Talk:Light gas gun

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I'm not an expert, but I think this needs to be discussed:

I'm not convinced by the argument that higher projectile velocities are achieved solely by increasing the speed of sound through the gas, or that it's a limiting factor. While it may be true that pressure differences are transmitted at the speed of sound in most conditions, I was under the impression that it was different above a certain pressure threshold. Otherwise, surely such things as blast shockwaves (which are by definition supersonic relative to the working medium) wouldn't exist. Granted, the speed of sound within the immediate influence of the shockwave is changed, but I don't think that this is what the article is saying.

I suppose this all depends on what you're defining as the speed of sound however. Either way, I think the way the article describes things is a little misleading. It might be better to describe the physics in terms of molecules and their interactions, rather than the speed of sound being a reason for the limit in velocity.

As I say, I'm by no means an expert, but I'm just going on what seems intuitive to me.

Keep in mind that the speed of "sound" is the speed of compression waves in the medium. There are ways to propogate energy faster than that, but they don't rely on pressure waves. Above a "certain pressure threshold" things do change radically, as the pressure is high enough to condense the medium, which will radically increase the density and therefore the speed of sound in that medium.

As for explosive shockwaves, they do travel at very high velocities, but if the explosive relies on pressure to detonate, then the detonation velocity must be limited to the speed of sound in the explosive itself--that's just how fast pressure waves can propagate. If the detonation relies on thermal energy, then the speed of sound can be exceeded, as the heat energy travels at the speed of light in the medium, and that will then be the limiting factor of the explosion (the reason explosives don't have light-speed shockwaves, and in fact most have subsonic shockwaves, is that it takes time to transfer enough energy to initiate decomposition. In nuclear explosions, the much of the initial energy transfer is of radiant energy, so it will happen at supersonic speeds, but after a brief while, the dissipation of the energy is such that the thermal energy is insufficient to superheat the air, and the shockwave drops to sonic speeds.

Now it is possible to achieve >5000 fps velocities with shrapnel from bombs, and that is because the shrapnel is in physical contact with the solid explosive as it detonates, so the limit then is the speed of sound in the bomb components, which are solids (in the case of true shrapnel, it would be the speed of sound in the explosive, in the case of shell fragments it would be the speed of sound in the shell casing). Harness this in a gun, and it would give you a velocity increase of nearly an order of magnitude over current firearms, but the trick is to use detonating propellants without turning said gun into a bomb. So far the only solutions to this problem I've encountered are explosive shells and claymore mines. scot 16:07, 26 September 2005 (UTC)

One more addition: you wrote in your first edit this:

Unless I'm misunderstanding the way it's been explained, the article seems to ignore the bulk movement of molecules in the gas, to which there's no fundamental limit, and has to be a significant factor, right?

I'd like to address that, since I think it is a very good question. A particle accelerator, for example, has no problems accelerating particles up to a good fraction of the speed of light, and thos particles can be bounced off of an object to impart there momentum. The problem is, for this to work, you have to have all the particles going in the same direction--sort of a "matter laser"--and that's not the case in a chemical reaction. The molecules have no significant intertia--ignited in a vacuum, they'd just form an expanding cloud, but the center of mass would remain stationary. The pressure is what does the work and the pressure is limited by the nature of the replusion between the particles of the gas--this is why pressure doesn't impact the speed of sound (the replusion is still the same) but temperature does (higher temps mean more replusion, which is also why higher temp gasses have higher static pressures). scot 19:24, 26 September 2005 (UTC)