railguns n stuff

[willbanks]

I want to build a railgun, and I have a question about cap choice. I noticed a lot of people use multiple caps for the electrical discharge. Why would they use all those caps when they could just buy a single ultra capacitor? it'd take up less space, and seems cheaper, and can deliver  the same if not more joules of energy than multiple caps.
plus, it'd be easier to charge, right? a 3000F 2.7V cap only needs to be charged to 2V to deliver lots of energy, which seems a lot easier than having a bunch of circuitry to step up voltage to charge multiple caps.
I feel like I'm thinking wrong here though, so I wanted to ask here.

[jh15]

aren't they all discharged in sequence?

Guess we should buy up all the remaining disposable cameras from the photo
 shops before they go away.

[willbanks]

aren't they all discharged in sequence?

Guess we should buy up all the remaining disposable cameras from the photo
 shops before they go away.

i assumed they all discharged at once

[rs20]

I don't know anything much about rail guns, but transferring a lot of energy at a low voltage over a short period of time requires a huge huge amount of current, since (making all sorts of simplifying assumptions) I = E/(T*V). You're absolutely stacking every variable in favour of having to deal with enormous currents, way higher than a conventionally designed railgun, and those require huge thick cables because they're already dealing with such huge currents:



As you increase current (I), power (I²R) losses rise quadratically.

[willbanks]

ohhh ok that makes sense. thank you!

[T3sl4co1l]

They also use film caps, because of two reasons:
1. Speed of discharge.  They have low enough ESR to be useful.
2. Reversal.  The closed circuit has resistance and inductance, and it completes one or two cycles before the energy is used up.  Electrolytics can't tolerate this (it's destructive).

If enough ESR is added, the inductance can be dampened so that no reversal occurs.  This is fairly typical of electrolytics anyway, but that still doesn't mean it's safe to use them: they aren't usually made to withstand the intense forces (both the mechanical force of surge current, and the stress of rapid thermal heating) of short circuit discharge.  Not to say they can't be, but that's added cost to make custom ones that can; and you might be better off using commercial film capacitors by then.

The minimum energy required to fire a railgun is apparently some kJ.  It's not a problem that scales down well.  You must have enough current flow (I >> 10kA) to create a strong enough magnetic field (H >> 10^5 A/m) to have enough pressure (p = mu_0 * H^2 / 2) to overcome friction, let alone to be anywhere near worthwhile.

The main thing you save is voltage (EMF, ultimately dH/dt), because the rail is shorter.  But that's its own difficulty, because the impedance is that much lower.

You can see already that the Navy's railgun impedance is very low indeed.  Those cables: they aren't simply solid copper.  They are kickless cables, where the positive and negative are braided together -- like Litz cable, but both polarities intermingled (still fully insulated), for the least possible stray inductance.  And they have a lot of inductance to deal with, since that barrel will only be a few uH at full length, and they've got maybe a hundred feet between the gun and the furthest shipping container-load of capacitors.  That would be a lot of stray inductance otherwise.  (Plus, as the name suggests, the "kick" is confined inside the cable -- all that force is distributed over many strands, hence why the sets of cables don't explode away from each other like the shot itself does.  Yeah, they flop around still, but that's just the rail's recoil.)

And, on top of that, they've got tons of cables wired in parallel.  The total transmission line impedance will be fractional ohms, maybe 10s of milliohms even.  If that thing's charged to a modest, say, 1000V, the surge current can be hundreds of kiloamperes, maybe a megampere or more.

Which, if the inductance is on the order of single digit uH, the capacitance would be on the order of whole farads.  (This comes from the relation: Vpk / Ipk = sqrt(L / C).)  Hmm, hard to say: that's a truly preposterous amount of film capacitors, if so.  But if it's 1000V, you'd need 120F to store the claimed 60MJ, so it's probably that the voltage is higher, and the amount, well, it's still preposterous...

Tim

[rstofer]

There was a toy railgun project in Nuts and Volts Magazine some time back

http://www.nutsvolts.com/magazine/article/electromagnetic_coil_launcher_project

Not only can't all coils fire at the same time (that would lead to net zero force given an object somewhere near the middle of the rail), they are probably timed to give a particular acceleration curve.

[stj]

http://www.powerlabs.org/
 

[woodchips]

Eric Laithwaite (gyros!, for those that remember) published a book on linear electric motors, and gave examples for launching aircraft.

Didn't seem at all easy, the magnetic forces strip the electrons from the air molecules and make an impressive fire storm.

[T3sl4co1l]

There was a toy railgun project in Nuts and Volts Magazine some time back

http://www.nutsvolts.com/magazine/article/electromagnetic_coil_launcher_project

Not only can't all coils fire at the same time (that would lead to net zero force given an object somewhere near the middle of the rail), they are probably timed to give a particular acceleration curve.

Coilgun != railgun

Tim

[ConKbot]

Sure, get a pulse rated super cap, and keep your ESR, wire, rail, switch, and sliding contact to the projectile under 100-200 microohms, and you'll be in the 10s of KA range that you need.  4/0 (107mm^2) is 160 uohm/meter, so you're going to need very, very large bus bars.

[tecman]

I worked on a rail gun project (research project-gov funded) years ago.  It used large "homopolar" generators that stored energy in their inertia, https://en.wikipedia.org/wiki/Homopolar_generator.  The firing creates a plasma that is the basis for the gun.  The "electrodes" ran the length of the barrel.  The unit would shoot a 1" x 1" x 4" piece of lexan through 2" of steel plate.  In this project high voltage was not used.  the generator open circuit voltage was under 100 volts, but current was huge.  In other designs high voltage caps are used to store the energy (1/2 VC^2).  When discharged, the plasma has nearly 0 ohms so high current is limited by the resistance of the wiring, caps, etc.

paul