Date: Thu, 14 Sep 1989 18:54-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: Subject: Space-tech Digest #33 Contents: Henry Spencer Re: Mars mission, space elevators Tom Neff Re: Mars missions Tero Siili Re: Mars missions Kevin Ryan Re: Mars missions Ralph Marshall Re: Mars missions Marc Ringuette Re: Mars missions Roger Arnold Re: Mars missions Kevin Ryan Re: Mars missions Matthew Francey Re: Mars missions John Roberts Re: Mars missions David Rickel Re: Mars missions Paul Dietz Re: MPD Thrusters ------------------------------------------------------------ From: henry@utzoo.uucp To: uunet!cmr.ncsl.nist.gov!roberts@cs.toronto.edu Cc: cs.cmu.edu!space-tech@cs.toronto.edu Subject: RE: Mars mission, space elevators Date: Tue, 12 Sep 89 12:45:34 EDT > ... If the mass of material going up the elevator and the mass > coming down did not balance, the elevator would tend to wrap around the earth, > If you could use multiple tethers or tethers and compressive components to > anchor the elevator rigidly to the earth, then material moving up or down > the elevator would merely slow down or speed up the rotation of the earth. The classical space elevator has a nice heavy counterweight at the top, a little beyond Clarke orbit, to put the whole thing in tension. This essentially amounts to anchoring the thing rigidly to the Earth. A mass moving up will tend to decelerate the tether slightly, but this will induce side forces on the counterweight and the anchor point until the rotation of the Earth-tether-counterweight system has slowed to match. The structural and resonance properties need to be investigated carefully, but the tether remains vertical on average. Non-anchored variants, like the "rolling tether" schemes, need to balance upward and downward flows of mass, use some similar trick, or have high- exhaust-velocity-low-thrust rocket engines to maintain orbit. Henry Spencer at U of Toronto Zoology uunet!attcan!utzoo!henry henry@zoo.toronto.edu ------------------------------ From: tneff%bfmny0@uunet.UU.NET (Tom Neff) Date: Mon, 11 Sep 89 21:51:36 EDT X-Mailer: Mail User's Shell (6.5.6 6/30/89) To: SPACE-TECH Mailing List Subject: Re: Mars missions Clearly the artificial gravity afforded by a spinning tethered-capsule spacecraft becomes more Coriolis-free and thus more useful as the length of the tether increases. Assume we perfect a really long, light, strong line, so that we can catscradle several of them into an effective tethering system a mile or more in length, for a nice slow spin of a sizable capsule. In this case it seems unwise to put symmetrical habitable modules on each end of the tether as suggested by some correspondents. You would have two isolated houses with arduous transportation between them. It seems better to use a heavier counterweight at one end and the habitable capsule at the other, for an asymmetrical COM on the tether. Then everyone is together. What goes in the counterweight? The reactor, for instance. That way the core might be jettisoned in case of danger, though I don't know how practical or necessary this would be. What goes at the 0g center? Probably a whole orthogonal axis of stuff, including the main engines, the landers, science experiments etc. plus a small intermittently-inhabited research module. How do you adjust for mass moving up and down (even deck to deck) in the habitable module without causing oscillation? Hang an additional, smaller counterweight outboard of the manned capsule and reel it in and out continually under computer control in response to mass distribution changes in the system as a whole. -- Tom Neff UUCP: ...!uunet!bfmny0!tneff "Truisms aren't everything." Internet: tneff@bfmny0.UU.NET ------------------------------ Date: Tue, 12 Sep 89 11:33 EET From: Tero Siili Subject: Mars mission scenarios To: space-tech@cs.cmu.edu X-Vms-To: IN%"space-tech@cs.cmu.edu" I don't want to spoil anybody's enthusiasm in brainstorming for a Mars mission; however, some people at JPL have been thinking about these things and I assume, that a report has also been published. A rotating spacecraft has been proposed, but looking like three spokes; the spokes are tunnels, NOT tethers (and with three spokes tethers would be complicated, if not impossible). When contemplating artificial gravity solutions, one must keep in mind, that - the positive or preventive effects of artificial gravity have not been demonstrated yet - we don't know, what level of AG - g/2, g/3, g/6 - would be sufficient to prevent adverse effects - tests have indicated, that humans do not feel comfortable, if the rotation rate exceeds approximately 2 rpm; this together with the required gravity level will dictate the diameter of a rotating spacecraft. The open questions must be studied carefully before a humanity embarks on a Mars mission. For these studies a special laboratory will be needed, a Variable Gravity Research Facility (VGRF). This problem has quite recently been studied by the students of the International Space University in Strasbourg, France this summer. A report will most probably be out by January 1990. To give you an idea of the costs, this prerequisite for the Mars mission would cost 30 000 M USD! Concerning radiation, some type of "safe haven" is probably mandatory. If the spacecraft is designed with the spoke concept, every spoke can have its own shelter. Tero Siili Finnish Meteorological Institute / GEO P.O. Box 503 SF-00101 Helsinki Finland ------------------------------ Date: Tue, 12 Sep 89 10:58 EDT From: KEVIN@A.CFR.CMU.EDU Subject: Mars Missions To: space-tech@CS.CMU.EDU X-VMS-To: CMCCVB::IN%"space-tech@cs.cmu.edu" I've seen one or two of the seriously proposed mars mission ships in model or drawing form. The one I remember best was a NERVA design - central spindle of rotating ship (along axis of rotation), reactor on aft end behind a shadow shield, with the spindle forming a diagonal of a (roughly) square and flat shape. The bow and stern of the spindle were two corners, two partial G pods formed the others, with tether/strut assemblies forming the edges. Landers and other mass were on the spindle in low-G positions, and part of the area of the square was filled with radiator panels suspended from the spindle to provide cooling for the reactor. I don't recall any despun sections. Passage to and from the two crew pods was through shafts going directly to the spindle. I suspect that a present day ship would use something other than NERVA, such as ion or MPD drive, but the design should still hold pretty well. This model was named PILGRIM. I found out about it years ago through an article by J. E. Pournelle (Revell?), but it's been out of production for entirely too long, and I couldn't get one for myself (sigh). Kevin Ryan kr0u@andrew.cmu.edu ------------------------------ Posted-From: The MITRE Corp., Bedford, MA X-Alternate-Route: user%node@mbunix.mitre.org Posted-Date: Tue, 12 Sep 89 09:27:37 edt Date: Tue, 12 Sep 89 09:27:37 edt From: marsh@linus.MITRE.ORG To: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU, space-tech@cs.cmu.edu Subject: Re: Mars missions I have a suggestion for applying thrust to your set of rotating pods. Rather than have some incredibly complex software system that tries to make it all work without introducing oscillations, why not change the problem? Winch the two pods into the center of the cable so that they form a solid mass. The thing is going to be whipping around at an amazing speed, but if you put the astronauts in a non-rotating ball at the center for the duration of the thrust you should be able to withstand it. Once the thrust application is over, just let the pods go out the ends of the Kevlar thread again. Ralph Marshall marsh@darwin.mitre.org ------------------------------ Date: Tue, 12 Sep 1989 14:15-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: Re: Mars missions marsh@linus.MITRE.ORG (Ralph Marshall) writes: > ...Winch the two pods into the center of the cable > so that they form a solid mass. The thing is going to be whipping around > at an amazing speed, but if you put the astronauts in a non-rotating > ball at the center for the duration of the thrust you should be able > to withstand it. I don't think your idea is practical. The structural strength of the capsules would have to be very high. This conflicts with the the strong desire not to drastically increase any weight requirement. One of the reasons that a spinning system at 1 g can be considered is that the capsules will be strong enough to withstand atmospheric pressure and probably a multi-gee launch anyway. But withstanding more than maybe 5g would require stronger, heavier structure, and the cost would rise. I wonder if we can keep the G's to 5, by having a couple of "barbells" stick out farther and endure the greater accelerations, but still be stiff enough to allow out-of-axis accelerations.... OOH BABY! Got it! Use tension to maintain stiffness. Check this out: have a triangular arrangement of capsules with cables at high tension connecting each to the other two, stiffening the structure: ------------------------------- \ --_ _-- / \ --_ _-- / \ --_ _-- / \ --_ _-- / \ X / \ | / \ | / \ | / \ | / \ | / \ | / \ | / \ | / \|/ Accelerations off the axis just increase tension on some of the cables. This has the really great feature that all stiffening is done by tension. This allows the use of lightweight cables rather than stiff structural materials which would undoubtedly be much heavier. This reminds me of Buckminster Fuller...I wonder if there's a more efficient arrangement than the one I describe? The parameters can vary, and this may change the configuration. Let's hold the 1 g radial acceleration fixed. If a low-thrust system is used, side acceleration may be limited to, say, .01 g; so the cables at the "edges" of the triangles above needn't be very large. If a high-thrust chemical system is used, it may be impractical to keep the system spinning at all; or maybe some sort of weird pentagon will have sufficient rigidity to handle off-axis accelerations. Hmm. I haven't worked out any numbers, but I'm pretty happy with this triangle deal for low accelerations. Fun! [ Aside: I hope we can discuss some other aspects of a mars mission design: - Is laser communications practical? How much dispersion is there, and if it is low, how do you aim the lasers? - What kind of power systems are good for this kind of mission Could we use a rotation-stiffened mylar mirror to magnify sunlight to solar cells or a thermionic generator? - How do MPD thrusters work? - How many exercise bikes should you bring along? Can you think of anything else? -- Marc ] ---------------------------------------------------------------------------- | Marc Ringuette | mnr@cs.cmu.edu | Never lick a gift | | Carnegie Mellon Comp. Sci. | 412-268-3728(w) | horse in the mouth. | | Pittsburgh, PA 15213 | 412-681-5408(h) | | ---------------------------------------------------------------------------- ------------------------------ From: telesoft!roger@ucsd.edu (Roger Arnold @prodigal) Date: Tue, 12 Sep 89 10:36:41 PDT To: UNHH!K_MACART.BITNET@VMA.CC.CMU.EDU Subject: Re: Mars mission Cc: space-tech@cs.cmu.edu Regarding the "rotating dumbell" (capsules joined by tether) vs. a rotating cylinder: the issue is rotation rate. NASA studies (using rotating rooms) indicate that the rotation rate must be kept below 3 RPM, and preferably down to about 1 RPM, to avoid inducing motion sickness. To get one gravity of acceleration at 2 RPM (probably tolerable for a selected crew), elementary physics tells you that you need a radius of about 250 meters. If you're talking a rotating cylinder with a radius of 250 meters, you're talking a major space colony, not a Mars ship. Strength of the tether, or likelyhood of its failure, is not an issue. Even with a 5:1 safety margin, the tether would represent less than 10% of the mass of the ship. And even if it broke, the relative velocity of the two capsules would be only 100 mps. It wouldn't be hard to provide enough storable onboard maneuvering fuel for either capsule to rendezvous with the other after such an event. - Roger Arnold ..ucsd!telesoft!roger ------------------------------ Date: Wed, 13 Sep 89 14:31 EDT From: KEVIN@A.CFR.CMU.EDU Subject: Mars mission To: space-tech@CS.CMU.EDU X-VMS-To: CMCCVB::IN%"space-tech@cs.cmu.edu" In regards to manuvering a rotating spaceship... From the details on the strength of Kevlar cabling, supporting the pods on a rotating spaceship should be relatively easy, with the cabling amounting to ~1-2% of the mass of the pods. Note that the pods are not likely to contain more than crew/working quarters, with the majority of reaction mass and extras on the spine of the ship. Some folks have expressed concern about the ability to turn the ship, for manuvering purposes. Given a sufficently stiff structure minor precessing of the rotating ship should be doable. (Note that this is a low-thrust propulsion scheme we're talking about here.) The structure can be stiffened by, for example, adding the proper combination of tension and compression elements. Take a ship with a long spine, and two pods rotating attached to the middle. Add two struts at right angles to both (if pods are x-axis, and spine is y-axis, put struts in z-axis). Run taut cables from pods and tips of struts to tips of spine, and more cables from tips of struts to the pods themselves. You now have cables tracing out the edges of an octahedron. Note that the struts do not have to extend as far out as the pods to do a good job of stiffening the ship. Very stiff, very lightweight, and the edge cables need only be strong enough to handle precessional forces under normal manuvering. For major manuvering, such as flipping the ship to reverse thrust, I suspect the method of choice would be to despin the ship, but that's only my guess. You would certainly want this capacity for launching landers and such (and especially for docking them). I just wanted to give an idea of how you could make a lightweight but stiff ship design without using a lot of compression elements or a full torus. Comments appreciated. Kevin Ryan kr0u@andrew.cmu.edu ------------------------------ To: space-tech@cs.cmu.edu Subject: Re: Mars Missions Date: Tue, 12 Sep 89 18:35:38 EDT From: mdf@ziebmef.mef.org In-reply-to Tom Neff's message of Mon, 11 Sep 89 18:30:18 EDT: > It does seem to me that midcourse corrections in a tethered system > would be potential nightmares. The oscillations you could set up > are mind boggling. Some sort of computer coordinated thrust from > all components would be essential. Why not just de-spin the system during the thrusting? An extra 100m/s delta-v for a planned number of despins may be easier to handle than a complicated (read: perhaps error prone) thruster system. -- Name: Matthew Francey Address: N43o34'13.5" W79o34'33.3" 86m mdf@ziebmef.mef.org uunet!{utgpu!moore,attcan!telly}!ziebmef!mdf ------------------------------ Date: Wed, 13 Sep 89 21:18:14 EDT From: John Roberts Disclaimer: Opinions expressed are those of the sender and do not reflect NIST policy or agreement. To: space-tech@cs.cmu.edu Subject: Re: Mars mission >From: Marc.Ringuette@daisy.learning.cs.cmu.edu >Subject: Re: Mars missions >OOH BABY! Got it! Use tension to maintain stiffness. Check this out: have >a triangular arrangement of capsules with cables at high tension connecting >each to the other two, stiffening the structure: > ------------------------------- > \ --_ _-- / > \ --_ _-- / > \ --_ _-- / > \ --_ _-- / > \ X / > \ | / > \ | / > \ | / > \ | / > \ | / > \ | / > \ | / > \ | / > \|/ Of course, the outer cables would be bowed out, by an amount depending on the relative mass of the pods and the cables. Some sort of control system would be needed if you wanted to keep all the cables taut. Do the inner cables still serve a useful purpose? The arrangement and interconnection of the cables can be arbitrarily complex, up to and including a disk of fabric (!) >[ Aside: I hope we can discuss some other aspects of a mars mission design: > - Is laser communications practical? How much dispersion is there, and > if it is low, how do you aim the lasers? Lasers would be a "luxury item" (i.e. for high data rate), since radio has been demonstrated to work well at this distance. I think that with the current state of the art, while laser beams disperse much less than radio beams, it is also *much* harder to amplify the received signal, and the signal to noise ratio is much worse. According to a July 17 posting by Peter Yee on the SPACE digest, it is possible using large astronomical instruments (projecting backwards through a large telescope) to put a beam on the moon about 1 mile in diameter. I think one can assume a fairly linear expansion beyond that point. > - What kind of power systems are good for this kind of mission Could we > use a rotation-stiffened mylar mirror to magnify sunlight to solar cells > or a thermionic generator? I think solar power would still be usable in space at the orbit of Mars (solar intensity ~40% of that at Earth orbit). As has been previously discussed, it would probably not be practical on the surface of Mars, at least with current systems. John Roberts roberts@cmr.ncsl.nist.gov ------------------------------ [ The inner cables could still hold most of the weight. Kevlar cables can stretch out pretty straignt in 1 gravity! That means the outside cables can be a fraction of the size of the radial ones. // Marc ] ------------------------------ Date: Thu, 14 Sep 89 01:37:27 PDT From: sci!daver%gungnir@Sun.COM (Dave Rickel) To: space-tech@cs.cmu.edu Subject: Mars mission It seems that .38 g's would be a better gravity for the centrifuge, to avoid adapting problems while they're on Mars. Unfortunately, we don't have any data showing whether or not .38 g's is enough to stop the decalcification and other problems of low gravity. I haven't seen the idea of an Earth/Mars shuttle mentioned yet (maybe everyone knows about it and have discounted it). This is where you have living quarters arranged in such an orbit that they approach Earth and Mars periodically. I don't remember what the period of such an orbit would be, or how often it would approach Earth and Mars. It seems that it would have to approach close enough to one or the other (or both) for its orbit to be perturbed considerably, since the ratio of the periods isn't a nice fraction. Anyway, the advantages are that you don't have to boost your life support system into Earth/Mars transfer orbit and out again with every trip to Mars. And you can incrementally increase the cozyness of your shuttle's habitat. The disadvantage is that you need another life support system at Mars, to tide you over till the shuttle comes back. You're SOL if you get off at Mars, and find out that the life support system waiting for you is out of order. It seems that if you are truly serious about going to Mars (multiple times) such a shuttle would be the way to go. Unless you want to wait for the infamous fusion torchship to be invented (which could be a problem. The infrastructure for the shuttle system would be fairly large; it would be awfully disheartening to be halfway through building it when someone invented something that made interplanetary travel fast and cheap). I haven't seen much yet about environment. It seems pretty clear that you'll want to recycle air and water. I dimly recall something about how bubbling contaminated air through superheated water would break down most of the nasty compounds. Anyway, it seems that life support would probably be some sort of algae to handle 95% of the recycling/detoxification, and some non-biological system (involving heat, electricity, chemicals (hmm, sounds like something you'd use on Godzilla), filtration, or whatever else is handy) to deal with the rest. Any ideas of how good you'd get? How much per man such a system would weigh, how many kg of consumables you'd have to carry per man per day? I've no idea what the state of affairs is regarding long-term life support systems, i suspect that we don't know nearly enough yet to get to Mars and back (i also suspect that something good enough could be put together in five to ten years, if we were really serious). How do they get around once they're finally on Mars? CO/O2 rockets seem feasible, and all the raw materials are readily available. H2/O2 is better, of course, but possibly the raw materials are harder to get (and the finished materials are harder to handle). How much time are we talking about for a mission? It looks something like 3/4 year to get there, 3/4 year for the planets to realign, and another 3/4 year to get back (that's assuming a Mars year is two Earth years--since it's less, i think it'll take a bit longer for the planets to realign). 830 days, give or take a bunch. Non-hohmann ellipses could shorten the mission considerably. david rickel decwrl!sci!daver ------------------------------ Date: Wed, 13 Sep 89 13:28:45 EDT From: dietz@cs.rochester.edu To: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU Cc: space-tech@cs.cmu.edu Subject: Mars missions Marc asked: how does an MPD thruster work? A coaxial MPD thruster consists of two concentric electrodes. The inner electrode is a rod, the outer a cylinder. The inner electrode may be longer or shorter than the outer. A gas is introduced between the electrodes at one end. A strong current passes from one electrode to the other through the gas. The currents in the electrodes produce a magnetic field which circles the inner electrode. The Lorentz (JxB) force propels the ionized gas down the barrel. You can think of this as a railgun that accelerates a plasma rather than a solid object. Beyond the electrodes, the plasma can continue to carry current. If properly designed, the magnetic field can be such that the JxB force has an inward directed component (pinch force), which helps reduce the lateral expansion of the plasma. In continuous mode, an MPD thruster needs megawatts of power to operate at high efficiency. If lower average power (and thrust) is desired, you can operate the thruster in a quasisteady mode (plateaus of current with low duty cycle) or in a pulsed mode (even shorter pulses). Solid-fueled MPD engines have been investigated for use in stationkeeping of satellites, since they can produce strong, short pulses of thrust (good for maneuvering rotating satellites) and are smaller than ion engines. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ End of Space-tech Digest #33 *******************