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Engine work, big vehicle work

August 2, 2003 Notes

August 2, 2003 Notes


Engine Work


We are seeing reduced catalytic activity in our test motor, which is worrisome.  There are several possibilities:


It may be poisoning from the stabilizers in food grade peroxide.  If true, it should be possible to clean.


It may be soot deposits from the propane preheating.  That should also be cleanable, and possibly avoidable with lean preheat or hydrogen.


It may be a degrading of the mechanical structure, allowing more channeling around the outside.  Welding in the anti-channel rings should help.


Worst case, it is an actual stripping of the platinum from the support base.  We may have aggravated this with some preheats that got out of temperature range, but if it is stripping due to fluid flow, all we can do is make sure we even out the flow as much as possible.


We welded two stainless AN fittings to the top of the engine to allow us to plumb in the propane or hydrogen and forced air without going through any pipe taped connections.  This turned out to be a bad idea, because the air and fuel did not mix well before flowing through the spreading plate, resulting in a very rich mixture on the side by the fuel, a very lean mixture on the side by the air, and a very hot stoichemetric mixture near the middle, which melted some screens.  We capped the fittings and went back to mixing the air and fuel together above the engine.


Our current spreading plate has 64 x 1/16” diameter holes.  Making this in 0.030” thick stainless broke a lot of drill bits, so we are having our next set made by a chem-etch company.  We are moving to 208 x 1/32” diameter holes, with the middle blanked off, which should give us more even distribution.  We may move to even smaller holes in the future, although plugging is an issue.  We always manage to collect a surprising amount of Teflon pipe tape on top of our spreading plates.


It was interesting to see in our gas flow preheat tests that even with three 20 mesh screens between the spreading plate and the catalyst, there were very obvious hot spots corresponding to the spreading plate hole pattern.  Replacing the screens with three discs of nickel foam removed the hot spots, giving an even glow during the preheat.


We tried again to use hydrogen for the preheat, because it should be much cleaner burning, and it removes the need to flame the catalyst at all due to room temperature catalytic burning.  Hydrogen flames just seem to instantly jump wherever they can.  Spreading plate orifices and even porous foam orifices don’t seem to stop it at all, it just flares up immediately on the other side.  It isn’t clear if this is actually a flashback through the orifice, or if it is the heating of the orifice material causing autoignition on the other side, but the results are the same.  The only way we could use hydrogen / air would be if we welded a cross on top of the engine, and just let it go ahead and start burning up there.  We would have to stand our valves off far enough that the thermal conduction through the plumbing doesn’t hurt them.


We changed the propane flow meter to a smaller size to get enough accuracy to work out the proper lean-burn mixture ratio for an equivalent preheat temperature.  With 4 SCFM air flow, 6 indicated SCFH (0.1 SCFM) gives a 1900F preheat, and no fire out of the catalyst.  The temperature varies by 50F as our air compressor cycles between its pressure limits, so we may add an in-line low pressure regulator to make it more consistent.  If the propane pressure falls off due to a nearly empty bottle, this arrangement safely goes cooler, rather than getting hotter as when we were doing a rich preheat.  We only consume ¼ the propane that the rich preheat did, which is a nice bonus.


Samples of the rolled foil catalyst did increase in activity when cleaned in nitric acid, so we tried to clean the entire engine.  We put a sample of the nickel foam in some nitric acid to see if it had a problem, and there wasn’t any apparent reaction after several minutes, so we went ahead and poured nitric acid (30%) into the engine.  A couple minutes later we looked at the foam sample again, and it was basically GONE, leaving the nitric acid a bright green color.  We dumped out the engine, but when we later took it apart, we found only scraps of the foam remaining between the catalyst rolls.  This may or may not have ruined the catalyst underneath it.


We have a very large batch of catalyst coming in a couple weeks, which will allow us to do many direct comparison tests.


Big Vehicle Work


We welded extensions to the computer ring mounting pegs on the top flange bulkhead.  We built this plate a couple months ago to allow us to move the isolation mounted electronics bulkhead directly from the subscale vehicle to the full size vehicle, but it annoyingly turned out to be a tiny bit too short to fit a pressure transducer in the middle of the flange underneath the electronics.  Rather than running a short length of tubing and strapping the transducer down off to the side, we raised up the board so the transducer can be screwed directly into the flange.


We pulled everything off the base of the vehicle and rewelded the lifting peg to try to fix our leak.  There weren’t any obvious cracks, but Russ put down a lot more weld to close up any porosity.  We haven’t re-leak tested it yet.


We wired up all the engine valves, and successfully ran all of them from both the manual switch box and the actual flight computer.  For my future reference, the per valve wiring order is: red, black, white (pot feedback), green, white (motor drive).  The cable wiring order is: red, green, blue, orange, white.


We built a nice welded aluminum distribution manifold that replaces the 2” NPT flange on the downstream side of our master cutoff valve.  We are trying to avoid NPT connections as much as possible now.  Unfortunately, it turns out that there isn’t clearance to connect a hose to one of the ports because of the central lifting peg.  We couldn’t clock the valve differently because of the engine mounting plates, so this is a fundamental issue.  We will put a 90 on that side for now, but we will probably make a deeper manifold with two outlets on each side in the future.


http://media.armadilloaerospace.com/2003_08_02/manifold.jpg  (interesting how the blue anodize turned black after welding)



We rigged up an electric actuator for the Sea-Catch release by mounting one of our KZCO valve actuators as a bolt turner.  The SeaCatch squib chamber had to be tapped out the rest of the way and slightly counterbored so a bolt could thread all the way down to the release point, then the bolt head was ground square so it fit the valve stem coupler on the actuator, then a mounting bracket was fabricated to hold it all together.  The limit switches on the KZCO actuator were bridged over to allow it to continuously rotate, rather than just 90 degrees.  This worked out as expected, but it takes about two complete bolt turns to pop the release, so this actuator takes almost 7 seconds to open at standard voltage.  Speed increases basically linearly with voltage, but we would prefer to run all of our actuation systems at 12v, so we are seeing if we can get faster gearboxes from KZCO.




The tank-conformal fiberglass base for our parachute box was laid up.  After it cures, we will be making a custom packing box with a matching contour to allow it to be pressed in.




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