Space-tech excerpt: Orbital debris [270 lines, fall '89] The topic: how to reduce or eliminate orbital debris, which may become a serious practial problem in low orbit? ------------------------------ From: Steven Deterling I am working on a project here at A&M this semester and am wondering if this group has any suggestions or information to offer. We are trying to design a mission (or set of mission) for the purpose of removal of orbital debris. Any ideas would be greatly appreciated. ------------------------------ From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU My first idea is to have a very-large-surface-area setup where you try to vaporize small bits of debris by putting an obstacle in front of it. For instance, a mylar sheet or some sort of ultra-light foam. The critical factor for that kind of scheme would be whether a thin film would do something useful with the debris - presumably you'd like to vaporize it so it wouldn't have a destructive impact if it hit somebody. Maybe vaporizing it would also decrease its decay time. Actually, I have a better idea than a single film: two or three mylar films spaced far enough apart that when debris strikes one, if it breaks up at all, the fanned-out secondary debris hits the second sheet, and again for the third. It may allow you to destroy much larger chunks of stuff because it forces each object to come into contact with a larger surface area of the sheet (as opposed to just punching a tiny little hole). This is assuming objects break up, as opposed to just lose a few molecules off the surface. I wonder if you take your average small object and run it into a film at a few km/s, what happens? What's a representative sample of debris? (My guess: chips of paint, metal shavings, bits of rubber, metal bolts, entire assemblies; probably biased toward the really small stuff, but you care more about the big stuff). === Or maybe you were thinking of bigger pieces of debris that you can track and predict the orbit of. One option would be to chase the debris with a low-acceleration tug and put it in your garbage bag. But this is probably prohibitively expensive except for the very largest objects - it may take weeks or months per object, and each craft needs propulsion, power, and communications. A second tactic: put something in the object's way that vaporizes it. I wonder how much foam you'd have to put in the way of a chunk of metal before it was destroyed? The better you can predict the orbit, the more stuff you can stack in its path. If you can't get the orbit down pat, maybe you can use on-board radar and shoot a gun at it - a small solid projectile may be able to break up a large object; I wonder if this isn't worse than nothing. === Another line of thought: how do you cheaply contain the debris before it is generated? For instance, some sort of foam or glue or something that prevents an object from breaking up before it burns up completely. You carry a small amount of this gunk along on your mission, and wire it so that when you eject something, it foams up. But probably most debris is generated by accident, in small quantities. Or is it? Do you have figures? How about putting a plastic bag around the vehicle after it injects into orbit? :-} ------------------------------ From: Steven Deterling We also are basically keen on the idea of vaporizing debris. I do not know enough about hypervelocity collisions to say whether or not a small piece of junk running into a mylar sheet will generate enough heat to be destroyed. We have considered some form of encapsulation for bigger pieces of junk. Not too sure about what to yet, however. The scenario we are starting to look at real closely is to have some sort of craft with an engine on the back and a high power laser of some sort attached to vaporize small particles. The front of the craft would be a collector for larger, "non-vaporizeable" particles. When the collector was full, it could be started on a trajectory toward the Sun and the rear of the craft could be separated for re-use. We are looking at possible using an ion engine for our craft. We are not too concerned with the time span, a mission of 5 years or so would not be bad. As long as we are decreasing the amount of debris in orbit, we are being beneficial. Comments on this scheme from everyone would really be helpful. ------------------------------ From: Joe Beckenbach As for encapsulation: for anything too big to vaporize but too small to grab easily, perhaps spray on some foam concrete or let it plow through layers of foam metal, something either to increase its size or to slow it down so it will either embed or decay. I think this is the basic idea that everyone's been trying to figure out how to do. ------------------------------ From: KEVIN@A.CFR.CMU.EDU (Kevin Ryan) Cleaning orbital junk, eh? If only the atmosphere did a better job of slowing them down, at least for a little while... Enter the little men in white shirts and wire-frames, with little plastic pocket protectors. They smile at the brass hats. "We have a solution to the orbital debris problem. Detonate a large, clean (relatively clean, of course - all things are relative) thermonuclear device in the upper atmosphere. It will cause a large and temporary 'hump' in the atmosphere, thus greatly slowing the orbital junk, which will soon reenter. After the 'hump' subsides, relaunch the satellites of your choice. This cleans out debris in _all_ orbits which intersect this rather large 'hump.' If you don't get them all, use a larger device, or do it repeatedly." Slowly, the brass hats start to smile. This would mean getting to use some of their BIG toys... I wish I was spinning this out of imaginary cloth. I have seen serious (!) suggestions for creating such an atmospheric 'hump' to slow/divert/destroy ICBM's - and if it works for high suborbital missiles, it should work for LEO debris, which should be at the very least less aerodynamic. Before I'm flamed, let me emphasize - "Not on MY planet, monkey boy!" Just thought I'd chuck it in for amusement... ------------------------------ From: henry@utzoo.uucp (Henry Spencer) Jordin Kare has observed that the kind of 1MW laser that would be used as a feasibility-test system for a laser launcher could also be quite useful in sweeping up debris. It could vaporize very small pieces, and could de-orbit larger ones by blowing pulses of gas off their leading surfaces (a laser retrorocket). ------------------------------ From: neufeld@helios.physics.utoronto.ca (Christopher Neufeld) Over the past couple of weeks we've seen a few ways to clean dust and grit out of low earth orbit, where it could damage satellites, shuttles, or the space station. Two of the more memorable ones were the ice cube in an opposing orbit, and the giant flypaper. I submit that there is an easier and more selective way to do the same thing. According to some calculations I made this afternoon, and which I'm still having trouble believing, it's very easy, assuming that most of the grit is going spinward, in the direction of most satellite launches. This grit goes from west to east as seen by an observer on the ground. A mirror is placed in the sunlight in the east as seen by a terrestrial observer. The mirror reflects sunlight across the sky, from east to west, so that it is shining directly into the path of the orbiting grit. The scenario I used was a mettalic flake 1mm in diameter, and 0.1mm thick, in a circular orbit 300km above the surface of the earth. It turns out that the photon pressure on the flakes lowers the perigee of the orbit to 100km, at which time it can be said to be braking in the atmosphere and out of our way, in only 50 hours of exposure. If we have 5% coverage, this is 1000 hours real time, or roughly six weeks. The advantage to this approach is that it works best on small objects. A communication satellite would suffer a delta-v of only about 1m/s, which I presume is within the tolerance of the onboard thrusters to compensate. An alternative solution is to put a giant sunshade which blocks light reaching orbit as they cross from day to night, while still letting the particles get the sun in their faces as they go from night to day. I favor the first approach because it is easier to stabilize the mirror than a sunscreen, since solar pressure on the mirror acts to oppose the earth's gravity, while solar pressure on the sunshade adds to the earth's gravity. Also, a mirror can be easily aimed to sweep different orbits, while a sunshade or a retrograde ice cube would require a lot of effort and time to do the same. Here are the calculations: I used the following parameters for the solution of the great cosmic vacuum cleaner: Particle is a cylinder: 1 mm in diameter 0.1 mm thick Particle's specific gravity: exactly 7x10^3 kg/m^3 Particle orbiting at exactly 300 km in a circular orbit Mass of the earth: exactly 6x10^24 kg Earth has no higher order gravitational moments. Gravitational constant: exactly 6.67x10^-11 N m^2/kg^2 Gravitational acceleration at the earth's surface: exactly 9.81 m/s^2 Radius of the earth: 6.387x10^6 m Radius of the orbit: 6.687x10^6 m Orbital velocity: 7.736x10^3 m/s For purposes of momentum transfer from the particle: I used the effective area of the particle as 1/2 the area of an end cap, and assumed that all radiation incident on the (tumbling) flake was absorbed. This is actually a conservative estimate, since the actual figure goes from 1/2 for a perfectly absorbing slab to 2/3 for a perfectly reflecting slab. This under-estimation of the area will absorb any inefficiencies in the mirror, since I am still using a power flux at the particle of 1.4 kW/m^2, the solar flux in space at one astronomical unit. Force on the particle is Psolar/(speed of light) * area of particle. This gives an acceleration of 3.333x10^-4 m/s^2 for as long as the particle is in the beam. Now, it is necessary to find the delta-v on a particle orbiting at 300 km to drop the perigee to 100 km. This turns out to be about 60 m/s. See the note at the end of this article for the math behind this calculation. The acceleration will provide this impulse in only 50 hours. If we have 5% coverage, this is 1000 hours real time, or roughly six weeks. Now, I have to justify my assumption that hitting the particle several times will result in the lowering of the perigee, but will not change the apogee, which will stay at 300 km. Assume that the orbit is initially circular. I hit it with the beam as it traverses some 15 degrees of its orbit. The particle slows down by some small amount, then continues in its orbit as a free particle. From classical mechanics, a gravitational orbit is closed (no precession). So, the particle must return to the point at which it received the initial impulse. This argument then repeats for each orbit. So, after giving it a delta-v of 60 m/s, the apogee is at 300 km while the perigee is at 100 km. It is now hitting atmosphere, and will quickly be removed from worry. For a Clarke orbit, the delta-v is 1500 m/s, which takes quite a bit longer, but the algebra is essentially the same. In this case, though, the mirror has to rotate to track the sun as it moves relative to the orbit over a period of one year. The mirror must shine into the orbits always at apogee to get the efficiency I've postulated, and apogee will precess with respect to the earth and sun, since it will always point to the same fixed stars. ------------------------------ From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU I like this approach a lot! However, I'm concerned about two questions: 1. How big a mirror do we need? What is the size of the cross section through which most of the grit goes? 2. What are the magnitudes of the forces involved on the mirror itself, and how much of its time can be spent usefully? My guess is that the mirror would have to be very large and that it would have somewhat less than a 25% duty cycle because it would probably want to remain in a single orientation throughout its orbit. However, I don't really trust my guesses on this at all. ------------------------------ From: Christopher Neufeld I'm still working out the orbital dynamics for the mirror, but I usually have a pretty good feel for the orbits without doing the math (that's why I suspected my initial erroneous results). The situation I'm looking at is a dynamically unstable SOLAR orbit leading the earth by a bit. I would choose the position of the mirror and its angle so that the light pressure from the reflection exactly balances the earth's pull. If it drifted away from the earth a bit, the pull would be weakened, and it would tend to drift further, so the mirror would have to be furled slightly to lower the outward solar pressure and bring it back into line. If it drifted toward the earth, the pull would be strengthened, and extra mirror kept in reserve for that eventuality would be unfurled until it is back where it belongs. The feedback scheme shouldn't be impossible. Anyway, sometime tomorrow I'll work out the details of the sail: its mass per unit area, position with respect to the earth, and a typical size. I expect that the mirror can be shining in a useful direction at least 90% of the time. More details as they become available. ------------------------------ [ End of excerpt ] ------------------------------------------------------------ Space-tech excerpt: Launch Loops [660 lines, Feb. '90] From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU I'm sure many of you have heard of Keith Lofstrom's concept of the Launch Loop. I think it's incredibly cool, and I'll take a crack at describing what the principle of it is. 1. Kinetic Structures ===================== The physical basis of the concept is something which I call a kinetic structure. I'll explain it by example. Shoot a stream of water from a garden hose up in the air. It forms an arc. There is no need for material strength in the water: there's no tension or compression going on, but rather just the water's free-fall motion along the path prescribed by gravity. Imagine shooting a stream of water into a very high arc: it could go higher than you could build the tallest skyscraper, since it's not limited by the strengths of construction materials. Now imagine balancing a pie plate on top of the arc of water, so that it is supported by deflecting the water downwards slightly. The plate is suspended there, higher than you might have thought possible, by the force from the continuing deflection of the stream of water. 2. The Launch Loop ================== If you replace the stream of water with a segmented ribbon of iron, achieve the deflection of the stream by using magnets, and have two 'stations' suspended by the ribbon rather than a single pie plate, you have Lofstrom's launch loop. It is a structure about 2000km long and 80km high. The loop of iron runs along the earth's surface in one direction, is deflected upwards by magnets at an earth station, back parallel to the earth's surface by a station 80km high, downwards by another station, and back along the earth's surface. A --------->------------->------------->--------------- B / \ / \ / \ C ---------<----------<------------<--------------<---------- D ===============================Earth==================================== The Launch Loop. A and B are stations, 80 km high. C and D are deflector stations, 2000 km apart, on the ground. The segmented iron ribbon moves at 14 km/s. The horizontal sections, and the earth, are actually convex, not straight as shown. The whole loop of iron segments whizzes along at 14 km/s inside a vacuum sheath. The stations at A and B are held up by the force generated by magnetically deflecting the ribbon downwards; they are anchored to the ground by cables, which are needed for stability and to counteract the horizontal forces. 3. Say What? ============ I should head off your initial skepticism. This is no joke. The guy has worked out details of how you deflect the ribbon, what materials are required, how to anchor the stations, and all the other details. The idea has been reviewed by a lot of people, so if you think you see a glaring flaw, it's probably because I haven't conveyed the idea properly. The paper I have, AIAA-85-1368 (from an AIAA conference in 1985), has lots of numbers for everything. Some details that I should mention: - the iron ribbon consists of 200cm x 5cm x 1cm segments, which are slotted to fit into each other. - the ribbon undergoes no stress whatsoever; it is just a passive holder of kinetic energy. - starting up the launch loop is a difficult task, involving spinning up the ribbon while it is floating on the surface of the ocean. 4. Using the Launch Loop ======================== Once you have spun up this thing, what do you do with it? The stations themselves are useful things: they're outside the atmosphere, yet they are anchored to the surface by cables. You could put an observatory on one of them, and commute to it up and down the 80km cable. But the main use of the loop is to launch vehicles, weighing about 5 tons, including passenger vehicles. The idea is that the vehicle sits on the top section of track, and uses magnetic coupling with the moving ribbon to accelerate along the 2000km top portion until the desired velocity is reached (which could be orbital velocity or escape velocity). So to get into orbit, you winch yourself up a cable to station A, hop in a car, get accelerated up to orbital velocity on the cable, and let go. Because the loop is so massive, accelerating a vehicle doesn't decrease its velocity much; the velocity is added back in by the ground-based magnets. The result is that ground-based electrical power has been used to send a payload into orbit. 5. Practical Objections ======================= Lofstrom talks of the project as if it might be real, and even gives some guesses as to construction costs ($2 billion total). My evaluation of this whole thing is: incredibly cool physical concept, incredibly impractical engineering problem. Particularly, the ocean-based leg of the system is a 2000-km-long vacuum sheath which must be flawless. Spinning up the thing involves getting the entire 4000-km-long loop going perfectly on the first try, dealing with weather and all sorts of unpleasantness, and gradually lifting the stations up to their correct positions. 6. My preferred version: the hula hoop ====================================== I think the ground-based section of this thing is the worst part. So I propose having the loop go all the way around the earth, in low orbit. The ribbon will be moving faster than orbital velocity, so that it can be deflected by the stations to hold them up. I'd say there should be about 60 stations spaced around the equator, each of them fastened by cables to the ground. The ribbon moves in a shape somewhere between a 60-sided polygon and a circle. In between stations, it flies in free fall. It's still a really complex device, but at least it isn't in the weather. To spin up this structure, you could start with the stations in low orbit, and gradually decelerate them to a standstill as the ribbon is spun up and starts to support them. I notice that Lofstrom references some articles in the L-5 news and JBIS which discuss this idea. ------------------------------ From: Tom Neff I thought the point of the Launch Loop was that it could be powered from, and launch payloads from, the ground. The Hula Hoop might be easier to construct, but how do you power it and how does it get anything launched? ------------------------------ From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU This isn't a problem: the ~60 stations are all _stationary_ with respect to the ground, approximately 100km up, and are anchored by cables. You can run power lines and elevators up the cables. ------------------------------ From: dietz@cs.rochester.edu (Paul Dietz) One of my complaints with the launch loop is what happens when it fails. Should the loop break anywhere, or should the airtight jacket spring a leak, all the levitated sections fall back to earth, spread over thousands of kilometers. IMHO, the launch loop idea is more feasible on the moon. The levitation magnets can be anchored to the lunar surface, and can be externally powered. The loop provides a nice way to store energy over the lunar night. And, the required loop velocity is lower, if all you want to do is get to lunar orbit. Some work at Argonne National Labs has been inspired by the launch loop. A fellow there named Hull and some coworkers have looked into magnetically levitated rings for energy storage here on earth. The concept is nice, since (for fixed centripetal acceleration) the energy stored scales as R^2, and the energy stored per ring + magnet mass scales as R. Hull found a nifty *passively* stable attractive magnetic levitation scheme (Lofstrom used active stabilization). The idea works like this. Let o and + denote cables carrying currents into and out of the page, and let - represent the iron loop. Then, there are two positions where the magnetic field of the currents support the loop against gravity: + o + o - - In the first position, the loop is stable against vertical perturbations but unstable against lateral perturbations. In the second, the opposite is the case. Hull noticed that if you alternate sections of the two types, then the net effect, if the loop is moving in the right range of speeds, is to make it stable in *both* directions. This is the principle of Strong Focusing, which is vital to the operation of modern particle accelerators (where alternating gradient quadrupole magnets focus particle beams). ------------------------------ From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU I guess I'd be more interested in launch loops on the moon if I actually cared about being able to launch things from the moon. But it seems like a tidy little mass driver would be a better bet there in any case. == Paul correctly points out a really big drawback of the design, namely the fact that the thing has to work flawlessly, all the time, or the whole thing falls down. Any system which must be in continuous operation is far, far less practical than a system which just goes BANG and is done. ------------------------------ From: Jordin Kare The "full orbit" launch loop has been proposed several times. I (re)invented it in about 1980, but later found a fairly detailed analysis, I think by Hans Moravec. It is simpler than the Lofstrom loop, but takes much more material, and would be much more subject to cutting by LEO debris/meteorites. Stabilizing it is also nontrivial, although not necessarily more difficult than for the Lofstrom loop. There are serious problems starting the thing up. However, the big problem is getting the mass up there in orbit to begin with -- at least the Lofstrom Loop gets built on the ground. Jordin Kare ------------------------------ From: Lou Adornato I think a surface based loop would be a better plan for lunar colonization because of the lower materials and construction demands. Keep in mind that the size and speed of a lunar loop would be a lot lower than for a terrestrial loop. Also, for a lunar loop you wouldn't need a vacuum sheath, and there wouldn't be any concerns about weather. There are several advantages of the loop over a mass driver. According to Lofstrom, the accelerations of a terrestrial (and possibly a lunar) mass driver would be so high that it could only be used for inanimate payloads. Also, there would be a lot of problems providing the required peak energy demands. A loop would have a fairly constant energy demand, and the peak acceleration to escape velocity (at least to get to an L point or the moon) would be 3g's, and (if my understanding of the the math is correct), about 0.5g for a lunar loop. From what I understand of the design of this monster, if higher acclerations are allowed, the loop can be made smaller. ------------------------------ From: "Edward V. Wright" The Kevlar slingshot promises to be cheaper than the mass driver for launching lunar payloads. The Kevlar slingshot is just what it sounds like -- a Kevlar sling attached to a mechanical arm. Attach a payload to the end of the sling. Spin the sling until the tip reaches orbital velocity, then release the payload. The idea is almost absurdly simple, but apparently will work on the Moon. The idea was developed by Dr. Jerry Pournelle and Dr. Marvin Minsky and was mentioned by Pournelle in one of his columns. I think they gave a paper on this somewhere, but I don't have exact references. ------------------------------ From: Bob Munck The Hula Hoop (loop entirely around the earth, I assume moving faster than orbital velocity) has the obvious drawback of weighing on the order of 150,000 tons (for a 1x5 cm loop) and needing to be in orbit before it's usable. If we could put that much up, we probably wouldn't really need it. How about a 2.7 cm diameter Kevlar cable with a 1 cm iron core, breaking strength about the same as the Launch Loop (about 300T?)? That's down to a "mere" 50,000T in LEO. Does it really need that kind of strength? Would 50T be enough (cable weighs 8,000T)? That's down under 100 shuttle missions, a distinct possibility. The iron core could be discontinuous, say 1 cm pellets at 2 cm intervals. OK, where have I wandered? A 40,000 km Kevlar hoop (i cm diameter) with a (possibly discontinuous) iron core (j cm diameter) around the earth at k km altitude, traveling at x m/s. Stations at y km intervals capable of accelerating the hoop by magnetic coupling to the iron with power coming up a (superconducting) cable from the ground. Some of the stations would have an elevator capable of lifting an z ton payload and hanging it on the hoop, which would accelerate it up to orbital (or more) velocity. I like it. Possible values: i=1.5, j=0.6, k=75, x=15, y=2000, z=5. Starting up is fairly easy (but k might be low) and the broken cable mode flings cable outwards and drops stations straight down, relatively safe. What factors have I missed? How stable is it with wind and random payloads on the spokes? -- Bob Munck ------------------------------ From: Bob Munck Come on, folks, help me out. In a previous message I rambled on about a Kevlar/iron hoop around the Equator in LEO (or lower) spinning faster than orbital velocity. Stationary facilities would "ride" it magnetically (hence the iron component) with Kevlar tethers down to ground level bringing up electricity to keep the hoop spinning and payloads on the order of 5T. The payloads would couple (also magnetically) to the Hoop and, letting go of the station, be accelerated to orbital velocity and beyond. The Hoop and tethers are all on the order of 5 sq cm: the Hoop must withstand whatever strain is generated by its higher-than-orbital speed, its mass, and the weight of the stations; the tethers must support their own weight and the payloads. I *think* that the total is within reason for us to boost into orbit -- a hundred or so Shuttle launches or a lot of little mass driver shots. Start-up is easy: assemble in LEO at orbital velocity, spin it up a bit with strap-on rockets, fly up a couple of stations and reel down their tethers. If the Hoop snaps, it throws itself all over the Solar System and drops the stations straight down. (Humm. It might shotgun all our comsats.) WHAT'S WRONG WITH THIS IDEA? Does the Hoop have to be spinning so fast that it can't possibly hold together? (I haven't the foggiest how to calculate the strain on such a Hoop for a given velocity.) Is it unstable? Are the tethers beyond our current strength-of-materials capabilities? Am I orders of magnitude off on the launch mass requirement? Is the whole idea of holding up the stationary facilities crazy? (But isn't that what a Lofstrom Loop does?). HELP!! -- Bob Munck, MITRE McLean ps. I'm struck by the thought of standing on the Hoop in a 1g field with my head toward Earth, going around every 45 minutes. Hence the "Ringworld." ------------------------------ From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU Bob - I'm starting to like this a lot! I think we're looking at a new form for the hula hoop: use tension to hold the structure together, rather than deflector stations. Magnetic deflection is only used to hold up the launch station, and magnetic coupling is still used to launch payloads and re-accelerate the ribbon. It requires much smaller deflectors than the original scheme, and may have better reliability, since the loop can 'idle' with no deflectors operating. To sum up the idea: put a Kevlar ring in low orbit, then spin it to create tension. Float a station on it as for the Lofstrom Loop, and launch vehicles using magnetic coupling with iron pellets in the cable. The tensile strength works out to be challenging but not out of the question (see below). We still need to work out the dynamic properties of the system, even if roughly, to see if we can hold up a station and succeed in launching vehicles, while staying within tension bounds and remaining dynamically stable. Here are my calculations for the tensile strength question: What's the tension force on a spinning loop? Do induction on chains of k masses cabled together in a loop: 3 in a triangle, 4 in a square, etc. For all values of k, the total mass sums to M. The limit of the tension on the cables as k approaches infinity is the tension on the loop. k = 3 : k = 4 : O O---------O /|\ | | / |T\ <--- Theta = pi/2 - pi/k | | / | \ | | / \ | | / \ | | O-----------O O---------O 2 2 F M v M v F = lim c = lim ----- = -------- t k->inf ------------ k->inf r 2 pi r 2 cos Theta ------------- 2 k sin pi/k ================= What's the maximum spin velocity (in excess of orbital velocity) of a loop with these parameters? .... X = tensile strength of cable Y = density of cable c = cross-sectional area of cable r = radius of loop M = total mass of cable 2 G(r) = gravitational acceleration at radius r = Vo / r Vo = orbital velocity at radius r 2 2 2 M | v | 2 pi r c Y (v - Vo ) 2 2 F = ---- |--- - G(r) | = --------------------- = c Y (v - Vo ) t 2 pi | r | 2 pi r 2 2 F = X c = c Y (v - Vo ) t max max 2 V = SQRT( X / Y + Vo ) max ================= For Kevlar 29 as used for oil rigs, tensile strength = X = 2.76E9 N/m/m density = Y = 1.44E3 kg/m/m/m V = 120 m/s (in excess of orbital velocity of ~ 8.3 km/s for LEO) max This isn't too useful, but with 10-50x stronger materials, this increases to 1-3 km/s. ============== This design has the advantage that we need only one station, plus some guy cables spaced around the equator. ------------------------------ From: John Sahr Some commentary on the calculations by Marc. Summary: the Kevlar Hula hoop is likely to be very unstable if it is "anchored" and the loop travels at "reasonable" speeds. The loop must spin at "unreasonable" speed in order to become stable. For a lineal density of 1 kg/m and tension of 100T, we can calculate the speed of the transverse waves along the loop, namely v = sqrt(T/rho), ( rho = M/(2 pi r) ) = 1 km/sec. Thus, an observer on a stationary earth would observe perturbations travelling along the loop at speeds of 9.3 and 7.3 km/sec, both in the same direction of the loop. Factoring in the earth's rotation, and assuming prograde spinning of the loop in the equatorial plane, the velocities of the waves observed from the earth's surface would be 7.7 and 5.7 km/sec, still in the direction of the loop. This is a real problem, as any "stationary" perturbations (anchoring cables, suspended stations, advertising signs, and other whatnot) will be generating waves along the loop, only downstream, and none upstream. This situation is analogous to the operation of certain microwave tubes which rely upon "fast" and "slow" waves to amplify perturbations in the electron beam density. The difference is that this is a periodic problem (it is a loop, after all), and it is possible that there is a stable, noncircular, and probably (mathematically) nonlinear solution involving rather large amplitude perturbations of the loop (I'll have to think about it). A way to combat this problem is to find a way to increase the velocity of waves along the loop, so that one of the waves can travel "upstream." This can be done by increasing the T/rho ratio by a factor of about 35. From an equation above, T == F_t is proportional to the mass density, and we can write T/rho = F_t/(M/(2 pi r)) = v^2/r - G In other words, the only way to increase the wave speed is to increase loop speed. In fact, because of that pesky G, v_wave can never exceed the speed of the loop. However, the Earth is spinning with an equatorial speed of about 1600 km/hour = 450 m/s = V_e. So, the slowest loop speed V_s that will satisfy the stability condition satisfies V_w + V_e >= V_s; where V_w^2 = (V_s^2 - rG). Solving this for the minimum possible loop speed gives V_e^2 + rG V_s(min) = ------------ = 75 km/sec 2 V_e The loop must make a complete orbit every 9 minutes or less. This is a rather large velocity; it could be reduced substantially by either letting the stations drift prograde, or by spinning the Earth up so that its day was shorter, say 4 hours instead of the current 24. In the absence of a stable nonlinear large amplitude solution, or an ambitious dynamic active correction of perturbations, or a loop material which is very good at damping out transverse motions, this strikes me as a pretty fundamental limitation to this idea. note: Someone should double check the statements I have made. -john ------------------------------ From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU The question remains: how to make the Hula Hoop stable? John's wave calculations were very informative, and I will assume that it is necessary to damp out any waves that occur. Then the key to achieving stability is to have a means of damping out the waves downstream of the perturbation. A factor in our favor is that the main perturbation, the station, is stationary, so there can be an anchored damping mechanism downstream from it, which applies a continuous damping force to the cable. My best idea for dealing with the perturbations caused by the launch vehicle, which are not stationary, is to use a series of cables to the ground which actively damp out perturbations by adjusting their downward force. I have no idea if this will work. Can anyone fill in any details, or think of a passive way to achieve the damping? ===================== Thinking further about it, I conclude that it's important to distinguish between two separate issues. One is how to get the hoop to move in a circle: the idea of a Kevlar cable was introduced in order to achieve the desired curvature through tension rather than by using a whole lot of deflector stations. The second issue is how to aim the cable precisely where you want it. In the original loop proposals, the ribbon must be aimed at its destination with incredible precision, since there is no internal strength in the ribbon. This involves very aggressive active control. However, when we use a Kevlar cable, it is tempting to use the strength of the cable to help guide it to its destination, in order to reduce the control problem. I believe that it is these transverse guiding forces which introduce waves, and if we can't deal with the waves, we can always go back to strict active control a la Lofstrom. ===================== I'm trying to do some clearer thinking about what the shape of the hoop would look like. I've been imagining, in my weaker moments, that we just hang the station from the hoop, and that the hoop bends down in a 'U' shape. This is totally wrong! A 'U' shape would indicate that the tension of the cable is holding up the station. But unless we totally change the concept of the thing, it's magnetic deflection of the moving cable that provides the necessary force. I think a good mental picture to start from is the following. Imagine that the earth is a point mass, and that we want to support two stations 180 degrees apart, using only the iron-pellet type loop. We can do this by firing the pellets back and forth in curved paths: __-------__ -=__ O __=- ------- The faster the pellets are going, the straighter the path. If they go too fast, they run into the Earth (which isn't really a point mass after all). ====== Now, let's come back to reality. If we try this two-node solution using iron pellets around the Earth, we can't fire them very fast at all or their paths would pass through the Earth. But if we use the Kevlar hoop, the tension of the cable can pull it in a more tightly curved path, so it misses the Earth even at higher velocities. At least, I think this works. If somebody could work it out in more detail, it would be a good thing. And is a two-node setup appropriate, or more, or less? A 1-node solution is sort of asymmetrical, but perhaps a setup with 1 big node and a dozen smaller ones (cables to the ground) or something. But do we agree that any version of this will have the station supported on a 'peak' of the hoop rather than a 'valley'? I think this is right. ====== Some general reflections: I'm starting to think that we haven't really solved any fundamental problems by making the loop out of Kevlar. My feeling is that it should be able to operate in a more passive mode than the original loop, but it doesn't seem to be working out that way. ------------------------------ From: John Sahr >How about sixteen spokes anchored to the earth, one every 2500 km? >The mechanism that couples to the Hoop could include some way to pull >down harder or loosen up on the Hoop over a couple of meters of play >and with tenth-second response time. Would that do to damp the >perturbations? For a passive support system for this kinetic structure, it is possible that evenly spaced supports would be about the worst choice. Because the transverse wave speed is slower than the loop orbit speed, ordinary standing waves couldn't form; however, since the loop itself is periodic, the n=8k (n = number of wavelengths, n, k are integers, for both fast and slow waves) modes would stand between 16 spokes. A better solution would be to have aperiodic spacing such that no modes, or very few, were allowed. This could be used to filter out the long wavelength waves. In a linear world, it would be enough to have two supports, such that the ratio of the two distances between them is irrational. However, this cable is going to stretch, have nonlinear restoring forces at large amplitudes, and will be excited by the spokes them selves. With 16 suitably spaced spokes, you might be able to effectively filter out the waves of wavelength greater than a few hundred kilometers, and one or two spokes might be able to have damping mechanisms for shorter wavelengths. I could never recommend building this structure if it wasn't passively stable. I had an idea for stabilizing the loop, though, whose dynamics I have not yet had time to work out. It goes like this. The problem with the loop is that transverse waves both travel "downstream," a "negative energy mode" situation. It seems to be the case that the loop must spin terribly fast in order to allow the loop tension to be large enough to have one wave travel upstream in the frame of a rotating Earth. Notice that a retrograde loop would have the same problem, but in the opposite direction. Therefore, what would be the dynamic stability properties of two loops mechanically coupled to each other, one spinning prograde, one retrograde? To couple the two loops, they could be put side by side and connected by rungs like a ladder (probably won't work), connected like a spiralling ladder (like DNA, and probably most feasible), or connected concentrically (one inside the other; best mechanical connection, most esthetically pleasing: how to spin that inside loop, though?). Each cable separately provides one direction of wave modes; perhaps the composite structure will have 2 or 4 modes. I suspect that this structure might have very good dynamic stiffness. But it might also explode into a million tiny pieces. It may not be necessary for the retrograde loop to have the same momentum as the prograde loop. There are a few other nice properties as well; residual loop-spoke friction can be canceled (as far as the the spoke is concerned). It's just an idea. Even if this doesn't work for a launch loop, it might provide a nice frame for space structures. ------------------------------ From: Bob Gray Check out: "Orbital rings and Jacob's ladders" by Paul Birch. Vol 36, Journal of the British Interplanetary Society. pp 115-128 (1983) The article describes how to bootstrap the system by starting with a thin cable, and using that to lift the materials to build the main orbital ring. Eventual cost to orbit was estimated at $0.05/Kg. He then describes how more than one ring can be used to reach any point on the Earth's surface, and how the ring could be used in conjunction with a Lunar ring to provide a very high speed shuttle service from the Earth's surface to the Lunar surface in a few hours. ------------------------------ [ End of excerpt ] ------------------------------------------------------------ Space-tech excerpt: Robots on the moon [330 lines, Mar. '90] ------------------------------ From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU I was just thinking of trying to sketch out an idea: a robotic, teleoperated "construction team" of about 50 little (say, 20-pound) robots designed and operated in completely distinct ways (for redundancy and flexibility), which would operate on the moon and be able to put together payloads shipped there in kit form. They would be teleoperated from an earth station, and use common communications equipment.... ------------------------------ From: Gord Deinstadt Marc, I have been thinking along similar lines for some time. My ultimate goal was a system of tiny robots that could assemble the next generation of larger robots, and so on until they are big enough to do "real" work. But, even if they were less capable they could still do a lot. The biggest problem I see is power. A 20-pound robot can't store much energy relative to its size, at least not with batteries. I'm not sure how constraining this is, since a lot of the work can be done while standing still, which makes a power cord acceptable. You still have to have a lunar power station, though. Hmm- microwave transmitters a few thousand miles apart, locked in phase, should be able to beam power from Earth to a smallish rectenna on the Moon. Anyhow, I'm convinced you can do a whole lot with fairly stupid robots on the Moon, provided you're willing to do it slowly. But slowly is relative; we could put up a bunch of robots next year, so they'd have a big head start over a manned base. In fact I think this is a great way to build the infrastructure for a manned base, not to mention useful chores such as assaying minerals. ------------------------------ From: Joe Beckenbach > The biggest problem I see is power. A 20-pound robot can't store much > energy relative to its size, at least not with batteries. Why not simply let each be solar-powered? True, a little additional stuff will be necessary to allow the 'bots to turn off at nearly the same time, but that could be done in a small plain area by having each one turn off when the sunshine level drops below a critical value. So think of it as a 50% duty cycle through each month. 1/3 :-) Put up instruments, a couple of small propulsion experiments, a commsat or two, and a batch of robots all on one bus. Put this in orbit around Luna, and then send each item on its way, like blowing dandelion seeds. ------------------------------ From: GOTT@wishep.physics.wisc.edu (George Ott) How large and intelligent does a robot have to be to be useful for setting up a lunar base? Would a semi-intelligent bulldozer do the trick or do we need something with some sort of arms? What are the early tasks that need to be accomplished? Or that could accomplished using "off the shelf robots" or at least robots built from off the shelf parts.... One bobcat One Macintosh Si One satellite dish (for comm.) One SNAP generator for power Three grey scale CCD cameras for seeing ------------------------------ From: henry@zoo.toronto.edu (Henry Spencer) Remember that you need either (a) plenty of power to run heaters, (b) a heated garage, or (c) robots built to soak at cryogenic temperatures during the lunar night. The Surveyor landers tried to survive using small battery-powered heaters in crucial places plus durable hardware; not all of them made it through one lunar night, and I don't think any made it through two. The Lunokhods and the Apollo instrument networks did better, but they had isotope power. If I were trying to use off-the-shelf hardware, I think I'd opt for the heated garage. ------------------------------ From: gwh%ocf.Berkeley.EDU@lilac.berkeley.edu (George Herbert) Besides using a non-space rated brain [a mac SE???] that robot would be using a thirty watt RTG [oy] for power. Do you know how little you can do with thirty watts? ------------------------------ From: GOTT@wishep.physics.wisc.edu (George Ott) Some mention has been made of running the lunar bobcat on solar power. What are the advantages of solar power over radioisotope power in this case? Which has a higher power per lb. launched from Earth? How do the systems compare with regards to reliability? ------------------------------ From: Gord Deinstadt My guess is that you could manage about 100 grams/sq metre for lunar solar cells (thin-film type). This yields on the order of 100 watts/sq metre, or 1 kilowatt/kilogram. I don't know the numbers for radioistope thermal generators, but they are considerably worse. An RTG needs a radiator plus shielding, both of which are relatively massy. Space probe designers use solar cells whenever possible, falling back on RTGs when the probe is going to the outer solar system. Both solar cells and RTGs have a serious problem when used on the Moon; dust! Every moving vehicle is going to stir up dust, and that is going to coat any nearby solar cells or RTG radiators. For the RTG, we can just wipe off the radiator every once in a while. For solar cells, we have to worry about scratching the surface. The obvious solution is to put a hard coating on the solar cells (say, diamond film) but this is unlikely to have the optical properties we need for best efficiency. If we want to use solar cells (which I believe we do, based on power density), we may have to locate them in a suburb far away from the dust of the lunar base. And we'll still have to worry about dust thrown up high by rockets taking off and landing; maybe the only way to escape this is to build a hard-surface pad. Or employ a lot of robots wiping solar panels. Now they're stupid, shivering robots doing menial labour! :-) ------------------------------ From: Vincent.Cate@SAM.CS.CMU.EDU If you go to the North pole of the moon you can get a 6 month day. Solar power will work well even at the low angle because there is no atmosphere. You can drive around for 6 months. Then getting, off to a timed and planned start, you can spend 14 to 28 days migrating south for the winter (curve around as you go south). The moon is small enough, and there are no fences, so far. Net result is that we could drive a remote control bobcat year round. I think of it more as a remote control space buggy than a bobcat, since it will probably be very light and take awhile to push dirt around - but hey, we will have the time. If you have two of these with some remote control arms such that you can use one to work on the other you may even be able to keep them both working for some time. -- Vince ------------------------------ From: "Edward V. Wright" Jerry Pournelle's Lunar Society spent considerable time working on a lunar bulldozer very much like the proposed Bobcat. They called it the Go-cart. It was an ATV-type vehicle with a waldo on the front. It was designed to be teleoperated from Earth: autonomous most of the time, but call for help when you encounter something that you don't understand. As far as migrating from one pole to another, I think you may be overestimating the distance a vehicle can cover, over unexplored terrain, in a day. The Moon is about 6000 miles in circumference, we're talking a 3000-mile trek, so 100 to 200 miles per day. That doesn't sound very fast, but as I recall the proposed Martian rovers cover only a fraction of that distance in a day. You could probably improve on that some, but only at the risk of losing vehicles. (I'd plan on some attrition, anyway. There are a *lot* of holes on the Moon.) It might be better just to land two groups of robots, one at either pole. Operate each group of robots for six months at a time. Or build a tower that's tall enough for its top to be in sunlight all the time. The robots spend 6 months of the year roaming free and 6 months tethered to the base of the tower, doing work around the base camp and getting their power from extension cords. ------------------------------ From: Gord Deinstadt I see a need for three types of robots. 1. A truck (solar powered, see above). 2. A backhoe (more flexible than a bulldozer - used for all kinds of heavy lifting and moving). 3. A "monkey", ie. a small thing with high-dexterity effectors for doing fiddly tasks like repairing other robots. >Personally, I favor the "semi-autonomous" theory of operation, where if the >fairly stupid robot checks in with his mentors whenever he encounters anything >beyond his scope of "reasoning." I hate to see projects stalled because we're too ambitious in our software designs. I don't know what AI is capable of these days, but for Lunar work and a short design cycle I'd even settle for simple remote control, which is why I said the robots could be slow. Semi- autonomous would be better if it is available more or less off-the-shelf. ------------------------------ From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU I'd like to say more about my vision of a Robot Workshop, and along the way make some comments about the discussion so far. My goal is to extend a virtual human presence into an area the size of, say, a large room: the Robot Workshop. In orbit, this might be a way to do Space Station assembly; on the moon, it could be the means of constructing and maintaining a small mineral processing plant or mass driver. I prefer an octopus configuration, with one central power/communications system, plus ten or twenty small tethered robots. This has major implications for keeping each robot simple: they need be no more complex than robots in a lab on Earth, with power and control connections, motors, cables, and little else. Preferably, they would have some decent force or touch sensors to provide enough information to the operators. I think it is essential that each robot NOT be required to have its own power source, radio link, processing unit, etc. As I envision it, the central system has solar cells or an RTG, communications with Earth including several video channels and teleoperation control channels, and hardware to control which of the robots is being used and which video channels to ship back. I see the robots operating in a 95% to 100% teleoperated mode. My reading on the usefulness and flexibility of AI techniques for operating the robots is that for the various tasks you'd run into in a workshop, a skilled human operator can beat AI almost all the time, even with a several-second round trip delay. The only roles I would look for software to play is are to monitor for problems and do an 'emergency freeze' if an exceptional condition occurs, and to provide local control of simple short-duration operations such as sanding or polishing. I'm pretty solid in this conviction, being an AI guy myself. If you CAN teleoperate, you SHOULD teleoperate. What kind of robots would we need? I imagine anchored robot arms, arms on tracks, arms on wheels, arms with snap-on hand attachments such as wrenches and screwdrivers, cameras on arms, cameras on tracks, fully mobile dune buggies with clips for attaching payloads or cords for hauling, arms with three-finger hands, arms with grippers, arms with hooks, toolkits, glue, duct tape... How do we use these most effectively? I suggest a team of very talented operators, each with separate controls, but working in a common command center where they can yell at each other and help each other out. Not all the robots would be operating at once, particularly if the video bandwidth is a severely limiting factor, which I believe it might. There should be mock-ups of the environment so they can work out problems in the mock-up if they need to before trying it in real life. ============= What does my proposal provide? - A simple, flexible way to project human presence into a workshop. It is relatively low-tech and straightforward. - A heterogeneous and flexible system with minimal hardware requirements. - A system able to do simple assembly and repair. ============= On to some specifics people have mentioned: On temperature control: I bet using heaters really isn't that expensive. Isn't a vacuum bottle what you keep things hot in? Maybe we can get some figures from outer-solar-system probes. I figure all we have to do is make sure that the heat flow is good enough that no part gets left in the cold. Maybe lighting the area with some major floodlights would help. And rather than using electric heaters, perhaps small RTG pellets used for pure heat, rather than power generation. On self-reproducing robots: no way, no time soon, and CERTAINLY not with lunar materials! It would be nice to have robots that could do simple repairs on each other, and standard parts would be nice, but that's about it. Heterogeneity is a big goal. I would really like the robots to be able to be developed and operated completely separately, for instance by university projects in several places. Coordination is a BIG hassle, so let's just skip it by making the interface simple - control lines in, video out, and each project can do its own thing. Hedges your bets, too. Gord Deinstadt is definitely right that we should expect things to happen VERY SLOWLY. I think that's OK. There are a lot of hours in a month. On migrating south for the winter: definitely not - you only move across terrain if you have to. Not to mention that my proposed system can't move at all. On RTG's versus solar cells: I think solar wins by a lot, in terms of weight, but RTG's provide power 100% of the time. I don't know which wins: if power isn't the dominant weight requirement, it might be worth getting a 100% duty cycle by using RTG's. I don't mean to ignore suggestions for, say, a mobile, independently powered bulldozer. I think it's a fine idea. However, I think a lot of the power of my proposal comes from the fact that a simple system of video cameras and teleoperated arms is very easy and very useful. Amortizing the cost of the rest of the system -- power, communications -- over a number of cameras and arms makes them a lot cheaper. ------------------------------ From: Tom Neff Two refinements to the Robot Workshop: - You'd want at least two separate power busses and radio circuits available in the workshop (even if both are not constantly in use), so that repairs to a failure in either one can be done using the other, without automatically incurring a human repair call. - Although teleoperation beats AI at Earth-Moon distances for many things, I think it would make a lot of sense to give the robots enough sense to obey "GRASP X", "PUT A ON B", "UNSCREW" and so forth. The feedback loop would tend to interfere with smooth operation otherwise. And it is something we can do, unlike "FIGURE OUT WHAT IS WRONG AND FIX IT AND REPORT BACK" as some of our more eager beavers would go for. - How about five arms and three eyes per station. The teleoperator picks an eye-pair for stereo vision, but can switch pairs at will for a change in perspective without having to move or wait (also an assistant can monitor his work from the side). The five (or whatever) arms include a "bottom arm" with a support platform attached to hold small work, two general purpose manipulator arms and two hold/transport arms. == This concept could work in orbit or on a planetary/satellite surface. In all cases you need a reliable automated way of getting new stuff TO the workshop. In orbit the Progress system works. On the Moon you have other problems. An automated soft landing + breakdown technique, combined with a teleoperated 'drag rover' might work. But this won't be simple to achieve. ------------------------------ [ End of excerpt ]