idea posted on the newsgroups. The latest post by Ron Baalke "People Are Robots, Too. Almost" hints at the plausible explanation of this lack of creativity... > PS. I have been frequenting the sci.space.tech and a few other > newsgroups before the World Wide Web was invented. Until now I > have never seen any novel idea posted on the newsgroups. The > latest post by Ron Baalke "People Are Robots, Too. Almost" > hints at the plausible explanation of this lack of creativity... I believe I can easily match your creativity if I choose to. There are two problems though. There is a difference between useful creativity and undisciplined imagination. Most people find the latter boring if it does not affect them personally. Me included. I do wonder about Cherenkof radiation effects on the physical extremeties in some of your posts. Time is a resource best spent wisely. If transportation to LEO is not made cost effective, projects beyond it won't happen. This includes your current one. I do question how hard you have been looking for creativity, and your definition of same. > AN> Having a small momentum wheel is useful for fine adjustment > AN> of the angular velocity of the lunavator. If you do the [quoted text clipped - 6 lines] > and reeling make it flex and shake. Finite element analysis > is the only way to model the rotating tether. Yes, I understand. This is how my tether simulator works. While a winch can cause waves, it can also help get rid of them. Also you can use it in such a way as to not cause a problem wave. So I don't think that using a winch means you need to be firing rockets. I am not convinced that a momentum wheel is any real use for a rotating tether. But I am happy to agree to disagree. > I agree as long as we ignore the tether mass. I can now simulate a winch, so we can put real numbers to the winch stuff. > The gravity method is useful only > when gravity is significant in comparison with the centrifugal > force. I will have to simulate this winching in and out to get some real numbers on how fast you can gain or loose angular momentum at what distances from the moon. But I think you could do it at any realistic centrifugal force level, just that it could take a bigger winch motor. > Unfortunately, this condition shakes the tether. But I don't think "shaking the tether" is going to be a big problem. Winches with simple things like a "maximum force limit" or a "power limit" will reduce waves. You want to pull in during the weak parts of the wave and not so much when the tension is high, and these can do some of this. A real computer control algorithm could do very well at getting rid of waves. But with even simple thing, it has not been a real problem on my initial tests. > PS. I have been frequenting the sci.space.tech and a few other > newsgroups before the World Wide Web was invented. Until now I > have never seen any novel idea posted on the newsgroups. The > latest post by Ron Baalke "People Are Robots, Too. Almost" > hints at the plausible explanation of this lack of creativity... Just checking, you are saying that this thread is the first with some novel ideas? --- Vince Henry Spencer wrote: HS> Another option to consider: it could be better to HS> use a smaller rotating tether centered at the lower HS> end of a non-rotating tether. Downsides are a more HS> complex system and higher G-forces on the rotating HS> tether (because at constant tip velocity, centrifugal HS> force scales inversely with radius). Upsides include HS> independent control of rotation rate and altitude (by HS> independent length control of the two tethers). Interesting idea. The independent control of rotation rate and altitude is very useful. The main problem is control of the non-rotating tether. Perhaps winches and weights riding on the non-rotating tether can dampen its vibrations. There are many other options, for example guns or catapults on the lunar surface. 11 years ago I proposed a loop with winches encircling a planet or a moon. I call it orbital loop. PS. I would rather use tethers where they are needed the most -- low Earth orbit. There is lots of massive junk there. Hook up a rotating electrodynamic tether to the space station, and you have a cheap space transportation system launching a few small payloads a day to low Earth orbit and beyond. ======================================================== VC> Just checking, you are saying that this thread VC> is the first with some novel ideas? Yes. The first that I have seen. I define novel ideas as ideas that may solve important global problems, and require testing to prove that they are wrong. ======================================================== johnhare wrote: j> I do question how hard you have been looking for j> creativity, and your definition of same. As defined above. What I am missing in sci.space.tech is an in-depth discussion of novel ideas pertaining to reducing the cost of access to space. Most men hate novel ideas. Their computer hardware (Mac or PC), programming language, spoken language, dress, local sports team, favorite space transportation system, etc. are their religions. If we were open minded, there would be long lasting threads leading to a consensus, web sites, and sharing of related work (library research, programming, computer graphics, etc.). I added the ability to simulate a winch pulling in a tether to my simulator. You can have the winch limited by any combination of power, force, speed, distance, and time. I simulate picking up 2 Kg, winching in for a bit to get higher tip speed, then tossing 1 kg backwards (and by Newton's equal and opposite reaction we must get some thrust from the 1 kg that we tossed). So we are simulating a tether flinging regolith for thrust. Also found one big problem with my applet. It does not work under some microsoft JVMs. Hope to have this fixed soon. See example 89 for winching in applet at: http://spacetethers.com/spacetethers.html -- Vince |
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