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Railgun fired!

A place to ask general spud cannon related questions.
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Unread postAuthor: jackssmirkingrevenge » Sun Feb 22, 2009 12:25 am

rp181 wrote:Naa, I don't want a injector at all


Why not? I'm given to understand that a good injector makes a big difference when it comes to performance.
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Unread postAuthor: rp181 » Sun Feb 22, 2009 12:37 am

It actually does not. Yes, It does give more power because of the imparted energy, and increases efficiency a very small amount. A injector just adds a whole new system. It also makes good projectile contact hard, It has to be loose enough to slide. Thats why the NAVY uses a hydraulic press to push the projectile in.
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Unread postAuthor: jackssmirkingrevenge » Sun Feb 22, 2009 12:40 am

rp181 wrote:Thats why the NAVY uses a hydraulic press to push the projectile in.


The NAVY has a slightly larger capacitor bank ;)

Why not have some sort of flexible/spring loaded bush at the sides of the *cough* projectile as contacts?

edit: temporary blink in the brain-finger interface
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Last edited by jackssmirkingrevenge on Sun Feb 22, 2009 3:32 am, edited 1 time in total.
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Unread postAuthor: rp181 » Sun Feb 22, 2009 1:15 am

by sides of the capacitors, if you mean the projectile, then yes, i have thought about it. This is why i didnt:
- much harder to make, i had enough trouble getting a V projectile with a band saw =p
- lower contact area - this is going to make alot more plasma, and an ideal railgun should have no plasma
- higher resistance - the spring would be the main conductor through the projectile, and unless its very thick, it would probably just vaporize

It would actually probably be a good idea to make the actual rails on springs, and being pushed inwards, but the way the garolite was machined, it would be impossible to incorperate, and i cant machine it (school probably could, but no way im going to risk it).
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Unread postAuthor: MakerOfToys » Sun Feb 22, 2009 7:03 am

Don't spring the rails! Even if you can get accurate machining to do so.


Those cables that separated, even though they were zip-tied? Same thing will happen to your rails, making contact-pressure lower and rail/projectile erosion worse.

In fact, shorten those cables as much as you can, twist them together, and anchor them really well-- like with conduit clips screwed to a board every 3 inches or something-- to reduce inductance. . . you'll get a faster rise-time and therefore better energy transfer. Use the PVC version of conduit clips. . .

Also, might I suggest that instead of the 'bandsawn' projectile you mentioned above, a piece of appropriately sized aluminum U-channel with some form of insulating, non-compressible filler inside the U might be a better projectile. (Easy way: make tape dams on the open sides and fill with 5 minute epoxy. Or body filler, but body filler tends to be more compressible/flexible.) You'll need to remove the anodizing on the hardware-store variety of U-channel, of course. . .

Under NO circumstances use the same material for rail and projectile--galling and severe rail wear will result. Also, using a projectile with a lower melting point than the rails will tend to reduce rail erosion.

A thought just occurred: 'spring' the sides of the u-channel outward a bit, then fill with silicone RTV . . . there's your 'spring' for your projectile. Figuring the right amount of distortion will take some cut-and-try, but hey, you'd be looking for excuses to play with this thing anyway, right?

Rubbing down the rails with a thickish coat of graphite between shots might be worth experimenting with, too. . . The oil carrier on the most common forms of anti-seize tends to insulate, and might make erosion worse rather than better.


Edit: Stupid spelling mistakes, and an ommited comment on projectile material. No point in double posting. . .
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Last edited by MakerOfToys on Sun Feb 22, 2009 7:19 am, edited 2 times in total.

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Unread postAuthor: Ragnarok » Sun Feb 22, 2009 7:06 am

rp181 wrote:It would actually probably be a good idea to make the actual rails on springs, and being pushed inwards

I'm going to argue otherwise - given the rails are under considerable electromagnetic forces which push the rails apart during firing, I doubt spring-loading them is a good idea - they'll likely start resonating, and you'll almost certainly just ensure internal arcing and rail damage.
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Unread postAuthor: rp181 » Sun Feb 22, 2009 10:21 am

Same material for both the projectile and rails works very good. The erosion if abit more, but efficiency goes much higher. A person reached 12% with no injection (there was a sacrificial SCR), and a .25" copper projectile. Power was 20kJ.
For the projectile with springs, I thought jason rollete had a good idea. He does a standard U projectile, and then drills a hole in the tail. A close fitting carbon rod is put in, with a spring connecting to the carbon.
Im not really worried about the rails separating, I hammered the rails in, so there is no wiggle room. Also, the plastic is very stiff. I was tightening a bolt too much, and the bolt broke in half, the plastic had no damage what so ever.
The corrosion shield i put is made for conductive joints. It is a petroleum based product with copper in it. After the shot, It seems all the copper is gone, and the oil is still there.

@jack:
Its more your projectile's compatibility with the Injection and Rails that makes a difference. Since i can only go 50 PSI, I had to keep the projectile loose. In the end, I got it so the injection would push the aluminum right to the end of the rails, then stop.

Later i plan on trying augmented rails (ND magnets), series and parallel agumented, and injection with sulfur hexaflouride, or atleast helium.
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Unread postAuthor: psycix » Sun Feb 22, 2009 11:18 am

and injection with sulfur hexaflouride, or atleast helium.

sulferhexafluoride is a very heavy gas
helium is a very light one
Do you want light or heavy?!

I'd inject with 500 psi if I were you. MOAR power.
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Unread postAuthor: Fnord » Sun Feb 22, 2009 11:39 am

A person reached 12% with no injection (there was a sacrificial SCR), and a .25" copper projectile. Power was 20kJ.


Sounds interesting.
Got a link?


Edit @ Rag:
What does the scouter say about his spudbux level?


Ha. You've been saving that for months, haven't you? :)
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Unread postAuthor: maggotman » Sun Feb 22, 2009 12:01 pm

what if you forced it then their with a small pneumatic ram it could serve as a breech aswel.;
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Unread postAuthor: Ragnarok » Sun Feb 22, 2009 1:33 pm

_Fnord wrote:Ha. You've been saving that for months, haven't you?

Not really, I was more expecting to have to use it for JSR's post count - but never one to avoid a daft joke, I thought "Hell, why not?"
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Unread postAuthor: rp181 » Sun Feb 22, 2009 2:40 pm

Here:
http://4hv.org/e107_plugins/forum/forum ... 45561.post
It doesn't say 12%, but i was talking to him a little while back. Make sure you look at his high speed video.

I said sulfur hexafluoride because it is a very good arc suppressant. I plan to increase injection power as bank power goes up.
@maggot:
That would work, if the ram moved fast enough.
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Unread postAuthor: CpTn_lAw » Sun Feb 22, 2009 3:31 pm

that is in fact near the speed of the electron flux...not gonna happen ^^. Pneumatic injection is good because it pushes the aluminium (or whatever-metal-you-use) sparks out of the gun, enhancing its durability. (btw, any chance of a chronograph speed estimation?)
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Unread postAuthor: TurboSuper » Sun Feb 22, 2009 3:47 pm

That is totally awesome! Just one question, and I'm not sure if you've answered this already: Are the capacitors you're using specially designed for being rapidly discharged?
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Unread postAuthor: rp181 » Sun Feb 22, 2009 3:59 pm

@ CpTn
You wish =) How much pressure do you think just the sparks cause? I don't know, but alot more then 50 PSI =p Look for the cloud of smoke in the video.
@turbo
There as far from pulse rated as you can get. Pulse capacitor's are expensive, these are the cheap alternative.
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