General > Discussion
Particle Beam Weapons: More Than Just Flashlights?
DanielOR:
DoctorJ,
I am begining to see your point. One place where I disagree, however:
"Reduced pressure means less acceleration available to the bullet - it reaches the highest speed it will reach well before it gets to the muzzle [the point at which the pressure is dispersed]." - that does not ring true to me. And this is why. yes, it is true, that as the bullet travels down the barrel the volume occupied by the heated gas increases and pressure decreases. However, as long as the pressure in front is lower than the pressure behind, there will be a force acting on the bullet, accelerating it. Simply, Force = Pressure * Area; also, Force = mass * acceleration. Yes, Pressure decreases, so the applied force at the tip of the barrel is much smaller than the applied force at the chamber. BUT: the bullet is still being accelerated all the way to the tip of the barrel (at which point, as you correctly state, pressure equalizes and the bullet is on its own - all inertia). Therefore, the fastest the bullet goes (highest momentum (mv) and kinetic energy (mv^2/2)) is at the muzzle of the barrel. Hence the reason muzzle velosity is reported in gun ballistics. Think about it - if highest speed was at the chamber, barrel length would only matter for accuracy, not distance of flight. And we know that longer barrels mean longer flight.
Now, onto the idea of gas/liquid instead of powder. The problem of reduced pressure is still there - let's say you generate hot plazma instead of hot gas. Pressure is way higher, true (thank goodness for those strong future polymers, having a gun blow up is no fun!). However, as the projectile moves down the barrel the volume still increases, and, IF THE TEMPERATURE REMAINS THE SAME, the pressure will drop. Combined gas law flatly states that [P * V / T = const]. And here is where your idea if a liquid or plasma is brilliant: WE NEED NOT KEEP TEMPERATURE CONSTANT! With gunpoweder, you're sort of screwed - once it goes bang you have no control. But imagine you can keep heating the plazma in the barrel just as the projectile moves - then temperature increases to compensate (or even overtake) the expansion of volume behind the bullet, so that the pressure (i.e. accelerating force) remains the same all the way down the barrel. This would produce a much, MUCH faster bullet. Brilliant!
DanielOR:
Afterthought: such a weapon would produce a stream of very hot plazma out of the barrel. I would have a range of a foot or two, I imagine (maybe less, total guess here). It has two effects:
1. cannot fire too close to a friendly or near a wall - plasma would burn shooter if reflected and anyone else near the barrel
2. Can be a close quarter weapon. In a pinch - even with no projectile at all.
Complex weapon, has to generate plasma, then keep VERY rapidly heating it as a bullet travels down the barrel. Either heating elements in the barrel (i dislike this solution, because a bullet would rub against them and useful life would be a shot or two, not to mention it is hard to transfer heat from heating coils to gas fast enough) or, perhaps, using a laser of the right frequency to excite the plazma. The effect is similar to Optical Pumoing - frequency of light is matched to a major line in the spectrum of the gas that is used to generate plasma.
Doctor J:
--- Quote from: DanielOR on June 06, 2008, 06:47:41 pm ---However, as long as the pressure in front is lower than the pressure behind, there will be a force acting on the bullet, accelerating it.
--- End quote ---
That is indeed a good point that i had failed to consider. Nonetheless, when we incorporate this in the next approximation, we see that the rate of acceleration would continue to slow the further the bullet traveled from the chamber. The ETC effectively maintains a constant rate of acceleration.
--- Quote from: DanielOR on June 06, 2008, 06:47:41 pm ---The problem of reduced pressure is still there - let's say you generate hot plazma instead of hot gas. Pressure is way higher, true (thank goodness for those strong future polymers, having a gun blow up is no fun!). However, as the projectile moves down the barrel the volume still increases, and, IF THE TEMPERATURE REMAINS THE SAME, the pressure will drop. Combined gas law flatly states that [P * V / T = const].
--- End quote ---
I think you might have missed a point here: as the bullet travels down the barrel, the ETC continues to generate more plasma to fill the increasing volume. I also must offer a correction to the formula: [P * V / T = n * R], where 'R' is the universal gas constant and 'n' is the number of moles of gas. It is 'n' that is increasing, as you have already noted that otherwise the heater element would have to operate ridiculously fast to keep up with the bullet. This is why the Army ETC program is working with electrically ignited powder. Rather than flashing it all at once, they can gradually increase the amount undergoing combustion.
Doctor J:
--- Quote from: DanielOR on June 06, 2008, 09:06:58 pm ---Afterthought: such a weapon would produce a stream of very hot plazma out of the barrel. I would have a range of a foot or two, I imagine (maybe less, total guess here). It has two effects:
1. cannot fire too close to a friendly or near a wall - plasma would burn shooter if reflected and anyone else near the barrel
2. Can be a close quarter weapon. In a pinch - even with no projectile at all.
--- End quote ---
#1 is true enough, though it would not extend very much beyond the edge of the 'tile' or square that the shooter occupies. By comparison, a typical rocket launcher has a back blast area 8 meters wide by 15 meters long. If we can safely ignore that, i'm willing to not worry about that half meter plasma ejection area.
As to #2, designs i've seen store the fluid in the cartridge - you wouldn't be able to generate plasma when out of ammo.
DanielOR:
DoctorJ,
you are right - P * V / T = const is true for a closed ssytem, i.e. for a fixed amount of material. if indeed you increase n, the amount of gas - that is another way to maintain pressure as the volume goes up.
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