Date: Thu, 15 Feb 1990 22:41-EST From: space-tech-request@cs.cmu.edu To: "~/st/lists/stdigest" Subject: Space-tech Digest #46 Contents: Randy Appleton Re: SSX and Propellants George Herbert Re: SSX and Propellants Paul Dietz Re: SSX and Propellants Joe Beckenbach Re: SSX and Propellants Henry Spencer Re: SSX and Propellants Mike Van Pelt Re: SSX & Staging Mike Van Pelt Re: SSX & Staging Bob Munck Earth Hula Hoop / Launch Loop / Ringworld Henry Spencer Re: Earth Hula Hoop / Launch Loop / Ringworld James Smith Re: hula hoop Henry Spencer Re: hula hoop Marc Ringuette Kevlar hula hoop John Sahr Re: Kevlar hula hoop Marc Ringuette Re: Kevlar hula hoop Kevin Ryan Re: Kevlar hula hoop Lou Adornato Re: Kevlar hula hoop Bob Munck Re: Kevlar hula hoop Marc Ringuette Re: Kevlar hula hoop ------------------------------------------------------------ Date: Tue, 13 Feb 90 23:02:18 EST From: Randy Appleton To: gwh@ocf.berkeley.edu, dietz@cs.rochester.edu, van-bc!mindlink!a752@cs.rochester.edu Subject: Re: SSX and Propellants Cc: ewright@convex.com, gwh@bigbang.berkeley.edu, henry@zoo.toronto.edu, space-tech@CS.CMU.EDU What exactly was RP-1, and why would anyone choose it over Kerosene? Thanks Randy ------------------------------ Date: Tue, 13 Feb 90 20:44:44 PST From: gwh@ocf.berkeley.edu To: dietz@cs.rochester.edu, gwh@ocf.berkeley.edu, randy@ms.uky.edu, van-bc!mindlink!a752@cs.rochester.edu Subject: Re: SSX and Propellants Cc: ewright@convex.com, gwh@bigbang.berkeley.edu, henry@zoo.toronto.edu, space-tech@CS.CMU.EDU RP-1 is a highly refined kerosene derrivative. Among other things it's optomized for rocket use. ------------------------------ To: Randy Appleton Cc: space-tech@CS.CMU.EDU Subject: Re: SSX and Propellants Date: Wed, 14 Feb 90 11:32:11 -0500 From: dietz@cs.rochester.edu > What exactly was RP-1, and why would anyone choose it over Kerosene? ^^^ RP-1 is still used, I believe, in the Atlas and the first stage of the Delta rockets. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ To: space-tech@CS.CMU.EDU Subject: Re: SSX and Propellants Reply-To: jerbil@csvax.caltech.edu Date: Tue, 13 Feb 90 15:27:31 -0800 From: Joe Beckenbach > A vehicle like SSX typically has a burn time of about 400 secs. > A very rough calculation (assuming constant horizontal accleleration) > shows that the vehicle enters orbit about 940 miles downrange. > So if you launch from Edwards AFB, the ascent will never be over > the densely populated East Coast of the United States. ... which would, going east and somewhat north, put you in orbit above Denver. :-) Deorbiting immediately and tracking "ballistically" still brings the craft down somewhere near the Mississippi River. I have little clue as to where a proper great circle with the launch site as northernmost ground-track point would place these -- I think (again eyeballing a mentally visualized Earth) puts it above north-central Mexico and down again along the eastern coast of Mexico. A completely 'vertical' launch would lay the ground-track due east, to orbit over northern New Mexico and come down in the Ozarks (Arkansas) or eastern Texas. By waiting an orbit or so, you can choose anything along your track. > it can land in any flat area the size of a parking lot. [Just don't pull any funny stuff over Europe; Russia was embarassed enough having a _Cessna_ park uninvited in Moscow.... :-] Joe Beckenbach --- Joe Beckenbach jerbil@csvax.caltech.edu (818) 356-6767 ------------------------------ From: henry@zoo.toronto.edu Date: Wed, 14 Feb 90 13:14:06 EST To: DAISY.LEARNING.CS.CMU.EDU!mnr@zoo.toronto.edu, space-tech@CS.CMU.EDU Subject: Re: SSX and Propellants > The problem with using Florida is the weather. Also, if you > abort and can't make it back to the launch site, you end up > in the drink. Indeed, if you look at the proposed test tracks for the NASP -- a large vehicle full of liquid hydrogen, probably just as big a crash hazard as an SSX -- they work very hard to stay over land as much as possible, to avoid the chance of losing the thing in the event of an emergency landing. North America isn't really quite big enough to test the thing, which is somewhat of an inconvenience! :-) The *low-speed* test route starts from Edwards and does a 180 around KSC to go back. The medium-speed route does a 90-degree turn around KSC instead, and another turn around Maine. The high-speed one accelerates all the way to KSC and then cranks in a left turn, swinging out in an enormous semicircle over Bermuda that hits land again over eastern Canada -- it's still reasonably safe because the bird is never outside of gliding range of land. You can glide a long way when you start out at Mach 20 or so -- the normal engine shutdown on that track is somewhere around Michigan, for a landing at Edwards! Henry Spencer at U of Toronto Zoology uunet!attcan!utzoo!henry henry@zoo.toronto.edu ------------------------------ Date: Wed, 14 Feb 90 10:50:49 PST From: Mike Van Pelt To: space-tech@CS.CMU.EDU Subject: Re: SSX & Staging >> Why should a vehicle like SSX be single stage to orbit? >> >> If you can make an SSTO land vertically on rockets, you >> could also make a first stage do the same thing... >Possible, but significantly more cumbersome. The real problem with >recovering spent stages is that they cut loose quite a ways downrange >and are moving the wrong way at high speed and high altitude. Unless your first stage's "downrange" extends back to the launch site, like an "Abort Once Around" trajectory. I don't know what the economics of this would be as far as fuel goes; you're taking the first stage almost into orbit. If you had nicely distributed spaceports around the globe, you could fly your first stage from one to the other. This only works if there is roughly equal traffic from all the spaceports. ------------------------------ To: space-tech@ames!cs.cmu.edu From: Mike Van Pelt Subject: Staging Date: 16 Feb 90 00:31:12 GMT Reply-To: Mike Van Pelt Suppose your first stage simply goes straight up, and carries the second stage above the atmosphere? For recovery, it would just come straight back down to the launch site. It doesn't help the second stages get to orbital velocity, but at least it doesn't have to force its way through the atmosphere. Would this provide enough advantage to make it worthwhile? -- Mike Van Pelt "Beware the first release, my son, Headland Technology/Video 7 and shun the frumious 1.0" ...ames!vsi1!v7fs1!mvp ------------------------------ [[ It wouldn't help much. What you really need is a whole lot of velocity. --Marc ]] ------------------------------ File-Id: Munck.Office-386.space.4871 To: Space Technology Reply-To: munck@mwunix.mitre.org Return-Receipt-To: munck@mwunix.mitre.org Subject: Earth Hula Hoop / Launch Loop / Ringworld Date: Wed, 14 Feb 90 15:03:09 -0500 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: henry@zoo.toronto.edu Date: Wed, 14 Feb 90 16:57:35 EST To: munck@mwunix.mitre.org Cc: space-tech@CS.CMU.EDU Subject: Re: Earth Hula Hoop / Launch Loop / Ringworld > 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.) ... Actually, the hoop doesn't even have to be a solid body; it can be a pellet stream. In fact, this may be preferable, because one big problem with any large structure in orbit is space-debris collisions. (Some folks looking at using tethers as launch aids, for example, plan triple-redundant tethers with cross-links and repair mechanisms.) > ... Are the tethers beyond our current strength-of-materials > capabilities?... Is the whole idea of holding up the stationary facilities > crazy? ... If you can build the hoop and keep it running, there isn't anything fundamentally wrong. It's been proposed before; check back issues of JBIS for some fairly similar notions. The launch mass is a problem, as is the debris-collision issue. Henry Spencer at U of Toronto Zoology uunet!attcan!utzoo!henry henry@zoo.toronto.edu ------------------------------ Date: Wed, 14 Feb 90 19:15:26 -0500 From: 6079 Smith J To: space-tech@CS.CMU.EDU Subject: Re: hula hoop Henry said that a pellet stream is preferable to a solid hoop due to space-debris collisions. Isn't a hoop preferable for that very reason? Picture a few pellets, moving at major velocity, getting knocked just slightly off course and plowing right into one of the stations... | James W. Smith, University of Arkansas | jws3@uafhcx.uark.edu | | There's a long, hard road and a full, hard drive | | And a sector there where I feel alive | | Neither NASA nor the U of Ark. is responsible for what I say. Mea culpa. | ------------------------------ From: henry@zoo.toronto.edu Date: Thu, 15 Feb 90 12:25:04 EST To: space-tech@CS.CMU.EDU Cc: UAFHCX.UARK.EDU!jws3@zoo.toronto.edu Subject: Re: hula hoop > Henry said that a pellet stream is preferable to a solid hoop due to > space-debris collisions. > Isn't a hoop preferable for that very reason? Picture a few pellets, moving > at major velocity, getting knocked just slightly off course and plowing > right into one of the stations... Picture a hoop, cut by debris, plowing right into one of the stations. We're not necessarily talking about small stuff here; the debris can and does get quite large. For that matter, the stations are going to have to be armored against it anyway, so a pellet or two is not going to be that significant. The significance of using pellets is that the damage from a debris collision is localized. Henry Spencer at U of Toronto Zoology uunet!attcan!utzoo!henry henry@zoo.toronto.edu ------------------------------ Date: Wed, 14 Feb 1990 21:27-EST From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: Kevlar hula hoop Bob - I'm starting to like this a lot! Henry - you missed the point, I think. Bob is suggesting 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 OK -- even with standard Kevlar like you'd use to anchor an oil rig, you can spin it to 1 km/s above orbital velocity, which is probably enough. 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 of a loop with 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 2 M v 2 pi r c Y v 2 F = -------- = -------------- = c Y v t 2 pi r 2 pi r 2 F = X c = c Y v t max max v = SQRT( X / Y ) 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 = 1.4 km/s (1.0 km/s with 2x safety factor) max [[[[ Note -- see following posts for a correction --Marc ]]]] ============== ============== ============== ============== ============== This design has the advantage that we need only one station, plus some guy cables spaced around the equator. Assuming we choose a 100T total cable tension and a 2x safety factor, the cable is 7 square cm in cross section, and weighs 1 kg per meter. The total weight of the cable is about 40,000T. Ouch. How big a station can it support? Each 100km of the cable weighs about 100T. Spinning at 1 km/s, which is 1/8 of orbital velocity, a 100km segment of cable should be able to support about 1/8 of its mass, 12.5T, if the tension in that 100km segment is relieved and the deflection force used to hold up a station. If we assume that we can utilize most of the support available from, say, a 100-1000km length of the cable, then this cable size and speed could support a station weighing maybe 10-100T, and perhaps launch 5-10T vehicles. If we have higher strength than industrial-grade Kevlar cable available, then the mass of the cable can be reduced for the same functionality, and perhaps the cable velocity could be bumped up to escape velocity, to support planetary missions. With these latest guesses, I've been going pretty far out on a limb, but this combined tension/magnetic launch system probably qualifies as another on my list of conceivable launch schemes: - chemically propelled rocket/spaceplane - laser propelled rocket - nuclear-bomb propelled rocket - EM launcher - chemically propelled gun launcher - fixed skyhook - rotating skyhook - launch loop - tension/magnetic kevlar hula hoop Does anybody want to take a crack at some of the dynamics or other properties of this system? /////////////////////////////////////////////////////////////////////// /// Marc Ringuette /// Carnegie Mellon University, Comp. Sci. Dept. /// /// mnr@cs.cmu.edu /// Pittsburgh, PA 15213. Phone 412-268-3728(w) /// /////////////////////////////////////////////////////////////////////// ------------------------------ Date: Thu, 15 Feb 90 00:11:51 EST From: John Sahr To: space-tech@CS.CMU.EDU Subject: re: Kevlar hula hoop 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. Marc calculates the tension in a spinning loop (like a riata), and arrives at 2 M v F = ------ , t 2 pi r M is the total loop mass, v is the loop speed, and r is the loop radius. which is correct for a loop in the absence of gravity. If I understand correctly, the Earth (or another planet or moon) would be centered in the loop, which modifies the formula to 2 M | v | F = ---- | --- - G(r) | , t 2 pi | r | where G(r) is the acceleration of gravity at the radius of the loop. All this says is that at orbital velocity, the loop tension is zero, sort of what you'd expect. Marc's loop velocity of 1.4 km/sec, given some design specs for Kevlar material, would fall down in low Earth orbit. However, the spirit of the calculation was correct. Correcting for gravity, the loop speed (for the same tension) is about 8.3 km/sec (which is faster than the orbital speed by about 200 m/s). Because Marc supplied the mass (lineal) density and the tension, we can calculate the speed of the transverse waves along the loop, namely v = sqrt(T/rho), ( rho = M/(2 pi r) ) using T = 100 tonnes = 10e6 N, and rho = 1kg/m, we get a wave speed of v = 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 ------------------------------ Date: Thu, 15 Feb 1990 02:00-EST From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: re: Kevlar hula hoop Hey, spiffy job, John. Thanks for catching my (major) error. Just a quick shift into a rotating reference frame... 8^) The revised equation for the maximum velocity of the hoop should be 2 V = SQRT( X / Y + V ) where V = orbital velocity, ~ 8 km/s max o o X = Tensile strength Y = Density With strength/density for Kevlar being 2*10^6 with no safety factor, Vmax works out to 120 m/s over orbital velocity. Useless. But with a material 10-50x stronger than Kevlar, we can improve that to 1-3 km/s over orbital velocity. That kind of strength probably isn't out of the question, so we're still in the ball game so far. Then the question remains: how to make the thing stable? Your 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? \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ \\\ Marc Ringuette \\\ Carnegie Mellon University, Comp. Sci. Dept. \\\ \\\ mnr@cs.cmu.edu \\\ Pittsburgh, PA 15213. Phone 412-268-3728(w) \\\ \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ ------------------------------ Date: Thu, 15 Feb 90 10:16 EST From: KEVIN@A.CFR.CMU.EDU Subject: Re: Earth Hula Hoop / Launch Loop / Ringworld To: space-tech@CS.CMU.EDU X-Envelope-to: space-tech@CS.CMU.EDU X-VMS-To: CMCCVB::IN%"space-tech@cs.cmu.edu",KEVIN I don't have my books here (here being at work), but I recall that Larry Niven discussed the stability problems of ringworlds in one of his anthologies. A ringworld consists of a ring-shaped structure, a ribbon something like 10^5 km wide, with 10^3 km high edges, spinning around a sun with a ring radius of 10^8 km fast enough to provide 1G. Silly S-F stuff, as the ring must spin at ~770 km/sec, have more material than our solar system, and a tensile strength near the binding force of atomic nuclei. Look it up in _Ringworld_, by Niven, if you're interested in the details. At any rate, there seem to be problems with a spin-stiffened ring shaped structure around a gravity well. Apparently some MIT folks calculated that it would be unstable, and tend to drift off center until it hit the source of the gravity well. Active stabilization would be required. I'm uncertain how actual spin rate affects this, but I would _guess_ that a stiff ring shaped object, constrained to move at the same velocity at all points in its orbit, would be positionally unstable. I'll try to look up the reference, but in the meantime could anyone who has the necessary math think about it? ................................................................................ internet kr0u@andrew.cmu.edu / bitnet kevin%cmcfra@cmccvb.bitnet ____ _/ decnet {cu20b, nyu20, or vassar}::cmcfra::kevin / /| 0\ phonenet 412-268-3461 or 412-371-3681 /___/-^---[ US mail 7235 Penn #4B, Pgh, PA 15208 . . . . _/ | \_ ................................................................................ ------------------------------ Date: Thu, 15 Feb 90 09:32:27 CST From: Lou Adornato To: space-tech@CS.CMU.EDU Subject: Kevlar hula hoop One additional refinement on the hula hoop idea - send out several robot missions to capture some of the closer (high iron) asteroids and move them into Earth orbit. A solar mirror furnace could be used to produce the iron pellets in orbit (kind of a sloppy version of sputtering deposition), so that the total mass to be boosted decreases significantly. I would think that an ion engine would be the likely choice for thrust on the return leg, as it can be counted on to be stable at the end of the outbound leg and greatly reduces the boost weight. Other problems to be considered would be attitude control, guidance and navigation, the problem of locating an asteroid and docking with it, and some means of anchoring the engine platform to the asteroid. These are left as an exercise for the reader (I've _always_ wanted to say that!). Lou Adornato | Statements herein do not represent the opinions or attitudes Cray Research | of Cray Research, Inc. or its subsidiaries. lfa@cray.com | (...yet) ------------------------------ File-Id: Munck.Office-386.space.4889a To: Space Technology Reply-To: munck@mwunix.mitre.org Return-Receipt-To: munck@mwunix.mitre.org Subject: re: Kevlar hula hoop Date: Thu, 15 Feb 90 15:43:20 -0500 From: Bob Munck THIS IS GREAT! Sometimes the Internet is really magic (or a technology sufficiently-advanced to appear so). Send out a half-baked idea and it gets fully-baked or incinerated as it deserves by a bunch of bright minds all over the place. I think the Kevlar hula hoop idea hasn't yet been incinerated. Major hot-spots so far: 1. Too heavy. 4E4 tonnes in orbit = 2000 shuttle payloads (Jordin Kare). Hum, I dis-remembered 100 T for a shuttle payload. Maybe that's a Soviet BDB payload. (I've sent a memo to VP Quail suggesting that we trade our farm surplus for the Soviet space effort in its entirety. I used simple words.) Lou Adornato suggested going out to an asteroid for the iron, but I think that's really overkill for only 4E4 T or so, and we still need the Kevlar. HOWEVER, both are pretty cheap here on Earth and very rugged. How about just shooting iron-cored Kelvar baseballs with a linear accelerator or rail gun to LEO? That should be fairly inexpensive, and the on-orbit facility to spin them into the Hoop would probably be a single shuttle payload. 2. Unstable in the plane of its orbit (John Sahr) (VERY impressive calculating!) 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? Or would wind pressure on the spoke and its own natural oscillations just make things worse? Marc, what does this do to your strain calculations? My thought is that alternate spokes could carry power and payloads. I'd like to see payloads of at least 1T, figuring that to be a reasonable manned carrier. 3. Too slow with current materials (Marc Ringuette) "with no safety factor, Vmax works out to 120 m/s over orbital velocity. Useless." When I first read that, I thought "Shucks, it's no good." However, I've decided I don't know why Marc said "Useless." (Admittedly, I didn't follow his calculations of station/payload size in his first message.) Even with just a few tens of meters/second over orbital velocity, it's still passing the spokes at 8 km/s, and (I think) can still hold them up. After all, the force that's straining the Kevlar to its limit is also the force that holds up the stations. Just because the hoop is moving at a leisurely 55 mph faster than something in orbit at the same altitude doesn't mean it's too slow. Does it? A payload leaving the top of a spoke would "grab" the Hoop just hard enough to accelerate at about 1.5 gee. That would get it to orbital speed well before the next spoke 2500 km downstream. (Using the same acceleration to get up the spoke and we're in orbit in 15 minutes, gently enough that I should be able to take it for another 30 years, until I'm 75. That's my outside limit on getting this thing built.) 4. Frequent severing by debris (Henry Spencer, Jordin Kare). Multiple strands with cross-links? The iron on the outside, maybe with a layer of foamed iron? With it going so slowly relative to nearby orbits, repair robots could hang around until needed to fix a (non-severing) cut. Of course, they'd have only about 5 minutes on average until the next spoke comes around. The Hoop could "hide" below significant amounts of atmosphere before getting so low that friction is a problem. Suppose it's only 30 km up; that might be high enough that friction doesn't kill it and low enough that most debris doesn't get to it. Of course, nothing could be in free orbit nearby any more, and getting it started is tougher. But spokes weigh less. How about SDI lasers on the spokes to zap incoming meteors? Or Henry's pellets made "brilliant" with guidance electronics and steering jets? Spokes every 2500 km makes the pellet stream easier. Thanks to everyone who responded! I hope the idea has stayed interesting through the discussions so far. There are still "real-world" factors that nobody has considered, such as cable stretch, power requirements and transmission up the spokes, how the power stations can keep the Hoop up to speed, how to move payloads up the spokes, wind shear, finding sites to anchor the spokes, etc. etc. But it still looks possible!! -- Bob Munck, MITRE ------------------------------ Date: Thu, 15 Feb 1990 18:11-EST From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: Re: Kevlar hula hoop Kevin: the ring is indeed gravitationally unstable, but the magnitude of the forces involved are tiny compared with the other factors, such as the weight of the station and waves in the cable. We would almost certainly need spokes (cables to the ground, hanging magnetically off the cable) periodically around the equator. Bob - you forgot something about what happens if the hoop moves too slowly. As a launch vehicle approaches orbital velocity, it nearly catches up with the hoop. It would take too long to accelerate that last bit. True, you could accelerate to 90% of orbital velocity and then use a rocket, but that's no fun. =============== I think 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. \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ \\\ Marc Ringuette \\\ Carnegie Mellon University, Comp. Sci. Dept. \\\___ /// mnr@cs.cmu.edu /// Pittsburgh, PA 15213. Phone 412-268-3728(w) /// /////////////////////////////////////////////////////////////////////// ------------------------------ End of Space-tech Digest #46 *******************