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]

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.

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.

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].

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]

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. 

#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.

[blondandy]

PV=nRT

only for ideal gases. the larger volume your molecules are, the dodgier it becomes.

is it me, or is this topic riding hard on a steed named tangent?

[DanielOR]

PV=nRT

only for ideal gases. the larger volume your molecules are, the dodgier it becomes.

Is it larger or "more willing to interact with each other"?  Ideal gas approximation is more about molecules behaving like steel balls.  You are correct eithe way - plasma particles would be highly charged.  Not sure how much the ideal gas law would be modified.  Compared to the minor issue of plasma guns and what not, maybe substituting ideal gas for plasma is excusable?    At least we are not approximating an Ortnoc by a sphere...

is it me, or is this topic riding hard on a steed named tangent?

Yeeee-haw!

[SirMoric]

With regards to todays normal projectile, the gunpowder nowadays is actually slow-burning.... which means that instead of an near-"explosion" in the chamber the pressure behind the bullet steadily increases and thereby increasing speed of the bullet even more. This also has the effect that you can save weight, since you don't have to contain a massive explosion.

This was what made the German 88 deadly during the war at least.

Just a thought.

[DanielOR]

SirMoric,

thanks for the heads-up.  means that what we plan to do with plasma is alreay being done today with slow burning powder.  Cool.

[Darkpriest667]

Doctor im glad that someone else knows about ETC besides me.


yes one of the major problems is worrying about the plasma ejection...  also take into account its extremely bright (the sun) less than 3 feet from your face.. the kind of sungoggles you would need for this weapon would make night fighting an impossibility.


Also DoctorJ i heard another serious hurdle they have had to deal with is that the chemical reaction causes some sort of radiation or the plasma does im sorry im not a scientist but I overhear things :-p.   so basically if you walked where you fired and stood there for any length of time there was a possibility of radiation exposure.

is this incorrect or no?

[DanielOR]

DarkPriest,

I only read the Wikipedia.  Did not get a sense that there was a radiation risk from that.  Is there another source?

As a rule, I imagine standing next to any gun while operating is not the healthiest thing to do, and this one is no exception. 

[blondandy]

in ufopedia it says the plasma is encapsulated. so this is not an issue.

indeed, if it was not encapsulated, then it would diffuse quickly. ie have the range of a blowtorch. hotter particles move faster, so diffusion is quicker.

if the plasma is hot enough for nuclear fusion, then you would get some radioisotopes produced.

[Darkpriest667]

if its not hot enough for fusion then the projectile isnt going to go very far....  Also im interested in the material they are using to encapsulate the plasma, whatever it is must not be cheap or easily obtainable.

DarkPriest,

I only read the Wikipedia.  Did not get a sense that there was a radiation risk from that.  Is there another source?

As a rule, I imagine standing next to any gun while operating is not the healthiest thing to do, and this one is no exception. 

yes there is another couple of sources... The united states air force, the united states army, the department of defense, and i believe also the CIA has worked on ETC a little.. Good luck getting any info from those 4 sources.

[DanielOR]

DarkPriest,

suppose I might, but they, in return, will know everything about me, including what I had for breakfast two weeks prior. 

[blondandy]

if its not hot enough for fusion then the projectile isnt going to go very far....  Also im interested in the material they are using to encapsulate the plasma, whatever it is must not be cheap or easily obtainable.

indeed. by the definition of plasma, all electrons have been stripped from atoms. as materials are held together by electrons interacting, it is impossible to contain a plasma with any material.

i suggest a magnetic field to keep the particles going in a circle.