ETAv2.1 High Velocity Launcher

[inonickname]

To give you an idea of the forces involved here, I'm using a 6-8mm ID chamber with a 60mm OD-hardened steel chrome plated bar. This is not actually strong enough, and it will fail eventually. A lot of the survival of the chamber will be to do with it's inertia.

holy crap , how much pressure do you expect it to generate

For a short duration, in the millions of psi. After that, a steady few hundred thousand. I also have the advantage of DYI of energy density. 1cc Of state of the art caps will store about 1.7 joules. 1cc of the "motivation" I'm using will store 9877 joules of energy.

I'm pretty sure I'm retarded though. Just tried hand tapping M24 in a piece of rather hard steel. Hopefully I can get it done on a mill on Monday, if not I'll have to redesign or even worse..do the threads on a lathe :s

[MrCrowley]

What's the legal classification of these (ETGs) in America and Australia anyway? Knowing Aussie, it'd be classed as a firearm.

I'm pretty sure I'm retarded though. Just tried hand tapping M24 in a piece of rather hard steel.

[ramses]

DYI, what are the specs of your caps? Where did you acquire them? What material are you using for capillary tubes?

Excellent job, BTW. you seem to be approaching the "high velocity" required by newton's impact depth approximation.

[DYI]

@rp181: While that sounds worth investigating in theory, I can't think of any way to do it that doesn't require tools and materials which would be far beyond my means to acquire. Considering that you seem to be able to get all the supplies for your projects for free, why don't you try it?


@ino: How well lined up with the long axis of the rod do these threads need to be? To cut the 1"-8 internal threads for my ETG, I held the workpiece in a three-jawed chuck, lined up the indent on the back of the tap with the point of a live center in the tailstock, then moved the tailstock toward the chuck until the tap was held in between the live center, and the hole it would be tapping. I locked the tailstock in place, moved the live center forward a little to hold the tap tightly, then set up a pipe wrench so that it was gripping the tap, and braced on the bed of the lathe. By turning the chuck (using another pipe wrench holding the workpiece) and also keeping the live center pressed as hard as I could get it onto the tap, I was able to tap the threads very straight, and without too much difficulty. They're not entirely perfect, but more than close enough for my purposes.

Also, several hundred thousand "continuous" (don't worry, I know what you mean by that) seems overly optimistic, from what I remember of my calculations on the same topic. I thought it was more like 100k-200k. I'll have to redo them and compare...

@MrCrowley: In Canada, the definition of a firearm requires that it is, or can be used as, a weapon. As my ETG is entirely immobile, and can only be aimed at a wall roughly 3' away due to the restrictions imposed by the length of the main cables, it doesn't fall under that definition. I don't remember the exact wording of the U.S. definition, but at worst it would be considered a muzzle loader, and not subject to a whole lot of legislation in most places. If I remember correctly, Australian law technically classifies everything from an automatic rifle to a toaster as firearms. It would be a safe bet that ETGs fall somewhere in between, although a toaster would likely be a more effective weapon than my ETG...

@ramses:
Well, you already know the capacitance. The maximum voltage for each is 16kV, with a lifetime of 10⁵ shots at 20% voltage reversal. They weigh roughly 130 pounds each, and are regular geometry steel-cased Maxwell models (with two insulated terminals, the case is not live).

The capillary tubes are, and have always been, ABS. Although it's hard to tell, since the back of the plate bulged, I wouldn't be surprised if I exceeded the ~2.2mm penetration depth that Newton's approximation would yield.

Also, I'm sorry I didn't get around to answering your other question, regarding where I obtained them, before. I completely forgot about it. The simple answer is that the capacitors were obtained by [removed] from an [removed] [removed], [removed] by a [removed] in [removed] called [removed], who was [removed] them for [removed] [removed] .

[inonickname]

I tried the center today. I broke my 1 foot long tap handle after little more than a cm of threads. I'm currently designing and building a low energy version to use more "gentle" propellants, and will get the large chamber threaded later.

[Fnord]

DYI:
I was using 400v 3900uf bus capacitors. I could barely shoot though paper with 4" barrel and a steel bb. This was the first crude prototype which leaked like hell. Chamber volume was perhaps 0.25ml.
Electrode(steel bolts) corrosion was minor, but the fiberglass housing gathered carbon very quickly.
Mk 2 was made from a block of garolite with 9/16" threaded copper electrodes and intended to be used at full power, but I never got the chance.

Also, after discharging, approximately 60v remained in the capacitor, so about 98% of the avalible energy was used.

[rp181]

lyctics are never very good for this kind of stuff, especially at 400v.

Make sure that 60v isn't from ringing, put a protection diode on there.

DYI: I would, but I am way too busy on the helicopter UAV (funded by the railgun :p).

[Fnord]

What type of capacitors do you suggest? Keep in mind I want to be able to pick it up and move it, and also not have to replace damaged caps that often.

[DYI]

I tried the center today. I broke my 1 foot long tap handle after little more than a cm of threads.

Are you referring to a tap wrench as a "tap handle" here? I was only cutting about 50% thread depth, and I used an 18" pipe wrench. What kind of thread depth are you trying to cut?

@Fnord: Sounds like you'll have to find a high energy density capacitor, rated for at least a few kilovolts to achieve that. It won't be particularly easy, as the kind you'll need are fairly new on the market. Electrolytics are very far from being optimal, as you could probably guess, for both performance and reliability. At 5kJ, my design wouldn't even fire if I increased the capacitance to 200uF (I've tried), much less the values you'd be looking at when using electrolytics.

[inonickname]

I tried the center today. I broke my 1 foot long tap handle after little more than a cm of threads.

Are you referring to a tap wrench as a "tap handle" here? I was only cutting about 50% thread depth, and I used an 18" pipe wrench. What kind of thread depth are you trying to cut?

@Fnord: Sounds like you'll have to find a high energy density capacitor, rated for at least a few kilovolts to achieve that. It won't be particularly easy, as the kind you'll need are fairly new on the market. Electrolytics are very far from being optimal, as you could probably guess, for both performance and reliability. At 5kJ, my design wouldn't even fire if I increased the capacitance to 200uF (I've tried), much less the values you'd be looking at when using electrolytics.

A fair amount I'll knock it down to ~60% and try again with a pipe wrench, seen as I broke my tap wrench..

[ramses]

I almost forgot to ask, how are you switching this monster? trigatron?

[DYI]

I almost forgot to ask, how are you switching this monster? trigatron?

The switch consists of two copper electrodes (1/8" x 2" flat bar) which a 1.25"x1.25" aluminum arm is pulled down between (by a gravity-driven setup similar to a trebuchet). The electrodes are angled apart somewhat to allow the arm to wedge smoothly between them, although the discharge is long gone by the time physical contact is made between the components (despite the high speed of the arm, likely sufficient to break a wayward finger without much difficulty). I've never experienced any problems with the contacts becoming even slightly stuck together, although I have lost well over a gram of the switch arm, which now forms a heavy coating of Al₂O₃ on the inside of the switch's containment vessel.

@inonickname: I was able to cut fairly shallow female threads in the outer casing components because the 1"-8 male threaded parts were made entirely from threaded rod. I don't currently have the tooling to cut external threads over 1/2" for myself. Just be careful to get good thread depth on the external threads.

[rp181]

Check out this concept:
http://www.coilgun.info/mark5/home.htm

May not work with that high voltage, but it could still be better.

[ramses]

I built a pneumatic one for my disk launcher.

3/4" QEV, etc. basically a potato gun that shot a wedge.

It still welded shut, although I was using around 2000uf, which is around 20x slower than what you're using.

[DYI]

Well, the pulse in my design lasts roughly 20μs. Keep in mind that I already incorporate "catching" the switch arm with slightly angled contacts, so the only benefit that could be gained would be by closing the switch fast enough that the switch arm's surface had not yet started to boil when direct physical contact occurred. Some back-of-the-envelope calculation shows that the arc will start when the arm is about 7mm from the electrodes, and that the switch arm would have to approach the electrodes at a mere 350m/s to make contact before the pulse was complete. Now it does take some small amount of time for breakdown to occur in the air around the electrodes, and the wedge shape alters things somewhat, but it would also be necessary to make the connection in something like 1-2μs after the arc started.

So, how fast do you recommend I move the wedge toward the electrodes when I try what you propose? 1000m/s? 4000m/s? This is supposed to reduce switch erosion, right? I can't help but get the impression that the switch may deform somewhat when impacted by a multi-pound copper slug carrying similar quantities of energy to a kilogram of high explosives...

One more question: What should I use to drive the wedge into the switch? An ETG, perhaps?