March 3, 2003 Notes

 

We didn’t have any meetings this week, because there was an ice storm here in Dallas, and I was out of town over the weekend.

 

Paraphernalia

 

Due to popular demand, Anna has set up a PayPal account for Armadillo T-Shirts, sweat shirts, and our “Armadillo Droppings” – miscellaneous bits of various things we have broken or discarded in our work.  Note that she will be out of town this week, so no orders will be fulfilled until next week.

 

Machining

 

I finally got around to making the retaining plate for our 12” diameter engine.  I started with a ½” thick plate of 316 SS, and milled out an eight spoke wagon wheel retainer.  It took almost ten hours, and now that it is done, I am wondering if I made the spokes too thin for high-pressure operation.  Our production engines will use the welded anti-channel-ring support scheme, so they won’t need a retaining plate, but we need to be able to pull the engines apart for the initial tests, so we need a plate strong enough to take the pressure drop across the entire pack.

 

When I first got the CNC mill, I had visions of just writing the programs and letting the machine busily mill away for hours while I did something else.  With a high pressure coolant flood system it would probably work out like that, but I don’t want coolant spraying all over my garage, so I only use a minimal amount of manually applied cutting fluid, and I wind up baby sitting the mill quite a bit to clear chips away, especially in deeply milled slots.  I also still have occasional problems plunge-milling into the stainless plate.  I am using three flute stainless-optimized carbide end mills, but if I don’t get a good shot of cutting fluid under them, they still sometimes jam up while trying to plunge 0.1” down.  I am considering drilling pilot holes in the future.

 

It is a low priority project, since it isn’t on the X-Prize critical path, but we are still putting things together for a 1000lbf peroxide / kerosene biprop engine.  I was going to drill all the flange holes this week, but it turns out that even with the mill table cranked all the way down, and the spindle all the way up, the chamber only just barely slides under, so there is no room for a drill.  I was able to sneak a center drill in there and spot all the holes, but we are going to have to drill them on a manual drill press.  In the future, we will make sure that anything with a flange will fit on the mill.

 

Another research direction that we have taken occasional stabs at is a “plate nozzle” composed of lots of little nozzles instead of one big one.  We tried doing that with a retaining plate turned into a bunch of nozzles a while ago, and it didn’t work (subsonic flow), but we are considering trying it again with one of the welded engines that don’t need a retaining plate, so we can have a gap between the screens and the nozzles.  If it works out, it would let us easily make almost arbitrarily large engines.  The contracting cones on top are easily made with a 60 or 90 degree countersink, but the expansion cones should be much narrower for good performance.  I just tried using a carbide bur with a 30 degree angle, which would have been perfect, but I rapidly wore down the tiny little cutting edges pushing it into a piece of 316 ss with a hole drilled through it.  I think what I need is a tapered end mill, but I haven’t seen anything like that in the 24 to 40 degree included angle range.  Anyone have suggestions?