Author Topic: Everything wrong with amateur railguns and how to make them better  (Read 9813 times)

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Offline Honda Rider 271Topic starter

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First, I am here for guidance not to pontificate so please add to my post and correct any and all mistakes and misconceptions.

Awesome comprehensive reference reflecting state of the art in the late 80s: 300+ pages I haven't finished it yet: https://apps.dtic.mil/dtic/tr/fulltext/u2/a192102.pdf

OK so tons of individuals have built railguns and these railguns all have a few things in common:

1) Relatively small, high cost capacitor bank power supply
2) Terrible efficiency below 1%
3)  Because of 1 and 2, really weak projectiles

The most powerful individual railguns that appear to have been built are on the order or 30kj of input energy (edit: apparently sam barros had one with 100kJ). Even 10% of that in terms of kinetic energy would be pretty impressive, on the order of a high power hunting rifle. Unfortunately from firing demonstrations it's clear that the actual projectile energy these designs deliver is on the order of 100 joules or less: somewhere between a small-bore air rifle and a .22. Man I live in Texas, surely we can do better.

In order to make an AWESOME railgun one or more of the above issues needs to be solved. First the pulse power supply:

While it's possible to find relatively large high power capacitors for salvage this leaves you at the mercy of what's available. Usually salvage cap lots are significantly limited in number. So you may be able to assemble a modest bank for relatively little money, but you probably won't be able to build a huge one. Conversely, if you want to build a big bank, you can buy as many capacitors as you want from the production market but this is extremely expensive with even a 100Kj bank costing on the order of several 10s of thousands of dollars. Maybe it's possible to find a physics lab scrapping their trashcan sized caps but this strikes me as especially impractical.

Building your own capacitors for any reasonable bank size is not an option (correct me if I'm wrong).

Building your own high energy inductor IS a realistic option and it is probably possible to create an inductor storing on the order of 1MJ energy for a few thousand dollars. I'd settle for 100kj. The material of choice would probably be aluminum.

So the inductor should probably be the choice of energy storage for amateurs who want to build a badass railgun--NOT capacitors unless you are so lucky as to come across a reasonably priced, gigantic salvage lot.

Charging the inductor is an open question. Since the charge time is on the order of 100x longer than the discharge, maybe it's possible to use a relatively small bank of high C-rate lithium polymer.

Now the second part, increasing firing efficiency.

Why could amateur railguns be so inefficient? A few guesses:

--Improper dimensions and geometry of rails and projectile
--Not taking skin effect into account, leading to much more energy wasted thru resistive heating than expected
--Pulse width too short (cap bank left to its own devices can easily dump all the energy nearly 2 orders of magnitude faster than it takes the projectile to leave the rails even with short rails). This could be tuned with an inductor in series with the cap bank (but we don't want a cap bank, we want POWER)
--Friction. Even if you have a loose-fitting projectile and rely on plasma to bridge the gap your projectile could still be intermittently spot-welding or sticking to the rails. I don't know how much loss this would be responsible for but maybe a high pressure low temperature gas could help. Totally speculative.
--Reliance on single-stage rails. It's known that adding additional lateral rails in series can increase efficiency. I don't know how important this is or even what such a design would look like in practice. I think this one is mostly irrelevant for the time being since it should be possible to achieve efficiencies higher than <1% even with a single pair of rails.

Action plan:
--Obtain solid grasp on what makes an efficient railgun
--Figure out muzzle energy level desired (let's say 5kj, on the order of a 12G shotgun slug)
--Figure out what efficiency is reasonable with all realistic optimizations
--Design inductor capable of delivering (muzzle energy / efficiency), inductor supply
--Design switching for inductor charge and discharge
--Design rails and supporting structure, projectile and electrical interconnects
--Design loading / injection system

Anyway if you read all that maybe you have something to add. Let's make amateur railguns BADASS instead of pitiful exercises in wasting money and energy.
« Last Edit: December 27, 2019, 09:34:46 am by Honda Rider 271 »
 

Offline Honda Rider 271Topic starter

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #1 on: December 27, 2019, 01:02:34 am »
Update!

First I bought some capacitors. But before that some newly found low-hanging fruit:

1) Amateur projectiles (the armature and anything it drives) are too massive for the small electrical energy levels involved. Kinetic energy is .5MV^2 so other things equal reducing the projectile mass by a factor of 10 should increase kinetic energy by a factor of 10 even after accounting for new 90% smaller mass. Miniaturizing the projectile to a reasonable mass (<10g) might include conductivity and or thermal problems reducing the theoretical energy gain. The small scale high velocity laboratory railguns I've found in academic research all shoot extremely low mass projectiles.

2) Staging. Low power railguns will need such a small projectile to achieve high velocities that a plasma armature might be most practical. Staging is thought to work better in plasma armature designs and many research railguns appear to use staged input vs dumping the power in all in one location. I will try both cap bank + coil for pulse elongation and staged inputs to see which works better.

Capacitors:

I found 28 450v 4400uF caps locally for cheap. Cap energy is .5CV^2. This theoretically gives just under 12.5kJ maxed out but accounting for a 10% lower actual capacitance and 420V working voltage so nothing blows up 10kJ is a more realistic working energy. I paid around 30-40 joules / usd. If I can find large quantities of big caps at similar prices a 100kJ bank would be affordable. Even so, I think a large homemade inductor may become affordable somewhere before 1MJ. That's a $30K cap bank.

Pulse lengthening inductor:

Depending on the actual resistance of wiring, rails, armature and switching if installed the 28p cap bank can discharge itself anywhere on up from a few tens of microseconds. This is way too short for a reasonable rail length so I will try a relatively small inductor in series. Reasonable-size conductors may have too much resistance but the inductor can be bathed in an insulated box full of liquid nitrogen which is cheap and readily available. Staging is also a solution.

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.
 

Offline I wanted a rude username

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #2 on: December 27, 2019, 05:16:03 am »


For maximum efficiency, I suspect you will need to build a pulse-forming network.

If you want to get really exotic, you could use a light-gas gun as your injector ...
 

Offline T3sl4co1l

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #3 on: December 27, 2019, 05:33:11 am »
If you want to get really exotic, you could use a light-gas gun as your injector ...

Better yet, instead of charging a capacitor, run a compressor to charge an air tank -- far greater efficiency, plus the lower peak power requirement means you can run it on a few LiPo packs instead of a fuckoff massive capacitor. :)

Absolutely serious -- this is all about transforming energy, right?  Why choose literally the absolute worst method when you could make a practical device instead? :P

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #4 on: December 27, 2019, 05:51:08 am »
Or one of those convenient, primer-actuated, deflagration-powered, remote lead delivery systems with a bore diameter not exceeding 12.7 mm. But the thing to remember is ... practical is inversely proportional to cool.  ;D
 

Offline i_am_fubar

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #5 on: December 27, 2019, 06:46:32 am »
In theory, EM launching can hit higher velocities than any gas powered system. Be it rail or coil guns. But that's assuming a light weight projectile. Added advantage that the projectiles are now much smaller and don't require 30-90% extra mass of explosive to provide the propulsion.

However, amature launchers will always be limited in power compared to military R&D units. Due to, well million and billion dollar budgets.

There are some notabley not awful ones out there. Sam Burrows power labs has a pretty good stab.

Also, stuff doesn't need to be practice to be fun :)
 

Offline T3sl4co1l

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #6 on: December 27, 2019, 06:54:06 am »
Regarding the railgun, the fundamental problems are:
1. You don't get much pressure.  A flux density of 1 tesla is equivalent to a mere 4 atm (~60 PSI) tops (pressure goes as B^2).
2. With core materials, you can't go much higher in flux density (and that's if you have space to put in a core at all, which you really don't).
3. Copper or silver isn't nearly conductive enough relative to do the job.  This is harder to express, but there is effectively a bulk time constant for a given material, which also depends on the size of that material.  Hence why transformer cores are made of laminated iron (small dimension) and why shields are made of thick material (a cm-thick bar of copper will effectively shield into the low Hz, as an NdFeB magnet will viscerally attest to!).
4. Secondarily, the inductance isn't high enough to give a "slow burn" (100s us to ms), even with rather large capacitors.  (Note it doesn't matter very much whether the waveform reverses or not, as long as most of the ringdown/decay is going into mechanical work.)

The power density is also high enough that you can't avoid electrical arcing, or indeed, much of the armature and rails turning into plasma, which drops a modest voltage in the process (say 20V+) and which ends up receiving much of the thrust.  Also, depending on how the barrel is constructed and fed, vaporized metal may well be a sizable fraction of the motive force, i.e. just by sheer gas pressure alone -- essentially making it an arc-flash gun.

Which, on that note, again -- you're transforming electrical energy into, in this case heat, and maybe some chemical energy comes along too (say if you use an aluminum armature so that it combusts in the air fed to the barrel).  That can generate quite sizable forces by itself, without much electromagnetic push.

#2 is practical with a longer track, in which case you can use more conventional means -- dragging eddy currents along, rather than inducing repulsion or attraction.  In other words, a linear motor, and these indeed are quite practical with the newest (US?) aircraft carriers sporting EM launch tracks.  Kicking an entire fighter jet up to ~stall speed is no small piece of kinetic energy!

#3 basically means that it can work at scale, assuming the other problems are solved (erosion, capacitor volume, electrical power requirements).  That's the biggest reason why amateurs can't do it -- they literally can't think big enough (or, not so much think as purchase, or have a firing range to test at, or...).

#4 is helped with scale, but a longer barrel also means a longer discharge time is desirable.  I'm not sure offhand how well that works out, say with the Navy's model.  They have a lot of uF's, though (think I estimated something like 10kV and 1F?).

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #7 on: December 27, 2019, 07:16:37 am »
There's been a few rail gun threads here over the years and you can find them just searching for 'rail gun'.
One that springs to mind:
https://www.eevblog.com/forum/projects/my-electromagnetic-bb-gun-project-130-ms-velocity-20-rpm-rate-of-fire/
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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 #8 on: December 27, 2019, 07:37:11 am »
Thanks for all the replies. The guys who sold me my cap bank provided some more information on where to get giant trashcan sized caps from utilities. I'm looking into that to assemble a really BIG bank with decent energy.

For maximum efficiency, I suspect you will need to build a pulse-forming network.

If you want to get really exotic, you could use a light-gas gun as your injector ...

Unless I misunderstand the pulse forming network is basically what I'm trying to do by putting an inductor (or inductors) in series with the cap bank. Right?

A light gas gun is on my radar, but is a different kind of project. I would like to make one later.

Better yet, instead of charging a capacitor, run a compressor to charge an air tank -- far greater efficiency, plus the lower peak power requirement means you can run it on a few LiPo packs instead of a fuckoff massive capacitor. :)

Absolutely serious -- this is all about transforming energy, right?  Why choose literally the absolute worst method when you could make a practical device instead? :P

Tim
Pneumatic cannons are cool but I want to get a round into kilometers / second territory. I'll need more stored energy than 10kJ but that will come.

Or one of those convenient, primer-actuated, deflagration-powered, remote lead delivery systems with a bore diameter not exceeding 12.7 mm. But the thing to remember is ... practical is inversely proportional to cool.  ;D
Yeah this is AMERICA we have no shortage of guns. Long live FREEDOM! This is a totally different type of project though and eventually I hope to accelerate a projectile to much higher velocities than a conventional firearm.

There are some notabley not awful ones out there. Sam Burrows power labs has a pretty good stab.
Yeah, the Powerlabs railgun was an early inspiration. It's not clear how much speed he actually achieved though and while he has an unusually high voltage his total bank energy is only 16kJ. He does have what appears to be a lot of good information particularly on injection.

Regarding the railgun, the fundamental problems are:
1. You don't get much pressure.  A flux density of 1 tesla is equivalent to a mere 4 atm (~60 PSI) tops (pressure goes as B^2).
2. With core materials, you can't go much higher in flux density (and that's if you have space to put in a core at all, which you really don't).
3. Copper or silver isn't nearly conductive enough relative to do the job.  This is harder to express, but there is effectively a bulk time constant for a given material, which also depends on the size of that material.  Hence why transformer cores are made of laminated iron (small dimension) and why shields are made of thick material (a cm-thick bar of copper will effectively shield into the low Hz, as an NdFeB magnet will viscerally attest to!).
4. Secondarily, the inductance isn't high enough to give a "slow burn" (100s us to ms), even with rather large capacitors.  (Note it doesn't matter very much whether the waveform reverses or not, as long as most of the ringdown/decay is going into mechanical work.)

The power density is also high enough that you can't avoid electrical arcing, or indeed, much of the armature and rails turning into plasma, which drops a modest voltage in the process (say 20V+) and which ends up receiving much of the thrust.  Also, depending on how the barrel is constructed and fed, vaporized metal may well be a sizable fraction of the motive force, i.e. just by sheer gas pressure alone -- essentially making it an arc-flash gun.

Which, on that note, again -- you're transforming electrical energy into, in this case heat, and maybe some chemical energy comes along too (say if you use an aluminum armature so that it combusts in the air fed to the barrel).  That can generate quite sizable forces by itself, without much electromagnetic push.

#2 is practical with a longer track, in which case you can use more conventional means -- dragging eddy currents along, rather than inducing repulsion or attraction.  In other words, a linear motor, and these indeed are quite practical with the newest (US?) aircraft carriers sporting EM launch tracks.  Kicking an entire fighter jet up to ~stall speed is no small piece of kinetic energy!

#3 basically means that it can work at scale, assuming the other problems are solved (erosion, capacitor volume, electrical power requirements).  That's the biggest reason why amateurs can't do it -- they literally can't think big enough (or, not so much think as purchase, or have a firing range to test at, or...).

#4 is helped with scale, but a longer barrel also means a longer discharge time is desirable.  I'm not sure offhand how well that works out, say with the Navy's model.  They have a lot of uF's, though (think I estimated something like 10kV and 1F?).

Tim

So to understand you, railguns work better (more efficiently) if they're bigger and more powerful. Eventually I want to get into "serious power" territory but I will need more caps or that giant inductor.

For my 10kJ "learning experience" version it seems almost a foregone conclusion that I will have to rely on a plasma armature. Maybe that will work better with the bank reconfigured to provide higher voltage than ~400v peak?
If I were to try this kind of project I would use a mixed approach. The projectile would be launched by traditional smokeless powder for 1/2 of the barrel, then railgun for the next 1/2. Something tells me this would lessen the size of the capacitor bank for a given output projectile speed

It also simplifies the triggering since the high voltage can be applied 100% of the time to the half end of the barrel

Yes, a fast injection is supposed to solve a lot of issues. I won't discount a chemical injection but will try pneumatic first.

There's been a few rail gun threads here over the years and you can find them just searching for 'rail gun'.
One that springs to mind:
https://www.eevblog.com/forum/projects/my-electromagnetic-bb-gun-project-130-ms-velocity-20-rpm-rate-of-fire/
Don't get me wrong I may not even reach 130M/S with a 10kJ bank and hack engineering but eventually my goal is to get kilometers / second. Thanks for the heads up, I will check out the threads and see what I can learn.

 

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #9 on: December 27, 2019, 07:58:41 am »
There's been a few rail gun threads here over the years and you can find them just searching for 'rail gun'.
One that springs to mind:
https://www.eevblog.com/forum/projects/my-electromagnetic-bb-gun-project-130-ms-velocity-20-rpm-rate-of-fire/
Don't get me wrong I may not even reach 130M/S with a 10kJ bank and hack engineering but eventually my goal is to get kilometers / second. Thanks for the heads up, I will check out the threads and see what I can learn.
Really ?  :o  :-//
So you have this sort of budget ?

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Offline i_am_fubar

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #10 on: December 27, 2019, 08:25:06 am »
I wonder how much that cable bundle twitches when It fires. Serious current there. How much you figure? 10s of kA?
 

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #11 on: December 27, 2019, 08:38:27 am »
Quote
The United States Naval Surface Warfare Center Dahlgren Division demonstrated an 8 MJ railgun firing 3.2 kg (7.1 lb) projectiles in October 2006 as a prototype of a 64 MJ weapon to be deployed aboard Navy warships. The main problem the U.S. Navy has had with implementing a railgun cannon system is that the guns wear out due to the immense pressures, stresses and heat that are generated by the millions of amperes of current necessary to fire projectiles with megajoules of energy.

(Emphasis added)
 

Offline Psi

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #12 on: December 27, 2019, 08:41:48 am »
I wonder how much that cable bundle twitches when It fires. Serious current there. How much you figure? 10s of kA?

See for yourself
https://youtu.be/eiUDdAGCht0?t=14
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Re: Everything wrong with amateur railguns and how to make them better
« Reply #13 on: December 27, 2019, 08:49:33 am »
I wonder how much that cable bundle twitches when It fires. Serious current there. How much you figure? 10s of kA?

See for yourself
https://youtu.be/eiUDdAGCht0?t=14
Following on from that a news item on the commissioned rail gun:
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Offline mzzj

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #14 on: December 27, 2019, 08:54:18 am »

1) Amateur projectiles (the armature and anything it drives) are too massive for the small electrical energy levels involved. Kinetic energy is .5MV^2 so other things equal reducing the projectile mass by a factor of 10 should increase kinetic energy by a factor of 10 even after accounting for new 90% smaller mass. Miniaturizing the projectile to a reasonable mass (<10g) might include conductivity and or thermal problems reducing the theoretical energy gain. The small scale high velocity laboratory railguns I've found in academic research all shoot extremely low mass projectiles.
Where the f** did you get this idea from?



 

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #15 on: December 27, 2019, 08:55:53 am »
Further interesting video:
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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 #16 on: December 27, 2019, 08:58:58 am »
The Naval railgun is completely different scale of project with repeat fire requirement...by the way it unloads a projectile weighing SEVERAL THOUSAND TIMES more than I'll be working with. Mine will be at most a few grams. Budget is a few thousand usd. At this scale 1km/s should be doable with under 100kJ. Many small railguns firing a few grams to kilometers a second have been built in university labs.

Where the f** did you get this idea from?

Is this a nice way of correcting whatever I got wrong but you forgot to include that part? Please be helpful. I am trying to learn.

edit: Powerlabs had a more powerful version of the gun with 100kJ cap bank on their youtube. No idea how fast it went. Looks like it was too powerful for the design:

« Last Edit: December 27, 2019, 09:37:23 am by Honda Rider 271 »
 

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #17 on: December 27, 2019, 09:48:26 am »
What experience do you have in ballistics ?
Are you a current reloader of traditional powder propelled firearms ?

Even this ^ is a big learning curve to get consistently accurate results despite the 100+ years of reloading knowledge available.
IMHO rail gun territory is beyond the capability of even a well equipped and knowledgeable mechanical and electrical engineer, however good luck and keep us informed of your progress.
There's a few keen shooters here that I'm sure would be interested in your developments.  :popcorn:
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Offline T3sl4co1l

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #18 on: December 27, 2019, 11:24:44 am »
I wonder how much that cable bundle twitches when It fires. Serious current there. How much you figure? 10s of kA?

They use "kickless cables", so called because they don't... you know. ;D

A rather obscure type, but it's just another style of transmission line.  With both poles brought along in the cable, there's no net external magnetic field, or force.  They're typically made of zillions of twisted pairs, inside a jacket.  Very low inductance (a bigger priority here), or more generally speaking, very low impedance because it's so many transmission lines in parallel.

I've used some not-quite-wrist-sized stuff before, measured less than 200nH for a 2 or 3m length.  Water cooled, good for one or two thousand amperes.  Understandably, they were prone to arcing breakdown, as the combination of close clearances (both terminals come out at each end, in this case separated by about 1/8" of G10) and immersion in water isn't exactly a recipe for success.
Example: https://www.flexcable.com/all-products/resistance-welding/kickless-cables

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Offline David Hess

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #19 on: December 27, 2019, 02:54:26 pm »
Unless I misunderstand the pulse forming network is basically what I'm trying to do by putting an inductor (or inductors) in series with the cap bank. Right?

In the extreme case, the capacitors and inductors form a distributed element transmission line so the discharge pulse has a lower peak current, limited by the impedance of the transmission line, and a discharge time controlled by the electrical length of the transmission line.

What experience do you have in ballistics ?
Are you a current reloader of traditional powder propelled firearms ?

Even this ^ is a big learning curve to get consistently accurate results despite the 100+ years of reloading knowledge available.

Most shooters and firearms are not accurate enough that reloading presents any limitation to accuracy.
« Last Edit: December 27, 2019, 02:56:00 pm by David Hess »
 

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #20 on: December 28, 2019, 05:50:41 am »

Where the f** did you get this idea from?

Is this a nice way of correcting whatever I got wrong but you forgot to include that part? Please be helpful. I am trying to learn.

I don’t know where did you get the idea that 10 times lighter projectile would accelerate to 10 times higher speed with same input energy.

Afaik ETG or electrothermal gun has shown most promise of any hobbyist projects:

Numbers are there for someone to calculate muzzle velocity and efficiency..
 
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Offline LukeW

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #21 on: December 28, 2019, 10:22:18 am »
The railgun is an Australian invention, thanks to J.P. Barber and Richard Marshall at ANU in the early 1970s, where velocities of 5.9 km/s were achieved.

The trick was to use not a capacitor bank, but a large homopolar generator as the pulsed-power source - it used to be the largest homopolar generator in the world.

Interesting reading:

https://repositories.lib.utexas.edu/bitstream/handle/2152/33197/PN_057_Marshall.pdf

https://link.springer.com/chapter/10.1007%2F978-1-4684-1048-8_25

https://sci-hub.tw/10.1109/20.911872
 

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #22 on: December 28, 2019, 05:30:28 pm »
Nobody is born an expert. You trying to discourage him now might well rob us of our next expert.

Obviously what has been tried before isn´t working. He want´s to try a different approach and I think that is much better than going with the same old stuff that hasn´t worked.

I like the inductor idea but there have also been huge advances in capacitor technology. You can build your own super caps out of activated carbon, foil sheets and an electrolyte.

I would suggest building a device to cover foil strips in a layer of paint made from activated carbon, graphene and glue. Wrap into jellyrolls with tabs for contacts and shove into PVC pipe and top off with some electrolyte solution. The materials are all extremely cheap and you can make them pretty quickly if you build a few tools.
 

Offline ogden

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #23 on: December 28, 2019, 05:47:31 pm »
Many small railguns firing a few grams to kilometers a second have been built in university labs.
You mention many railguns built in university labs, yet provide zero pointers. Give at least few.
« Last Edit: December 28, 2019, 08:17:23 pm by ogden »
 

Online Marco

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Re: Everything wrong with amateur railguns and how to make them better
« Reply #24 on: December 28, 2019, 06:51:18 pm »
Why are the armatures on the sabots generally at the back? Wouldn't it be much more stable to have them at the front? Pulling the weight instead of pushing it.

I wonder if it would make sense to have the electrode rails in pairs and precharge the air between a pair of rails with plasma (a given pair would be at the same potential during launch, the armature would be between one pair on one side and another pair on the other to make the conductive path). Give the arc which needs to form to the armature a bit of a head start.
« Last Edit: December 28, 2019, 06:58:22 pm by Marco »
 


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