Date: Thu, 2 Mar 1989 11:31-EST From: space-tech-request@cs.cmu.edu To: "~/st/lists/stdigest" Subject: Space-tech Digest #27 Contents: Paul Dietz Re: RV velocity & railgun Roger Arnold Re: RV velocity & railgun Tom Neff Re: RV velocity & railgun Jim Meritt Re: RV velocity & railgun Paul Dietz Re: RV velocity & railgun Ron Fischer Re: Laser-based model rocketry Mike Van Pelt Re: Laser-based model rocketry Jordin Kare Re: Laser-based model rocketry Ron Fischer Laser-based rocketry fuel alternatives? Jordin Kare Re: Laser-based rocketry fuel alternatives? Peter Scott EML sites Nick Szabo gun-launch time criticality Paul Dietz EML Conf. Papers -- January 1989 IEEE Trans. on Mag. Jordin Kare Re: EML Conf. Papers Paul Dietz Railgun EML ------------------------------------------------------------ Date: Fri, 24 Feb 89 16:32:49 EST From: dietz@cs.rochester.edu To: well!tneff@uunet.UU.NET Cc: jtk@mordor.s1.gov, space-tech@cs.cmu.edu Subject: RV velocity & railgun Tom Neff and Jordin Kare made some comments about loss of mass/velocity due to air resistance making space bombardment infeasible. My reference is Kaloupis and Bruckner "The Ram Accelerator: A Chemically Driven Mass Launcher", AIAA-88-2968, July 1988. They consider a 2000 kg vehicle launched at an elevation of 4 km above sea level. With a muzzle velocity of 9 km/s and a launch angle of 30 degrees above horizontal, their vehicle loses less than 1 km/s and 0.7% of its mass on the way up. Mass loss is lower at higher launch velocities. I assume the vehicle is stored in a highly elliptical orbit. This is because I am not considering use against time critical targets, like tanks. Deorbited at apogee, the vehicle reenters near vertically, so it will traverse less atmosphere during reentry than during launch, even if the target is at sea level. Moreover, we can design the vehicle to jettison low density components, like empty fuel tanks, before reentry, leaving only a high density impactor -- lead or tungsten coated with an ablator, say (I do not assume terminal guidance, although perhaps it would be possible). In contrast, the mean density of the ram accelerator projectile at launch is roughly 1 g/cm^3. Comparisons with meteorites are suspect, because meteorites are roughly spherical, while manmade objects can be highly elongated and be carefully shaped to reduce drag. The ram accelerator's projectile's length is about ten times its diameter; it has a sharply pointed nose. (It is also unstable, unfortunately.) Maybe Kaloupis and Bruckner have grossly underestimated the drag and ablation on their projectile. I don't know. But their figures contradict the assertions that drag and mass loss will be intolerable. Perhaps if Jordin's reference to RV's is inaccurate it is because nuclear RV's are not designed for very low drag? Tom Neff suggested this system in inferior to nuclear weapons. He is perhaps confused about the intended area of application. It is not intended, as in SF stories, as a means to knock out missile silos. Rather, it is intended for use against third-world countries, where the use of nuclear weapons is unthinkable. Would the British have found such a system useful in the Falklands? The US in Vietnam? (No moral judgements implied here.) Nuclear weapons were not used in either case, of course. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ Date: Fri, 24 Feb 89 17:42:15 PST From: telesoft!roger@ucsd.edu (Roger Arnold @prodigal) To: dietz@cs.rochester.edu Subject: Re: RV velocity & railgun Cc: space-tech@cs.cmu.edu On the subject of intercontinental, non-nuclear bombardment, there has been military interest in use of OSC's "Pegasus" launch vehicle for that purpose. I've lost the reference, but it was from a military special interest newsletter that appeared in the newsletter excerpts section of High Technology Business about 6 months to a year ago. At $10 million per shot, minimum, it would be limited to use against high priority targets. I believe that the article quoted some general talking about delivery accuracies on the order of 1-10 meters, based on use of GPS data, and possibly optical lock-on in a terminal guidance phase. One of the military developments that could have a BIG impact on both manned and unmanned launch capabilities would be a decision to rely on intercontinental bombardment for defense of Western Europe after a US pullout. Such a strategy would be dependent on the feasibility of delivering clusters of small, smart reentry vehicles capable of spotting and killing tanks. Without getting into issues of technical feasibility of the RVs, I think it's clear that economic feasibility would require a much cheaper way to deliver the clusters than anything we currently have. Railguns or ram-cannon are perhaps possibilities, but somebody mentioned that intercontinental bombardment is the province of the Air Force. I think the Air Force would prefer a more flexible, less vulnerable, and more "Air Force-ish" alternative: a hypersonic flyback booster cabable of delivering the equivalent of half a dozen or so Pegasus upper stages. It would, of course, also make a dandy booster for a low cost launch vehicle. I don't expect this to happen. Makes more sense just to arm Europe with enough shoulder fired tank-killing mini-missiles to make it an unattractive target for invasion. If you believe it isn't already. - Roger Arnold ..ucsd!telesoft!roger ------------------------------ Date: Sat, 25 Feb 89 14:34:14 PST From: well!tneff@apple.com (Tom Neff) To: uunet!cs.rochester.edu!dietz@apple.com Subject: Re: RV velocity & railgun Cc: jtk@mordor.s1.gov, space-tech@cs.cmu.edu Paul Dietz's Friday message rebuts the assertion that kinetic bombardment projectiles would lose too much energy on their way up through the atmosphere and back down again. He posits a highly elliptical orbit, since deorbiting at apogee would allow nearly vertical re-entry. The problem I have with this is that you lose the element of surprise. Once launched and "stored" these things would surely be watched. If you do a retroburn at apogee you let your opponent in on the game nearly half an orbital period ahead of time. On the crudest level, there is plenty of time to launch a retaliatory counterstrike and inaugurate Armaggedon before the projectile even enters. The objection is also offered that this kind of system would only be for use against Libya or the equivalent midget enemy-of-the-decade. The twofold answer is this: even third world gadflies have superpower patrons unwilling to sit by and watch the other side wreak havoc outside its accepted sphere (and conversely, any troublemaker so unmitigatedly friendless among nations as to be a risk-free target for a kinetic bombardment strike could presumably be dealt with more cheaply, not to mention comprehensively, by a conventional attack). Second, all this is too expensive to waste on Libya and its ilk. If that's all it's good for, it's a waste of money. ------------------------------ Date: Mon, 27 Feb 89 08:06:39 EST From: jwm@stdc.jhuapl.edu (Jim Meritt) To: dietz@cs.rochester.edu, well!tneff@uunet.uu.net Subject: Re: RV velocity & railgun Cc: jtk@mordor.s1.gov, space-tech@cs.cmu.edu >Tom Neff suggested this system in inferior to nuclear weapons. He is >perhaps confused about the intended area of application. It is not >intended, as in SF stories, as a means to knock out missile silos. >Rather, it is intended for use against third-world countries, where >the use of nuclear weapons is unthinkable. Would the British have >found such a system useful in the Falklands? The US in Vietnam? (No >moral judgements implied here.) Nuclear weapons were not used >in either case, of course. For grins, look at the latest in the Destroyer series. A "third world country" deploys such a system against the US (with no terminal guidance). It uses EM launching and shoots a (snicker) train engine.. Jim Meritt ------------------------------ Date: Mon, 27 Feb 89 14:28:54 EST From: dietz@cs.rochester.edu To: well!tneff@apple.com Cc: jtk@mordor.s1.gov, space-tech@cs.cmu.edu Subject: RV velocity & railgun > The objection is also offered that this kind of system would only be > for use against Libya or the equivalent midget enemy-of-the-decade. > The twofold answer is this: even third world gadflies have superpower > patrons unwilling to sit by and watch the other side wreak havoc > outside its accepted sphere (and conversely, any troublemaker so > unmitigatedly friendless among nations as to be a risk-free target for > a kinetic bombardment strike could presumably be dealt with more > cheaply, not to mention comprehensively, by a conventional attack). (1) I suppose the superpower patronage explains why, after Reagan bombed Libya, the russians nuked us. (2) As for conventional attack being cheaper: I don't think this is necessarily so. Conventional attack places at risk expensive aircraft and personnel. We lost a bomber in the Libya raid, remember, and the russians lost lots of aircraft to Stingers. Recall that this argument started after it was claimed in that article that the system could be built for $300 M -- less than the cost of one stealth bomber. The claimed cost of $10-40/lb to orbit would be hard to match with conventional explosives delivered by cruise missile. > Second, all this is too expensive to waste on Libya and its ilk. If > that's all it's good for, it's a waste of money. We spend a large amount of money countering and intimidating Libya and its ilk. If it is a waste of money, then so is much current military spending (I'm not saying it isn't). I suggest another mode of operation. After we place a weapon in orbit, we sell it. This is no different from the current sale of weapons, except that the customer takes control remotely. We might maintain veto power (a destruct charge in each weapon, say) to prevent use against ourselves. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ Date: 24 Feb 89 17:22:37 PST (Friday) From: "Ron_Fischer.AISNorth"@Xerox.COM Subject: Re: Laser-based model rocketry To: jtk@mordor.s1.GOV cc: dietz@cs.rochester.EDU, space-tech@cs.cmu.EDU Question: are there practical lower limits on the size of a laser launch system? E.g. would it be possible with several hundred dollars to create a laser model rocketry platform? Perhaps we could ignore the obvious FAA, OSHA, etc. problems for a moment. Clearly, constructing the laser and aiming device would be all of the work. This may not be out of the question however. Back in late high school I purchased plans for a 20 watt IR laser. How much power would a very small (perhaps 2 ounce or so) target need? Just lofting it a few hundred feet in the air would be interesting. (ron) ------------------------------ Date: Sat, 25 Feb 89 11:57:39 PST From: v7fs1!mvp@apple.com (Mike Van Pelt) To: ames!Xerox.COM!"Ron_Fischer.AISNorth"@apple.com Subject: Re: Laser-based model rocketry Cc: space-tech@cs.cmu.edu >Back in late high school I purchased plans for a 20 watt IR laser. >How much power would a very small (perhaps 2 ounce or so) target need? >Just lofting it a few hundred feet in the air would be interesting. Jordin is probably going to answer this in more detail, since it's one of his favorite topics. But according to a talk I heard by Dr. Kantrowitz a few years ago, the thrust for laser-launching is generated by a "laser propagated detonation" in the gas next to the target. This happens at a flux density of about (I think) 2 megawatts per square centimeter, which isn't practcal with a 20 watt laser. (He also said that this phenomenon is why people who say that lasers can be defended against with mirrors just don't understand lasers.) Also according to Kantrowitz, in regards to the problem of IR absorption by water vapor, it should be possible to avoid this problem by a scheme involving double pulses. A first, lower power pulse would excite the water vapor into a higher energy state where it is transparant to infrared. Then the second pulse, at much higher power, would punch through a column of air that had been "bleached" by the leader pulse. This was several years ago, and I don't know what the current state of the art is. You ought to hear Jordin's song about laser launchers. I heard him sing it at Westercon in Sacramento once -- outstanding. It's worth sitting though a whole night of variations on "Banned from Argo" to hear the occasional song like this one. ------------------------------ Date: Mon, 27 Feb 89 13:32:09 PST From: Jordan Kare To: "Ron_Fischer.AISNorth"@xerox.com, jtk@mordor.s1.gov Subject: Re: Laser-based model rocketry Cc: dietz@cs.rochester.edu, space-tech@cs.cmu.edu A laser model rocket is probably not practical -- though it's a fun thought. Typical laser thrust levels are a few grams per kilowatt (e.g., we're trying for 10 kg thrust/Megawatt at high specific impulse, and 100 kg/Megawatt in air-breathing mode, where you have unlimited mass). So even a 10 g model rocket will need >100 watts of laser, plus optics capable of tracking the vehicle. My own lab setup has a 100 J laser, but it only fires every minute or so; finding pulsed lasers with respectable average power is hard. On the other hand, it would be a great project for someone/some group to do... Be warned -- it would be NOISY... Jordin Kare ------------------------------ Date: 28 Feb 89 14:28:23 PST (Tuesday) From: "Ron_Fischer.mvenvos"@Xerox.COM Subject: Laser-based rocketry fuel alternatives? To: jtk@mordor.s1.GOV cc: dietz@cs.rochester.EDU, space-tech@cs.cmu.EDU Are there alternate fuels that might be more volatile and require less energy delivered by the laser? Or is 2mw/cm sq. a minimum for any form of "laser propogated detonation?" I'm tempted to open this up to an examination of using Amateur rocketry to experiment with alternative launching techniques. This would be a hobby I could respect. (ron) ------------------------------ Date: Wed, 1 Mar 89 14:52:50 PST From: Jordan Kare To: "Ron_Fischer.mvenvos"@xerox.com, jtk@mordor.s1.gov Subject: Re: Laser-based rocketry fuel alternatives? Cc: dietz@cs.rochester.edu, space-tech@cs.cmu.edu RE alternative propellants -- you can evaporate (ablate) materials with fairly low fluxes -- 10^5 w/cm^2 or less; it takes upwards of 10^7 w/cm^2 to propagate a laser-supported detonation (supersonic shock) wave. In between, depending on the conditions, you will either just get faster and faster ablation as the flux increases, or you may form a (subsonic) laser-supported plasma. CW laser rockets, for instance, use such a laser-supported plasma sitting in the throat of a nozzle to heat a continuous gas stream. Going to volatile propellants usually doesn't do you much good, esp. in a "real" (high Isp) laser rocket -- the laser is adding much more energy than is present in the chemical bonds in the propellant in any case. Usually the problem is the reverse -- the propellant evaporates too easily and "dribbles" away between laser pulses. But there might be regimes where a low-Isp ablation-mode rocket would benefit. There are schemes around for "laser steering" using a laser to trigger burning of chemical propellant (small explosive charges, in fact). Amateur laser rockets, amateur gas guns, etc. would be neat, but most advanced techniques I know of scale badly -- i.e. it's hard to make a small version that is not dominated by very different problems from a full-sized one. And safety is always a concern. But I would be interested in seening suggestions. Jordin Kare ------------------------------ Date: Tue, 24 Jan 89 10:33:20 PDT From: Peter Scott Subject: Re: EML sites To: space-tech@cs.cmu.edu Hi, I just joined the list and read the condensed discussion of electromagnetic launchers. dietz@cs.rochester.edu (Paul Dietz) writes: >They >found 50% of the velocity loss occured below 6 km, while mass loss >from the ablator peaks at 25 km. This strange situation occurs >because at lower altitudes the dynamic pressure is higher, so the >blowing layer of vaporized carbon is more optically dense and blocks >radiation from the shock more effectively. I'm sure there are a bunch of 6km high mountains near the equator (Mount Kenya, 5200m, overlaps the equator line in my organiser's world map; Kilimanjaro, 5895m, less than 5 degrees away; in Ecuador, Chimborazo Peak, 5267m, less than 3 degrees away, and in Peru, Huascaran, 6768m, less than 10 degrees away). Not having personally visited any of them, I don't know how easy it would be to construct an EML on any of them, but *surely* it'd be worth it on at least one of them. Maybe in return for the foreign investment they'd waive the environmental impact report :-). I'd like to hear informed opinion from anyone who's visited any of those mountains, as well as discussion on the pros and cons of constructing an EML there. The principal advantage seems to me to be not the energy savings but the chance to avoid using expensive high-temperature casings. Peter Scott (pjs@grouch.jpl.nasa.gov) ------------------------------ Date: Sat, 25 Feb 89 16:02:19 PST From: szabonj@minke.cs.washington.edu (Nick Szabo) To: space-tech@cs.cmu.edu Subject: gun-launch time criticality How much extra energy is needed by EML or gas-gun launchers given time constraints? For example, to deliver mail from Pike's Peak to Tokyo, how much extra energy is required to get it there within 2 hours, instead of putting it in a highly eliptical orbit? Nick Szabo szabonj@larry.cs.washington.edu ------------------------------ Date: Wed, 1 Mar 89 17:38:51 EST From: dietz@cs.rochester.edu To: space-tech@cs.cmu.edu Subject: EML Conference Papers -- January 1989 IEEE Trans. on Mag. The 1/89 issue of IEEE Transactions on Magnetics contains the papers presented last year at the Electromagnetic Launch conference in Austin. Very interesting. Most of it is low level grunt work aimed at small-scale railguns, but there are some papers about earth-to-space launchers. The lead off paper, an invited paper by H. Fair et. al., states: "... specific examples are given to demonstrate significant recent progress in hypervelocity atmospheric transit and hardening of launch vehicle components to withstand higher launch accelerators [sic] by several orders of magnitude. ... Based on technical achievements in all the critical supporting technologies, we believe it is now feasible to demonstrate a credible electromagnetic earth to space launch system." Miles Palmer and A. Dabiri have a paper on a cut-rate railgun based system. This is, I believe, the system described in the Discover article. Their discussion of ablation and heating tends to make me think that the analysis of the ram accelerator was too optimisitic (oh well). The article describes the behavior of conical nosed projectiles with small radius nosetips. This leads to severe heating, so the nosetip must be actively cooled (transpiration) for best performance. They suggest using lithium as a coolant. The hypersonic drag coefficient for their design is .019 with a nosetip half-angle of four degrees (vs C_d ~ 1 for conventional cylindrical projectiles). A most interesting part of their design was the power supply. To power their railgun they want to use a single large inductor charged by lead-acid batteries(!). The inductor, an aluminum coil in concrete, is cooled in LN2 and stores 80 GJ. The battery system supplies 610 MW. The vehicle mass is 600 kg, accelerated to 8 km/s over a distance of 250 meters. The inductor cost is $56 M, the batteries, $67M. Because the railgun is short it can conceivably be slewed to different orientations, which adds operational flexibility. They state: "The low cost battery based EMSL systems could be amortized even given present launch requirement, unlike previous systems." [referencing previous studies of railgun launchers] Finally, there is an interesting invited article by Ribe and Barnes reviewing impact fusion; specifically, magnetically insulated impact fusion. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ Date: Wed, 1 Mar 89 17:27:12 PST From: Jordan Kare To: dietz@cs.rochester.edu, space-tech@cs.cmu.edu Subject: Re: EML Conference Papers -- January 1989 IEEE Trans. on Mag. >From: dietz@cs.rochester.edu >Message-Id: <8903012238.AA29584@kochab.cs.rochester.edu> >To: space-tech@cs.cmu.edu >Subject: EML Conference Papers -- January 1989 IEEE Trans. on Mag. > >Miles Palmer and A. Dabiri have a paper on a cut-rate railgun based >system. ...Their discussion of ablation and heating tends to make me >think that the analysis of the ram accelerator was too optimisitic (oh >well). This has been my impression for some time. > The article describes the behavior of conical nosed >projectiles with small radius nosetips. This leads to severe heating, >so the nosetip must be actively cooled (transpiration) for best >performance. They suggest using lithium as a coolant. The hypersonic >drag coefficient for their design is .019 with a nosetip half-angle of >four degrees (vs C_d ~ 1 for conventional cylindrical projectiles). This strikes me as pretty optimistic; enough so that I wonder if they're doing the drag calculations right. But I'll have to get the paper & run it past some aero types.... > >A most interesting part of their design was the power supply. To >power their railgun they want to use a single large inductor charged >by lead-acid batteries(!). Why (!)? This is becoming downright popular, and is certainly a sensible approach. A good truck battery can supply something like 10 kw for 1 cent/watt -- cheapest peak power available on time scales of seconds to minutes. I once designed a lunar laser launcher and assumed the energy storage was 2 MW-hours of car batteries to get a worst-case mass figure. Came out with a perfectly reasonable number -- prime power was still heavier. Incidentally, tests have shown that Sears Die-Hards are by far the best for powering things like railguns (seriously!). Jordin (Hard Charging) Kare ------------------------------ Date: Thu, 2 Mar 89 09:13:38 EST From: dietz@cs.rochester.edu To: jtk@mordor.s1.gov Cc: space-tech@cs.cmu.edu Subject: Railgun EML >>Their discussion of ablation and heating tends to make me >>think that the analysis of the ram accelerator was too optimisitic (oh >>well). > This has been my impression for some time. Actually, on rereading Palmer&Dabiri's paper, it appears that coolant requirements are minimal for large projectiles. For a 500 kg mass launched at 12 km/s at 15 degrees, they calculate that 2.1 kg of lithium coolant are required (nosetip radius = 5 mm). >> The hypersonic >> drag coefficient for their design is .019 with a nosetip half-angle of >> four degrees (vs C_d ~ 1 for conventional cylindrical projectiles). Correction: I meant the *nose* half-angle is 4 degrees. The nosetip is spherical. > This strikes me as pretty optimistic; enough so that I > wonder if they're doing the drag calculations right. But I'll > have to get the paper & run it past some aero types.... They give the following empirical formula for hypersonic drag coefficient, drawn from F. Regan, "Re-entry Vehicle Dynamics", AIAA education series, 1984, p. 230. For small nosetip halfangle theta, and for speeds greater than mach 15 (4.5 km/s), ~ C_D = 2 * theta**2 + (rnose / rbase)**2 The two terms are body pressure drag and nosetip pressure drag. This assumes theta >~ 2 degrees. They say that at smaller angles, skin drag becomes significant. Their initial operational payload has theta = 4 degrees, rnose = 2.2 cm, rbase = 22 cm (so the vehicle is a cone about 3 meters long). >>A most interesting part of their design was the power supply. To >>power their railgun they want to use a single large inductor charged >>by lead-acid batteries(!). > Why (!)? This is becoming downright popular, and is >certainly a sensible approach. Well, it seemed a neat hack from my position of relative ignorance. In all fairness I should have recalled a note that you (?) posted on the subject last year. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ End of Space-tech Digest #27 *******************