Sabots, Darts, and hello!

[StevenRS11]

So, I've been making things that shoot things on and off for a very long time. I was a here lurker when Larda punched some serious holes in steel with that insane 200x hybrid. I made a few combustions, some simple pneumatics and a decent piston pneumatic that I still have. Now, I have a job that leaves me enough time to tinker a bit once more.

Plus, you know. Tools.

I found a bag of these little flechettes that I bought years ago and whipped up the simplest, dirtiest combustion cannon to shoot them. It worked surprisingly well-

This is a 2x6 piece of pressure treated pine, pretty tough stuff. Dart when all the way through. You can see the impressions left by the plastic sabot as it separates, too.



This is a steel door from a heavy duty locker, about 1.2 mm thick. A .22 caliber air rifle won't penetrate it, and it's shooting a significantly heavier (16 grains vs 9) projectile right under the speed of sound. Here you can really see how nice and symmetrical the sabot separations are.



This is the ammo in the sabot. It has 3 petals that separate and a pusher plate/obdurating seal. Total mass is 15.7 grains.



This is the silly looking cannon-thing responsible putting a hole in my wall. Half inch bore, 36 inches long.




What I want to do now is see how fast I can make one of these things go. I am thinking either a 50x+ hybrid or some sort of piston/light gas gun. Right now I am leaning towards a hybrid fueled with either propane/oxygen/helium or a rich hydrogen oxygen mix. Asking the people here with more experience than I, what sounds like the best way to do this? I can weld, mill, and lathe(sorta). No gun-drill, but I know a guy. I have some very heavy steel laying around, including 4130 round bar stock in 1" and 4" OD so that's not really an issue.

Or, I could go 180 degrees and make a lower velocity belt fed repeater. Might be more fun, who knows? Could cast the tricky to machine components, maybe cannibalize a piston engine.

Help too many choices!

[farcticox1]

Looks like you're going to be busy

[StevenRS11]

Looks like you're going to be busy

That’s exactly what I’m hoping for! That said, I want my busy-ness (definitely not business) to produce something worthwhile, so I want to be smart about this. I have tentatively decided on a 20x hybrid, with the barrel and chamber safe for up to 200x if it seems promising. That way I can start with a much easier fueling setup. Should I start a separate thread in the hybrid section, keep it here till it’s done, or will a mod move it?

I already started on the fueling system. It will be two side by side pistons, one larger than the other. The difference in diameter will determine the air fuel ratio and allow me to charge it to arbitrary pressures without having to do any maths each time. I figure an adjustable dead space will allow for fine tuning the ratios.

Ignition will be a spot welder that I made years ago. It never spot welded anything, but it should turn a bit of wire into plasma just fine.

[D_Hall]

Keep in mind that if you go H2 + O2 you are almost certainly going to be experiencing detonations. There's even a very real possibility of spontaneous ignition during fueling. Translation: I wouldn't recommend such.

[StevenRS11]

Keep in mind that if you go H2 + O2 you are almost certainly going to be experiencing detonations. There's even a very real possibility of spontaneous ignition during fueling. Translation: I wouldn't recommend such.

If I was going to take any one person’s advice, it would probably be yours. I’m going to stick with propane and air for now, and eventually look into a helium/oxygen/propane mix. Hydrogen’s ignition energy is just so low.

Right now I’m having issues with fueling- I may stop futzing around with my ‘clever’ piston setup and just fork out for an accurate pressure guage. It really makes me respect the engineering that went into carburetors.

On the plus side, I managed to get my 3D printer to add a shear skirt/burst disc to the sabot’s base that’s actually air tight. It’s not failing at a super consistent pressure, but that’s probably the trade off I will have to make. Or it could be just the properties of nylon. I wonder if I could design something that fails based on rate of pressure change instead of static pressure?

[jackssmirkingrevenge]

Great job, love the sabot separation! Also a great illustration of what a penetration multiple APFSDS style projectiles are and why they are so successful as armor defeating projectiles, as well as extreme long range projectiles where they are essentially "penetrating" the atmosphere.

If accelerating fancy projectiles is your goal, I think a propane fueled burst disk hybrid will do the job quite nicely, if you haven't dabbled with hybrids before you will be amazed with what can be achieved even with relatively low mixes, especially with larger bores, and as D_Hall says staying away from pure oxygen is highly recommended.

[StevenRS11]

O.O

Wat.

Just experienced my first hybrid shot. Before I started machining, I figured I should test out my fueling system and sabots first. Threw together some black iron pipe, fired it at 5x and saw the light. According to HGDT, that little dart was moving at right under 2000 fps. According to my ears it made that very distinctive zip/tear noise that small supersonic things make. Now, without my chrony (which I will get set up tmrw) I have no way of knowing for sure, but I'm confident I sent something going faster than the speed of sound with some pipe fittings and propane.

Now, time to get machining.

Anyway, here are some pictures- (none of the launcher itself, but its not much to look at)


This is the fueling system I came up with- it uses a blowtorch to meter propane and air at atmospheric pressure. The pump next to it uses 2 schrader valves with the springs ripped out as check valves. It has very, very little dead space and a .375 diameter piston so it should let me reach quite high pressures. The printed parts are polycarbonate so I'm not worried about them melting.


That's a 3/8" thick aluminum transmission housing. Its 365 casting alloy at T5 temper (it should be, atleast) and is very hard. Pellet gun just sorta marrs the surface, and you can't drive a nail into it with a hammer. Dart blew through it, only stopping because of the fins.

[jackssmirkingrevenge]

Beautiful... once you go hybrid, you rarely go back. Any chance of some high speed footage?

According to my ears

Remember that hearing damage is cumulative and permanent, good hearing protection is recommended.

[StevenRS11]

Whenever I'm in the shop, I got earplugs, safety glasses and a hat on. It was required dress code in the machine shop where I learned and it's stuck with me ever since. Had a carbide router bit crash into the fixture one time at ~12K rpm and launch a fragment into the wall past my head. Hole is still there, with a square of lexan over it to remind me that things that aren't meant to shoot things still can. I may be a safety sally, but it only takes that one time.

As to my hybrid adventures, a few questions. RIght now I have a single ignition point near the rear of my chamber. Would I benefit much from inducing turbulence near the ignition point? Say a short perforated tube running a third/half of the length of the chamber, with the spark inside it? Some experiments in initiating DDT mentioned short, sharp edged spirals so that seems like a bad thing.

I was actually working on an air arc flash a while ago that would work pretty well. Used a ZVS flyback and voltage multiplier to charge ~10nF at 20kv. I never finished the triggering mechanism though, so would need to do that.

Also JSR, you wouldn't happen to be hw97karbine, would you? If so, wow its a small world.

[D_Hall]

Based on the experiments I ran with VERA, it doesn't really matter after all is said and done.

Put it this way... Yes, inducing turbulence before ignition results in a faster pressure ramp, but once the gases start to move (read: valve opens) the flame propagation rate goes through the roof and the remaining fuel is consumed more or less instantaneously.

Seriously, some of VERA's first shots were with no projectile; just a burst disk. In the data I expected to see the data slowly (where slow is a relative term) ramp up and then - when the disk failed - to ramp down. What I saw instead was the pressure slowly ramp up...and then take a step function to essentially maximum theoretical pressures. Whether I was running the mixing fans or not changed the rate that the pressure initially ramped up, but the step function to max theoretical pressure remained the same.

[StevenRS11]

That's really surprising, huh. I guess intuition only goes so far with stuff like this.

Also, air gap flash is done. The 20nF, 30kV field distortion triggered spark is almost as loud as the hybrid. It should have a peak intensity duration on the order of a few hundred nanoseconds. The trigger is actually mechanical- a tiny strip of aluminum foil near the muzzle is deflected by the gasses to complete a circuit, dumping 300 volts through a trigger coil. I can vary the timing by changing the distance the foil has to deflect.

Hopefully, I will get some really crisp high speed photos once it gets dark.

Now, I know I probably don't have to say this to D_Hall and JSR, but DO NOT BLINDLY TRY TO DO THIS. At this voltage you represent a short circuit to the cap, which is capable of pushing hundreds of amps through you. Furthermore, 30kV snaps, sparks, and hisses everywhere. It can jump surprising distances, toss around small conductive objects, and surface track over contaminated surfaces even farther. Large metal objects nearby can store enough charge to shock your socks off.

[D_Hall]

That's really surprising, huh. I guess intuition only goes so far with stuff like this.

Yes and no. It's just a case of intuition not following through.... What happens when the gases start physically moving not due to simple expansion but due to moving towards the muzzle? Sudden increase in turbulence due sudden acceleration and any/all imperfections in the chamber.

It can jump surprising distances, toss around small conductive objects, and surface track over contaminated surfaces even farther. Large metal objects nearby can store enough charge to shock your socks off.

We've got some igniters at the office that require on the order of 1000 amps to function correctly.

[jackssmirkingrevenge]

I may be a safety sally, but it only takes that one time.

Better Sally than Carol

As to my hybrid adventures, a few questions. RIght now I have a single ignition point near the rear of my chamber. Would I benefit much from inducing turbulence near the ignition point? Say a short perforated tube running a third/half of the length of the chamber, with the spark inside it? Some experiments in initiating DDT mentioned short, sharp edged spirals so that seems like a bad thing.

For practical purposes I do not think you would see an increase in performance in any way proportional to the effort.

Also JSR, you wouldn't happen to be hw97karbine, would you? If so, wow its a small world.

That would be my YT channel, yes

[StevenRS11]

Been a while! Had some trouble with the air gap flash- apparently the iphone’s ‘Bulb mode’ is not a true bulb mode. If I can get my hands on a real dsrl camera I’ll get it working. It was fun working with high voltage again, though.

Anyway, what I’m going to do now is make my hybrid a bit more, uh, presentable. And a bit easier to load. I think, instead of machining the chamber from stock, I’ll use a scrap oxygen cylinder I have laying around.

What I’m really interested in is replacing the threaded union with a lever style action that moves the entire barrel forward, then clamps back down with enough force to seal the barrel/diaphragm/chamber joint. Is there any record of someone doing this? I am imagining a linkage similar to what you see on those glass bottles with a lid that clamps shut.

Also, I met up with some of my old friends from high school who shared in my... youthful destructive tendencies. We shot this thing, a trebuchet that flings incendiaries, a few other spud guns we had, and some firearms. It’s amazing how much fun this hobby is, and how it can bring people together.