Subject: Space-tech Digest #127 Contents: Nuclear thermal propulsion (3 msgs) Diamond film semiconductors (3 msgs) gas-dynamic end-caps for gun-launchers? (8 msgs) ------------------------------------------------------------ Date: Tue, 6 Oct 92 20:18:03 EDT From: Aaron R Wininger To: space-tech@cs.cmu.edu Subject: Nuclear thermal propulsion Sender: mnr@DAISY.LEARNING.CS.CMU.EDU Does anyone know what the current state nuclear thermal propulsion (US and World)? The US had the Rover program (cancelled in 72 due to stupidity) which seemed to be very promising (ie. manned Mars mission in early 80's). I assume w/ Bush Space Exploration Initiative, Rover would be resurrected, but have been unable to find much data on any new programs. Aaron Wininger Columbia Plasma Physics Lab arw4@cunixb.cc.columbia.edu wininger@cuplvx.ap.columbia.edu ------------------------------ Date: Tue, 6 Oct 92 21:38:56 -0400 From: Jon Leech To: space-tech@cs.cmu.edu Subject: Re: Nuclear thermal propulsion (& Mike Griffin's involvement) >Does anyone know what the current state nuclear thermal propulsion (US >and World)? Henry will doubtless answer this in great detail, so I'll confine myself to a related comment. Last Friday, SELECT carried testimony by Michael Griffin before the Subcommittee on Investigations and Oversight, House Committee on Science, Space and Technology. I caught part of this; I didn't attempt to transcribe it, but it was fascinating. The inquisitor - er, Congressman - (Wolper, I believe) was apparently trying to establish links between Griffin's activities during his tenure at SDIO, corporations involved in the Timberwind program, Griffin's involvement in the Stafford Commission's report, which advocated nuclear thermal for Mars propulsion, and upcoming recommendations that Griffin's SEI office will be making vis-a-vis Mars propulsion options. I can only guess at what heinous deed Wolper had in mind (my guess: some sort of kickback from aforementioned companies to Griffin in return for his recommending this particular nuclear propulsion technology). He also seemed curious about classified material re Timberwind being released in the Stafford report. Does anyone have insight into what's going on here, and if it's a total red herring? I'm inclined to believe so, but Griffin definitely had on a different face during this hearing. OK, so this isn't very -tech. Feel free to followup to talk.politics.space instead, if that seems appropriate. Jon __@/ ------------------------------ From: henry@zoo.toronto.edu Date: Wed, 7 Oct 92 11:27:25 EDT To: space-tech@cs.cmu.edu Subject: Re: Nuclear thermal propulsion >Does anyone know what the current state nuclear thermal propulsion (US >and World)? There is renewed interest in the US, but very little money so far. The main civilian line of effort so far is looking at a slightly-upgraded Nerva design. There are crazies :-) off on the sidelines looking at a variety of farther-out concepts, including gaseous fission. Heaven only knows what's under wraps within DoD. The only really known program, Timberwind, is looking at a particle-bed reactor, which could give higher performance than Nerva but has a lot of technical unknowns. There are fairly-knowledgeable people saying "Timberwind is not likely to work". It's also become a political minefield after some rather rash notions, e.g. suborbital test flights over Antarctica, were leaked. All such programs face political problems, and also technical fallout from the political problems. For example, it is no longer considered acceptable to test-fire such engines in the atmosphere. This means that ground tests are going to require big, expensive exhaust scrubbers, easily accounting for half or more of the cost of testing. The politics of using such engines are probably solvable *if* they are never lit below a "nuclear-safe orbit". 1500km? 2000km? I forget the exact number, but the idea is to guarantee an orbital lifetime beyond that of most fission products. This is a bit troublesome, because you'd like to use them to boost out of low orbit, at the very least. Timberwind was talking about using them as upper stages on orbital launchers, but that's a political non-starter if I ever saw one. The Russians had a Nerva-type program too, and claim to have developed some technology somewhat beyond the US's, but little is known and the US folks are a little skeptical of some of the claims. >... I assume w/ Bush Space Exploration Initiative, Rover >would be resurrected, but have been unable to find much data... Do remember that SEI is getting a very hostile reception, and no money, from Congress. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: Wed, 21 Oct 92 15:32:29 -0400 From: dietz@cs.rochester.edu To: ssi!lfa@uunet.UU.NET Subject: Re: Diamond film semiconductors Cc: space-tech@cs.cmu.edu Lou Adornato wrote (about diamond-film semiconductor electron sources): I've been trying to figure out some numbers on using these in ion engines, but I'm lost. I'm assuming that 1m^2 of emitting surface, yeilding 1e4 cm^2, for an output of 1e7 A. Multiply this by Coulombs constant to get the total number of electrons. The mass of the electrons (?) times the Emission velocity should give total thrust, right? The emission velocity should be related to the surface temperature of the emitter and the field potential, and this is where I get lost. Someone care to pontificate on this? Ion engines emit *ion* beams, not electron beams (well, they have an electron source that neutralizes the ion beam, but most of the energy goes into the ion beam). Currents in charged particle beams are limited by something called the space charge effect. Put briefly: the beam is nonneutral (at least in the space between the accelerating grids) and has a certain repulsive self-energy. The current cannot be increased beyond the point at which the density of this self-energy exceeds the energy density of the accelerating electric field. The implications of all this is that to minimize the size of an ion engine, at a given exhaust velocity, you want the mass/charge ratio of the particles being accelerated to be as high as is practical. This is one reason (the other being to minimize ionization energy per unit mass) why heavy ions like cesium, xenon, or mercury get used, and why there has been talk about using buckyballs (mass 720 AMU). The other thing you do in an ion engine is to make the accelerating gap be as small as possible, as the energy density in the field there goes up with decreasing gap size, at constant accelerating voltage. There are limits as to how far this goes (if the grids are too thin they get sputtered away too quickly). Generally you do *not* want light particles, like electrons, accelerated through your engine, as this reduces efficiency. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ Date: Wed, 21 Oct 92 13:38:14 PDT From: jerbil@ultra.com (Joseph Beckenbach {Adapter Software Release Engr}) To: space-tech@cs.cmu.edu Subject: Re: Diamond film semiconductors This from Paul Dietz: > there has been talk about using buckyballs (mass 720 AMU). M-x weird-thought-mode :-) Has anyone tried creating silicon-based buckyballs (Si60, mass 1680 AMU)? Or is silicon not quite "flexible" enough to form the necessary atomic structures? Wonder when it stops being worth adding mass? As long as you've got enough pebbles, you keep throwing them. Very slowly. :-) :-) M-x weird-thought-mode Joseph Beckenbach ------------------------------ Date: Wed, 21 Oct 92 20:26:41 -0400 From: dietz@cs.rochester.edu To: jerbil@ultra.com Subject: Re: Diamond film semiconductors Cc: space-tech@cs.cmu.edu I don't think you could make silicon buckyballs, as silicon doesn't have a graphite-like form. Perhaps boron nitride (I think that's been tried, but no positive results). There was a report recently of a titanium-carbon cluster molecule. Anyway, silicon clusters have been formed, with small numbers of atoms. The reason buckyballs were suggested was that they appear to be very strong and hard to fragment, yet not that hard to ionize. Light ions mixed with the beam will degrade efficiency. Paul ------------------------------ Date: 21 Oct 1992 21:05:02 -0400 (EDT) From: "GORDON D. PUSCH" Subject: gas-dynamic end-caps for gun-launchers? To: space-tech@cs.cmu.edu I've just re-read some of the postings on "distributed injection launchers" (a.k.a. *guns* :-) from two years ago, and the ones regarding the "end-cap" problem kicked loose an idea ... Back when I was still interested in air-breathing boosters, I spent some time learning a bit about supersonic gas-dynamics. One of the things that stuck in my mind was the concept of a "gas-dynamic window" for high-power gas lasers. The gist of it is that a supersonic airsteam flowing transverse to the beam could be used to form a "non-solid window" seperating the lasing cavity from the atmosphere --- kind of a supersonic generalization of Cockcroft's original "momentum curtain" concept for hovercraft: a curtain of rapidily-moving air could be used to seperate regions at different pressures. The gas-stream stream is ejected on one side of the laser's "muzzle" from a linear supersonic injection-nozzle (the "injector"); on the other side there's a linear supersonic intake diffuser (the "extractor"); a turbopump pumps gas out of the extractor and back to the injector. Such "gas-dynamic" windows were alleged to be able to support considerable pressure differences (on the order of the gas-stream stagnation-pressure?); in this case the gas-stream bends through an arc as it crosses from the injector to the extractor, with the pressure-difference providing the centripetal force. Gas-dynamic windows could also be used to (mostly) keep seperate dissimilar gasses. Finally, since they weren't solid, they couldn't be damaged by the laser beam ... It occurs to me that one (or more) such gas-dynamic windows (cascaded for redundancy) might be the ideal solution to the "end-cap" problem for gun-launchers. Since it isn't solid, there's no need to open, close, or burst it, and I wouldn't think a measly mach-2 crosswind would bother a mach-25 projectile (much). I would think that the ejector turbopump should also be able to do double-duty by helping to pump the barrel if one designed it correctly: residual gas in the barrel should get entrained into the gas-stream and ejected. By introducing a controlled flow of hydrogen into the barrel near the breech, the residual gas in the barrel will be mostly H2, reducing frictional heating. Unless a recirculation-zone forms near the last window to act as a "flame-holder" (this should be preventable by good design), there should be no danger of combustion in the H2-entrainment- zone within the window, since the flame speed for even stoichiometric H2/air is *VERY* much less than V_sonic at 300 Kelvin. The entrained H2 need not be wasted --- the mixture can be sent to a combustor and burned to drive the turbopump (think of it as a turbojet recirculating part of its own exhaust). Any comments? Gordon D. Pusch ! BITnet: TASCC A&D, Stn. 49a ! AECL, Chalk River Laboratories ! Phone: (613) 584-3311, X-4107 (off.) Chalk River, Ont. CANADA ! (613) 584-2368 (hm.) K0J 1J0 ! ------------------------------ From: ssi!lfa@uunet.UU.NET (Louis F. Adornato) Subject: Re: gas-dynamic end-caps for gun-launchers? To: uunet!crl.aecl.ca!PUSCHG@uunet.UU.NET (GORDON D. PUSCH) Date: Fri, 23 Oct 92 13:22:33 CDT Cc: uunet!cs.cmu.edu!space-tech@uunet.UU.NET X-Mailer: ELM [version 2.3 PL11] > I've just re-read some of the postings on "distributed injection launchers" > (a.k.a. *guns* :-) from two years ago, and the ones regarding the "end-cap" > problem kicked loose an idea ... > interesting stuff deleted< > It occurs to me that one (or more) such gas-dynamic windows (cascaded for > redundancy) might be the ideal solution to the "end-cap" problem for > gun-launchers. > more interesting stuff deleted < How large a gap has this concept been applied to? What problems would result from scaling it up to a few meters? BTW, sorry about that flag thing. Lou Adornato | The secretary (and the rest of the company) Supercomputer Systems, Inc | have disavowed any knowledge of my actions. Eau Claire, WI | "Eat Cheese or Die!" uunet!ssi!lfa or lfa@ssi.com | Wisconsin State Motto ------------------------------ Date: 23 Oct 1992 19:28:05 -0400 (EDT) From: "GORDON D. PUSCH" Subject: Re: gas-dynamic end-caps for gun-launchers? To: space-tech@cs.cmu.edu Paul Dietz writes (responding to his second point first): > ...On the other hand, what exactly is wrong with mylar diaphragms? > Nothing really; I'm just kicking out an idea. My main motivation was to come up with a "non-solid re-usable pressure-barrier," rather than, say, a mechanical shutter. A supporting argument for a "non-solid" end-gap might be that unlike a mylar diaphram, it does not have to be physically replaced after each shot, nor does the barrel need to be pumped back down from 1 atm; I would expect this to improve the rep-rate. (Granted, the "no pump-down" argument loses much of its force for DILs, where you have to pump out the combustion products *anyway,* but it should be relevent to coil guns.) Also, I recall what a minute fleck of white paint did to the window of a certain space-shuttle, once, so I'm a little worried about the conse- quences of hitting a sheet of mylar at mach-25; maybe this is a non-problem --- I don't know enough about hypervelocity impact effects to say; maybe it's no worse than hitting a wall of *air* at mach-25 ... > Why a curtain? Why not just use a ejector pump? That is: > direct a high pressure gas stream out the end of the barrel. > This will tend to drag any gas inside the barrel out with it. > The gas would be emitted from nozzles angled into the barrel, > near the end. One could stage these as well. > Sure; something like an "aspirator" near the muzzle, and an "inside-out" gaseous-diffusion pump further back. Such an arrangement would certainly be simpler to build than what I proposed, and less "finicky." (For example, after I'd sent out my post, I started worrying about the flow in the curtain being disrupted by the projectile, leading to an "unstart/restart" transient in the supersonic inlet; or maybe the shockwave might cause a compressor- stall in the turbopumps; etc., etc. ...) A worry: if I recall correctly, a diffusion pump must have a diameter only a few times large than the mean-free-path to pump effectively, and it is limited as to how much back-pressure it can handle --- it requires a "backing" pump. On the other hand, a "simple" ejector-pump is limited to relatively low vacua. While the two pumping modes are not entirely distinct --- they kind of blur together into a continuum in the mid- vacuum range --- I'm not sure they're well "impedance-matched," for lack of a better term; at least, I've never *heard* of someone using an ejector as a backing pump to a diffuser. Of course, that could be because mechanical pumps are simply much more efficient ... Lou Adornato writes: > > ... How large a gap has this concept been applied to? What problems > would result from scaling it up to a few meters? > Haven't the foggiest; I'm an *accelerator physicist*, not an aerodynamicist. It's just something that caught my attention once, and has been percolating ever since. Don't even have any references; sorry! ... Gordon D. Pusch ! BITnet: TASCC A&D, Stn. 49a ! AECL, Chalk River Laboratories ! Phone: (613) 584-3311, X-4107 (off.) Chalk River, Ont. CANADA ! (613) 584-2368 (hm.) K0J 1J0 ! ------------------------------ To: "GORDON D. PUSCH" Cc: space-tech@cs.cmu.edu, gwh@lurnix.COM Subject: Re: gas-dynamic end-caps for gun-launchers? Date: Fri, 23 Oct 92 18:43:00 -0700 From: gwh@lurnix.COM >> ...On the other hand, what exactly is wrong with mylar diaphragms? >> >Nothing really; I'm just kicking out an idea. ... >Also, I recall what a minute fleck of white paint did to the window of >a certain space-shuttle... My gut feeling is that the energy required to penetrate a half-mm of mylar is a lot less than to displace the first few centimeters of air that the vehicle will penetrate. As for impact, well, recall that you're going to be impacting _something_ at mach 25; either your mylar barrier, a mach 2 perpendicular stream of gas, or just the air outside if you invent a magic forcefield (hint: it's possible 8-) to keep air out. In any case, it's going to be uncomfortable, and it in fact will be at closer to Mach 30-32, so that you're still moving Mach 25 once you pass atmospheric drag. Only half of most projectile launcher design projectiles is payload. Some of that is fins, usually. Some (10%) is a kick motor to circularize. A lot is ablative insulation so that you don't melt while you're ascending. -george william herbert gwh@lurnix.com ------------------------------ To: space-tech@cs.cmu.edu Cc: gwh@lurnix.COM Subject: Re: gas dynamic end-caps for gun-launchers? Self_Organization: Retro Aerospace, (510) 849-4853 Work_Organization: Lurnix, (510) 849-4478 x229 fax (510) 849-0409 X-Funny-Message-Of-Day: my mind is going, dave, my mind... From: gwh@lurnix.COM (George William Herbert) Date: Mon, 26 Oct 92 11:12:34 -0800 Sender: gwh@lurnix.COM Phil Fraering writes: >How does the magic forcefield work? Easy. Ionize some air, or another gas (I don't know which one will be optimal). After the end of the gun tube, you have a continuation (also sealed to the tube) which has conductive floor and roof with a potential across them. Run a magnetic field across it with the lines of force perpendicular to the axis of the barrel and to the electric potential. We now have a textbook MHD device. The combined electric and magnetic fields establish a force on the ionized gas, which will be constant within the continuation (more or less); you just adjust things (voltage and B field) so that the force in the ionized gas pushes "out", along the axis of the barrel, and that the magnitude of the force is enough that the final pressure at the muzzle end is atmospheric pressure. Voila. You can kill the E and B fields instantaneously* as the projectile is about to launch through them, so they don't tweak with it. If you design the muzzle extension with the MHD section long enough, you can bring it back up and stabilize the pressure gradient again after the projectile goes through. * Ok, I'm lying here. You can't do anything instantaneous with a magnetic field, and I know it 8-) But that's not a problem, all you do is design the MHD section long enough that you can drop and then recover after the projectile passes through faster than the air outside can push its way in (at 330 m/s - epsilon). The term "magic forcefield" was sort of a tipoff. It's the common description UC Berkeley Physics Grad students had for MHD effects after taking a few classes in them 8-) -george william herbert Retro Aerospace, "A Force in a Different Direction" gwh@lurnix.com gwh@retro.com coming soon to a net near you ------------------------------ Date: 26 Oct 1992 20:02:58 -0500 (EST) From: "GORDON D. PUSCH" Subject: Re: gas dynamic end-caps for gun-launchers? Cc: space-tech@cs.cmu.edu George Herbert writes: > >Phil Fraering writes: >>How does the magic forcefield work? > >Easy. Ionize some air, or another gas ... [... description of textbook "crossed-fields" MHD device deleted ...] >... you just adjust things (voltage and B field) so that the force >in the ionized gas pushes "out", along the axis of the barrel, >and that the magnitude of the force is enough that the final pressure >at the muzzle end is atmospheric pressure. Voila. > This *might* conceivably work at very low pressures (i.e., molecular mean-free-path very much greater than the mean ion-cyclotron radius). At higher pressures (where the aforementioned quantities are comparable) the gas breaks down, and you'll get a propagating arc-discharge instead, traveling initially at the "ExB" drift-velocity; the arc will tend to act as a "snowplow," sweeping up the gas ahead of it. (Think of it as a rail-gun with a superposed B-field and no payload; alternatively, the "payload" is the residual gas in the tube... Isn't a railgun with an applied field sometimes called an "augmented" railgun???). (One might wonder why one couldn't simply coat the plates with an *insulator*, thereby preventing an arc-current from flowing, but leaving the E-field intact. However, I am assured by our electrostatic- deflector specialist that coating the plates with an insulator actually *INCREASES* the probability of breakdown (for reasons that are still somewhat poorly understood; it appears to have something to do with the surface properties of most known insulators), unless the insulator is thick enough to withstand the full applied voltage. H.V. insulators are actually based on the principle of forcing the arc to take a circuitous path through the *air* --- one long enough to keep it from breaking down...) So I don't think this idea'll work like a "Death-Star/STTNG shuttlebay"- type "static" pressure-curtain; it'll be an intrinsically non-stationary process... What one *MIGHT* be able to do is run it *cyclically*: think of the arc as a "virtual piston" in a piston pump; periodically re-initiate it to sweep out residual gas near the muzzle-end. I have absolutely *NO* idea if this is actually a viable concept, let alone how much power it'll eat... > >You can kill the E and B fields instantaneously* as the projectile >is about to launch through them, so they don't tweak with it. >If you design the muzzle extension with the MHD section long enough, >you can bring it back up and stabilize the pressure gradient again >after the projectile goes through. > > * Ok, I'm lying here. You can't do anything instantaneous with > a magnetic field, and I know it 8-) But that's not a problem, > all you do is design the MHD section long enough that you can drop > and then recover after the projectile passes through faster than the > air outside can push its way in (at 330 m/s - epsilon). > Not *instantaneously,* no; but various people are doing *wonderful* things with pulsed high-power technology --- for example, one can switch energy from an inductor to a capacitor in about a quarter of an LC-oscillation- period using plasma switches... I'm not sure whether this is even necessary under the "plasma-piston" concept, though; it would require some careful calculations regarding just how the plasma sheath surrounding the projectile might "tweak" the plasma-piston's driving fields. Since plasmas act (sort of) like perfect diamagnets over short time scales, the projectile would "snowplow" the magnet flux in the muzzle ahead of itself; from the projectile's viewpoint, this'll look like an additional backpressure; from the *gun's* viewpoint, there'll be a *BIG* EMF induced in the "plasma-piston's" field-coils. Also, the sheath-plasma will throw a short across the E-field plates. I suppose if one designed it properly, one could arrange for the plasma-piston to outrun the projectile once the projectile's sheath initiated it --- or perhaps even to drop back behind, and give it an additional impulse a-la a rail-gun??? (Actually, that last sentence is purest speculation; I really don't think projectile/plasma-piston interactions are susceptable to back-of-the-envelope analysis...) Gordon D. Pusch ! BITnet: TASCC A&D, Stn. 49a ! AECL, Chalk River Laboratories ! Phone: (613) 584-3311, X-4107 (off.) Chalk River, Ont. CANADA ! (613) 584-2368 (hm.) K0J 1J0 ! ------------------------------ Date: Tue, 27 Oct 92 08:28:03 EST From: Mark Flanagan To: gwh@lurnix.COM Subject: Re: gas dynamic end-caps for gun-launchers? Cc: space-tech@cs.cmu.edu George Herbert writes: >[ionized air curtain...] > > You can kill the E and B fields instantaneously* as the projectile > is about to launch through them, so they don't tweak with it. > If you design the muzzle extension with the MHD section long enough, > you can bring it back up and stabilize the pressure gradient again > after the projectile goes through. > My first reaction to this would be that your ionized gas would be continuously blown out of your barrel, both as the projectile passes through, and with minor static pressure variations at the muzzle. So...you either have a way to replenish it for the entire time the weapon is expected to be "ready to fire", or I'm missing something. (Never having studied MHD stuff, it would not be surprising...) Mark Flanagan mark@uab15.larc.nasa.gov ------------------------------ Date: 27 Oct 1992 13:35:24 -0500 (EST) From: "GORDON D. PUSCH" Subject: Re: gas dynamic end-caps for gun-launchers? To: space-tech@cs.cmu.edu George Herbert writes: > >Phil Fraering writes: >>How does the magic forcefield work? > >Easy. Ionize some air, or another gas ... [... description of textbook "crossed-fields" MHD device deleted ...] >... you just adjust things (voltage and B field) so that the force >in the ionized gas pushes "out", along the axis of the barrel, >and that the magnitude of the force is enough that the final pressure >at the muzzle end is atmospheric pressure. Voila. > This *might* conceivably work at very low pressures (i.e., molecular mean-free-path very much greater than the mean ion-cyclotron radius). At higher pressures (where the aforementioned quantities are comparable) the gas breaks down, and you'll get a propagating arc-discharge instead, traveling initially at the "ExB" drift-velocity; the arc will tend to act as a "snowplow," sweeping up the gas ahead of it. (Think of it as a rail-gun with a superposed B-field and no payload; alternatively, the "payload" is the residual gas in the tube... Isn't a railgun with an applied field sometimes called an "augmented" railgun???). (One might wonder why one couldn't simply coat the plates with an *insulator*, thereby preventing an arc-current from flowing, but leaving the E-field intact. However, I am assured by our electrostatic- deflector specialist that coating the plates with an insulator actually *INCREASES* the probability of breakdown (for reasons that are still somewhat poorly understood; it appears to have something to do with the surface properties of most known insulators), unless the insulator is thick enough to withstand the full applied voltage. H.V. insulators are actually based on the principle of forcing the arc to take a circuitous path through the *air* --- one long enough to keep it from breaking down...) So I don't think this idea'll work like a "Death-Star/STTNG shuttlebay"- type "static" pressure-curtain; it'll be an intrinsically non-stationary process... What one *MIGHT* be able to do is run it *cyclically*: think of the arc as a "virtual piston" in a piston pump; periodically re-initiate it to sweep out residual gas near the muzzle-end. I have absolutely *NO* idea if this is actually a viable concept, let alone how much power it'll eat... > >You can kill the E and B fields instantaneously* as the projectile >is about to launch through them, so they don't tweak with it. >If you design the muzzle extension with the MHD section long enough, >you can bring it back up and stabilize the pressure gradient again >after the projectile goes through. > > * Ok, I'm lying here. You can't do anything instantaneous with > a magnetic field, and I know it 8-) But that's not a problem, > all you do is design the MHD section long enough that you can drop > and then recover after the projectile passes through faster than the > air outside can push its way in (at 330 m/s - epsilon). > Not *instantaneously,* no; but various people are doing *wonderful* things with pulsed high-power technology --- for example, one can switch energy from an inductor to a capacitor in about a quarter of an LC-oscillation- period using plasma switches... I'm not sure whether this is even necessary under the "plasma-piston" concept, though; it would require some careful calculations regarding just how the plasma sheath surrounding the projectile might "tweak" the plasma-piston's driving fields. Since plasmas act (sort of) like perfect diamagnets over short time scales, the projectile would "snowplow" the magnet flux in the muzzle ahead of itself; from the projectile's viewpoint, this'll look like an additional backpressure; from the *gun's* viewpoint, there'll be a *BIG* EMF induced in the "plasma-piston's" field-coils. Also, the sheath-plasma will throw a short across the E-field plates. I suppose if one designed it properly, one could arrange for the plasma-piston to outrun the projectile once the projectile's sheath initiated it --- or perhaps even to drop back behind, and give it an additional impulse a-la a rail-gun??? (Actually, that last sentence is purest speculation; I really don't think projectile/plasma-piston interactions are susceptable to back-of-the-envelope analysis...) Gordon D. Pusch ! BITnet: TASCC A&D, Stn. 49a ! AECL, Chalk River Laboratories ! Phone: (613) 584-3311, X-4107 (off.) Chalk River, Ont. CANADA ! (613) 584-2368 (hm.) K0J 1J0 ! ------------------------------ End of Space-tech Digest #127 *******************