SpudFiles

jackssmirkingrevenge wrote:

POLAND_SPUD wrote:this is getting a bit annoying, but...

Surely for this one you mean plywood

Personally I'd be using sheet steel, thin enough to deform. Though with what's we've been discussing, I might have to use a hill to show spalling.

Zeus wrote:Personally I'd be using sheet steel, thin enough to deform. Though with what's we've been discussing, I might have to use a hill to show spalling.

Also an option.

Some inspiration from Bofors

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this thing is amazing... not meaning to sound like a spud noob but i dont quite understand how it works. Does the electric charge heat up the air or what?

pneumaticcannons wrote:this thing is amazing... not meaning to sound like a spud noob but i dont quite understand how it works. Does the electric charge heat up the air or what?

i think the electricity starts off the reaction,

The combustion reaction in question is:
4Al + 6H2O --> 2Al2O3 + 6H2

Does the electric charge heat up the air or what?

The first post wrote:Despite being very energetically favorable, this reaction is very difficult to initiate, due to the passive oxide layer which is always present on aluminum in contact with oxygen. The solution is, in this case, to boil and partially ionize it by passing an electric current through flakes of aluminum (roughly 10μm x 500μm x 500μm for this launcher) suspended in water. The current pulse is just under 40μs in duration and had a maximum magnitude between 50kA and 90kA, depending on the desired energy input. This current flow and boiling process is extremely violent and induces essentially instant mixing and combustion, converting a room temperature fluid-solid slurry into a 6000K plasma, which is then used as the propellant gas in the same manner as in a conventional combustion gun.

Electric current flow causes resistance heating in the aluminum foil which converts it to a gas and allows it to react quickly with the water. There is hardly any air space in the chamber. It goes from being full of the above mentioned solid-liquid slurry, to being full of a relatively high density plasma. I'll add that the chemical reaction is not the whole story - by varying the energy of the current pulse, I can obtain different final pressures and temperatures in the chamber.

I can see not wanting to read through a whole 16 page thread to find the answer you're looking for, but this was in the first post...


Also: I didn't take a more careful record of the spalling simply because I ran out of time. There are quite a few things I would have liked to try, but didn't get around to. I did find a piece of the back of the plate embedded in one of the 2x6s I was using as a target support, and a few scratches on the plexiglass sheet behind the target (on the aluminum ball shot). The fragments in question are very small, and have a pretty wide spread to them (you can see in very high speed "video" of impacts at orbital speeds that the spall expands in a roughly spherical pattern).

alright im understanding it more now thanks. Incredible science and effort behind this whole project

I was expecting to see a duct taped monster of a gun and a builder claiming it is the best damn ballvalve gun in the world...except for some DWV parts..


I was NOT expecting to actually see a thick steel plate pierced like is was hit by an asteroid.




Great work..

any chance of a handheld??

Can I ask one thing DYI? Since the airsoft bb went through aluminum, does that mean it could possibly go through kevlar? If you can, could you test that theory for me please? I just wanted to prove a point to a friend, that with the right amount of power that it is possible.

I'm just going to guess it'll probably go through an equal amount of kevlar twice as easily as that aluminum plate.

If you look up highspeed videos of kevlar tests you'll see it behaves in an entirely different way than aluminum. The energy from an impacting bullet is dissipated by stretching the cloth fiber instead of piercing it. This way kevlar's extremely high tensile strength can come into play when absorbing impacts.

However, when your projectile velocity starts to approach the speed of sound in your target material, it doesn't work like that. Solids behave more like compressible liquids because the velocity is higher than the rate at which the material's molecules can interact with each other. It's similar to how EFPs work.

Oh, an in this case, all that kinetic energy is going to create heat, which kevlar doesn't fair well against.

Can I ask one thing DYI? Since the airsoft bb went through aluminum, does that mean it could possibly go through kevlar? If you can, could you test that theory for me please? I just wanted to prove a point to a friend, that with the right amount of power that it is possible.

How much Kevlar?
I'll add to what Fnord said (which is quite accurate as far as I'm aware) that the low density of Kevlar works against it in this case - when the impact behaviour becomes essentially hydrodynamic in nature, the density of the target usually makes more of a difference than the mechanical properties. Also, Kevlar doesn't work too well even against low speed projectiles when it isn't properly supported - stretch it out across a frame, and nearly any respectable projectile will punch through. Pack it with fabric on top and underneath and sit it loosely on a soft target, and it does a great job.

I don't expect this would manage to perforate something like the type of vest normally used to defeat pistol rounds, despite having more energy (when firing an airsoft round, at least - the brass slug, for example, wouldn't have any trouble at all). However, think about the extreme case - at some point, the round is going to carry enough *energy* to destroy the target, regardless of what the round or target happen to be. For example, at a speed of c - 0.3nm/s (EDIT: this calculation is in error, the actual speed required would be much higher) , an airsoft round would carry more than enough energy to disassemble an Earth-sized planet...

An interesting side note, spacecraft shielding often employs kevlar and similar materials between the outer and inner layers of the Whipple shield - it's apparently a good choice for slowing the fragments coming off the top layer after it's perforated. It is, of course, affected by the high temperatures and ends up looking very charred, but I guess the high tensile strength : density ratio makes it superior to other materials that might be considered for that particular application. Note that my comment about density above doesn't apply so much here - Whipple shields are totally useless against conventional projectiles, and bank on anything hitting them being very high speed. They function by breaking up the projectile with a lightweight outer shell, allowing it to travel some distance after that to expand the debris cloud to more manageable areal densities, then stop it with a thicker inner shell.

For example, at a speed of c - 0.3nm/s , an airsoft round would carry more than enough energy to disassemble an Earth-sized planet...

If I remember my theory of relativity correctly, and airsoft round at that speed would weigh more than the moon until it slows back down shedding mass and energy and causing considerable damage to a small planet in the process.

Warhead, side note, NASA tested the Challenger (?) failure by shooting a piece of foam at the wing, and it got through.

DYI, with enough velocity can any material make it through any stronger material?

If I remember my theory of relativity correctly, and airsoft round at that speed would weigh more than the moon until it slows back down shedding mass and energy and causing considerable damage to a small planet in the process.

Your memory serves you well, Tech. All that extra energy would indeed make for an impressively massive airsoft round. The added mass would be the Lorentz factor multiplied by the rest mass, or roughly 111 billion tonnes for my example (using a 0.2g BB carrying ~10^31 J ). That would be a VERY small moon (only about 4km diameter at 4g/cc). Come to think of it...

(EDIT: the following statement is mostly incorrect, except the bit about the foam. The actual situation under consideration warrants a general relativistic treatment, and the round would NOT experience any sort of gravitational collapse, and would certainly NOT collapse into a singularity - any singularity must be such in ALL inertial frames of reference.)

This thing would be denser than a neutron star. Since that mass isn't held in the form of matter, it wouldn't exhibit any of the normal degeneracy pressures that keep white dwarves or neutron stars from collapsing further. Would what little matter there was collapse down to a singularity? If so, something like that would just punch through targets with little noticeable effect, picking up a small amount of matter along the way. Perhaps this is the limit on small projectiles destroying large targets - gravitational collapse of the projectile

(Also note that somewhere in the process of accelerating this round, there'd be a fairly energetic fusion reaction when it collapsed past a certain density, probably a few thousand times its starting density. I'm sure whoever designs the near-lightspeed accelerator to fire this thing will account for that though...)

So to answer your question, Gun Freak: No. Sufficient velocity will, in extreme cases, cause the material of your choice to first collapse into some sort of optimally packed crystalline structure, then into electron degenerate matter (where it becomes "metallic", regardless of what it originally was), then into neutron degenerate matter (which doesn't really have any elemental composition), and so on. Just remember that a 10g chunk of foam at a Lorentz factor of 0.1 is still most definitely a chunk of foam, and will release around 21kT worth of *BOOM* on impact (more or less depending on fusion reactions, and whether they're exothermic or not). If you're talking RHA equivalent penetration, probably several tens of metres.

Thank you for taking the answering of my question to epic extremes

It would be a little funny if the RCMP were reading this and arrested you for 'premeditated future destruction of the earth with an airsoft round'