My Lunar Trebuchet post from March 4 2001

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spacester

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Now mind you I don't stand by all the stuff in the post that follows. I was being playful to allow for a look at various issues but mostly to talk about trebuchets, which I think are just flat-out cool.<br /><br />So settle in for a long read, or skip the thread entirely if you don't like long posts.<br /><br />btw I'm going for the early record for longest post in the new era of sdc. Sorry, I like long posts, I'm a long post slut, what can I say? (I wonder if s*l*u*t will be censored?)<br /><br />Anyway, submitted strictly for your entertainment as I indulge my ego and prey on your nostalgia for the sdc days of yore . . . on with the earnestly silly Lunar Trebuchet Proposal.<br /> <div class="Discussion_UserSignature"> </div>
 
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spacester

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Preface:<br />Many discussions of the practicalities of establishing lunar colonies seem to neglect consideration of the mining of asteroids. Discussions can also tend to assume a significant build-up of lunar infrastructure. The concepts presented here embrace asteroid-derived resources, and deal with the initial establishment of a human presence on the moon. If once established on solid economic footing, lunar colonies can be expected to enter a growth phase, perhaps eventually rendering the infrastructure presented here obsolete. This obsolescence would constitute success of these ideas and would therefore be welcomed.<br /><br />If the reader determines this discussion is a serious projection of future space activity based on sound economic (market-driven) ideas, but leads to a rather whimsical application of medieval technology, the author will not be disappointed.<br /><br />A Lunar Colony: Establishing a foothold<br /><br />As much as this author would like to visit the moon, unfortunately there appear to be no significant, economically compelling reasons for establishing a lunar colony, other than providing the experience of “being there”. To this author and to many others, this reason is sufficient. However, the economic realities must be faced if it is to happen.<br /><br />Asteroid Mining<br /><br />A very good case for making huge profits from asteroid mining has been made, based on the huge abundance of metals and volatiles on these bodies. These resources are also more accessible than lunar resources of the same type. It is therefore reasonable to assume that initial commercial activities beyond low earth orbit will be primarily extraction of resources from the asteroids, combined with activities on the moon which will be dominated by adventure, entertainment and tourism. Further, asteroid-derived resources will find a ready market on the moon: if they result in profits after delivery to Earth’s surface, they would certainly be more affordable to a lunar <div class="Discussion_UserSignature"> </div>
 
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scottb50

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<<It has been postulated that there is water ice deposited within lunar craters at the poles.>><br /><br />It hasn't been proven, that's what postulated means!<br /><br />Your kind of putting the horse before the cart here. Now you want to fling postulated ice around the Moon? Helium 3 and such might someday be usable, if Fussion can be made to work, right now it is useless and while there are a lot of usable Elements abundant in the Moon the cost of extracting them, let alone processing them and building something with them is a long way down the road.<br /><br />Personally I think the Moon will be a nice place to visit, to say you have been there, and it would be great for very stable large scale sensing devices that could be shielded from the commotion and electrical interference of Earth, but for a practical purpose I don't see that much reason to go back to the Moon.<br /><br /> Asteroids might, and I stress might, be usable, but we don't know the real composition of those we can reach easily, ie. those in predictable near Earth orbits. What we have seen so far are pretty much rocks with masses too low to be that valuable. Those that have value, that we find with high metal concentrations my be fairly small and isolated, hard to find, like panning for Gold. After all if you consider the constant bombarment from debris from Space most of it is dust by the time it reaches the surface. Substantial meteors reaching the ground are fairly rare, look at the commotion in New Zealand a few months ago.<br /><br /> As a thought there is an Asteroid passing relatively close to Earth, as reported by Space.com, that could have been easily reached, it's orbit has been defined since the 1980's. It comes back this way every four years or so and is big enough that it could provide a lot of information realatively quickly as to what we could hope to find. Why haven't we sent a probe to meet it? <br /><br />Either way the Moon offers no advantage over LEO. To go to the Moon you ha <div class="Discussion_UserSignature"> </div>
 
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spacester

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This is<br />the <br />same<br />list<br />of<br />links<br />I put <br />on another thread<br /><br />The trebuchet was a silly intellectual exercise. If there are large deposits of ice at the lunar poles and you want to get it to the equator, I think you use trucks. And Earth-sourced water is the default option for now, that must be said.<br /><br />But then and now, I just wanted to get a discussion going by exploring some of the issues. One of the last two links has some actual NEO targets identified . . . <br /><br />I definitely still believe – and always will – that the primary resource provided by the moon is simply the wonder of being there. Its a huge resource.<br /> <div class="Discussion_UserSignature"> </div>
 
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igorsboss

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Sillyness acknowledged, but here's my reply anyway...<br /><br /><font color="yellow">The chief operational difficulty is seen as dealing with the incoming ice balls. ... they would arrive at a speed of 6653 miles per hour ... so presumably the ice would shatter.</font><br /><br />1 kg of ice at 2974m/s would have a kinetic energy of about 4.42MJ. 4.42MJ = 1055 kilocalories, about.<br /><br />Since impact cause kinetic energy to go to zero, all the kinetic energy is converted to heat energy.<br /><br />Even if the ice ball was chilled to 1K before being thrown, the ice ball would simply vaporize from the heat gain, into the vaccuum of space.<br /><br />That doesn't make the trebuchet impossible, however. Just forget about robots picking up ice shards. Instead, find a way to catch the cloud of water vapor rising from the impact site.<br /><br /><font color="yellow">Using theta = 45 degrees for maximum range in a vacuum yields...</font><br /><br />Theta=45 degrees yields the maximum ballistic range over a plane. The maximum ballistic range over a sphere is a circular orbit, with theta = 0 degrees.<br /><br />So, if you chuck the ice cube into a near-circular orbit with perigee at only about 200 meters above the lunar surface, what happens? Does the ice still vaporize on impact?<br />
 
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spacester

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Um, yeah the heat energy thing, that's a problem. <img src="/images/icons/laugh.gif" /> Use trucks. <br /><br />Not that <i>all</i> of the KE has to heat the ice. Whatever it lands on will absorb and briefly store some as distortion energy, so the catcher's mitt will heat up a bit. But still . . . <br /><br />I pondered over the 45 degree assumption and then quit worrying and went with it, waiting for correction from others. No one did the first time around. <img src="/images/icons/laugh.gif" /> <br /><br />The maximum range is going to be something approaching 360 degrees around the globe and take just a bit less than orbital energy for a hypothetical zero altitude circular orbit. An orbit cannot be achieved by flinging or shooting something from the ground. You always have to change your course at least once (velocity vector, deltaV required) even if you somehow get all the energy you need at launch. A flung ice cube will always strike the surface unless you can fling it at escape velocity plus a bit.<br /><br />The calculation of the launch angle per distance over the spherical surface is and was a bit beyond the scope of the intellectual exercise. But I'm glad you brought it up: perhaps a very low angle could allow for an extraordinarily long rolling stop and the ice wouldn't melt so much.<br /><br />Hmmm.. wait a minute, just as I was posting . . . how much energy could you dissipate in breaking the crystalline bonds in the ice, i.e. shattering it?<br /> <div class="Discussion_UserSignature"> </div>
 
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scottb50

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What if you substitute a cow for a block of ice. <div class="Discussion_UserSignature"> </div>
 
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igorsboss

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I can't find the specific heat of a cow in my CRC Handbook.<br /><br />I'll check "Joy of Cooking" and get back to ya...
 
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igorsboss

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Nope, "Joy of Cooking" didn't have it either...<br /><br />I found two appropriate references so far. One was from "Monty Python and the Holy Grail", involving the French, but did not achieve orbital velocity.<br /><br />The other one looked more promising, but I could only locate the abstract, not the entire article. In it, the researcher seems to have observed that certian female members of the family bovidae are able to make the leap to orbital velocity by themselves, without the assistance of a trebuchet. I didn't understand the reference to the feline violin, however.
 
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igorsboss

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I believe the minimum heat energy course will be an unobstructed suborbital path with a minimum apogee.<br /><br />Why? The higher the apogee, the more kinetic energy you must impart to the ice. (An unverified claim) Hence theta aproaches zero.<br /><br />The more kinetic energy you give the ice as you throw it, the more energy that gets converted to heat of melting later. Since the final velocity is zero, whatever you put in as kinetic energy will come back as heat.<br /><br />The velocity required for this suborbital path can be approximated by observing that it will be just slightly slower than the velocity required for the lowest unobstructed circular lunar orbit. So, I ask you, what is the velocity of the lowest unobstructed circular lunar orbit? We can use that velocity to approximate the kinetic energy that must be dissapated in order to stop the ice.<br /><br />Even if you break all the crystaline bonds of the ice, you still have to catch a fast moving cloud.<br /><br />Even the distortion energy of the catcher's mitt will be converted to heat, since the catcher's mitt's final kinetic energy will be zero. Just make sure you have a catcher's mitt with enough thermal inertia to keep cool.<br />
 
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mrmorris

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<font color="yellow">"perhaps a very low angle could allow for an extraordinarily long rolling stop and the ice wouldn't melt so much. "</font><br /><br />Instead of a catcher's mitt on the surface, picture an underground aquifer. The 'landing area' would consist of a very low friction (teflon coated?) funnel. The funnel would direct the incoming ice ball into a tube leading to the aquifer at an angle closely approaching that of the terminal trajectory. A set of airlock doors in the tube would be set to open and shut as the iceballs arrive (they can either be 'timed' based on the launch signal (without windage effects, time-to-arrival should be calculable to tenths of a second), or 'motion-activated' based on the iceball itself.<br /><br />Incoming iceballs then would come in to the landing area and be directed to the underground aquifer -- where their kinetic energy would be absorbed by the water (or ice) already there. The energy from the incoming iceballs would prevent the water in the aquifer from freezing. Even if it did freeze -- the next incoming iceball would simply expend its energy melting the ice. Any water vaporized by the impact should eventually recondense, as the airlock doors will prevent much of it from escaping the aquifer. It should be possible to time the arrival of the iceballs to keep the aquifer temperature fairly stable over time. In fact perhaps we should have *two* aquifers... one for hot water -- the other for cold. <img src="/images/icons/smile.gif" /><br />
 
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spacester

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mrmorris, that sounds pretty cool, nice design concept.<br /><br />One thing to think about: Lunar Gravity is lumpy. Mass is unevenly distributed enough to cause significant variations in the gravity field.<br /><br />Could this affect the repeatability of the landing spot? One would think not because, as you said, the conditions would be the same for each throw. But I wonder if the variations from the trebuchet might amplify and lead to greater dispersion at the target due to lumpy gravity. I'm not gonna learn the math to figure it out though. <img src="/images/icons/laugh.gif" /><br /><br />At any rate, I like the chute-and-pool idea. Fun stuff.<br /> <div class="Discussion_UserSignature"> </div>
 
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scottb50

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Hot and cold running water, no through in a nice patio and a small yard for the dog and we definitely have a concept here. <div class="Discussion_UserSignature"> </div>
 
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mrmorris

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<font color="yellow">"...if the variations from the trebuchet..."</font><br /><br />The gravity over a given area of the moon is unchanging, and isn't going to vary enough over *slightly* different flight paths to matter. However -- as you say, the trebuchet is the issue. Is it going to throw exactly the same every single time.<br /><br />I didn't actually slog through the math of your post, and I *certainly* didn't try to work out what the margin of error is. It might be that a .0001 % change in the trebuchet throw force would change the landing zone by half a mile. If so, and the tolerance on the throw force can't be tuned to +/- .0000001% (making completely uncalculated WAGs here) then the concept is unworkable. However -- the concept has several points in its favor:<br /><br />-- As you say -- even low-tech trebuchets tend to be extremely precise. A trebuchet designed using current technologies and machined to very tight tolerances should be capable of <b>extreme</b> precision in applying a set amount of force.<br /><br />-- Atmospheric randomizers are not in effect.<br /><br />-- The iceballs will be 100% water and therefore of constant density. As ice they can be made to very exact specifications. Variations due to the projectile therefore can be assumed non-existent.<br /><br />-- Expanding on the above point, computers built into the trebuchet should be able to detect any minute differences in weight between projectiles and alter the applied force accordingly.<br /><br />With all of this -- the landing zone *should* be fairly small. However -- to make it even more likely for the funnel mouth to always be correctly placed -- it should probably be elongated in the direction of the flight path instead of being circular. I can't *imagine* any lateral forces coming into play from the trebuchet. Any errors that creep in are likely to affect the distance thrown rather than the angle thown. Therefore we create a long, narrow funnel mouth.
 
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mrmorris

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*Just* as I posted I thunk a thought. What exactly are the effects of sunlight hitting this ice going to be while it's in flight? I approximated the flight duration based on ~3400 miles linear distance and your projected speed of ~6650 miles per hour. Because I'm lazy -- I'm treating the flight path as a 90-degree triangle with equal legs. The two legs (flight path) would then be 2*(3400/sqrt of 2) or ~4800 miles. Assuming any of this has a basis in reality, flight time would be ~43 minutes. So how much will the sunlight affect the ball of ice in 43 minutes? HeckifIknow. <br /><br />If this will cause portions of the ice to vaporize, then this will cause flight path variations. Just melting the ice <b>shouldn't</b> matter, as that would impart no delta-v. The water will hit the funnel and most should make it to the aquifer. Of course, if that portion of the funnel is in sunlight -- some water will likely vaporize. However -- most of the funnel should be in constant darkness, as it will be essentially a deep crater (in fact the funnel/aquifer would likely be located *in* a deep crater -- so a deep crater within a deep crater) and largely angled toward the pole.<br /><br />However -- the possibility of vaporization during the flight bothers me. We might have to send ice only when the flight path will be largely shaded by the moon.<br />
 
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igorsboss

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Agreed. Vaporization by sunlight would produce thrust, creating a curveball flight path.<br /><br />Also, mass loss by sublimation before, during, and after flight may be an efficiency problem due to the ultra-low ambient pressure.<br />
 
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spacester

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Dang it, you're probably right. I guess we'll have to wrap that rascal. <div class="Discussion_UserSignature"> </div>
 
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scottb50

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Industrial strength condoms. <div class="Discussion_UserSignature"> </div>
 
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mrmorris

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<font color="yellow">"I guess we'll have to wrap that rascal. "</font><br /><br />Wrapping the ice in some sort of protective coating *might* prevent vaporization. However -- it introduces several possible problems.<br /><br />1: Depending on the insulative and relective properties of the shell used, the ice/water inside might <b>still</b> reach its boiling point. If so -- you have a 'pressure cooker' situation and the iceball becomes a potentially explosive projectile. A thin aluminum shell polished to a mirror sheen *might* work.<br /><br />2. In addition to mining the ice you now have to mine and refine the shell material and presumably manufacture the shells on site. Also -- what exactly is this material going to be? How much will the additional complexity add to the overhead of the project?<br /><br />3. The shell will likely invalidate the underground aquifer idea. The energy of the iceball velocity is no longer <i>easily</i> shed as per my proposal if it's stored within an aluminum shell. It's <b>conceivable</b> that a shell could be constructed strong enough to contain the ice and possible steam pressure thereof, yet breakable enough that it will shatter on impact at the aquifer (the bottom of which would soon be littered with discarded shell pieces). However -- it's definitely complicating the situation further.<br /><br />I can think of a couple of possibilties for making such a situation more reasonable, but they (of course) introduce a lot more "What if's". <br /><br />Instead of the 'shell creation' operation being a sideline of the ice one -- make the two complementary endeavors. A factory near the poles is set up to mine titanium or aluminum, or iron, or a combination of all these and more from the lunar regolith. These are refined and made into either hollow shells or solid balls of refined metal. The trebuchet is used to send them towards the base. Either the trebuchet has to be considerably more intelligent and flexible or the missi
 
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mrmorris

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Well I was thinking this morning...<br /><br />BTW -- I'm putting <b>**way**</b> too much thought into this 'thought exercise'.<br /><br />There's a way to make the 'missile mill' overhead less of an issue with the polar ice/ore mining project. Namely -- there's no reason why the MM can't be used for <b>multiple</b> lunar resource projects. Make the 'landing area' spoken of in the previous post circular (possibly shades of the 'funnel and tube' idea from the aquifer). The end of the tube points towards the arm of the MM. At this point, missiles sent <i>from any direction</i> will be routed to the MM. Now it becomes possible for mining operations <i>in any direction from the base</i> to make deliveries of materials via 'Air Trebuchet'. It's now possible to have ice-mining operations at both poles deliving ice and ore. Operations mining the lunar regolith for He3 or *any* materials can be located in any direction from the base and package their wares in aluminum/titanium/etc. shells and fling them over.<br /><br />Trucking operations on the moon are never going to be simple or easy. Even *if* we had interstate-quality highways crisscrossing the lunar surface -- shipments will always be slow and dangerous. Vehicle breakdowns which would be annoying on I-10 can be deadly on L-2. There won't be a AAA-equivalent to call for some time.<br /><br /><i>As a side note though -- LAA will have a really nifty advertising slogan: "Sooner or later you'll break down and wish you'd called LAA... right up until the point that your air runs out."</i><br /><br />Trebuchet deliveries of materials however, will:<br /><br />- Risk no one's life trundling across the lunar surface.<br />- Require very modest equipment investment for the amount of material being delivered.<br />- Use very small amounts of energy on the sending end.<br />- <b>Generate</b> energy on the receiving end (I'm aware of no other delivery system that does this.)<br />- Can be operated 336x2 at the poles (14 days of 'light'
 
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nexium

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This tread is an excellent example of brain storming. We started with a simple idea with some problems. Solved most of the problems, but made it lots more complicated. Can we now simplify without creating a lot of new problems? Sorry I can only provide the cheering. Neil
 
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