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12" Engine, Pilot Bulkhead, CNC Adventures

12” Engine

December 17, 21, and 28, 2002 Meeting Notes


12” Engine


Our big engine housing is in.  This nicely shows the scaling behavior of monoprop engines – the catalyst packs only get slightly thicker to handle the thicker plates, while the nozzles scale up proportionately:




I need to make a retaining plate for it on the mill, and we need some more silver before we can completely put it together.  It should be good for up to 5000 lbf if we run the pressure up, but we are going to be working on optimizing for lower pressures.


The engine parts weigh 49 pounds without any plumbing or catalyst.  If we go with welded engines without flanges, and thin it out a bit more for lower pressure operation, that weight can probably be cut in half.  Catalyst pack weight will still be about 20 pounds, and the valve and plumbing is 10 pounds, so the total system will probably be 55 pounds or so.


Some data:


0.25 lb 12” 20 mesh stainless screen

0.14 lb 12” 32 mesh silver screen

1.23 lb 12” anti-channel ring


0.048 lb           5.5” 20 mesh stainless screen

0.031 lb           5.5” 32 mesh silver screen

0.070 lb           5.5” 20 mesh stainless screen with thick silver plating (over a nickel flash)

0.078 lb           5.5” anti-channel ring


We did a time-delay video of test samples of the silver screen and stainless screen left in a beaker of 50% nitric acid to see how quickly they were attacked by the acid, because the weak (10%) solutions don’t seem to clean the screens very well, but the strong solutions give off some disturbing red fumes when poured in an engine, and we weren’t sure how rapidly it was eating things away.  It took eight hours to completely dissolve the silver, but that was in an unstirred container and a 60 degree F room temperature.  An agitated container that is heating up from the action would probably go through it much faster, but 60 second washes is probably a fine cleaning time.



Pilot Bulkhead


We performed several more experiments with pilot acceleration couches.  We made a plywood seat frame and made another foamed set, this time forming an initial base layer, then a separate body contour liner that sits on top of that.  10 gallons of crushed Styrofoam with 3 drinking cups (need to measure the exact volume) each of the two part foam gives a good working mixture that can be shaped well, but foams into a solid shape after curing.


We have decided that we are going to do away with the seat frame sides, and just foam up the entire 40” diameter cone bulkhead around the pilot for our next test, avoiding any hard edges near the pilot.  We went ahead and made a “real” bulkhead out of 2” thick aluminum core fiberglass composite board from Techlam.  We cut the circle with a jigsaw, then used some clamps to allow us to use a table saw to cut an exact 10 degree angle along the edges.  We flipped out big aluminum cone upside down, and pressed the bulkhead into it, which helped round it out and reinforced it a lot.  We laid down a big, thick fillet of epoxy/flox on that side, and we will be doing the other side next week.  We will mount the racing harness, then make another foamed couch.  This bulkhead arrangement should be strong enough for drop testing with crushable noses, and pressure testing.



CNC Adventures


We have our used CNC mill fully operational in my garage now, busily cutting away on the quad-engine bulkhead while I type this.  The mill is a Sharnoa / CNC-Systems SD-850/4 with a Tiger IV controller.  If anyone out there has experience with one of these, and wouldn’t mind a question now and then, let me know.  For instance, the circular interpolation feed rates seem to be off by a factor of 50 for some reason, and I have no clue why.  I need to program feed rates like F200 to get it to move at a decent speed.


Some things I have learned:


Getting a laptop talking to the CNC took a lot of trial and error.  The formula turned out to be: Use a straight-through cable, not a NULL model cable.  Seven bit, even parity (that took a while to figure out!).  Translate newlines to carriage returns.  A slight pause is required after CR when sending programs.  I wrote a small utility to deal with all of this and add a few convenience features for hand-written G-code.  I am writing all the G-code by hand at the moment, but I will probably start using Turbo-CAD/CAM for more complex pars.


I am cutting without coolant to try and keep the mess down.  I initially tried cutting at 3000 rpm with a 3/8” end mill, thinking that the solid carbide mill shouldn’t have any problem with it.  It wound up leaving a frozen “wake” of aluminum behind it then rapidly welded aluminum all around the cutting edges and snapped in two.  I have been doing the rest of my cutting at only 500 rpm, which is probably very conservative, but I don’t care too much about optimizing the speed yet.


I was having some problems with the four flute center cutting end mill grabbing when plunging down.  Adjusting speeds and feeds didn’t help much, but manually putting a spot of cutting fluid down let it always go through.  I am probably going to start using two flute end mills, which should clear the aluminum chips better, and resist getting gummed up more.


For cutting bulkheads with multiple holes, workholding was a major problem.  The first test cases I made were stood off from the table with parallels, and held by clamps at the edges.  To cut the entire outer circle of the bulkhead, I had to manually remove and reposition the corner clamps as the mill went around, which takes much of the benefit out of unattended operation.  For the ½” thick, 24” diameter bulkhead, I tried using a complete covering of double sided carpet tape to stick the slab of aluminum to a plywood board, and clamped the entire thing by the corners, allowing the mill to chew about 0.050” into the plywood as it cuts around.  It probably wouldn’t work for anything ultra-precise, and it would probably break loose under aggressive feed rates, but it seems to be great for what we are doing at the moment.


You never have a big enough machine… This machine is pretty damn big, but it only has 19” of Y travel, so I need to cut the 24” bulkheads in two passes.  After a couple experiments, my current technique is to cut the first half, return to center, plunge cut down into the plate, undo the clamps, carefully rotate the plate 180 degrees using the spindle as a pivot point, then re-clamp.  It probably gets off a handful of thousandths, but it seems to do the job.




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