possibly stupid firearm physics question.

[Kaylee]

okay... this may be a dumb question, as it's been years since chemistry, but......


To go from a solid form to a gaseous form is an endothermic reaction, yes?

So.. would it be possibly to flash-sublimate some Unspecified Substance from a solid into a gas, thus providing the gas to push a projectile down the barrel, but pulling heat OUT of the system rather than dumping it in? And if so, would this solve one of the problems with caseless?


-K

 

[ronin308]

I think that the Rx you are referring to is not an endothermic one. An endothermic Rx is one where heat is being held in, the sign of this is that the object is cold. When an object is hot, its giving off excess heat, this is called an exothermic Rx. So I think that a solid-->gas Rx is an exothermic one. As far as your other question: Hmm...good question...

Disclaimer- I haven't taken a chem class in about 2 years

 

[tyme]

I'm not sure there's any compound that sublimates fast or instantaneously. Without the heat-induced pressure wave, I doubt it would be much more than a spit-wad launcher.
[edited again]
Sublimation is endothermic. I guess the only qualification is that energy is taken out of the environment, not that there's some multi-compound reaction as well.
[further explanation]
Although endothermic reactions often cool the substances involved because of chemical changes (e.g. KOH + H2O + Heat -> K+ + 2OH- + H+) (wow I get the feeling that's wrong somehow), the same thing is happening during sublimation. It takes heat (energy) to make something sublime, but the temperature of the compound doesn't change during a state change. It's deceiving: when dry ice sublimates, the gas doesn't feel as cold as the dry ice, but what you feel is mostly near-room-temperature O2, N2, and water vapor that's been cooled by the CO2 coming from the dry ice. Eventually, in a sealed room (danger: you'll run out of oxygen if you really try this) and with enough dry ice, room temperature would drop to the sublimation temperature.

 

[Peter Gun]

Co2 sumblimates directly at 1 atmosphere. It only has a liquid state under pressure. hence the smoke effect of dry ice. and it is endothermic.

 

[MeekAndMild]

I think Peter Gun has it. This is how paintball markers (guns which shoot paint balls)convert liquid CO2 under pressure to gaseous CO2 which then expands. But that presents the opposite problem, seen with paintball markers in cold weather, ie freezup to a low temperature low pressure solid, which then jamms the toy until it sublimates.

This is why some of the pro paintball players use compressed nitrogen markers. These guns are still limited by the universal gas law constant PV=nRT so the nitrogen cylinders get cold, but the gas lines don't freeze.

 

[40ozflatfoot]

(blank)-thermic notwithstanding, what you're talking about is a system whereby an amount of energy required to force a fast transition from a solid to a gas directly, with no liquid state in between. Your system must also produce enough gas to launch a projectile downrange at velocities comparible to those that use conventional propellants and methods. And, of course, in order to do so, you must put energy into the system to force sublimation. Such an energy source would be very cumbersome, and quite expensive to use.

Perhaps, as we gain a better understanding of state transitions, and develop new compounds, such an energy source could be produced.

But, if that happens, why use it to launch a projectile? After all, wasn't the secret of the phaser the power pack? A "projectile" with a muzzle velocity of the speed of light is somewhat more preferrable to one with a muzzle velocity that can allow high speed photography to catch it in flight.

Why not get a gyrojet instead?

 

[ronin308]

Good thing I quit pre-med!!!

 

[Long Path]

Sure, you get enothermic properties, but you can't get enough to absorb the attendent heat of friction (big one, particularly high-speed accel. from static to kinetic), metal expansion, and, um... heck, even chambering!

Except for releasing frozen gases in a vacuum into the chamber of your gun, I dunno how you're going to handle these super-solids.

Nifty trick from physics class was to attach a serious gasoline powered pump to a glass bell jar over a beaker of liquid nitrogen, and crank it on high at a vacuum. Of course the liquid N is boiling like mad, until the vacuum is near perfect, at which point it's just too dadgum cold, and what's left freezes. Pretty cool, and it DIDN'T all sublimate immediately upon removing the bell jar from over it. Some melted. Some took awhile to sublimate. I loved Doc Shepley's class.

 

[Bear Flare]

Kaylee:

Interesting concept. You are talking about changing from an exothermic combustion reaction to an endothermic phase change. Remember, energy is energy. To get that bullet to fly, the energy has to come from somewhere, i.e. heat.

The BTU's you are pumping out of an exothermic combustion reaction are expanding the product gasses, and those BTU's would have to be equalled if you were sucking heat out of a system to expand gas.

Your substance would have to have an extremely low vapor pressure. I'm neither a chemist nor a physicist (just a lowly Mechanical Engineer), but this material would need to be stored either under very high pressure or very low temperature or a combination of the two. A possible solution in use in airsoft and paintball for a while was chloro-flouro-hydrocarbons (Freon). This stuff is good for low velocity and low weight projectiles because the vapor pressure was pretty high at room temp (the temp at which the phase change is taking place for these guns), but it also had a high heat capacity.

Immediately upon firing, heat would be removed from the material and surroundings (barrel, bolt, air) and used to propel the bullet. The heat transfer is spontaneous because the phase change is not a chemical reaction with reactants and products and governed by a rate equation.

The heat transfer rate is dependent upon the difference in temperature between the material and its surroundings, and as the barrel and bolt are cooled by firing, the sublimation will slow. The barrel would have to heat back up to return the process to its original rate.

The cool barrel would be heated by the ambient air. So the cold barrel hits warm air, the air touching the barrel cools and drifts away being replaced by more air. This process is slower than contact conduction between the material and the barrel, also the air is not very hot. I don't see the gun being warmed fast enough for this process to work.

Also, as the air cools, moisture will condense on the gun. If the barrel goes below 32 degrees F, it will frost. Now you have a frost layer on moving parts causing jams.

I don't see it happening anytime soon.

Bear Flare