Talk:Light-gas gun

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)

In electromagnetism, propagation inside a waveguide can be quite different than in free space, including a different propagation speed. (In fact, the phase velocity can be much higher than the free space speed of light, although the group velocity limits the speed of information transfer to the free space speed of light). Since there are many (albeit imperfect) analogies between EM waves and pressure waves, I would think that pressure wave propagation inside a barrel (with a diameter comparable to the wavelength) would be very different than in free space, including the propagation speed, which could be higher than the free-space speed. I don't have time to work through all the details right now to see if this is the case. Can anyone comment on this? The wikipedia article on Muzzle_velocity doesn't mention pressure wave propagation speed as a limiting factor, but rather trade-offs between speed of combustion, barrel length, practical barrel burst strength, etc. I know that some normal rifles can reach as much as 4000 ft/sec muzzle velocity (1250 m/s or Mach 3.7 in free space). It is hard to believe that the temperature effect is enough to explain a factor of 3.7. According to the formula in the wiki article Speed_of_Sound, this would require a gas temperature of 3500 deg C, which seems too high. 209.6.248.221 04:25, 15 September 2007 (UTC)

Actually if Ia m reading the article right (and the article sources) the LGG works more like an airgun (a bb gun for instance) than say a firearm. A firearm is basically a combustive reaction that propells the bullet out, Airguns use compressed gas decompressing to fullfill the same role. —Preceding unsigned comment added by 168.103.116.242 (talk) 02:52, 23 December 2007 (UTC)

Yes, a light gas gun is a giant piston "air gun" using H₂ or He as a working fluid. You can actually use an He tank and a piston airgun and make what is technically a light gas gun by just filling the cylinder with the light gas. In practice, light gas guns use a piston propelled by chemical propellants (i.e. a firearm with the piston as the bullet) rather than a spring or gas strut. scot (talk) 17:05, 23 December 2007 (UTC)

Units of measure as they relate to individual guns

The section describing current operational light gas guns at Ames Research Center has been edited to reflect the original gun size designations. Throughout their 40+ year history, these guns have been identified by their bore sizes, in inches. The application of metric units was adopted from the astronomical community at Ames in the late 1950s, and was applied to velocity measurements exclusively. The four gun bores are .170", 0.5", 1.0", and 1.5".

Speed of sound of hydrogen

At conditions achieved in two-stage light-gas guns, the speed of sound of hydrogen depends on both pressure and temperature. This is because the hydrogen is compressed to densities where the interaction between the fluid particles can no longer be neglected. One publication that deals with the equation of state of hydrogen at such conditions is NRL Report 6675 from 1968, available via dtic [[1]]. Fig. B3 in this report plots sound speed vs. temperature for various pressures. From this graph it is obvious that "The speed of sound also increases with the temperature of the fluid (but is independent of the pressure)" (as stated in the article) is not correct for relevant conditions. There are newer references that confirm those findings.

Since I'm no native speaker, I would like to leave an update of the article to somebody more familliar with English.

By the way, Fig. B3 in NRL Report 6675 also shows that the speed of sound increases considerably with pressure. In high performance experiments with two-stage light-gas guns, more than 10,000 atm are achieved in hydrogen. According to the figure, this would correspond to a sound speed of roughly 5.5 km/s. 153.96.89.5 (talk) 20:27, 11 December 2014 (UTC)

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