Anyone here built a railgun? Tips and stories appreciated, it

>>1168807
https://youtu.be/NJRDclzi5Vg

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what the fuck

>>1168895
Electric fields and magnetic fields are inseparable.
The current flowing through the projectile creates a magnetic field.
This is electromagnetics 101.
Ever made an electromagnet? Disturbed a compass with a winding? Used an electric motor?
All exploit the exact same principle; electric current causes magnetic fields which attract or repel other magnetic (or electromagnetic) fields.

>>1168916
that's not the problem here
protip: check the area outlined in red

>>1168895
They just drew the one arrow the wrong way fag

>>1168895
somebody wanks left-handed

>>1168807
Tips...watch all the work that goes into generating a deadly current stored in capacitors positioned near your head and only capable of being able to pop a balloon on you tube before deciding to waste your time and risk your life to create something less powerful than a blowpipe.

Then make a blowpipe and darts.

>>1169168
This pretty much
If you want a 'cool' railgun that isn't a novelty, you basically have to spend a one or two digit thousands (depending on if you're going for used old gear or new gear rarity etc) to make a rail gun. Dem high voltage capacitor banks, dem high current and voltage SCRs. Control circuitry
And yeah it has to be high voltage because the energy of a capacitor goes up proprotional to V squared, whereas it only goes up proportional to C.
machining the rails and bolting them together strong enough as the rails will try to fly apart when accelerating the projectile

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>>1169175
>it has to be high voltage
Actually, after reviewing the prices of my local Jaycar as an example, I have come to the conclusion that since the capacitance of supercaps goes up far higher than that of HV caps, you can get more energy per dollar spent by buying LV supercaps.

>>1169200
>LV supercaps.

There are limitations with current-gen supercapacitors that make them unsuitable for any type of railgun.

It boils them down to them being poorly-suited for pulsed power delivery. They have high ESR, which greatly limits their current-delivery capabiility. Even if they're much cheaper in terms of dollar/joule, you need a substantial excess of capacity simply to get the necessary power out of them.

Look up the datasheets for a pulse-rated capacitor and a supercap, taking note of the ESR. The pulse-rated one will likely be in the low mΩ or even µΩ range, depending on how big it is. It's not uncommon to find smaller supercaps with resistances rated in _multiple_ Ohms.

>>1169229
Thanks for the heads up, didn't even know this was a thing. Looks like the newer ones are packing around 20mΩ for a standard low ESR supercap, and can go into the 100s of µΩ for a high price, but you're definitely right that this is the limiting factor.

I would build a chart comparing the watts/NZD if Jaycar had datasheets for all their caps, but they don't. I downloaded the entire Mouser passives catalogue for the purpose of having a larger dataset, but a 55MB PDF is nothing I can just throw into a spreadsheet and whip up the numbers on. It also doesn't list the price, so someone would need to write some software to browse the website itself to get that.

>>1169200
If you put your caps in series, they will provide more energy simply because you can put more voltage across them
the SQUARE of voltage my friend

>>1169745
If you treat two caps in series as one capacitor the total energy storage at their maximum voltage is the same as when they are seperate because capacitance in series adds like resistance in parallel (∑C = 1/(1/C1 + 1/C2 + 1/C3 + ...)). Each cap still stores the same amount of energy, so whether you put them in series or parallel is entirely up to whether you want more current or more voltage output. I hope this is intuitive.

>>1169748
Incorrect. The total capacitance is halved, but the voltage is doubled. which means the total energy is multiplied by four. Which means it offsets the fact that the total capacitance is halved.

>>1169750
The laws of thermodynamics called, they want their money back. Your comparison compares a single (say 1F, 1Vmax) capacitor to two of these capacitors in series, when if you compare the two in series to the two in parallel (effective capacitance of both is the sum of the individuals) the energy stored is the same. 1/2*0.5*2^2 == 1/2*2*1^2.

>>1169776
I don't know what i was thinking, I suck cocks.
(probably was thinking of the case where the capacitance is the same or only changing one of the parameters)

A compulsator might be a better option for small railguns. Have an electric hobby motor spin up a 10kg disc to 100krpm then have a motor you rewound for extreme currents connected to the other side with rectification diodes. When the projectile short the rails the second motor is shorted and sends the flywheel energy into it.

>>1170237
Never heard of doing that, sounds cool. Would it be better to put the permanent magnet(s) in the rotor and have the windings in the stator so you don't get commutative losses (and extreme inductive spikes between commutation)? Or is this what the diodes are for in the first place? Be a pain to make an entirely new motor, but possibly worth doing. Imagine the size flywheel you could wind up just with a road bike and its high gearing.

>>1170264
No you use brushless motors. The one spinning up the flywheel would be an inrunner with a very high RPM, usually remote control car or boat motors are suitable offering 50,000 to 100,000 RPM. On the other side of the fly wheel you have a large outrunner usually found in model helicopters or paragliders. All you have to do is rewire it with very thick wire, you may need to wrap the rotor in carbon fiber because you will be spinning it at 5x it's usual RPM, outrunner is used as gravity pushes the magnet into the rotor and the high pole count will deliver huge voltage even using thick copper with few turns.

You will have three wires from the compulsator they go through diodes for rectification to DC current. In theory synchronous rectification with transistors would be more efficient but it's too much work trying to deal with thousands of amps. Many diodes in parallel is just easier. The most difficult part will be connecting it, the stator will absorb heat from the windings but it's going to need to be either solid copper or silver for the circuit. Every milliohm will count.

Actually looking at how many joules flywheels hold the setup may not need to be so extreme. A 5kg 10cm disk at 40,000 rpm is 55000 joules.

>>1170304
I'm not up to speed (heh) on motor tubes aside from brushed DC and universal motors, so excuse any misunderstanding. By brushless do you mean a motor that uses electrical commutation via (hall effect) sensors and FETs? That sounds like it makes sense for the motor being used as a motor, but I don't know how the electrical commutation would work if it was being driven as a generator. Are you referring instead to brushless AC motors whose angular frequency is dependant on that of the frequency of the current? This makes more sense, do the RC devices that use these change the frequency of the voltage they send to the motor (probably a filthy square wave) to change the motor speed? Pretty clever, I guess these work in reverse, but doing the calculations for smoothing capacitors would be weird since the frequency changes, but you probably can't use caps anyway because of the very high voltage.

I was just thinking that while the fluid version of an inductor is a wider pipe and the capacitor is a rubber membrane, the mechanical versions are flywheels and (torsional)springs respectively. All junctions between drive shafts would have to be differentials to simulate Kirchhoff's current law. I guess you could make an "LC circuit" by putting a torsional spring (or two) on a flywheel and having it oscillate back and forth, but I'm pretty sure having a mass of water bounce to and fro around a circuit with a rubber membrane in it wouldn't last very long.

>>1168807
I was building a small one with disposable camera flash capacitors and was shocked badly. Never finished.

>>1168895
Nice catch

>>1170237
I always thought compulsators made more sense for large railguns, not small ones.