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ETG Help

All non-spudgun related discussion goes here such as projects, theories, serious questions, etc. All "off-topic" posts (aka useless posting, determined by moderators) will be removed.
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ETG Help

Unread postAuthor: kjjohn » Sat Jul 11, 2009 10:15 pm

I am possibly going to be building an ETG. I know how they work, all the technical details, I just need help with a few things. First of all, I have a fairly small cap bank. It consists of 2 400v(450 max), 470uf electrolytic caps in parallel, producing an output of 75.2 joules. I know this is practically nothing for an electrothermal, but I need to know how fast it will shoot, if at all. I am vaporizing a small piece of steel wool, and the projectile is a shaped .25 cal hot-rolled steel round. How fast (approximately) do you think this will shoot? I just want it to go faster than 200m/s.

Also, could I use thick PVC encased in steel as a barrel, because the trigger terminal design I came up with requires a non-metal inner barrel.
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Unread postAuthor: inonickname » Sat Jul 11, 2009 10:21 pm

You wont even get anywhere near that. Larda's was getting a bit higher, except he had a 28.8 kilojoule cap bank.

PVC will work, though make the insert replaceable as it will take some beating.
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Unread postAuthor: kjjohn » Sat Jul 11, 2009 10:33 pm

thanks for your input, inonickname
how big of a cap bank will I need to get, say, around 300m/s?
and approximately how fast would it shoot with my small 75j bank?
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Unread postAuthor: inonickname » Sat Jul 11, 2009 11:52 pm

kjjohn wrote:thanks for your input, inonickname
how big of a cap bank will I need to get, say, around 300m/s?
and approximately how fast would it shoot with my small 75j bank?


With that kind of projectile, it may barely even leave the barrel. With a lighter projectile you could get a fair velocity (though nothing stunning).

Perhaps 20 kilojoules.
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Unread postAuthor: kjjohn » Sun Jul 12, 2009 12:48 am

Sorry for all the questions, what kind of projectile do you suggest I use? Or possibly other propellants? I really want to maximize the performance out of this small bank, otherwise, I know where to get a 1kj bank for fairly cheap.

I thought of some other propellants. I could use water and flash it into steam, that would certainly be easier to vaporize than steel woll, but I still don't know if that would produce much kinetic energy.

I also know how to make some explosive propellants, but I cant discuss them on Spudfiles.
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Unread postAuthor: inonickname » Sun Jul 12, 2009 1:05 am

People have discussed etc's on here before- PM a mod/admin first and ask if it's ok.

Unfortunately, electro magnetic (etg, rail coil) are not very efficient at transferring energy from the bank to projectile, and the amount of energy is limited by the input energy. However, an etc does not have these limitations. Aluminum foil and powder is often used for etg's.
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Unread postAuthor: john bunsenburner » Sun Jul 12, 2009 4:15 am

Have a look on http://www.4hv.org, both larda and DYI are members on there, along with a few others form SF. Its a useful site, and has lots more members who know thier stuff on there. Also there are lots of coil, rail and electro thermal guns on there for you to see what others have come up with. They will also be able to help you out with you CB and projectile and so on, have a look around.
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Unread postAuthor: kjjohn » Sun Jul 12, 2009 11:42 am

I have worked with coil guns before. I tried to sign up for 4hv, but the confirmation email never sends.
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Unread postAuthor: kjjohn » Sun Jul 12, 2009 12:59 pm

is it safe to discuss propellants here that are liquids but have the potential of solid propellants, or should I ask a moderator?
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Unread postAuthor: spudtyrrant » Sun Jul 12, 2009 1:12 pm

try to make something similiar to this http://4hv.org/e107_plugins/forum/forum ... 62758.last if you want some real substantial power out of it its not that hard to build although it may cost you some dough but if you plan on making a powerful etg you should expect that
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Unread postAuthor: kjjohn » Sun Jul 12, 2009 2:28 pm

spudtyrrant wrote:try to make something similiar to this http://4hv.org/e107_plugins/forum/forum ... 62758.last if you want some real substantial power out of it its not that hard to build although it may cost you some dough but if you plan on making a powerful etg you should expect that


impressive setup

only problem with that is that I have no experience with SCRs. I know how to wire one up, but not the specifics. Also, I am planning on making this thing "fairly portable :lol: " , I want to be able to fit most everything in an enclosure the size of a 50cal machine gun, or slightly larger.

However, I do know where I can get some 243j electrolytic caps for 5 bucks apiece. I could buy two or three, at least get half a kj.
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Unread postAuthor: kjjohn » Sun Jul 12, 2009 2:38 pm

only problem with that is that I have no experience with SCRs. I know how to wire one up, but not the specifics.


Oops, actually I do know how to use low current SCRs (for switching >1000j). I might be lazy, but I just dont want to bother with all that stuff to prevent ringing in the system.
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Unread postAuthor: KineticAmbitions » Sun Jul 12, 2009 3:48 pm

Assuming a 2cm long round (you didn't tell us, as far as I know), your 300m/s goal would give 220J of kinetic energy. With a typical (read: horribly inefficient) design that vaporizes aluminum powder/foil to accelerate a projectile, you'd need anywhere from 4 - 20kJ of stored energy to do this. If you do some research, you can reach 10 - 15% efficiency (it has been done by an amateur, and I have seen it). At this efficiency level, you would need a maximum of 2.2kJ. However, achieving such an efficiency level is NOT a walk in the park - it requires a decent understanding of the internal ballistics involved.

A mechanical switch would be the best to use here. If you want high velocities, energy discharge capacitors would be better than electrolytics. At low speeds, I doubt that it matters a whole lot, as long as the circuit resistance and inductance are low enough to allow for full discharge before the projectile exits the barrel.

Look at some of the papers done on the subject by research labs - they know what they're doing more than most of us, due to their advanced simulation software. Soreq (an Israeli ballistics lab) has published some interesting papers on the subject, which were what the aforementioned 10% efficient ETG was based on (although Soreq's ETG is 30% efficient...).

A replaceable PVC chamber sheath is actually recommended strongly, but putting it in the barrel may cause blowby of the hot gases (not plasma, unless you're doing it wrong) as the sheath vaporises around the projectile. At lower energies, this shouldn't be too much of a problem.

To inonick - ETGs are being developed because they are MORE efficient than their powder-burning counterparts. Railguns are even better, having reached 50% efficiency already. As far as I know, efficiencies in that range are only surpassed by... pneumatic spudguns :lol:
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Unread postAuthor: kjjohn » Sun Jul 12, 2009 4:30 pm

yes, my round is between 2 and 2.5 cm. I know that it is easier to vaporize water than it is to vaporize aluminum. Would I be able to get significant results (>600fps) with a low power electrolytic bank vaporizing water?

I also thought of dry ice, as it can produce high pressures when flash vaporized.

Actually, I have heard of people shooting airsoft bbs at speeds in excess of 600fps with just 100j or less, but I suppose an airsoft bb is a lot different from a shaped steel round.
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Unread postAuthor: KineticAmbitions » Sun Jul 12, 2009 6:36 pm

Water is an effective working fluid for use in an ETG (it is, of course, what was used in the papers you should have read by now). You want something that will disassociate to produce a gas with a high speed of sound and low molar mass, for a high adiabatic constant value.

Of course, regardless of what your working fluid is, you still need the plasma to vaporize it. The plasma generator is an important part in any design - it can make all the difference between useless spark-shooter, and effective electrothermal launcher.

There isn't a whole lot of difference between an airsoft round and a steel rod in terms of propelling them, other than at the high end where the airsoft round mushrooms/shatters/vaporises, and the steel rod remains intact (although it will suffer the same fate eventually, you probably don't have that much power available...).
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