Tuesday, April 27, 2010

railgun R& D work

Because I am an ADHD/OCD type, I generally work on multiple projects at a time. This has held me back in the past, as I tended to start new projects when I had existing projects already going, and often I would turn aside when some obstacle slowed me down.

Now that I have a few handles on these tendencies - not to mention medicinal crutches, I'm finding I can finally achieve focus on a single project.

However, I've still got multiple projects open right now, so I still do Research & Development work on longer term projects (such as Railgun #2) while I'm doing fabrication work on things with completed designs (such as the Mad Scientist Light Switch project).

So, with regard to the RG #2 design... there are many design variables, and I've been trying to find answers which enable me to balance them against each other and determine the optimum values based on the materials I already have. Most of this I've done by reading white papers, doing a few back-of-the-envelope calculations, and a bit of model work in SolidWorks.

So here are some conclusions I've worked out recently on the design of RG #2:

* at 10kV, I've got all the voltage I need. The Dahlgren and Kirckudbright EML facilities seldom used voltages higher than 10kV. A fairly promising student-built "bench scale" EML at the Naval Postgraduate School performed well on 20kJ @ 10kV and that was without any of the performance-enhancing tweaks which have become common practice since that gun was built.

* the first bore is going to be right around .75" square

* My pre-accelerator is going to be scary dangerous all by itself - it will be able to operate reliably from just over 100 to well over 1,000 PSI. Dump valve is rated to 1,200 PSI maximum working pressure. I have yet to do any friction calculations for the armatures, but for sliding fits, the acceleration is genuinely frightening. At least the pre-accelerator section won't need to be very long.

* pre-accel gas will most likely be CO2

* the best looking pre-accel valve I can find, in terms of high flow and pressure numbers, is the Dema (manufacturer) model 458P. Funny thing, that looks to be exactly the same valve that Sam Barros used on his second gun. I couldn't find a better one - at least, not one I could ever afford. I've started an eBay search for this item.

* the dimensions of the rails I have are just fine for what I want to do. I'll actually be cutting one of them shorter! (they came as two different lengths)

* augmentation methods still look promising, especially on smaller ("bench scale") guns of the size I am able to build.

* I have found where to source extruded aluminum C-channel, at a low price, which has a nearly ideal cross section for easily fabricating C armatures

* bore liner material is still likely to be Delrin for the first try. Something with higher arc resistance may be necessary near the pre-accelerator / driving rail interface - G7 perhaps.

* the 1" x 4.75" x 36" LE phenolic material I have on hand will work beautifully for one of the clamping plates. I will need a very similar piece for the other clamping plate. This stuff is expensive, I was lucky to get the first piece for nearly nothing. I will have to shop carefully ( cut-drops from eBay or local plastics suppliers) to get the second piece for an affordable price. I've started an eBay search for the second piece of this stuff.

* the afore-mentioned LE phenolic clamp plate material is not flat. It appears to have been slightly bowed by heat. To address this, I can think of three options:

1. bolt it to a piece of milled-flat I-beam and adjust the bolts and nuts until it's pulled straight. Note that I have been thinking of picking up a chunk of I-beam to mount the gun on anyway.

2. attempt to straighten it by heating it while it lies on my surface plate.

3. mill it flat, removing a fair amount of thickness in the process (probably around .125 total)

* the clamping plate material I have on hand is stout enough to allow me to have various gun designs within a single set of plates, and should handle perhaps 4X the energy & power I can deliver now. If I get more caps later, I ought to be able to build a longer, more sophisticated launcher, still using the same expensive parts. I'll have to be careful with my machining decisions so I preserve the ability to change the gun design without changing those plates.

* I am going to compare performance using both:
1. standing start firings with really carefully machined armatures with clever geometries and precise pre-loading (the way the pros do it)
-as well as-
2. pre-accelerated firings with really basic C armatures and a more relaxed bore fit in the same gun.

So, a lot of design decisions are getting finalized, and I've even begun to construct a preliminary model in Solidworks utilizing the parts and materials I have on hand, assuming additional parts and materials are obtained, and utilizing an integrated pre-accelerator section within the same gun housing.

I'll post a pretty CGI model pic eventually, when it looks like it's closer to the final design. Don't hold your breath.

No comments: