bilbo.niket@LOCUS.UCLA.EDU ("Niket K. Patwardhan") (01/17/86)
Can somebody tell me what the disadvantage of ordinary guns (blob in a barrel propelled by hot gases under pressure) is as compared to the rail gun? It can't be length....Using the formula V**2 = U**2 + 2*a*s and 8000 metres/second as the final velocity (orbital) and 50 m/sec*sec (5G) as the maximum accelaration, one gets 640,000 metres as the necessary length. Thats 640 km or 400 miles! NO GUN OF THAT LENGTH could be feasible. However if you are trying to get just MASS up, you could increase the G-factor to say 5000 and wind up with a gun of 640 metres. If your payload was a chunk of solid steel 1 metre long you would require about 350 atmospheres of pressure to get this kind of accelaration, which is easy! (Steel ~7000kg/m**3). I have heard of some restriction due to the speed of sound in the gases.. but this should be easy to overcome by raising the temperature and selecting the right gas. (hydrogen????) PS What is the speed of sound in hydrogen at 3000K? What is the melting point of tungsten?
dietz@SLB-DOLL.CSNET (Paul Dietz) (01/19/86)
There are several advantages of an electric gun over a chemical gun. First, the chemical gun is limited by the velocity of sound in the gas; if the projectile moves faster than the gas particles it can't be accelerated. Unless exotic propellants are used (pressurized hydrogen, for example) velocity is limited. Second, the gas in a conventional gun fills the entire space between the breach and the projectile, meaning that gas has to be added while the projectile is being accelerated (hard) or the initial pressure must be very high. A conventional railgun shares the second problem (only it is magnetic flux, not gas, that fills the gun) but more current can be pumped in easily. Coilguns don't have that problem, and should have very high efficiencies. I wonder how hard it would be to design a hydrogen gas gun in which the gas is arc heated as in a railgun. It wouldn't be too efficient, but the major cost of launching is going to be capital costs, not electricity costs, so it might be worth looking into.
space@ucbvax.UUCP (01/20/86)
The rail guns being discussed are smaller than you think since they are designed to shoot small projectiles as weapons, not large payloads. A recent test was made with a plastic projectile (it must have some iron in it?) that puched its way into a solid block of aluminum. An issue of Spectrum covered rail guns and other weapons that might be used in a SDI system.
bilbo.niket@LOCUS.UCLA.EDU ("Niket K. Patwardhan") (01/21/86)
Your suggestion of arc-heating the hydrogen gas triggered another idea..... Why not work on a "rail-rocket"..... This would not have the speed of sound restriction, since by suitable nozzles the ejection velocity can be increased beyond the speed of sound. Essentially the "rail-rocket" would be a rocket that got its heating power from two rails that feed electrical power into an arc that was located where the combustion chamber would normally be. If the rocket was encased in an open barrel you would have a "rail-bazooka": and the rails and their support structure would be protected from the blast. BTW the speed of sound in hydrogen should be around 3900-4100 m/s at 3000K. Also the melting point of tungsten is 3370 or 3410 C depending on which source I look at (eshbach?? Engineering Handbook OR Encyclopaedia Brittanica).
dietz@SLB-DOLL.CSNET (Paul Dietz) (01/21/86)
A rail rocket would have to carry a lot of hydrogen, and have a VERY high mass flow rate (all the mass must flow out in a fraction of a second, or the barrel of the gun must be very long). Also, you'd have to commutate the thing somehow, which is hard at high velocity. Actually, a railgun can be thought of as an arc-rocket. Magnetic forces confine the exhaust gases at the base of the projectile, so the only mass flow is from an ablative coating on the back of the projectile, and that's just for thermal protection. I don't know if you want a light gas in the arc, or if any material will do. An alternative to the rail rocket would be a launcher in which the hydrogen fuel is heated by an intense laser or particle beam. A high voltage electron beam, for example, might be easier to generate (at a given power level) than the high current pulse a railgun needs.