Friday, April 30, 2010

project catalog, part 1 of N: Railgun #2

It is long past time for me to post an enumeration and explanation of all the various projects I have "open" at the moment - that is, projects I've started but not finished. This post could get long quick. In fact, I think I'll limit this post to one project and likewise write individual posts for the other projects.

Some of these have been in progress a long time, and have been repeatedly tabled and restarted again for various reasons, usually money and spare time, sometimes because I wasn't sure yet how to proceed and had to stop and do some planning or research.

So first of all...

Railgun #2 AKA "EML-2" (Electro-Magnetic Launcher #2), AKA MSRG-2 (Mad Scientist RailGun #2) etc.

Incidentally, I'd like a clever name or acronym for this device if anyone feels creative.

The pretty picture at left is only that so far - just a conceptual design, although it is slowly turning into a real model that I can make real shop drawings from which I can fabricate the real deal.

Originally planned to be a roughly 1 meter class device, the various potentially-expensive parts I already have on hand limit the actual accelerator rail length of the first version of this launcher to about 24" with another 12" or so of pre-acceleration gas gun on the breach end of the railgun section... for now.

Later, when I can afford to purchase high grade rails the full length of the casing, I will convert the gun proper to 36" of rails, bolted to a completely separate injector gun.  The materials get expensive if you have to buy them new, so I'm starting out with the various stuff I have already obtained cheaply, and designing so that I can add significant improvements later, re-using some of the more expensive materials.

The design of the launcher itself is still very much in flux, but many of the design decisions are now either made or are rapidly narrowing. Ferinstance, the bore of the first iteration will be approximately .75" square. Round bores are too much hassle and I don't need to build a round-bore device since I'm not interested in a practical weapon - this is strictly an unportable, short range, laboratory device. Oh by the way, the last time I contacted the BATF, the agent I spoke with said he didn't think EMLs / railguns met the criteria for a "firearm", although I never actually pursued the formal "determination of firearm status" (or whatever it's called) paperwork.

Like any other amateur railgun builder, I have little choice but to use a high voltage capacitor bank for the power supply. For over a decade, I have possessed a very large capacitor-based pulse-discharge machine which stores 18,000 Joules in six 60uF capacitors at 10kV max charge. The capacitors are big low-inductance pulse discharge devices, about 100 - 150 pounds weight each. They are significantly derated in terms of both expected current and voltage reversal, and I have all of the designer's paperwork on this machine. It was an experimental magnetic metal forming machine, built in the early 1970s.

It originally used six ignitron tubes to switch the current, one mounted on each capacitor. Ignitrons are rather slow as pulse switching devices go, but they are fine for railgun pulse times, they handle quite large charge transfers compared to triggered spark gaps, and they are rectifiers, which largely prevents voltage reversal across the caps. Furthermore, multiple switches - if they were still here - might have allowed me to play around with distributed energy storage and a segmented launcher some day. Unfortunately, the ignitor electrode assembly of an ignitron becomes very fragile once it has been used in crowbar or pulsed power switching. All six tubes were wrecked long before I got my mitts on it. Replacements are $1,500 each, which is $9,000 I certainly don't have just lying around.

So my plan is to replace all six capacitor banks with one of two triggered spark gap switches I have on hand, hopefully the smaller one I constructed, but if necessary, a larger railgap style switch is available, although it needs work.

In the absence of controllable switching, it's possible I'll need a pulse forming inductor to slow down the current pulse, or alternatively, an impedance matching transformer capable of handling a few hundred thousand amps. This is surprisingly do-able, and I've got a white paper that shows how. However, that thinking concerns standing-start launchers.

I do not plan to operate from a standing start unless I find it just isn't practicable any other way. I'm considering a way to use the armature to help with the switching, but not actually use the armature to do all of the switching.

The rails are copper, 0.5" thick x 1.0" wide. I have these, but have not machined them straight and flat yet. They were used as bus bars, so they have a few 1/4 inch holes that need filling. I plan to ream the holes and press pins (machined from 110 copper) into them. Some big bolt holes are already conveniently located at one end of each rail.

Bore liners will be fabricated from garden-variety Delrin for the first iteration. I do not have the liner material yet.

I'm still working out the breach connections to the rails but as the rails already have some .400 holes in them, you can bet that a 3/8" bolt will be involved somehow. No, I won't be passing current through the bolt.

Top and bottom clamping plates (resist the repulsion force of the rails, and indeed the rails mount directly against these plates) will be phenolic-linen material, 1" thick x 4.5 inches wide x 36" long.

15 - 20 clamping bolts, 1/2-24 x 5" gr.8, will hold the launcher together.

Armatures will be cut from aluminum "C" extrusions readily available from industrial suppliers. There are profiles offered which are startlingly similar to many armatures used during the middle of the railgun development days. I say that last bit because I think we're nearing the end of the R&D period of railguns (some 40 years!). A weaponized practical railgun should be fielded on a Navy ship by the close of 2012).

The injector will run off of high pressure CO2, using a small bottle similar to the one I use for MIG welder. The storage plenum for each shot will be a paintball gun tank. The valve between plenum and injector breach seems to be the Dema 458P.

One aspect of successful railgun design which seems significantly lacking from most other amateur's efforts has been instrumentation.  At minimum, we will want to know the pulse supply / rail current, the breach voltage, the muzzle voltage, and the armature muzzle velocity, recorded over the full duration of the shot.  Depending on how things work out with the armature-triggered switch idea, it may also prove necessary to have armature position sensing in at least one position - and preferably several - inside the launcher system.

This last desire is a rather challenging trick.  You and I can't (well, MOST of us probably can't, and certainly shouldn't) take high speed flash x-ray photographs of our launcher during the shot.

Various ideas have been tried by the pros, and only a few of them seem to be practical for amateurs.  For now, I'm thinking about various methods of optical probing with fiber links, so-called "B-dot" probes and even electrical contacts embedded in the liner walls.

Current pulse measurement will probably be done with a Rogowski coil and integrator.  Voltage measurements simply require resistive, non-inductive dividers.  Isolation and grounding will need to be watched carefully. I have no intention of blowing up my Tek 7934 with EMP, EMI, RFI, etc.

I've left out a great many of the details I've been sweating over. Any questions?

Lately I've begun to wonder whether there would be any point in setting up a PayPal fund for this project. It's going to be a long and expensive one. It already has been, and will continue to be. These things aren't easy or cheap, usually.

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