Marc.Ringuette@CS.CMU.EDU (01/28/89)
What follows is about 375 lines of information on high-velocity guns, with the focus being on firing projectiles at orbital velocity using current materials and chemical fuels. Surprisingly, the prospects look good! ------------------------------ From: Paul F. Dietz <dietz@cs.rochester.edu> I just read an interesting article: "The Distributed Injection Ballistic Launcher" H. Gilreath et. al., JHU APL Technical Digest 9(3), 1988, pp. 299-309. In a conventional gun, pressurized gas is injected once, and expands as the projectile travels down the barrel. As a result, acceleration drops off. The initial pressure is limited by the strength of the projectile and/or the barrel. Ideally, a gun should maintain constant pressure on the projectile. The DIL (approximately) does this by injecting gas behind the projectile from the sides at points along the barrel. This is a fairly old idea; the German V-3 guns in WWII used it (although they were never made operational). Just as a mass driver can be thought of as a linear electric motor, a DIL can be thought of as a linear internal combustion engine. Discrete injection of gas behind a flat-based projectile doesn't work very well. Instead, Gilreath et. al. propose to make the projectile boat-tailed -- that is, make its base be a long cone -- and inject the gas against the boat-tail as the projectile passes. If the boat-tail is sufficiently pointy (small boat-tail angle theta) then the axial velocity the gas must attain is reduced (by a factor of tan(theta)), and the system can operate efficiently even if the projectile is travelling much faster than the speed of sound in the gas. The limit the authors give is about 15 km/sec. The authors say (but do not justify) the mass penalties associated with launching directly to orbital velocities would be very great, due to the need for thermal protection. They suggest using the DIL as a first stage. They do say, however, that "complex electronics packages ... can easily tolerate accelerations of tens of thousands of g." That they say this isn't surprising, since JHU APL developed the first gun launched proximity fuse during WWII, and it tolerated 20,000 g, even though it contained five vacuum tubes. The article has an interesting picture of an extended-barrel 16" gun (conventional, not a DIL) that was operated in Barbados in the 1960s and early 70s. It could launch atmospheric diagnostic probes at 1.6 km/sec, with apogees up to 100 km (at a launch cost of few dollars per kilogram). The gun was used to launch scramjet test vehicles; they failed at launch, but theoretically they could have had a range of up to 3700 km with apogee at up to 1000 km. There is a picture of one test vehicle. It had a mass of 100 kg and burned 3 kg of triethyl aluminum (it is not clear if this vehicle had the stated range). It was designed to withstand accelerations up to 10,000 g, but suffered structural damage to its fins and skin in the test firing. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ From: attcan!utzoo!henry@uunet.UU.NET > ... "complex electronics packages > ... can easily tolerate accelerations of tens of thousands of g." > That they say this isn't surprising, since JHU APL developed the first > gun launched proximity fuse during WWII, and it tolerated 20,000 g, > even though it contained five vacuum tubes. Said packages do have to be specially built, though. The vacuum tubes for the proximity fuzes (a "fuse" is an electrical safety device) were quite unorthodox designs. They were placed on the axis of the shell to minimize centrifugal force, and the tube elements were made of the thinnest practical wire to get maximum benefit from the square-cube law. The tube envelopes, needless to say, were metal. I'm trying to recall a piece I saw some years ago on the electronics in the Copperhead laser-guided shell. My dim recollection is that they used ring-shaped circuit boards around a central core, supported the boards at both core and outer edge, and otherwise just used careful mil-spec circuit construction. > The article has an interesting picture of an extended-barrel 16" gun > (conventional, not a DIL) that was operated in Barbados in the 1960s > and early 70s. It could launch atmospheric diagnostic probes at 1.6 > km/sec, with apogees up to 100 km... Ah yes, HARP. (High Altitude Research Project.) A joint US-Canada project. The gun was two battleship barrels end-to-end. One hears occasional mutterings that work along those lines may have been continued for a while, on a smaller scale in secret. HARP used fairly straightforward methods: an extra-long barrel, smoothbore (it was actually 16.5 inches when they bored out the rifling, I think) with fin-stabilized projectiles, and subcaliber projectiles (that is, projectiles rather smaller than the gun bore, padded out to full diameter with a light-alloy jacket that falls off on departure from the barrel). These techniques are all standard, on a less ambitious level, for modern tank guns (although smoothbore guns weren't in HARP's day). "Distributed Injection Ballistic Launcher", indeed. :-) It's a booster cannon. (I think we can claim that Heinlein's name for it has priority, by about 40 years.) Henry Spencer at U of Toronto Zoology uunet!attcan!utzoo!henry henry@zoo.toronto.edu ------------------------------ From: dietz@cs.rochester.edu Following up on my last message about the distributed injection launcher, here are some other concepts I've read or thought about. All involve projectiles accelerated in a tube by gas pressure. Ram Accelerator --------------- In this concept, the space between the wall and the projectile forms an annular ramjet. Before firing, the tube is filled with some kind of gas (either oxidizer, fuel, a mixture or a monopropellant). The projectile front is shaped to compress the oncoming gas; the back is shaped to act as an inside-out nozzle. The projectile might carry fuel or oxidizer (perhaps in solid form). I recently read somewhere (on the net?) a report of a professor and students that built a model ram accelerator. Anyonme remember this? Travelling Charge Gun --------------------- Unlike a conventional gun, in which the charge is burned in the chamber, the charge in a TCG is attached to the back of the projectile and travels down the barrel. As a result, the pressure produced by the burning is applied where it does the most good. When the projectile velocity is high this is much more important than the loss in velocity due to the need to accelerate the charge. The TCG can be thought of as an inside-out solid rocket, where propellant burns inwards and the space between the propellant and the tube wall acts as a nozzle. The projectile could also have a spike projecting backwards to increase thrust. Unlike a conventional solid rocket, the projectile need not have a steerable nozzle or, indeed, any guidance at all. However, the fuel must burn much more quickly. One might also design a multistage TCG, equivalent to a multistage solid rocket. The first stage would help compress the exhaust from the second stage; this clearly isn't possible with ordinary rockets. One might also begin with the stages disconnected, perhaps with some buffer gas between them. This idea leads to... Multistage Light Gas Guns ------------------------- A large piston is accelerated by a conventional gun. It is rammed into a pump chamber filled with hydrogen or helium. The light gas is compressed and heated, and, after rupturing a diaphragm, accelerates a smaller projectile down a tube. The maximum muzzle velocity in a gun is, roughly, proportional to the initial speed of sound in the gas. The speed of sound is sqrt (gamma R T / M) where gamma is the ratio of specific heats, R is the universal gas constant, T is the initial temperature and M the molecular weight. So, it makes sense to use hot, light gases. There are light gas guns in operation that can accelerate gram sized objects to 7 km/sec or more. Naively, I think you could scale up guns while maintaining constant pressures and muzzle velocities. So, increasing the dimensions by a factor of ten would increase the projectile mass by a factor of a thousand. Scaling (for example) some numbers I saw for the Ames light piston gun (muzzle velocity: about 7 km/sec) to a 1000 kilogram projectile would give it a length of half a mile and a barrel diameter of four and a half feet. I'm not saying this would be practical, but it is interesting. A large light gas gun would use hydrogen, which is cheaper and has better performance. I wonder if it would be possible to combine light gas guns with traveling charges, so as to maintain pressure on the projectile at late times. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ From: dietz@cs.rochester.edu Continuing this thread... In traveling charge guns, the charge burns from the back forwards, not from the sides inwards. The pressure behind the projectile is already high, so using some sort of nozzle probably doesn't make sense. One thing that worries me about gun type launchers is abrasion between the projectile and the barrel wall. I assume this problem is solved by letting some of the gas leak around the projectile, forming a gas bearing. I talked to John Hunter at LLNL briefly. They are building a scale model of a light gas gun launcher. The full scale concept will launch projectiles with masses of several metric tons at 5 - 9 km/sec, at the rate of several per day. Their paper will be presented at a AIAA conference next July. Dr. Hunter said he would send me an abstract, and, if he does not object, I'll send more details to this list when I receive it. I find it encouraging that the professionals are actively investigating this topic. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ From: Andrew Higgins <ahiggins@uxe.cso.uiuc.edu> From: dietz@cs.rochester.edu (Paul F. Dietz) > I recently read somewhere (on the net?) a report of a professor and > students that built a model ram accelerator. Anyone remember this? You may be referring to the article "Impulsive Behavior" by Susan Sutphin in the April 1988 issue of _Space_World_ (Vol. Y-4-292 p. 18). This is certainly not a technical article, but it does give some good background information. A brief summary follows: Students at the University of Washington are working on a chemically propelled mass launcher. The project is headed by Professors Adam Bruckner and Abraham Hertzberg of the university's Department of Aeronautics and Astronautics. The vehicle is similar to the main body of a ramjet used in unguided missles. The vehicle travels through a stationary tube filled with premixed high pressure gaseous fuel and oxidizer. The vehicle carries no primary propellant of its own. According to Bruckner,"The concept is that we can accelerate a vehicle weighing several thousand kilograms up to about 10 kilometers per second using only chemical energy and readily available fuels." The project has produced a small scale model that uses a projectile weighing between 50 and 100 grams and achieves a velocity of 2,400 meters per second. They hope to increase this to 4,000 meters per second before having to move to a different facility. All design work is based on current technology. The university has signed a teaming agreement with Olin Corp. and has received a research grant from Langley Research Center to further the effort. Both Ames and Lewis Research Centers are showing interest. Bruckner, Hertzberg, and Bogdanoff currently have a number of patents pending. Also, according to the April 1988 issue of _Aerospace_America_, high velocity gun launch concepts were debated at the AIAA/Defense Advanced Research Projects Agency lightsat conference in Monterey CA, and at a similar conference at Utah State Univ. Someone may want to look into these. -- Andrew J. Higgins | Illini Space Development Society 404 1/2 E. White St apt 3 | a chapter of the National Space Society Champaign IL 61820 | at the University of Illinois phone: (217) 359-0056 | P.O. Box 2255 Station A e-mail: ahiggins@uxe.cso.uiuc.edu | Champaign IL 61820 ------------------------------ From: Ted Anderson <ota+@andrew.cmu.edu> I was the one who posted a note on the RAM Accelerator. The paper I have was published at the _37th meeting of the Aeroballistic Range Association_, Quebec, Canada, 9-12 September, 1986. The authors are A. Hertzberg, A.P. Brucknet, and D.W. Bogdanoff all from the Aerospace and Energetics Research Program, University of Washington, Seattle, WA 98195. On the issue of scaling up a light gas gun I'll mention that a John Hunter at LLNL was working on exactly this at least as recently as May. I have a draft of a paper he was preparing but I don't know what it's publishability status is. He determined that the system does indeed scale well. Ted Anderson ------------------------------ From: dietz@cs.rochester.edu From: Ted Anderson <ota+@andrew.cmu.edu> > The paper I > have was published at the _37th meeting of the Aeroballistic Range > Association_, Quebec, Canada, 9-12 September, 1986. The paper has now appeared in a journal: AIAA Journal, 26(2) (Feb. 1988), pages 195-203. It is slightly different from the conference version, but does not report the most recent experiments that reached 2.4 km/s. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ From: dietz@cs.rochester.edu I've received copies of some recent papers on the ram accelerator. The most interesting is: "The Ram Accelerator: A Chemically Driven Mass Launcher" P. Kaloupis and A. P. Bruckner. AIAA-88-2968. Presented at the 24th Joint Propulsion Conference, Boston, July 11-13, 1988. The paper describes the components of a system that can place a 2000 kg vehicle into LEO. The details are apropos to the discussion in this mailing list about gun based launchers. The launch vehicle has a diameter of .76 m and is 7.5 m long. It is first accelerated by a methane/air gun to 700 m/s. The ram accelerator contains nine different gas mixtures at 33 atmospheres (the gas mixtures tailored to have the correct properties for different ranges of projectile speed). The mixtures range from 0.5 CH4 + O2 + 3 CO2 (.7 to 1.1 km/s) to 8 H2 + O2 (7.2+ km/s). Peak acceleration is < 1000 g. The vehicles are made of graphite epoxy and have a total structural mass of 625 kg. The graphite epoxy is coated with carbon-carbon ablator. This heating is apparently not as bad as I had feared. Total mass loss for a 9 km/s launch velocity due to atmospheric heating (starting at 4000 m altitude) is only about 38 kg for a 16 degree angle of elevation, about 20 kg for 22 degrees. Velocity loss ranges from 10% (30 deg.) to 20% (16 deg.). It is claimed that mass loss from ablation decreases as muzzle velocity increases, because although the heat load is higher, the vehicle is not exposed to the heating for as long a time. It is also claimed that the heating is largely convective, not radiative. In-tube ablation due to passage through the propellant gas is less than 1 kg; ablation in the combustion zone, less than 3 kg. Unfortunately, the vehicle is aerodynamically unstable. They'd better add fins, I think. For a 9 km/s launcher, the launch tube is 5.1 km long and is made of 41,700 tonnes of AISI 4340 steel. The paper talks about orbital maneuvesrs. Solid rocket motors are ruled out because they could not withstand launch, and have insufficient performance. Instead, they propose using nitrogen tetroxide and monomethylhydrazine, pressurized by a gas generator using hydrazine. Isp = 297 sec, thrust = 10,000 newtons. Hardware mass of the propulsion system is approximately 200 kg. The vehicle is aerobraked down to LEO in one pass at 30-50 km without the use of special aerodynamic devices. This apparently does not present heating problems. Mass fraction to LEO ranges from 19% (8 km/s launch velocity, angle 22 deg) to 43% (10 km/s, 18 deg). However, they have apparently not addressed the problem of matching orbital planes, perhaps because they think waiting for precession to match planes is too time consuming. I find it encouraging that low launch angles lead to acceptable ablation. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ [ end of excerpt ] This material was posted to the space-tech mailing list a month or two ago. Space-tech is a mailing list which discusses the more technical side of space exploration. Topics have included EM launchers, solar sails, amateur satellite projects, and tethered satellites. If you want to join, send mail to space-tech-request@cs.cmu.edu with your name, Email address, and optionally a couple of lines saying who you are and what you do. 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