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 Post subject: airsoft rail guns...
PostPosted: Thu Dec 04, 2008 3:58 pm 
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You guys ever play air-soft?
http://en.wikipedia.org/wiki/Airsoft

I may have mentioned this here before, I don't recall, but I just had a "discussion" with a guy that thought he knows physics and it ended up in him pretty much calling me a moron without actually knowing the data so I figured I'd open up the fray here.

Concept -- airsoft gun. Fires a tiny plastic bb. But, let's say it's a gun that magnetically accelerates a small metal bb and incredible speeds. The force of impact of something 12mm in diameter travelling at an awesome speed would be devastating. And the fact that you could carry hundreds of rounds, and have zero recoil... that's when he jumped on me. He said, wait a minnet, zero recoil? You're living in a fantasy world dude.

His reasoning was that every action as an opposite reaction. Yes, that's correct, he is right. So he said that the force of moving that bb out at such an acceleration would cause an equal force back to the shooter making the gun like that unusable. That's where I called him on being wrong.

In a rail gun, basically what I'm talking about (a man portable railgun) the force of accelerating the bb is dispersed along the rings outward from the center of the barrel. Thus no "recoil" but, the barrel has to be built to withstand the force. And since we're handwavium a power source that can fit in the stock of your gun and a mechanism to accelerate the bbs why not material to withstand the force of recoil.

But, the SF discussion that I think would be interesting here is this:
Using a weapon such as this as the weapon in an SF game.

Thoughts?

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 Post subject: Re: airsoft rail guns...
PostPosted: Fri Dec 05, 2008 5:28 am 
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Well, first of all you have to be careful about mixing the definition of coilguns (a.k.a. gauss guns) and railguns. If you're talking about 'rings' then you're in coilgun territory. Two parallel rails and it's a railgun. The difference is that a coilgun is contactless.

What you're describing basically sounds like a gauss gun, except firing spherical rounds instead of 'needles' (although, IMO 4mm is a pretty thick needle). That's not good, because balls are not aerodynamically efficient, so you probably want to stick with boat-shaped objects.

Now, as for the recoil, we may be heading into very murky territory. Yes, every action results in an equal and opposite reaction. Does the reaction from the coils really get directed outwards rather than backwards? That sounds odd to me. Any engineering boffins out there care to chip inn? A railgun definitely has recoil, probably pretty hefty at the sort of velocities SF normally talks about. 'Felt' recoil is a very divisive subject, so I'm not even going to go there.

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 Post subject: Re: airsoft rail guns...
PostPosted: Fri Dec 05, 2008 5:55 am 
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It's "equal and opposite reaction", so if the round is propelled forward, then something has to recoil backwards.

There will always be a recoil force, no matter what. The gun could be built to withstand the stresses of recoil though, or have some mechanism to dissipate it mechanically maybe so that it might not be noticeable... but there will always be recoil, regardless.

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 Post subject: Re: airsoft rail guns...
PostPosted: Fri Dec 05, 2008 2:49 pm 
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Of course, in Traveller you have the option of messing about with gravitic dampers, as per PGMP-14s and FGMP-15s. I always assume that at reasonably high tech levels recoil (for personal weapons, anyway) will be handled by technological-means-which-I-don't-understand-or-I'd-be-writing-the-patent. ;)

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 Post subject: Re: airsoft rail guns...
PostPosted: Fri Dec 05, 2008 3:52 pm 
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EDG wrote:
It's "equal and opposite reaction", so if the round is propelled forward, then something has to recoil backwards.

There will always be a recoil force, no matter what. The gun could be built to withstand the stresses of recoil though, or have some mechanism to dissipate it mechanically maybe so that it might not be noticeable... but there will always be recoil, regardless.


I actually ran this past a physics forum I'm on too. The math shows that the force is lateral, not back towards the shooter. Yes, there is force, it's the direction here that's important.

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 Post subject: Re: airsoft rail guns...
PostPosted: Fri Dec 05, 2008 4:36 pm 
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bigtim wrote:
EDG wrote:
It's "equal and opposite reaction", so if the round is propelled forward, then something has to recoil backwards.

There will always be a recoil force, no matter what. The gun could be built to withstand the stresses of recoil though, or have some mechanism to dissipate it mechanically maybe so that it might not be noticeable... but there will always be recoil, regardless.


I actually ran this past a physics forum I'm on too. The math shows that the force is lateral, not back towards the shooter. Yes, there is force, it's the direction here that's important.


I am reasonably sure that this violates conservation of momentum - not to be postulated lightly.

The vector of all forces (direction and distance) must balance out to zero. Therfore, any outward force must be resisted by the coils (as you propose), however, the "forward" and "rearward" component of a lateral force is zero and is, therefore, incapable of balancing the forward momentum of the projectile.

Concider this, I want to shoot your gun at the back of my ship (a thick plate) and catch the bb to reload the gun and fire again. Every time the bb hits my ship, it pushes the ship forward, but you claim no backwards recoil on the gun. Therefore, my ship will accelerate without any reaction mass - you have invented the long sought after 'ractionless thruster'.

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 Post subject: Re: airsoft rail guns...
PostPosted: Fri Dec 05, 2008 7:53 pm 
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There will be recoil, but that force can be absorbed by the gun, or by the shooter.

When you fire a bullet from a conventional gun, there is recoil into the stock and into the shooters arm. It doesn't put a hole in your arm because the pressure exerted against you is not enough to break the skin.

The force may be the same, but the AREA of the bullet is so much smaller than the area of the end of the stock, that a bullet will penetrate the skin and the rifle stock will not.

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 Post subject: Re: airsoft rail guns...
PostPosted: Sat Dec 06, 2008 2:09 am 
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Ok, here goes.

If you have push only mag force then there is an opposite force from the direction of travel.
If you have pull/push mag (tube) force, much of the force is already in balance. (not taking in effect of atmosphere inside the barrel) Ie each booster ring/section pulls then pushes the object forward.
If you have an object doint the work as a pusher and puller, it also is in balance with it self.
But the objects it is using for the push/pulling must be of greater force (or mass) that the objects energy or they will be effected by the push/pull.

Last a rail gun is different than a mag accelerator, as already mentioned. But that aside, I understood what you were asking.

Dave Chase

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 Post subject: Re: airsoft rail guns...
PostPosted: Sat Dec 06, 2008 2:18 am 
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Vile wrote:

...
What you're describing basically sounds like a gauss gun, except firing spherical rounds instead of 'needles' (although, IMO 4mm is a pretty thick needle). That's not good, because balls are not aerodynamically efficient, so you probably want to stick with boat-shaped objects.

...


I am glad you said ball are not aerodynamically efficient. Why?
Because that is correct and
unless you make the ball spin in either a clockwise or counterclockwise motion to the direction of travel the ball will drop quickly toward the direction of the gravity pull.

It does not take much of a spin to counter this drop (you will not defeat the drop but hopefully you will hit your target before then). IF there is no gravity and IF there is not any type of atmosphere to interfere with the ball (or projectile) then it will work with out any spin.

Using a cylinder type projectile does not assist change the need for having spin. Quite the opposite. It needs more spin (or faster spin) to have truer trajectory. More so with an atmosphere enviroment versus the gravity field.

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 Post subject: Re: airsoft rail guns...
PostPosted: Sat Dec 06, 2008 3:44 am 
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Dave Chase wrote:
Ok, here goes.

If you have push only mag force then there is an opposite force from the direction of travel.
If you have pull/push mag (tube) force, much of the force is already in balance. (not taking in effect of atmosphere inside the barrel) Ie each booster ring/section pulls then pushes the object forward.
If you have an object doint the work as a pusher and puller, it also is in balance with it self.
But the objects it is using for the push/pulling must be of greater force (or mass) that the objects energy or they will be effected by the push/pull.

Last a rail gun is different than a mag accelerator, as already mentioned. But that aside, I understood what you were asking.

Dave Chase


maybe I'm missing something here but,

regardless of whether the ring/section is pushing -or- pulling, its reaction will be in the opposite direction of the bullet. If its pushing -and- pulling simultaneously, it won't accelerate the bullet as the forces would cancel out, right?
Don't all the force vectors added together have to equal zero to maintain conservation of momentum?
What would happen if you launched a volkswagen ... still no recoil at all? So an asteroid miner could launch iron ingots to a orbital factory without changing his own course/orbit?

I'm skeptical

balls in atmosphere... spin might not be just to counteract drop, but to curve a shot in sideways too ( within reason )... or how about sliders or knuckleballs or 'english' applied to riccochets .
would that be enough of an advantage to offset the energy losses from having worse aero-efficiencies? ( which would get worse still as velocity and thus reynold's numbers drop down... actually, I forget, but I think there's a fairly steep drop off on the drag coefficient for a sphere below a certain reynold's value )

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