SpudFiles

Here we are at the end of the summer, and like always, I haven't got quite as much done as I'd hoped to. The new ETG design was supposed to be nearly impossible to destroy in normal operation with any energy level I could subject it to. Were it not for two simple flaws, that would probably be the case.

Firstly, a diagram:


Dark gray parts are 1"-8 steel threaded rod (except the thinner one, which is part of a 5/8"-11 grade 5 bolt). The one at the far right is the chamber, the one on the left is the insulator casing. Light gray is 2", annealed 8620 round stock with 1"-8 internal threads. Light blue is the internal insulator, tan is the capillary tube and dark blue is the external insulator (destroyed in a 5kJ surface arc and replaced with petroleum jelly). Not shown in the picture is the barrel, a rather complicated (but very solid) assembly consisting of five separate machined parts which isn't important to this discussion.

Specifications:

Chamber volume: variable. Effective volumes between 0.3 and 0.6mL have been used.
Bore: 6.2mm
Barrel length: 200mm
Firing energy: tested up to 5kJ
Working fluid: water
Efficiency: 10-20%, based on very limited testing
Firing voltage: up to 14kV (for 5kJ) Don't try this at home kids
Muzzle velocity: IT'S OVER NINE THOUSAND! (ft/s)*

Now, as the title suggests (not the current title, but the original one...), the pictures.

The launcher:


Muzzle flashes like this one are rare, but highly impressive.


Performance against a plasticine block with an airsoft round, at 2.5kJ input. Round in crater is for size comparison.


Performance against 1/8" aluminum plate with an airsoft round, at 3kJ.
Very nearly made it through. The crater is rippled, hinting of melting, and its diameter at the surface is uniformly 8mm (from a 6mm projectile).


Steel ball bearings go through 1/4" aluminum easily and leave 5mm deep holes in 3/8" plate (with back bulged out), even though the ETG fails violently when firing them, wasting much energy in the process. This occurs somewhere under 3kJ, so increasing energy past that does very little for performance.



Due to an issue with video conversion software, the acoustics of the indoor testing area, and my inability to coax a muzzle flash out of it while the FC150 is recording, videos are neither forthcoming nor interesting.

The ETG is part of an ongoing attempt on my part to produce hypervelocity impacts, and I'm well on my way to success. Muzzle velocity on the final shot with an airsoft round was between 2200m/s and 3100m/s (corresponding to 10-20% efficiency), and I noticed an effect which, to the best of my knowledge, no member of our community has ever created, much less captured on video: an impact flash. Muzzle is on the left, with the accompanying small muzzle flash. Target is aluminum plate on the right. Apologies for part of the flash being off-frame.



To me, that single frame is the most important result of the year. It is a perfect finish to my summer, visible indication that my goal is finally within reach. How impressed you are by it depends greatly on your own preferences, but I will say that Spudfiles has never seen the like of this launcher before. In energy density⁺ and sheer muzzle speed, nothing here approaches it, not even HyGaC20.

Here's why it will never fire again.

Due to an immensely stupid design flaw on my part, the internal insulator is prone to firing out the back when the chamber pressure goes too high. It separates at the point where its diameter reduces, and the 1/2" of threads between that and the capillary tube are ripped out, allowing the newly freed fragment to escape, thus wasting huge amounts of gas and spraying the external insulator everywhere. Correcting this is simple, and will be done immediately at the start of next summer.

Also of concern is the tolerance of the hole that houses the internal insulator: as its bottom needs to be flat, an endmill is required to cut the last few millimetres. Because endmills do not self-centre, my 1/2" endmill expanded the hole to 0.52", creating enough extra volume to allow expansion, cracking, and general destruction of the insulator.

Next summer, there will be MUCH more. In the mean time, I appreciate any relevant comments, questions, and discussions. There are a few more pictures in my Photobucket gallery for your viewing pleasure, and more might appear over time.


*maybe
⁺Here defined as muzzle energy divided by total internal volume of the launcher

May I be the first to say congrats on recording an impact flash. That is probably a true first on Spudfiles. Well done.

That is awesome DYI. Job well done

That thing is absolutely amazing. I guess you'll hold many Spudfiles records until next summer. Why do you have to wait 'till next summer anyway?

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WHAT NINETHOUSAND? There's no way it can be that high... can iiiiit?



It is surely impressive.
Great job!

I'm still awaiting my pack of 1000 0.15g bb's, hopefully customs didn't nick 'em. Then I'll see if I can raise you

Awfully impressive how far the project has come. 9000 Feet per second is very impressive. It's given some reassurance that I might even get the BB to clear the barrel..ah well, we can dream.

Could you post details of your barrel? At the moment I'm thinking of an 8 od 6mm ID stainless steel polished tube sleeved in 1" hardened steel at the start, then 20mm for the rest. But if you have some magic solution, I'd like to see

Very. Nice. Work.

The barrel is a length of 6.2mm ID (8mm OD) seamless SS tubing from McMaster, rated for some ridiculous working pressure, in the 6-8 kpsi range. The first 63mm of the barrel is only differentiated from the chamber (1" diameter steel threaded rod) by the starting placement of the projectile (and the length of the paper chamber liner). The next 80mm (where the SS tubing starts) is sleeved in more 1" threaded rod. To ensure a secure fit in the sleeving, two methods are employed to hold the barrel in place.

Before being inserted into the sleeve, a very gradually tapered 1/4" steel rod is hammered into the much softer SS barrel from the breech forward. This expands the tubing to just over 5/16" on the outside. When this is then installed into the sleeve it doesn't quite fit, needing to be hammered into place and forming a near-perfect seal between the two pieces. The sleeve is then threaded into a 2" diameter 8620 round rod with four holes drilled from its surface through the center, and threaded to 1/4"-20. These line up with threaded holes on the barrel sleeve and bolts are inserted, gripping the barrel from the outside.

This arrangement isn't ideal. The breech ends up being of slightly larger ID than it should, and the barrel does slip by a millimetre or two on the more powerful shots (it NEVER leaks though). There are, however, some important advantages. Replacing a barrel is extremely easy, only requiring that a new section of the tube be cut, faced, and expanded, then fitted into the holder. This is ideal, because I don't yet have my own lathe. The slight increase in diameter at the breech means that there is never a lip between the two barrel segments. Any sort of protrusion at the joint could be problematic, as the round is likely going well over 1000m/s by the time it clears the first 2.5" of barrel.

I am certainly losing energy due to barrel choice - wadding improves things, but it's only so effective when the propellant gases are as hot and dense as they are here.

inonickname wrote:It's given some reassurance that I might even get the BB to clear the barrel..ah well, we can dream.

14kV discharges are fast, but they're not RDX-fast
I wouldn't get too excited yet, if I were you.

Also, remember that I'm using a VERY small chamber volume. You'd likely benefit from sleeving the entire length of the barrel, as you suggested.

I asked for a quote on 6mm id 8mm OD seamless stainless pipe from a UK supplier and still haven't heard back from them I suppose my BB's need to get here first anyway, assuming customs hasn't stolen them.

Any suggestions on chamber volume? I'm thinking of using the disposable liner idea, which also allows me different diameter chambers. I can currently make the chamber up to 12mm without redesigning.

My 0.1 celsius increment thermometer also got here today, and I picked up some parts for a ballistics pendulum when I can be bothered..It's gonna fuck me over bigtime if my plastic beads get stolen though. (These are for verification + experimentation).

If this supplier doesn't get back to me I might persevere and find some larger OD barrel tube to use, if I can find some thick enough I might be able to thread the barrel straight into the front threaded chamber plug. When the bb's and barrel are here, I can do some "low energy" testing with NC..

have you thought of different propellants? Liquid ammonia would probably work better than water. You can buy it OTC in car tyre inflaters, and all you need to do is cool the can down with some dry ice to make sure it stays as a liquid. It has a rather high tendency to stay as a liquid, so even in your hot room temperature chamber, you'll still have time to get it ready. Or anything else nice and volatile with a lower atomic mass than water.

Edit: Online metals sells 9.52mm OD 6.35mm ID CRS tube and 9.52mm OD 6.22mm ID stainless for $5 and $12 for a 3' length, respectively. The stainless should make a respectable barrel. They also sell suitable sizes for sleeving, but I have meters of round bar I can do that with.

Want to have a pow-wow for optimization/propellant use soon?

Nice.

I noticed you had some different capacitors. Information on them?

I did not think an impact flash was a major thing, I had gotten a couple with the railgun (though that was 12kJ). Ile try to find the video (hard drive failed two times since then...)

Looking forward to more progress.

EDIT:
Instead of jelly, may I suggest Scotch 70:
http://www.alliedelec.com/Images/Produc ... 174737.pdf
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 602_263622

Great stuff, and I think it would hold up fine.

rp181 wrote:I did not think an impact flash was a major thing

I think Larda got an impact flash on his ETG, although there is no "break" between the muzzle and impact flash like in your picture, the impact flash seems to be far greater than the muzzle flash.
http://www.spudfiles.com/forums/files/flash_138.jpg

I could be wrong though.

As impressive as the gun is (it really is), I question your statements regarding energy density. Or at least, you don't provide any numbers to back up such a statement. To do it right, you need to include the volume/mass/etc. of your power supply (capacitors, I'm guessing).

Here are the responses to your questions, in no particular order.

@rp181: I was under the impression that your railgun was never finished to a working state, that you never fired a proper projectile at a solid target, and that you never took high speed video of a shot. Or at least if you did, you kept me in the dark about it.

@MrCrowley: Larda's ETG didn't even exceed 300m/s, much less reach high enough speeds to cause such an effect. The "impact flash" you're seeing is the result of a very hot, directional muzzle blast being sprayed everywhere on impact with a solid obstacle.

@inonickname: NH₃ may be a superior propellant. However, water allows much greater flexibility in testing, and poses zero safety hazard. Also, I'm staying away from working with "chemicals" ( ) at the moment, for obvious reasons. No pow wows for at least a week, unfortunately. I'm moving in to my residence soon, and it may take a while to get fully set up and connected to t3h interwebz.

@D_Hall: I'm talking about muzzle energy divided by total internal volume of the launcher. Due to the low energy storage density of capacitors, even a spray'n'pray would likely beat it when they're included (and we won't even get into weight...). There's zero need for portability, or for use as anything other than a stationary test device, so I don't have much interest in making the storage system smaller. As your workplace deals primarily with weapons, I understand that your default assumption for the applicable definition of energy density would necessarily include the entire system. It's probably best to edit my original post to clarify that, regardless.

I was not very good with documentation
I did indeed finish it, and had some projectiles made. I shot it primarily at steel. I couldn't do much due to my location and other issues, just an occasional shot.

I do however think it was caused by your description of Larda's ETG, that would play nicely into a railgun. The molten metal becomes invisible until it has a chance to slow down.

What exactly is impact flash?

I asked for a quote on 6mm id 8mm OD seamless stainless pipe from a UK supplier and still haven't heard back from them

B and Q sell that but in alluminium, not sure if it helps.