Date: Tue, 20 Feb 1990 15:49-EST From: space-tech-request@cs.cmu.edu To: "~/st/lists/stdigest" Subject: Space-tech Digest #47 Contents: John Sahr Re: Kevlar loop stability Bob Gray Re: Launch loop, Hula hoop Jim Van Zandt Space Fountain Edward Wright Re: Space Fountain Mike Van Pelt Staging Henry Spencer Re: Staging Edward Wright Re: Staging Edmund Hack SSX Documents? Edward Wright SSX vs Phoenix Dave Rickel SSX fuel Edward Wright Re: SSX fuel ------------------------------------------------------------ Date: Fri, 16 Feb 90 11:56:45 EST From: John Sahr To: space-tech@CS.CMU.EDU Subject: re: Kevlar loop stability From: Bob Munck >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? 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. Although people are building "inherently unstable" planes these days, it is merely "oh, rats" if one tumbles over Nevada. They put several people on a firehose for the very good reason that if the hose got loose, it would wreak havoc. This structure is about the biggest firehose you could imagine. It must stay put if someone trips over the power cord, and, it really ought to survive the loss of one or more spokes. 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 Subject: Re: Launch loop, Hoola hoop To: space-tech Date: Mon, 19 Feb 90 11:00:33 WET "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. Bob. ------------------------------ Posted-From: The MITRE Corp., Bedford, MA X-Alternate-Route: user%node@mbunix.mitre.org To: space-tech@CS.CMU.EDU Subject: Space Fountain Date: Tue, 20 Feb 90 07:57:46 EST From: jrv@sdimax2.mitre.org > 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 > (Marc Ringuette) > - Kevlar slingshot (Edward V. Wright) I'd like to add one scheme to the list: the space fountain. The first description I saw was in "Starquake" by Robert L. Forward (a sequel to "Dragon's Egg"), and it was one of several devices described in "The Barsoom Project" by Pournelle and Barnes (a sequel to "Dream Park"). The space fountain is like a launch loop, but with discrete masses (rings) travelling up and down in the same vertical vacuum tube. There's a vertical accelerator on the ground, periodic stations along the tube to provide support by decelerating the rising rings and/or accelerating the falling ones, and something at the top to turn them around. To me, it seems easier to build than the launch loop. I like the idea of discrete rings. That lets you start with only a few circulating rings and add more as needed and/or divert and stop any that fail. It sounds easier than accelerating a 4000 km loop from rest. Finally, if there's a failure everything comes down in one spot rather than along path several thousand kilometers long. (Forward proposes three independent streams for redundancy. He describes a failure in which the streams get out of alignment and cut up the intermediate stations. Pournelle and Barnes propose to test the concept on Mars where a failure would have less severe consequences.) You couldn't get into orbit directly from an earth-stationary tower unless it was 36000 km tall, which would put you into a geostationary orbit. However, you could put an electromagnetic launcher for payloads inside the same vacuum tube. The payload could be launched into a high trajectory, with a small rocket to give a kick at apogee and perhaps aerobraking to get into LEO. You could reduce the apogee kick by making the tower lean (eastward, of course), although that would make it look _really_ silly. The advantage over a regular EML is that the payload can accelerate in a vacuum. With a length of 356 km and an acceleration of 9 g's (which when added to gravity would feel like 10 g's) you could reach a speed of 8 km/sec. Maybe the fountain wouldn't need to be taller than 20 or 30 km for inanimate payloads - but if they can withstand high accelerations at launch then they could survive an upward passage through the atmosphere as well. ----------------------------------------------------------------- Some thoughts on other topics... > ...the debris-collision issue... (Henry Spencer) > How about SDI lasers on the spokes to zap incoming meteors? (Bob Munck) The problem I see is with the fragments in the size range of about 1-10 cm diameter which are large enough to cause serious damage but are too small to track with ground based radar. The laser itself would have much too narrow a beam for surveillance. > Um, one thing I don't understand about this scheme is how you propose > to maintain the vacuum in a 2000km sheath when this ribbon is moving > in one end and out the other at 14km/s. (Peter Scott) The loop itself will tend to pull along any air inside the sheath. Maybe you could keep at least the upward bound side evacuated by just venting it somewhere above the atmosphere (like station A in Marc Ringuette's figure). > - rotating skyhook Gregory Benford describes one of these in "Tides of Light". > 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. (Lou Adornato) Paul Preuss describes a lunar EML carrying passengers in "Venus Prime - Maelstrom". He proposes a launching acceleration of 10 g's for 24 sec. This would give a final speed of (100 m/sec/sec)(24 sec) = 2400 m/sec, which is escape speed for the moon. The length of the EML would be v**2/2a = at**2/2 = .5(100 m/sec/sec)(24 sec)**2 = 28.8 km. With double the length, you would need only half the acceleration. ----------------------------------------------------------------- If you get the idea I enjoy science fiction, you're right. - Jim Van Zandt ------------------------------ Date: Tue, 20 Feb 90 11:58:45 CST From: "Edward V. Wright" To: jrv@sdimax2.mitre.org, space-tech@CS.CMU.EDU Subject: Re: Space Fountain >> - Kevlar slingshot (Edward V. Wright) Please don't give me credit I don't deserve. -:) The Kevlar slingshot was invented by Dr. Jerry Pournelle and Dr. Marvin Minsky. ------------------------------ 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 ------------------------------ From: henry@zoo.toronto.edu Date: Fri, 16 Feb 90 13:58:07 EST To: space-tech@CS.CMU.EDU Cc: uunet.uu.net!v7fs1!mvp@zoo.toronto.edu Subject: Re: Staging > 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? I'm not sure how the numbers would turn out. Getting above the atmosphere is worth something. Unfortunately, orbital velocity is the big problem area, so it might not help too much. (I.e., you might end up with a three-stage vehicle and have to figure out how to deal with the second stage.) If you can lob things nice and high, it might largely eliminate upper-stage gravity losses (which arise from having to accelerate upward, so that you don't reenter before you attain orbital velocity) as well; that would increase the benefit. Atmospheric and gravity losses together are typically something like 2 km/s -- this is from memory, my references aren't handy right now -- and that's starting to sound useful. Henry Spencer at U of Toronto Zoology uunet!attcan!utzoo!henry henry@zoo.toronto.edu ------------------------------ Date: Fri, 16 Feb 90 13:53:03 CST From: "Edward V. Wright" To: henry@zoo.toronto.edu, space-tech@CS.CMU.EDU Subject: Re: Staging Cc: mvp%v7fs1@uunet.uu.net >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? Probably not. The Phoenix design reference from Pacific American says the vehicle will boost vertically for 20 seconds before pitching to (near) horizontal. Total burn time is 400 seconds. Your booster would have to drop off after the first 20 seconds of flight and, assuming constant throttling, would reduce the orbiter's propellent load by only 5%. When you consider the additional hardware that must be built and the greater operational complexity, I think you'll find this is a poor tradeoff. Your idea is not completely without merit, however. There is on variation that might be tried some day. Mount the launch vehicle, on its side, on a rail-mounted sled. The rails run up the side of a nearby mountain. The sled contains propellents for the vehicle's initial acceleration and climb to the top of the mountain and small retrorockets to slow the sled after the vehicle separates. The vehicle's own aerospike engine is used for the climb. Recovery of the sled is very easy after launch: you just roll it back down the tracks. This type of sled was first described by Robert Heinlein in "The Man who Sold the Moon" and was part of the Hyperion design for a commercial launch vehicle proposed by McDonnell Douglas in the late 1960's. Later designs omitted the sled because it restricted the choice of launch sites. I admit to an emotional bias: I'd like to see a rocket climb the side of Pike's Peak! ------------------------------ Date: Fri, 16 Feb 90 10:05:38 PST From: Edmund Hack X-Vmsmail-To: AMES::"space-tech@cs.cmu.edu" Subject: SSX Documents? To: space-tech@CS.CMU.EDU All the recent discussion about the SSX proposals has me wanting to see the real thing. I think that someone mentioned in Space Digest a while back that he had a copy of a paper on it by one of the originators. If the paper is uncopyrighted (unlikely) or if an address is available to ask for permission to copy, I'd be willing to send out copies at cost to netland readers of space-tech. Cost would be based on how much a copy shop (Kinko's) would charge to repro it, envelopes and postage. Then, I'd send out a copy to anyone who sent me the requisite amount. On a side note, it was said the the SDIO office was going to issue an RFP for the SSX. I haven't seen _any_ rumblings of this in Space News (the new weekly about the space business), and they seem pretty up on what SDIO is doing. Anyone else got firm info on this? Edmund Hack Intelligent Systems Dept. Lockheed ESC hack@lock.span.nasa.gov "It was time to stop playing games. It was time to put on funny hats and eat ice cream. Froggie played his oboe" - Richard Scarry ------------------------------ Date: Mon, 19 Feb 90 13:08:27 CST From: "Edward V. Wright" To: space+@ANDREW.CMU.EDU, space-tech@CS.CMU.EDU Subject: SSX vs Phoenix Following a conversation I had with Max Hunter last week, I would like to correct and clarify some of the information that I posted previously. SSX appears to differ from Pacific American's Phoenix in several important respects. These include: 1) Propellents: Phoenix uses densified liquid hydrogen (hydrogen slush) and LOX. For SSX, Hunter prefers to use liquid methane, although hydrogen has not been completely ruled out. 2) Propulsion: Phoenix uses an aerospike engine concept to increase the performance (specific impulse) of its engines. SSX, contrary to my previous statements does not. Hunter prefers not to use an aerospike because the loss of a single combustor causes flow problems, which result in the loss of specific impulse as well as thrust. This makes the vehicle harder to save in the engine-out case. 3) Structure: The Phoenix airframe is primarily aluminum, with some composite materials used to save weight. Hunter envisions an all-composite airframe and fuel tank. (LOX tank would still be aluminum, due to the inability of most composites to withstand exposure to LOX.) 4) Thermal Protection and Reentry: Phoenix reenters base-first using a water-cooled heat shield. SSX reenters at a high angle-of-attack using an improved version of the quartz-blanket heat shield found on the upper surfaces of the Shuttle. ------------------------------ Date: Mon, 19 Feb 90 17:07:48 PST From: Dave Rickel To: space-tech@CS.CMU.EDU Subject: SSX fuel Sender: mnr@DAISY.LEARNING.CS.CMU.EDU I seem to remember that some of Hudson's SSTO designs weren't exactly SSTO-- they had auxillary fuel tanks clustered around the core. These tanks were dropped when their fuel was expended. These would solve some of the problems of weight gain during development, but you would lose some of the flexibility of where you could launch the things; you'd now have to worry about dropping junk downrange again (i'm guessing that the tanks would be dropped within the first 100 miles or so). I was wondering about the possibility of filling the drop tanks with some fuel other than hydrogen--maybe methane or kerosene. I don't know anything about aerospike engines; maybe it would be possible to have a multi-fueled engine and not pay too much in extra weight or performance losses (i tend to doubt it, though). Anyway, if you could burn multiple fuels, you would probably end up burning both hydrogen (from the internal tank) and methane (or whatever) (from the drop tanks) at startup. I think that the engine design calls for running hydrogen around the engine to cool the engine and to pre-warm the hydrogen for combustion. It seems pretty dangerous to use, say, methane as a coolant, and then switch to hydrogen--too much chance of the methane freezing in the plumbing on switchover. david rickel decwrl!sci!daver ------------------------------ Date: Tue, 20 Feb 90 11:20:58 CST From: "Edward V. Wright" To: gungnir!sci!daver@convex.com, space-tech@CS.CMU.EDU Subject: Re: SSX fuel >I seem to remember that some of Hudson's SSTO designs weren't exactly SSTO-- >they had auxillary fuel tanks clustered around the core. These tanks were >dropped when their fuel was expended. Perhaps you are thinking of Percheron, the expendable booster that was being developed by GCH, Inc. for Space Services. It did drop stages in this manner. But all of Hudson's designs for reuseable vehicles that I have seen, going back to Osiris in the early 1970's, have been true SSTO. ------------------------------ End of Space-tech Digest #47 *******************