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Amateur coil gun approach sound barrier

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Steve Conner

Sun Mar 05 2006, 08:53PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Wow, 300m/s is still a very impressive result, even if "der Schallmauer ist noch nicht geknackt"

I know just enough German to be amazed at the stuff pulslaser is doing, and if you don't know any, it's probably worth struggling with a machine translator.

I think spark gaps are still the best switch for this device though. The traces I saw on the website showed that the coil current was a high frequency oscillation, whose di/dt would probably be too high for a puck SCR. If you lowered the frequency to make more efficient use of the SCR, then I think the induction wouldn't work so well.

Madgyver

Sun Mar 05 2006, 09:49PM

Registered Member #177 Joined: Wed Feb 15 2006, 02:16PM
Location: Munich, Germany
Posts: 214

Holy cow, you said you were working on it earlier, but I didn't think to see something so soon. I am looking forward to see it for real.

Very nice. Are the caps from the "MIR" Shop in Schillerstrasse? I have seen some of these types there.

Besides, I am worried that you start to get into speeds were legal problems could occure. Any thoughts so far?

P.S. I thought railgun was too loud for your neighbours, would a super sonic boom be such a big difference?

pulslaser

Mon Mar 06 2006, 07:45PM

Registered Member #156 Joined: Sun Feb 12 2006, 07:04PM
Location:
Posts: 23

Hi Madgyver,
you're welcome. The machine is at my home in munic Sendling Liesel Beckmann Str. 2. That's in the south of munic, near the Luise Kieselbach Platz at the "Mittlerer Ring". You can reach it by U-Bahn U6 to Partnach Platz or S-Bahn S7 to Mittelsendling.
It didn't bought the capacitors from "Mir", I got them from some old ruby lasers. Because I had a lot of them I even offered some to "Mir" for selling but the weren't interessed.
I didn't though about legality until now. I think the law isn't well defined in these cases.
To my sadness I didn't reach speed velocity, so there no super sonic boom, but of course the is some noise, but not to compare with explosion from 10kJ capacitor discharge happening in plasma driven railgun

Quantum Singularity

Wed Mar 08 2006, 01:58AM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Congrats pulslaser, even though you might not have broke the sound barrier it is still very fast. Your coilgun is more like what I want to build. Spark gaps are what I am more familiar with (several tesla coils, exploading wire, can crusher, disk launcher, etc...) and it appears that in your configuration the spark gap switches seem to be good enough. I was thinking about building a traditional coil gun with steel projectile but it seems the induction type is more like what I have been experimenting with my disk launcher and is simpler in terms of timing and pulse length.

And tell me if I am correct - I am not an expert but when I was working on my disk launcher a few years ago I came to the conclusion that for this type of launcher you would want maximum coupling between coil & projectile and the greatest rate of change of the magnetic flux lines passing through the projectile? I guess what I am getting at is unlike the railgun discussion where you dont want such a quick pulse (using lower voltage caps all stacked in parallel) this type of launcher would fair better with higher voltage and less capacitance. With the practical limit being when too fast of a pulse causing inertia and air resistence to degrade performance. Does this sound correct?

I am still going to build a monster single stage but I might also work on a smaller multistage gun.

pulslaser

Wed Mar 08 2006, 05:52PM

Registered Member #156 Joined: Sun Feb 12 2006, 07:04PM
Location:
Posts: 23

Hi Tristan,
first of all thank's to youre congrats. The trigger gap is located behind the coil and the projectil triggers when it leaves the coil but trailing parts is still in the coil. I haven't implemented any delay circuit. I cannot measured the effective delay but I think it should be smaller than some microseconds. Of course You come in trouble with very high speeds, so one should place the trigger gap as close as possible behind the coil. I have tried to compensate the increasing speed by making the coils of the last stages shorter in the hope that even with late triggering there is some part of projectil still in the coil. I don't know if this makes any difference.
I think shortest possible pulse should be used, because the region where the magnetic field influence the projectil is very small. Althoug I haven't made any simulation I think field density is highest in/near the coil and degrades quickly with distance from the coil. But I have to admit that I didn't think about a upper limit by air resistance and inertia.
Like You I think that spark gaps are the best choice in this Voltage range, because there cheap and easily homemade. If I would have enough money to spend I would prefer to use ignitron, this kind I used in my railgun. But they are very expensive and seldom sold in ebay, over some years I just got two of them.
Best wished four your next gun

Quantum Singularity

Wed Mar 08 2006, 06:45PM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Ahh, I C. And for anyone wondering, pulslaser was answering questions I asked him in an email.

Another questions... you say your not sure about the delay in triggering especially with very high speed projectiles. You have it set to trigger as fast as it can by your design. Have you tried testing it by not running all the stages to tell the effect of each stage? Like run just the first stage and record data, then the first two stages, then the first 3 stages... and so on. You could calculate the energy transfer of each stage and determine maybe if the later stages with the faster velocity arent transfering as much energy to the projectile. And if it was found that triggering want fast enough could you put the trigger inside the coil slightly instead of after it? Like space out the last few turns just slightly and put the triggering in the small gap?

pulslaser

Wed Mar 15 2006, 11:31PM

Registered Member #156 Joined: Sun Feb 12 2006, 07:04PM
Location:
Posts: 23

I have made this measurements by recording the flash of the spark gaps with a photocell. Measuring the time the projectil needs to reach and trigger the next gap and knowing the distance I can calculate the speed increase by each coil. It's nice to see that the data compare well with the theoretical increase of speed by assuming a constant energie deposition at each stage .
Next I will change my projectil tube from some aluminium alloy to pure aluminium. A friend told me that there may be a possible increase up to 100% because of lower resistance.

sorry, I forgot to upload the images and didn't find possibilty to re edit the post

[Mod edit: double post merged.]

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