Everything wrong with amateur railguns and how to make them better - Page 1
Forum section: electronics / projects

author: Honda Rider 271
author_rank: Contributor
timestamp: December 23, 2019, 11:13:44 pm
content: 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.pdfOK 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 supply2) Terrible efficiency below 1%3) Because of 1 and 2, really weak projectilesThe 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 systemAnyway 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.

author: Honda Rider 271
author_rank: Contributor
timestamp: December 27, 2019, 01:02:34 am
content: 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.

author: I wanted a rude username
author_rank: Frequent Contributor
timestamp: December 27, 2019, 05:16:03 am
content: For maximum efficiency, I suspect you will need to build apulse-forming network.If you want to get really exotic, you could use alight-gas gunas your injector ...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2019, 05:33:11 am
content: Quote from: I wanted a rude username on December 27, 2019, 05:16:03 amBetter 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?Tim

author: I wanted a rude username
author_rank: Frequent Contributor
timestamp: December 27, 2019, 05:51:08 am
content: 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.

author: i_am_fubar
author_rank: Regular Contributor
timestamp: December 27, 2019, 06:46:32 am
content: 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

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2019, 06:54:06 am
content: 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

author: tautech
author_rank: Super Contributor
timestamp: December 27, 2019, 07:16:37 am
content: 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/

author: Honda Rider 271
author_rank: Contributor
timestamp: December 27, 2019, 07:37:11 am
content: 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.Quote from: I wanted a rude username on December 27, 2019, 05:16:03 amUnless 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.Quote from: T3sl4co1l on December 27, 2019, 05:33:11 amPneumatic 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.Quote from: I wanted a rude username on December 27, 2019, 05:51:08 amYeah 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.Quote from: i_am_fubar on December 27, 2019, 06:46:32 amYeah, 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.Quote from: T3sl4co1l on December 27, 2019, 06:54:06 amSo 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?Quote from: MasterTech on December 27, 2019, 07:11:27 amYes, a fast injection is supposed to solve a lot of issues. I won't discount a chemical injection but will try pneumatic first.Quote from: tautech on December 27, 2019, 07:16:37 amDon'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.

author: tautech
author_rank: Super Contributor
timestamp: December 27, 2019, 07:58:41 am
content: Quote from: Honda Rider 271 on December 27, 2019, 07:37:11 amReally ?So you have this sort of budget ?

author: i_am_fubar
author_rank: Regular Contributor
timestamp: December 27, 2019, 08:25:06 am
content: I wonder how much that cable bundle twitches when It fires. Serious current there. How much you figure? 10s of kA?

author: I wanted a rude username
author_rank: Frequent Contributor
timestamp: December 27, 2019, 08:38:27 am
content: Quote(Emphasis added)

author: Psi
author_rank: Super Contributor
timestamp: December 27, 2019, 08:41:48 am
content: Quote from: i_am_fubar on December 27, 2019, 08:25:06 amSee for yourselfhttps://youtu.be/eiUDdAGCht0?t=14

author: tautech
author_rank: Super Contributor
timestamp: December 27, 2019, 08:49:33 am
content: Quote from: Psi on December 27, 2019, 08:41:48 amFollowing on from that a news item on the commissioned rail gun:

author: mzzj
author_rank: Super Contributor
timestamp: December 27, 2019, 08:54:18 am
content: Quote from: Honda Rider 271 on December 27, 2019, 01:02:34 amWhere the f** did you get this idea from?

author: tautech
author_rank: Super Contributor
timestamp: December 27, 2019, 08:55:53 am
content: Further interesting video:

author: Honda Rider 271
author_rank: Contributor
timestamp: December 27, 2019, 08:58:58 am
content: 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.Quote from: mzzj on December 27, 2019, 08:54:18 amIs 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:

author: tautech
author_rank: Super Contributor
timestamp: December 27, 2019, 09:48:26 am
content: 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 mechanicalandelectrical 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2019, 11:24:44 am
content: Quote from: i_am_fubar on December 27, 2019, 08:25:06 amThey use "kickless cables", so called because they don't... you know.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-cablesTim

author: David Hess
author_rank: Super Contributor
timestamp: December 27, 2019, 02:54:26 pm
content: Quote from: Honda Rider 271 on December 27, 2019, 07:37:11 amIn 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.Quote from: tautech on December 27, 2019, 09:48:26 amMost shooters and firearms are not accurate enough that reloading presents any limitation to accuracy.

author: mzzj
author_rank: Super Contributor
timestamp: December 28, 2019, 05:50:41 am
content: Quote from: Honda Rider 271 on December 27, 2019, 08:58:58 amI 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..

author: LukeW
author_rank: Frequent Contributor
timestamp: December 28, 2019, 10:22:18 am
content: 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.pdfhttps://link.springer.com/chapter/10.1007%2F978-1-4684-1048-8_25https://sci-hub.tw/10.1109/20.911872

author: excitedbox
author_rank: Regular Contributor
timestamp: December 28, 2019, 05:30:28 pm
content: 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.

author: ogden
author_rank: Super Contributor
timestamp: December 28, 2019, 05:47:31 pm
content: Quote from: Honda Rider 271 on December 27, 2019, 08:58:58 amYou mention many railguns built in university labs, yet provide zero pointers. Give at least few.

author: Marco
author_rank: Super Contributor
timestamp: December 28, 2019, 06:51:18 pm
content: 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.

author: German_EE
author_rank: Super Contributor
timestamp: December 28, 2019, 08:10:17 pm
content: 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.

author: Honda Rider 271
author_rank: Contributor
timestamp: December 28, 2019, 08:54:20 pm
content: 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_gradetdsalso 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.

author: Marco
author_rank: Super Contributor
timestamp: December 28, 2019, 09:24:22 pm
content: 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.

author: Honda Rider 271
author_rank: Contributor
timestamp: December 28, 2019, 11:07:35 pm
content: 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.

author: 13
author_rank: Contributor
timestamp: December 29, 2019, 02:40:08 am
content: Quote from: Honda Rider 271 on December 27, 2019, 01:02:34 amI 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.

author: Honda Rider 271
author_rank: Contributor
timestamp: December 29, 2019, 05:13:50 am
content: 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.Quote from: 13 on December 29, 2019, 02:40:08 amThe 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2019, 06:00:04 am
content: 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).Tim

author: Marco
author_rank: Super Contributor
timestamp: December 29, 2019, 06:39:33 am
content: 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.

author: Marco
author_rank: Super Contributor
timestamp: December 29, 2019, 09:26:40 pm
content: Lets do a bit of an order of magnitude calculations here using the formulas fromhere.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.

author: mzzj
author_rank: Super Contributor
timestamp: December 30, 2019, 12:30:17 am
content: Quote from: Marco on December 29, 2019, 06:39:33 am1MJ flyback transformer

author: Psi
author_rank: Super Contributor
timestamp: January 02, 2020, 01:34:02 am
content: 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?

author: excitedbox
author_rank: Regular Contributor
timestamp: January 05, 2020, 09:27:30 pm
content: 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 themSo 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.