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Railgun injector

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Railgun injector

Unread postAuthor: rp181 » Sun Feb 08, 2009 9:57 pm

Finished the injector. Works well (enough). The valve is a triggered burst disc in a 3/4" union. Chamber is dual 2.3ft .5" PVC. The injection part of the railgun will only be 1 foot long.
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The aluminum attaches to the railgun. Question's and request's welcome.
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Unread postAuthor: Necrosis » Mon Feb 09, 2009 4:55 am

how will you stop the current from flowing throufg the aluminum attachment?

And for what it's worth. it looks kinda cool. but with some nice flat black paint it'd look way better. : D

I'm building a HPA injector for my own railgun so i'd like to know a few things.

I will attempt to add you to my contact list. Well anyhoos. cool injector!
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Unread postAuthor: Fnord » Mon Feb 09, 2009 9:56 am

How many people have railguns under construction here anyway?
I've been stuck with the rail housing partially done because I can't find any 1/4 polycarb around here.

I would have personally made an injector out of copper with a qev or something, as these really need to be used at ridiculous pressures. I made a 900 psi pump just for the injector.
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Unread postAuthor: rp181 » Mon Feb 09, 2009 6:17 pm

I wanted to do copper too, but it would have been too expensive. The aluminum wont contact power parts (it will be 1 foot away), and even if it did, it wouldn't matter.
Necreosis- Contacts on what? I have yahoo messenger =)
as someone said (i forgot who...), it needs to be a augmented railgun, not a augmented air cannon. The railgun should do most of the acceleration. I wanted to paint it flat black (my brother kept saying it would be stupid...), i might get some when i go get some bolts. HPA injector would be great, i would love to talk to you.
@_fnord
I bet alot of people wan't to make railgun's, but don't have the money /resources. I have been planning for a year, and stuff started pouring in just last month. I would much rather have the projectile start in the rails with no injection, but i have no money for a SCR that big or SCR's to use as sacrificial SCR's.

The donated plastic part's shipped out today. (G-11 parts machined to .0002" tolerance, 2 .25x 12x18" polycarbonate pieces for the capacitor bank, some teflon fake rails, and teflon to use in a plasma armature).

I have a couple of question's.
1) I have been trying to figure out a way to chronograph it ( i do not want to use sound). Any suggestion's?
2) I just came from school machining some aluminum. And it sucks. The aluminum i was going to use to attach to the injector is slanted. I can always drill the holes on the other side slanted, but i don't want to even try. Any one have any ideas?
The injector part (in above pic) will be flush with the back of the railgun. There will be holes 1 inch away from the edge of the garolite.

I finished the capacitor bank. If you want to see pics, go to my website (in sig) railgun, build log.
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Unread postAuthor: Fnord » Mon Feb 09, 2009 7:25 pm

it needs to be a augmented railgun, not a augmented air cannon. The railgun should do most of the acceleration.


Unfortunately, railguns work better the faster the projectile is traveling when it enters the rails. So ideally you'd want a lot more than 900 psi, though that's more trouble than it's worth for most of us.

I have been trying to figure out a way to chronograph it ( i do not want to use sound).


Put some thin cardboard maybe 5 feet in front of the muzzle and shoot through it. This should keep sparks out of the chronys' light sensors.

2) I just came from school machining some aluminum. And it sucks. The aluminum i was going to use to attach to the injector is slanted. I can always drill the holes on the other side slanted, but i don't want to even try. Any one have any ideas?


Kind of hard to picture what you mean exactly. Will sanding it do anything?
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Unread postAuthor: Ragnarok » Mon Feb 09, 2009 7:43 pm

rp181 wrote:I have been trying to figure out a way to chronograph it ( i do not want to use sound). Any suggestions?

Ballistic pendulum? They're accurate and reliable, if not foot-per-second precise.

Before anyone tries to call me on it, yes, there is a difference between accuracy and precision. Accuracy is more important, because precision is useless without it.

Say a velocity is 824 fps. Method A gives a velocity of range of 815-830 fps, and method B gives a velocity of 683.18 fps
Method A is accurate, B is precise - more significant figures does not guarantee that the results are accurate.
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Unread postAuthor: rp181 » Mon Feb 09, 2009 8:47 pm

@fnord
Sanding doesn't make sense =) I am asking a way to make 2 flat surfaces stay together firmly. I was also contemplating having several thin boards, with a optical detector between each. That way, i have reference to see if it is accurate, and backup.
@rag
pendulum is more involved, and i want to be able to see what kind of damage it does. That is my last option.

I just had a idea. Any one know what those switches are called? They are the ones that have a metal thing, and that pushes the actual switch. The metal plate is usually bent, and i have found one in a heavy lead acid flash light. Here is what it looks like:
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Unread postAuthor: Ragnarok » Mon Feb 09, 2009 8:58 pm

rp181 wrote:pendulum is more involved, and i want to be able to see what kind of damage it does.

Nothing to stop the velocity and damage being recorded on separate shots.

What you're looking for is generally known as a microswitch.
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Unread postAuthor: TurboSuper » Mon Feb 09, 2009 9:01 pm

Uhh...how about the old pair of light sensors wired up to a PC's sound card? Or is it going so fast that the sampling rate won't catch it?

I know what kinda switch you're talking about...I think it is indeed a microswitch.
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Unread postAuthor: Ragnarok » Mon Feb 09, 2009 9:06 pm

TurboSuper wrote:Or is it going so fast that the sampling rate won't catch it?

The problem is that railguns create a very large muzzle blast of plasma, which means optical sensors can't detect the projectile amongst it.
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Unread postAuthor: daberno123 » Mon Feb 09, 2009 9:10 pm

Simplest solution would be to just move the chrony back 5-10 feet (depending on the size of the muzzle blast). With this though you run the risk of hitting your chrony, which is a sizable investment as they tend to run about $80 for the most basic model.
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Unread postAuthor: rp181 » Mon Feb 09, 2009 9:11 pm

Sound cards are more than fast enough, low end ones are 20khz (i think). The problem with the light sensors is with a railgun, you get a significant amount of smoke, sparks and other stuff flying out, in a big stream, blocking out the projectile for detection.

This is why the cardboard was mentioned, to block some of it, but some websites ive visited has been saying that it hast worked all that well.

I was considering haveing 4 Aluminum foil sheets, in pairs of 2, pretty close togehter. As the projectile passes through, it should rip the foil, and if it had some slack, would stay connected. I could hook up theese to the soundcard, each Al foil pair has some resistance. So you when 1 triggers, you get a small peak, and when the second triggeres, the resistance would lower (parrallel resistors) and the peak would jump.
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Unread postAuthor: MakerOfToys » Thu Feb 12, 2009 4:18 am

Sorry for the bump, but:

From someone who has actually worked on a railgun trying to record various parameters, some of which had to be picked off the rails themselves, consider the following:

Muzzle flash is significant. I made several video takes where the flash inside just the breech shows the internals of the breech against room light through multiple inches of G10 and FR4.

No, I can't post the video, so please don't ask.

Railguns create HUGE EMI/RFI problems for the data acquisition system. For velocity, break-beam optical with a tight optical bandpass filter would probably work, especially with a shade between the sensor and the muzzle. . . but given the relative lightness of the 'ideal' railgun slug, I'd make the shade substantial with a hole in it just larger than the projectile, rather than depend on the projectile making its own hole. Other than that, shielding the sensor-electronics connection and providing good, low impedance signal paths out to the recording device should help. A soundcard might have latch-up trouble with RFI, or might luck out and be too slow to catch the worst of it. . . just have to try it and see.

Relatively high currents in the foil-sheet apparatus RP181 mentioned above will help. (like multiple amps with low-value resistors. High DC currents tend to drown RFI transients.)

a break-wire or break-foil device would be my choice, with the wire carrying significant current; and you look for DOWN steps in the signal trace as the projectile snaps each wire/foil. (again, think AMPS)

An interrupting circuit topology has some reliability things going for it; to wit: you can non-destructively test its readiness, it has fewer parts, and fewer switch-bounce or fail-to-make problems. Also, all other things being equal, a falling transition will be faster and more distinct than a rising transition on the same circuit. (power supply stiffness doesn't factor into a falling edge, for instance.)

I have no feeling (not having tried it) whether a commercial rifle chronograph would function near the muzzle of a railgun.
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Unread postAuthor: rp181 » Thu Feb 12, 2009 8:09 am

I have had success with the electronics about 10 feet away, and the chrono right up to it. M first choice. The machined garolite pieces arrived yesterday, and im working on getting everything to fit. The garolite was machined to close tolerances, and the copper/teflon is too thick.
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Unread postAuthor: rp181 » Sun Feb 15, 2009 3:33 pm

http://www.rp181.110mb.com/index.php?p=1_30_Build-log
Heres a pic of the railgun so far, I still need 2.5" bolts to hold the injector on better.
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