Subject: Space-tech Digest #67 Contents: Henry Spencer Re: fuel handling in free fall John DuBois silicon chip cantilever accelerometers Bill Davidsen Re: silicon chip cantilever accelerometers Henry Spencer Re: Corner reflector Jim Van Zandt Re: guidance of model rocket to the moon Lou Adornato Re: Insurance co. underwriting prizes Don Lindsay Mars Conveyor John DuBois Re: Mars Conveyor Henry Spencer Re: Mars Conveyor Paul Dietz Cyclers vs. Nuclear Propulsion Bill Davidsen Re: Cyclers vs. Nuclear Propulsion Paul Dietz Re: Cyclers vs. Nuclear Propulsion Marc Ringuette Re: Cyclers vs. Nuclear Propulsion Phil Fraering Space-tech as newsgroup? Jon Slenk Re: Space-tech as newsgroup? Marc Ringuette Space-tech status & statistics ------------------------------------------------------------ From: henry@zoo.toronto.edu Date: Sat, 23 Jun 90 19:53:46 EDT To: space-tech@CS.CMU.EDU Subject: fuel handling in free fall > > This brings up a question I've been dying to ask -- just how do you move > > liquids in microgravity? ... The only > > thing I can think of is that there's a flexible barrier between the pressure > > agent and the liquid, so that the whole thing acts like a piston. > > Correct, at least for Voyager. The attitude thruster tanks are > pressurized with a flexible membrane between the gas and the propellant. There are about five ways, including some cheating, to deal with this particular problem. One way is to rely on small rockets to apply enough acceleration to settle propellants for larger ones; that's not uncommon in launcher upper stages, but of course it just defers the problem rather than solving it, because then the small rockets have to deal with it. Launchers typically use solid rockets for the job unless restartability is required. The orthodox way is the one Jon mentioned, with either a rubber (well, elastomer of some sort) bag around the propellant or a diaphragm or bellows between propellant and pressurant. The Soviets use this for the maneuvering systems on Mir, it's the usual approach for liquid rockets on unmanned missions, and everyone uses it for reaction-control jets at least. (A popular combination for bigger rockets that need restart is an RCS using a membrane -- such a system has to be present anyway, to maintain attitude control so the big motor is pointing in the right direction -- which is fired to settle propellants before the big motor lights.) Another method, which I think has seen some limited use in space, is to use capillary action in mesh screens and the like to keep the feed lines and the outlet region of the tank full of liquid. This gets you enough liquid fuel to apply acceleration and settle the rest. Spin-stabilized satellites have centrifugal force available for fuel management, and never really have to deal with the problem. Finally, if you're using Amroc-style hybrid rockets, you can cheat. With no big concerns about mixture ratio and so on -- only one of the propellants is liquid -- you just pressurize with propellant vapor and don't care whether you have bubbles in the feed line initially. Even half-gaseous propellant will get you enough thrust to settle the rest. The idea gives me the creeps but I'm told it should work. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu uunet!attcan!utzoo!henry ------------------------------ From: John DuBois To: space-tech@CS.CMU.EDU Subject: Re: Rocket to the moon. Date: Sat, 23 Jun 90 20:44:38 PDT >I have a friend at BOEING in black box areas that was telling me about >a new inertial position system that uses no moving parts, and is the size >of a matchbox. I expect that the real workings are VERY secret, and would >be enormousllllly expensive. But knowing that something CAN be done is >half the battle. > John. I've heard of work being done on tiny silicon-chip cantilever accelerometers. In principle you could put a three-axis accelerometer on a single chip. In fact you could have a dozen accelerometers for each axis, to cover different ranges with maximum accuracy, along with attendant electronics. Perhaps the Boeing device is using these. John ------------------------------ Date: Mon, 25 Jun 90 13:43:39 EDT From: davidsen@crdos1.crd.ge.com To: space-tech@CS.CMU.EDU Subject: Re: Rocket to the moon. > > I've heard of work being done on tiny silicon-chip cantilever > accelerometers. In principle you could put a three-axis accelerometer > on a single chip. Yes, the only hard part about doing this is getting the scale effect to drop the size. A full size system was possible to high school kids in 1960, and worked reasonably well as a vibration detector. ------------------------------ From: henry@zoo.toronto.edu Date: Sun, 24 Jun 90 16:30:34 EDT To: space-tech@CS.CMU.EDU Subject: Re: Corner reflector > It seems to me that there are fundamental problems with communication by > light at great distances... While you might use lasers at > both ends for communication between a fairly low earth orbit and the moon, > it's hard to picture a scenario in which lasers are useful for interplanetary > communications. JPL is seriously investigating the idea for deep-space communications, actually. They consider it very promising in the long run, although I'm not familiar with the detailed tradeoffs. > ... There is a lot of "background noise" at optical > wavelengths, and the detectors are not too good... ??? The detectors are good enough that it's routine to count individual photons. (The lunar retroreflector work, for example, typically gets only one or two back from each pulse.) The background noise is rather less significant, as I recall, than the radio background noise. > ... light has too much energy per photon, so a tremendous amount of > transmission energy is required to assure reception of a reasonable number of > photons, as compared to radio wavelengths... Energy is ultimately what you detect, however; low-energy photons are more difficult to detect than high-energy ones. One major advantage of working at optical wavelengths is much tighter beams, which deliver a much larger fraction of the transmitter energy to the receiver, other things being equal. (Of course, other things are *not* equal, and one complication of tighter beams is a requirement for more precise pointing.) > How about this: the tiny spacecraft brings along a gram (or less) of > antimatter. At a prearranged time, this is allowed to react with normal > matter. That ought to produce a detectable signal! :-) :-) Only if we do it after the Gamma Ray Observatory is up! :-) Matter- antimatter reactions tend strongly toward high-energy output. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu uunet!attcan!utzoo!henry ------------------------------ Posted-From: The MITRE Corp., Bedford, MA X-Alternate-Route: user%node@mbunix.mitre.org To: space-tech@CS.CMU.EDU Subject: Re: Aim For The Moon - model rocket contest Date: Mon, 25 Jun 90 11:26:23 EDT From: jrv@sdimax2.mitre.org Lou Adornato writes: > BTW, the _best_ we could do would be a ring laser gyro. Marc.Ringuette notes: > ...Much like the EM launchers > we discussed last year, we don't want to go into any sort of low Earth orbit; > I think we want to have a continuous burn into an almost straight trajectory > heading towards where the moon will be, say, two days after launch. Military missiles have to work in bad weather, and LEO satellites have to maintain orientation while the sun and moon were periodically blocked by the earth. A probe in a direct ascent trajectory to the moon could avoid both these restrictions. Why couldn't it determine its orientation by optically tracking the moon and sun? With wide field of view optics, the system might not even need moving parts. How about peephole optics for the sun? (Maybe for the moon too - it might actually be easier to find the centroid of a fuzzy image.) - Jim Van Zandt ------------------------------ Date: Mon, 25 Jun 90 14:12:09 CDT From: Lou Adornato To: space-tech@CS.CMU.EDU Subject: Re: rockets to the moon Vincent.Cate@sam.cs.cmu.edu writes [regarding the use of insurance co. to underwrite space exploration "prizes"] >If the prize is winable then the insurance company is going to charge you >more than the amount of the prize. Why have them involved at all? >For the prize money I do not see the usefulness of an insurance company. The insurance co would act as a trustee over the prize money, sort of like the guy who holds the bet during a pool match. I beleive this was the way that the human powered flight award was handled, but I'm not sure. Another reason to bring in an insurance co. is the idea of "cash flow underwriting". The idea behind this is that the insurance company invests the cash flowing through its books, and can therefore pay out more than the premiums that it takes in (we engineering types are financially crippled by our early exposure to the conservation of energy/momentum/thermal equilibrium. There is _no_ such thing as a law of conservation of money [unless you're talking armed robbery]). Lou Adornato | Statements herein do not represent the opinions or Cray Research | attitudes of Cray Research, Inc. or its subsidiaries. lfa@cray.com | (...yet) ------------------------------ Date: Sat, 23 Jun 1990 17:12-EDT From: Donald.Lindsay@MATHOM.GANDALF.CS.CMU.EDU To: space-tech@CS.CMU.EDU Subject: Mars Conveyor There's an old idea, which I have always called the Mars Conveyor. It's quite simple: we put a large structure into an orbit that (approximately) visits both Earth and Mars. Once established, we leave it that way, and use our delta-v in getting to it/from it. I could have gone into this idea at greater length. However, before spending the bandwidth, I'd like to ask: why haven't I heard of this idea in years? Was there a fatal flaw? Don ------------------------------ From: John DuBois X-Mailer: SCO System V Mail (version 3.2) To: space-tech@CS.CMU.EDU Subject: Re: Mars Conveyor Date: Sat, 23 Jun 90 20:57:32 PDT Neil Armstrong calls these "cycling spaceships". I believe he's done a lot of work on them, optimizing orbits with planetary assists etc. to get the things back to a useful place in minimum time. I think they make excellent sense for moving materials that require lots of shielding, support, etc. (the most obvious example being humans) on well-travelled transfer routes. John ------------------------------ From: henry@zoo.toronto.edu Date: Sat, 23 Jun 90 19:37:01 EDT To: space-tech@CS.CMU.EDU Subject: Re: Mars Conveyor > ... we put a large structure into an orbit that > (approximately) visits both Earth and Mars. Once established, we > leave it that way, and use our delta-v in getting to it/from it. > ... why haven't I heard of this idea in years? Was there a fatal flaw? From the cynic's point of view, it has one fatal flaw: it assumes a gradual buildup of infrastructure for sustained spaceflight, which doesn't satisfy the Mars-right-away crowd. It is a rational thing to do if your objective is persistent exploration of Mars, and many proposals for such a program have included a "conveyor" like this. It is largely wasted effort if your objective is a one-shot mission with no followon plans. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu uunet!attcan!utzoo!henry ------------------------------ Date: Sat, 23 Jun 90 22:00:15 -0400 From: dietz@cs.rochester.edu To: space-tech@CS.CMU.EDU Subject: Cyclers vs. Nuclear Propulsion Henry Spencer wrote (about Mars/Earth cycler spacecraft): From the cynic's point of view, it has one fatal flaw: it assumes a gradual buildup of infrastructure for sustained spaceflight, which doesn't satisfy the Mars-right-away crowd. It is a rational thing to do if your objective is persistent exploration of Mars, and many proposals for such a program have included a "conveyor" like this. It is largely wasted effort if your objective is a one-shot mission with no followon plans. The proposals I saw for cyclers either had low duty cycles (you get to use each cycler once every five years or so), fairly high delta-V requirements to adjust the station's orbit, or some other undesirable characteristic. A technology that would help a whole lot more is nuclear propulsion. Frankly, any plan for manned exploration of Mars using just chemical rockets should have a stake driven through its heart. I did read the article in Analog last year, BTW. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ Reply-To: davidsen@crdos1.crd.ge.com Date: Mon, 25 Jun 90 13:50:17 EDT X-Mailer: Mail User's Shell (6.5 4/17/89) From: davidsen@crdos1.crd.ge.com To: space-tech@CS.CMU.EDU Subject: Re: Cyclers vs. Nuclear Propulsion > A technology that would help a whole lot more is nuclear propulsion. > Frankly, any plan for manned exploration of Mars using just chemical > rockets should have a stake driven through its heart. I did read > the article in Analog last year, BTW. For practical systems we will have to have not only a better source of power, but a way to harness it. If the device to grab individual atoms and kick them to relativistic speed for exhaust can be built with a practical size, mass, cost, and safty, then we can harness a lot of power. Throwing reaction mass out at low speeds just doesn't allow continuous acceleration. -- bill davidsen (davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen) "Stupidity, like virtue, is its own reward" -me ------------------------------ To: davidsen@crdos1.crd.ge.com Cc: space-tech@CS.CMU.EDU, dietz@cs.rochester.edu Subject: Re: Cyclers vs. Nuclear Propulsion Date: Mon, 25 Jun 90 15:51:01 -0400 From: dietz@cs.rochester.edu For practical systems we will have to have not only a better source of power, but a way to harness it. If the device to grab individual atoms and kick them to relativistic speed for exhaust can be built with a practical size, mass, cost, and safty, then we can harness a lot of power. Throwing reaction mass out at low speeds just doesn't allow continuous acceleration. Not at all. Even relatively low performance, near state-of-the-art nuclear rockets would be very desirable, and superior to chemical rockets, especially using ET propellants. Throwing reaction mass out at low speeds doesn't rule out continuous thrust, as long as the thrust is sufficiently low. Low thrust nuclear fission thermal rockets could operate with low chamber pressure and reactor power, yet still maintain high Isp (which is a function of gas temperature and expansion ratio). Indeed, low chamber pressure could permit the use of a pressure fed, rather than pump fed, system. A low thrust system loses when you want to maneuver around planets (since you want to make your delta-V deep in the gravity well), but for other applications (say, asteroid exploitation, or maneuvering a cycler) could be superior. Paul ------------------------------ Date: Mon, 25 Jun 1990 15:43-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@CS.CMU.EDU Subject: Re: Cyclers vs. Nuclear Propulsion > For practical systems we will have to have not only a better source of > power, but a way to harness it. If the device to grab individual atoms > and kick them to relativistic speed for exhaust can be built with a > practical size, mass, cost, and safty, then we can harness a lot of > power. Throwing reaction mass out at low speeds just doesn't allow > continuous acceleration. I think we already HAVE a way to harness the energy, if it's in the form of electricity: electric (ion) propulsion systems do just that. Maybe I'm reading this wrong, but I want to point out that just because you CAN eject material at high velocity doesn't mean you WANT to. If you work it out, you'll find that the most efficient use of a given amount of energy is to eject a LOT of reaction mass, rather than a little. This is because the momentum gained scales linearly with the exhaust velocity, but the energy required scales with its square. So high-velocity exhaust gives you more delta-V per unit reaction mass, but requires FAR more energy. What this means is, when you decide on your exhaust velocity, you want to balance the amount of reaction mass you want to carry vs. the amount of energy you have available. If you have energy to burn, you can bring very little reaction mass. But in almost all cases, you'll want to bring reaction mass that's some nontrivial fraction of your entire spacecraft mass, because then for a given amount of input energy you can get more thrust. In my opinion, current electric propulsion is actually pretty close to the ideal propulsion technology. You can choose the optimum exhaust velocity by adjusting the voltage applied to the exhaust ions. I think. I haven't seen all that much literature on existing electric propulsion units, though...if anyone has seen any good writeups on the state of the art, please let me know. --Marc ------------------------------ Date: Mon, 25 Jun 90 13:25:21 -0500 From: Fraering Philip To: space-tech@CS.CMU.EDU Today I logged in to check the weekend's accumulated mail and there were about 20 or so messages. Perhaps this mailing list would function better as a newsgroup. Phil |dlbres10@pc.usl.edu ------------------------------ Date: Mon, 25 Jun 90 16:02:33 -0400 (EDT) From: "Jon C. Slenk" To: space-tech@CS.CMU.EDU Subject: Re: Lets make it a newsgroup I wholeheartedly support the idea to try and make this a newsgroup. -Jon. ------------------------------ [ Whoa! Let's not start taking a vote or anything! Read the following message, then send your comments, if any, to space-tech-request. --Marc ] ------------------------------ Date: Mon, 25 Jun 1990 15:57-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@CS.CMU.EDU Subject: Space-tech status & statistics Phil, I don't like the idea of making space-tech a newsgroup. I think having it as a mailing list helps maintain better quality by weeding out people who won't go to the trouble of signing up. If any of you disagree, though, let me know. Please send comments to me and I'll summarize to the list if an interesting issue comes up. If any of you are tired of getting so many messages, just ask me to switch you to the digest, and you'll get packages of 10-15 messages at a time, collated by subject. Remember to send administrative mail to me, or to space-tech-request@cs.cmu.edu, not to the whole list. Some statistics: size of space-tech list: 151 members size of space-tech digest: 69 members in operation since: Sept. 2, 1988 total number of messages: 698 Excerpts are available for the following seven topics: EM Launchers Tether Propulsion CMU Mars Rover High Velocity Guns Mars Mission Ship Design Orbital Debris Launch Loops Robots on the moon They're heavily abridged by me down to maybe 30% of their original size. In my opinion, they're the highlights of the discussions through this April. All old space-tech traffic, and the excerpts, are available by anonymous ftp from my machine, daisy.learning.cs.cmu.edu (128.2.218.26), or by sending me mail. Enjoy. ---------------- | Marc Ringuette |------------------------ | mnr@cs.cmu.edu | Cucumber Science Dept. |---------------------------------- | 412-268-3728 | Cranberry Melon Univ. | 'Tis not her coldness, father, | ----------------| Pittsburgh, PA, 15213 | That chills my labouring breast: | ------------------------| It's that confounded cucumber | | I've ate and can't digest. | ---------------------------------- ------------------------------ End of Space-tech Digest #67 *******************