Author Topic: Everything wrong with amateur railguns and how to make them better  (Read 9809 times)

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Offline German_EE

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #25 on: December 28, 2019, 08:10:17 pm »
Some may question the wisdom of this thread but I do not see a problem, sometimes you feel the need to build something a little out of the ordinary. Elsewhere on the board you will find my quest to build a 100A meter shunt that is accurate to 6.5 digits, an almost impossible task, but as the quality of German TV is so low I need something to occupy my mind.

If our friend here wants to build a railgun and shoot off lumps of metal at speeds approaching Mach 1 then I wish him luck, just send me a link to the Youtube video showing first firing.  :popcorn:
Should you find yourself in a chronically leaking boat, energy devoted to changing vessels is likely to be more productive than energy devoted to patching leaks.

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Offline Honda Rider 271Topic starter

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #26 on: December 28, 2019, 08:54:20 pm »
Research continues. Since the propulsive force in a railgun is equivalent to current squared there is a current / time discharge  "dead zone" where railguns in fact suck. My calculations lead me to believe that anything below 100KA for <1ms will definitely suck. With the logarithmic discharge curve of a capacitor bank you need to stay in the higher part of the voltage range for the entire shot to extract good power. ~500kJ expended energy is about where fun things start happening with regard to hyper velocity shots. Anything less simply can't sustain a high enough current for long enough to get >1km/sec with a realistic armature. This implies a ~750kJ total energy cap bank if you want voltage to remain at half when discharge is finished.

You can calculate this yourself with F =.5L'*I^2 with L' being the inductance gradient of gun (.5uH/meter a common value). https://scholar.colorado.edu/cgi/viewcontent.cgi?article=1184&context=asen_gradetds also has relevant data.

In other words, a capacitor bank is probably out of the question. Now I need to learn more about how I could reasonably charge a giant inductor with nearly 1MJ.
« Last Edit: December 28, 2019, 09:02:10 pm by Honda Rider 271 »
 

Online Marco

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #27 on: December 28, 2019, 09:24:22 pm »
At constant resistance losses are also I^2'd, so I don't find that a convincing argument in and of itself.

I imagine that with increasing voltage/current the resistance of the arc between the rails and armature drops though.
 

Offline Honda Rider 271Topic starter

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #28 on: December 28, 2019, 11:07:35 pm »
At low currents, for very short periods of time, your armature + projectile (if any) needs to have extremely low mass (<1G) to be accelerated to KM/s even with extreme conductivity. There are too many problems trying to pass gigantic currents through or behind a really small object, and in addition, the rails need to be a certain minimum distance apart meaning your low mass object now needs to be low density to fill the space between the rails. Even if you could make a good low power design with small rail gap, really small directly conducting armatures that are non superconducting have too much resistance. Extremely low mass plasma-armature payloads will probably have difficulty surviving. What could be used as a resilient extremely low mass low density plasma-driven payload? It is possible to accelerate plasmas by themselves to several KM/s with relatively low energy since the mass is extremely low.
 

Offline 13

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #29 on: December 29, 2019, 02:40:08 am »
Injection system:

Pneumatic injection seems easiest. Ideally the charged cap bank will be switched to the open circuit of the rails and triggered by the projectile making contact. I'm not sure how much efficiency will be lost with armature switching vs using a big thyristor.

I would try to stay away from a pneumatic injection system. The rail erosion will limit efficiency and lifetime of the gun. To get around this you could use a solid state switching scheme made of SCRs. There are quite a few "cheap" SCRs on Ebay that are sold as NOS because the seller can't find the data sheet.
 

Offline Honda Rider 271Topic starter

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #30 on: December 29, 2019, 05:13:50 am »
At the moment the main problem is figuring how I will get 10e5-10e6 currents for 1ms or greater.

Options:

0) Really huge lithium battery bank. This would appear doable for a few thousand based on the ACIR 13.2 mohm and max charge voltage of cells like the Samsung 25R. Used Tesla modules appear to be initially less expensive but may have inferior resistive losses at pulse current due to the fused interconnects. Not sure about practicality of either option. Over 1000 high discharge 18650s would be required in parallel. This option would almost certainly preclude a plasma armature due to the inherently low voltage without an impractically large bank size.

0.5) Supercaps? I don't know anything about supercaps.

1) Homopolar generator. I would have to design and build a really big spinning wheel and brushes that carry the foregoing amps which is not trivial. Or buy a used one from a physics lab.

2) Inductor. How do I charge a really big inductor? Probably from one of the previous two options, so it might be more practical to just use one of them. Any suggestions are welcome.

3) Pulse alternator. This option looks impractical from an engineering standpoint since it is even more complicated than option 1. On the other hand, compensated pulse alternators or compulsators are apparently a very attractive means of compact pulse power supply and also entirely unknown outside of research.

4) Cap bank. Already ruled impractically expensive, unless I can find a physics lab throwing out their old particle accelerator.

\There are quite a few "cheap" SCRs on Ebay that are sold as NOS because the seller can't find the data sheet.

The whole point of pneumatic injection was to avoid the problem of putting in a switch somewhere else. But it looks like pneumatic injection may be undesirable anyway.

I've been looking for SCRs on ebay and it's not immediately clear which ones will do ludicrous current. They all look kind of small. At least I would have to parallel a handful of SCRs, which is fine.
« Last Edit: December 29, 2019, 05:18:42 am by Honda Rider 271 »
 

Offline T3sl4co1l

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #31 on: December 29, 2019, 06:00:04 am »
Not sure what you're thinking with the inductor.  You're charging an inductor with an inductor, so how do you charge the inductor?  Only makes things worse...

I think you are missing some idea of just how much power is involved here.  It's not just amperes, it's volts too.  Typical figures are in the hundreds to low kV.

Supercaps are useless, they have a far longer time constant (seconds) than the regular kind.  Batteries have an even longer time constant (minutes-hours).

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Online Marco

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #32 on: December 29, 2019, 06:39:33 am »
You could have an inductor with a lot of opening switches which switch the inductor from series to parallel. An aircore inductor for 10+ kJ with RL times in excess of 10 msec seems doable.

I suspect that an aircore inductor with such a massive amount of energy will be troublesome to handle from the stray magnetic field effects even if build as a toroid though.
 

Online Marco

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #33 on: December 29, 2019, 09:26:40 pm »
Lets do a bit of an order of magnitude calculations here using the formulas from here.

To get 10 grams up to mach 1 over a meter will take around 600 Joule and also 600 Newton, which will take around 60 kA and cause an acceleration of 60k N/m taking 6 ms to accelerate the projectile. Lets say the system resistance is 1 mOhm then you want your inductive energy storage to have on the range of 6 uH to not drop current too much, which will require it to store around 10 kJoule, so 6% efficiency. Lots of 6's there ...

This shows a problem with one big inductor to store energy, you are losing most energy because you want to keep acceleration constant high, leaving lots of energy behind in the inductor which you can't use.
« Last Edit: December 30, 2019, 01:05:23 am by Marco »
 

Offline mzzj

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #34 on: December 30, 2019, 12:30:17 am »
You could have an inductor with a lot of opening switches which switch the inductor from series to parallel. An aircore inductor for 10+ kJ with RL times in excess of 10 msec seems doable.

I suspect that an aircore inductor with such a massive amount of energy will be troublesome to handle from the stray magnetic field effects even if build as a toroid though.
1MJ flyback transformer  >:D
 

Offline Psi

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #35 on: January 02, 2020, 01:34:02 am »
One thing i do wonder about.  Material strength doesn't scale, so you would think a smaller rail gun would be easier to make than a huge one.
But you never really see small ones with any real power.

Anyone know where my reasoning is flawed?

Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline excitedbox

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #36 on: January 05, 2020, 09:27:30 pm »
I think there are economies of scale that make it easier to build a certain size. There is also more interest in building powerful rail guns so for military applications.

My thinking with the supercap was basically that you can produce them for incredibly cheap and even if you are only getting a small amount of current you could mass produce them to scale up. A bottle of activated carbon costs about $3 and can make hundreds of caps. I think if you got a 10lb bag of graphite and 50lb of activated carbon you will be under $1k and have enough material to make 10s of thousands of caps. Then you just need to get a shit ton of foil and steel airport toilet paper rolls (thin as hell) for the separator.

Maxwell makes 3000F caps with a 9,300 Amp short circuit current and 1500 Amp operating current. So we know it is possible to get high current out of them

So if you build a machine to dunk or paint foil tape in a activated carbon and graphene mixture and stacks small strips with separators etc. A small team of people should be able to stuff that into some PVC pipes, add the contacts and fill with electrolyte on a few thousand of them in a week or 2. You will kill the life of the caps short circuiting them but how many shots do you really plan on fireing?

The discharge speed is not as important if you make up for it with quantity.

If you have time then building caps is the way to go. Even if you build electrolytic caps using scrapped cans from a bunch of frat parties. I bet you could collect a ton per week at a good sized university. Then you cut them length wise, dip in miuriatic acid for a quick etch and stack with sheets of the cheapest toilet paper you can steal. Stuff these sandwiches in some container or dip in resin and inject with electrolyte solution.
 


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