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Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039
Marko wrote ...
It went with a blinding flash and a really loud bang, and there was practically nothing left. I guess this is why aluminium powder works better still.
How does one put a sustainable arc through aluminum powder though?
Marko
I know for a fact people use AL powder in ETG. Maybe thu pack it? Maybe they just bank the capacitors to have near 1KV to make arcing more possible. Aluminum powered as said above is rather loud when exploded. After all, that's what a lot of fireworks seem to use mixed with other elements.
It was right that I said my barrel wasn't shooting. But I found it was because there's a weld seem running down the whole inside causing my projectile to stay pit as the pressure goes around it. I now have 3mm thick iron extruded tubing with a smooth bore. Works good.
Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039
True. Will do this weekend. Foil I think would be best cost wise, just because even a dollar store amount can get hundreds of shots. But I'll see what Mg does for fun. I don't use my ribbon anyways
Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039
Just a simple concept, by taping stuff together. Need to drill cut the pipes a bit, clean them, paint them, etc. I used a 1:1 scale handgun airsoft pistol, and the trigger will be a switch to fire the gun. The barrel portion wa sput on the front, whch I will probably for fun, stick a green laser inside. Wish I had a mill, rather nice PVC is soft and I can just stick a shaving bit into my drill press.
Registered Member #1917
Joined: Fri Jan 09 2009, 02:38AM
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Posts: 62
Looks like there's a LOT of misunderstanding about ETGs still out there. Allow me to explain.
I know for a fact people use AL powder in ETG. Maybe thu pack it? Maybe they just bank the capacitors to have near 1KV to make arcing more possible. Aluminum powered as said above is rather loud when exploded. After all, that's what a lot of fireworks seem to use mixed with other elements.
Most people who build ETGs have no idea what they're doing, as regards both the electrical and the propulsion components. The attempted use of a METAL as a propellant gas should be ample evidence of this.
Aluminium foil also burns quickly when blasted into pieces and ignited by the discharge, which adds more fuel.
The amount of air left in the chamber is a perhaps a milligram, totally insufficient to cause any noticeable combustion effect until the gaseous aluminum exits the barrel. THEN you get that pretty flash and a big cloud of Al2O3 smoke.
Marko was correct in his assertions about the ineffective design of most hobbyist ETGs, but for the wrong reasons. As anyone who has ever built a cannon powered by high explosives knows, they are quite effective as propellants, in some cases even more so than typical modern propellants. Why aren't they used? Containing them requires a cannon that weighs ten times as much and needs to be totally replaced after a hundred shots. Fast pressure generation is no detriment to propulsion itself, and these esoteric and completely baseless theories about "shock wave reflection" have no relation to gun technology. Shock waves may chew up the chamber with great efficacy, but the pressure applied to the projectile is still there, pushing it like it would if it had been generated over 10 seconds instead of half a millisecond or two nanoseconds.
The reasoning behind slow presure generation in modern firearms is a simple one - if you start off at 900MPa and allow the gas to expand pushing the projectile out the barrel, what you've created is essentially a pneumatic burst disk gun - efficient, but requiring proportionally high starting pressure. The efficiency may be very high, depending on the launch speed and propellant gas, but chambers can only be built to contain so much pressure (about 2.5GPa, with the best modern steels before plastic deformation occurs). This 900MPa gun with instant pressure generation might be pushing the projectile with 50MPa by the time it hits the muzzle. Compare with a gun using a relatively slow burn that pushes the projectile at a starting presure of 450MPa which eventually drops down to perhaps 300MPa by the time the round reaches the breech. If the two show the same performance, but one is twice as heavy due to the chamber strength requirements, it is obvious which one will be chosen for combat use.
This brings us to hobbyist ETGs. None of the posters in this thread have anywhere near the capability to push modern materials to their tensile limits in a reasonably dimensioned chamber. For that matter, I hardly have such a capability - my best efforts so far have resulted in perhaps 700MPa, still within the limits of a good high-tensile steel. As such, this concern about the rate of pressure generation is not well-founded. My current ETG generates that 700MPa over the course of roughly 50 microseconds using a 15kV discharge, and manages 15% efficiency with heavier projectiles (here considered to be those that do not exceed 1km/s), and about 10% efficiency even at 3km/s. Could it do better with a more suitable waveform? Sure, but it works very nicely as it is.
There are, as I see it, about four serious problems in current hobbyist ETG designs, listed below. 1)Use of metallic propellant gases - it is NEVER a good idea. EVER. Water is readily available and vastly superior (for those of you who can't tell a compressible fluid from a stress-energy tensor, high sound speed and low thermal conductivity are highly desirable qualities in a propellant gas). 2)Direct use of exploding wires or similar for main pressure generation. Highly inefficient. Use capillary plasma generators instead, as almost ALL of the published work on the topic already does. 3)Use of extreme low voltage, sometimes under 1kV - it severely limits one's options, and is generally not suitable. There are better was of achieving nicer pressure generation features, if one is really that desperate. 4)Insistence upon use of inadequate construction materials and inaccurate construction techniques. ETGs are relatively high pressure launchers, and require good tolerances in their construction. Plumbing pipe and fittings are not in any way suitable, nor are shoddily machined parts. Bad tolerances results in gas loss and ablation of chamber and barrel materials. Brass or aluminum do not count as adequate construction materials. Use steel.
Following the above advice works, in a spectacular fashion. By the end of my ETG topic here in summer 2009, I'd shown results of almost 20% firing efficiency at 1200m/s. With an upgraded design using high-tensile steel parts, and larger capacitors, I've now achieved nearly 3000m/s at 10% ballistic efficiency. Here's a link to that improved (yet still inadequately built) launcher, showing some initial results before I had to pack up my operations at the end of the summer.
Building an ETG rifle has potential to be a huge, impressive, fascinating project. With continued ignorance of all relevant information, it will be a frustrating money sink that never amounts to much. Dedicate some time to perusing the literature on the topic, and take a look at the progression that finally led me to an effective design (most of which is posted on either Spudfiles or 4hv). I have free access to all articles on IEEEexplore and most other such sites, so PM me if you can't get a hold of one. I'm happy to help.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi DYI - thanks for this post...
Marko was correct in his assertions about the ineffective design of most hobbyist ETGs, but for the wrong reasons. As anyone who has ever built a cannon powered by high explosives knows, they are quite effective as propellants, in some cases even more so than typical modern propellants. Why aren't they used? Containing them requires a cannon that weighs ten times as much and needs to be totally replaced after a hundred shots. Fast pressure generation is no detriment to propulsion itself, and these esoteric and completely baseless theories about "shock wave reflection" have no relation to gun technology. Shock waves may chew up the chamber with great efficacy, but the pressure applied to the projectile is still there, pushing it like it would if it had been generated over 10 seconds instead of half a millisecond or two nanoseconds.
I must admit that I've never heard of a cannon powered by high explosives. I think a cannon that could launch a projectile with velocities significantly higher than speed of sound in air might be so valuable that some might actually bother using a few times fatter barrel and a propellant that supersonically combusts - but they ended into building railguns instead.
By the way if shock waves are efficient in eating up the chamber, doesn't energy expended on this also result in poor overall efficiency? I admit I don't really know to what extent that is a problem. If you build the chamber solidly enough with right materials, it might not waste much energy on deformation at all.
Also, from reading your post, it seems like you have pushed your projectile velocities to an extreme. If you're using water vapour, it only has speed of sound of like 400m/s and you're launching your projectiles at mach 10. Not only that's an awesome work but also quite far away of scope of what I was talking about. In your case, I can imagine you have no choice but to use fast capacitors and try to get as much of "supersonic combustion" effect as you can, to drive a tiny projectile at high speeds.
What I'm trying to figure out is more of a conservative design, using air as a propulsion gas and speeds under mach1 with relatively heavy projectiles.Th And I have much more questions regarding that:
The other possibly more major problem I now see with a typical air-driven etg is simply very uneven heating of the chamber gas. Apart from the detonation problem (which you point might not be a problem at all), with a single discharge into a tiny bit of foil, the most of the gas in the chamber might just be a dead volume, and we end basically using metal vapor, as you said.
That's why I advocated using a thin wire stretched along the chamber to heat as much air as possible at once. The wire is to act just as a seed for an arc, and not to contribute significantly to gas volume.
One thing I don't understand regarding this, why would one want a gas with low thermal conductivity though? This is against the above reasoning, at least for an air driven gun.
If one is using water on the other hand, chamber only needs to hold this small amount of water and it's easy to deliver energy to it uniformly, over short time. I haven't exactly got what is the purpose of the capillary tube. Does it shape the discharge in some specially favorable way?
Finally, when building a this kind of gun, one has to decide whether he wants it to maximize efficiency or velocity, and I suspect that most handheld guns with capacitors on them would want to be high efficiency and relatively low velocity. Considering this, I thought that modelling the rifle sorts of like an internal combustion engine might help to get the max out of efficiency, at least for subsonic speeds. Using water might still be far superior though, and that's on killa-x to find out :P
Registered Member #1917
Joined: Fri Jan 09 2009, 02:38AM
Location:
Posts: 62
I must admit that I've never heard of a cannon powered by high explosives. I think a cannon that could launch a projectile with velocities significantly higher than speed of sound in air might be so valuable that some might actually bother using a few times fatter barrel and a propellant that supersonically combusts - but they ended into building railguns instead.
And I must admit that they're not very common. Me and a few others of similar interests looked into them a few years ago. It was a fascinating project, but the legal and safety concerns surrounding HE prompted us to drop the project and concentrate on electric launchers. "Normal" firearm propellants in use today are capable of about 1800m/s with reasonably sized projectiles (i.e. the 120mm Rheinmetall L55). The only advantage that might be gained by using HE is higher efficiency at high speeds - due to the aforementioned prolonged burn desired in modern propellants, they're actually worse at low speeds.
By the way if shock waves are efficient in eating up the chamber, doesn't energy expended on this also result in poor overall efficiency? I admit I don't really know to what extent that is a problem. If you build the chamber solidly enough with right materials, it might not waste much energy on deformation at all.
The low deformations necessary to destroy a chamber don't really require much energy input, because the chamber material doesn't move very far or fast, especially with the very heavy walls required for survival (every shot causes some plastic deformation, as it's not physically possibly with our materials to build a chamber which could survive the detonation pressures in the tens of gigapascals). Much more can be gained by better thermal insulation of the inner barrel surface, an intriguing possibility only present in smoothbore guns (considering that about 30% of the chemical energy in a rifle cartridge goes to heating the barrel, this is definitely worth looking into).
Also, from reading your post, it seems like you have pushed your projectile velocities to an extreme. If you're using water vapour, it only has speed of sound of like 400m/s and you're launching your projectiles at mach 10. Not only that's an awesome work but also quite far away of scope of what I was talking about. In your case, I can imagine you have no choice but to use fast capacitors and try to get as much of "supersonic combustion" effect as you can, to drive a tiny projectile at high speeds.
The propellant gas in my ETG is a mixture of water, dihydrogen, dioxygen, and probably a few free hydrogen and oxygen atoms in the most heavily heated spots. Due mostly to the high temperatures seen, around 5000K in some cases, the speed of sound in the chamber is actually more like 4000m/s at the current best performance, as a lower bound. I'd also like to clarify that "supersonic combustion" is a relative term - in something like an oxygen/hydrogen/helium mix, a deflagration wave moving at 800m/s would not be considered a detonation.
What I'm trying to figure out is more of a conservative design, using air as a propulsion gas and speeds under mach1 with relatively heavy projectiles.Th And I have much more questions regarding that...
Firstly, if you want to use air as a propellant, you're in for a very challenging build. Simply starting with atmospheric pressure air is unfeasible due to the high temperatures reached in relation to the pressure generated - losses due to heat exchange with the barrel and chamber walls increase tremendously. Using compressed air comes with its own set of challenges - you now need to incorporate a rupture diaphragm and a fill valve resistant to the firing pressure, as well as an airtight chamber. Also, air isn't a great propellant gas anyway. I wouldn't recommend it. I actually devoted several months of work to this approach before I saw the error in it. It works, and becomes relatively feasible at outright huge input energies, but is otherwise undesirable.
Low thermal conductivity tends to translate to lower heat losses and less heat required to reach high temperatures, and thus high sound speeds. Its only beneficial effect is maintaining a more constant temperature through the barrel, but my CFD ETG model isn't yet advanced enough to see how important that is.
The capillary tube approach allows very efficient plasma generation, leading to the potential for much higher performance once this plasma (typically polyethylene or similar) is used to heat a propellant gas. I rediscovered it after several months of experimentation with the more typical style of hobbyist ETG (the kind that doesn't work well...), immediately noticing a threefold improvement over the best I'd achieved before. I soon after discovered why it worked so well, and never went back. Even with only the chamber air and polyethylene for propellants, it still hit about 8% efficiency at the relatively low speed of ~1000m/s.
You make a good point, and this does indeed seem to be the trend in hobbyist builds. Happily, efficiency can be very high even at reasonably high launch speeds. Unfortunately, ETGs are nasty beasts to model, and "optimizing" for anything is horribly difficult. I'm working on a proper CFD model for the post-discharge process now which may shed some light on the matter. Using short, fat capillary tubes and very low voltage capacitors as are typical of the handheld hobbyist builds, one might reasonably expect 5-10% efficiency at low speeds (only a few hundred meters per second - the low voltage has the nasty habit of causing arc quenching in the tube when one tries to push too much power through).
I've never had any interest in handheld electric guns - they may become feasible someday, but they're not today. In the case of handheld, cartridges would be necessary to make the capillary tube approach work. Regardless, the reload rate's going to be awful without cartridges, and the machining will be a very difficult task. I'll concentrate for now on getting them to work before I try to make them fun to use
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Rifle based ETG
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