SpudFiles

rp181 wrote:the thing with electrothermal is large energies are still required to vaporize the fuel. Try it, it will work, but you might not get the speed you would expect. 70 joules is pleanty for a coilgun, try that with cylinder projectiles, you will like it, its also compleatly silent.

Yup, enough for a coilgun, but too less for an ETG.

st.croixsurfer wrote:has anyone tried to get the used disposable cameras from your local cvs or wallgreens?

Guess where my hundred capacitors are from.

rp181 wrote:1) if there in series, dilectric breakdown will probably be exceeded, ESR will be high
2) if there parrallel, they will unevenly charge unless theres balancing resistors.
3) a cap will probably fail and it will be hard to find it/repair.

Oh ye of little faith.

10 series banks of 10 parallel. Approximately 3.3-4Kv and 100uf.

I'm not about to stick 100 in series or parallel alone and certainly not in one massive daisy chain

Larda wrote:

psycix wrote:2880 joules, 70 joules, whats the difference?

Actually i used 28 800 joules.

i started to think about that after i posted it. 28.8*1000 is not 2880

Oh ye of little faith.

10 series banks of 10 parallel. Approximately 3.3-4Kv and 100uf.

I'm not about to stick 100 in series or parallel alone and certainly not in one massive daisy chain

Lol, 3Kv? that will defintly exceed the dilectric breakdown. Each cap is designed for 300v, the last caps in the bank will be seeing 3Kv. look into buying caps. At my OEM store, there are 500J caps for 10$.

It would absolutely exceed the dielectric breakdown of ONE capacitor if I put 3Kv across it.

Click a piezo lighter with one end connected to such a capacitor and the spark goes right through it.

However in a bank the end capacitors are still looking at a difference of 300V across their terminals.

If you check out http://4hv.org/ you'll find that madness like this happens and works on a daily basis.

im just going to go shopping on digikey

by the way Larda what were the specs on your capacitors

I've actually tried launching airsoft bbs with 60 joules, and the results weren't very impressive. fine aluminum powder should work as a propellant, but you'll only need a very small amount. I think lead would be a fairy idea metal, but unfortunately lead vapor is toxic.

Something I didn't think about until just now: If your propellant gasses are 10,000+ F, the particle speed is going to be ridiculously high. When gasses reach such high temperatures, they don't seem to mind choked flow* very much. But, for people using them to launch projectiles, this means that any small gap around the projectile will let a lot of propellant gas escape compared to, say, a room-temp pneumatic.


* if you don't believe me, try playing around with disk diameters in hgdt.

I was checking out the site hotwired gave, and came across a guy who made an ETG that worked with an 80 J cap bank. Worth a look.
http://4hv.org/e107_plugins/forum/forum ... .php?30934

It would absolutely exceed the dielectric breakdown of ONE capacitor if I put 3Kv across it.

Click a piezo lighter with one end connected to such a capacitor and the spark goes right through it.

However in a bank the end capacitors are still looking at a difference of 300V across their terminals.

If you check out http://4hv.org/ you'll find that madness like this happens and works on a daily basis.

Yes, i am active on 4hv.

The voltage wont be past the limit on one cap, but as soon as you compleate the circuit, the voltage would add past each cap, like a avalanche. They will surivive, but not for long. I say buy some bigger caps.