August 2, 2003 Notes
We are seeing reduced catalytic activity in our test motor,
which is worrisome. There are several
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
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 dont seem to stop it at all, it just flares up
immediately on the other side. It isnt
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 doesnt 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
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 wasnt 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 werent any obvious cracks, but Russ put down a lot more
weld to close up any porosity. We
havent 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,
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 isnt clearance to connect a hose
to one of the ports because of the central lifting peg. We couldnt 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
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