July 19, 2003 Meeting Notes
Unequivocal Mixed Monoprop Success
On Tuesday, we received our third style of catalyst from
Catalytic Products International. This
catalyst is a 3.5 tall roll of platinum plated corrugated foil. Unlike the random bale metal we tested last
week, this material has a solid bond that doesnt rub or wash off, and it seems
as active as the ceramic monolith catalyst.
The catalyst we got on short notice has 200 pores per inch, but it can
also be fabricated at 400 ppi.
Corrugations give slightly more surface area to volume ratio than square
pores, but the 200 ppi sample should still be presenting less surface area than
the 400 ppi ceramic monolith.
The physical characteristics of the foil catalyst are nicer
than the ceramic monoliths, because it wont crack or chip. We had to cut a small ramp into the engines
to allow us to squeeze fit it in, but it made a nice, tight seal, and it was
plenty rigid to be self-supporting at this diameter.
We preheated it and made a test run, with little
success. We were rather expecting this,
due to the straight shot from the top of the catalyst out to the bottom.
Half the team is at LDRS (the big sport rocketry event) this
weekend, but on Saturday those of us remaining set up a test facility to
characterize exactly how we were preheating the catalysts. We put gas flow meters in the propane and
air feeds, and drilled a hole in an old catalyst so we could inert a
thermocouple deep inside while we adjusted the mixture ratios. This was very educational. The temperatures could vary over 1000
degrees F with a twist of the knob, so there was no way we were getting
repeatable results in our previous tests, and it wasnt surprising that we were
cooking screens and catalysts sometimes.
We settled on 4 scfm of air and 0.38 scfm (23 scfh) of
indicated propane flow (not corrected for density), which produced about a 2000
F pack temperature, and soaked it pretty well after five minutes. I need to buy one size larger flow meter so
we can increase the total flows more to get larger engines heated in a similar
or lower time. There is still a little
bit of a procedure to get the preheating started, because if you just turn on
the gas flows and light it at the nozzle, the flame actually starts away from
the base of the catalyst. If you take a
torch with a long extension and press it directly against the base of the
catalyst in a few places while the gas is flowing, the combustion will start
near the top, getting everything heated well along the way. The mixture ratio we are running is very
rich, so there is excess propane still coming out the nozzle, which we leave
burning. The flame changes from yellow
to blue as the pack gets more thoroughly heated.
We noticed something interesting
when we were doing the open-air heating tests with the ceramic catalyst. After I shut off the flow of propane, if I
left the air flowing through the engine, a flame continued burning above the
catalyst for almost 30 seconds, and the catalyst pack continued to glow red hot
in places for another 30 seconds after that.
We believe that the ceramic catalyst must have been absorbing propane
during the preheat. The metallic catalyst
did not display this behavior.
The propane flow control does
need to be fiddled with a bit if the propane tank is nearing empty, because the
gas pressure drops when there isnt as large a mass of propane to spread the
heat of vaporization over. We cooked
one screen when we let the propane flow drop down, which goes closer to
stoichemetric, and a lot hotter. It
wasnt a problem with a full propane tank.
We set up the engine with the new
catalyst and the new preheat system, brought it fully up to temperature, and
fired it again. We had the flow jetted
way down with a 0.120 orifice, so we could try to get it clean first, then
build up the flow. It again failed to
produce any real thrust, and left a pool of peroxide under the engine. However, looking up into the engine nozzle
(with the mirror-on-a-stick we have been using a lot lately), the entire
catalyst pack was glowing bright orange, much brighter than it had been from
the preheating. The propellant was
definitely reacting on the catalyst, so the liquid seemed very likely to be
just channeling straight down the center with a gas cushion along the sides.
To combat the theorized straight-through
channeling, we cut the catalyst into thirds, and assembled the engine with a
few screens between each section of the catalyst, forcing any flow to make at
least a couple turns before exiting. We
tried cutting it with the band saw, but it kept grabbing on the foil, so we
wound up cutting it by hand with a hack saw, which was no fun at all. The ragged edges and rolled over cells are
also good hedges against channeling, but not terribly repeatable. It is possible that precisely fabricated
slices of the same thickness may perform worse without the ragged edges.
It was a little more difficult to
get the preheat flame started at the top without the straight shots, but the
entire engine still heated up at about the same rate. Our mixture ratio by volume for these tests is 4000 : 900, which
is a bit richer than we were running the last set of tests.
This time, the run worked
perfectly. We made a total of six runs,
going through 30 liters of propellant, and they all worked great. Thrust curves were smooth, except for some
shaking from the trailer. Interestingly,
there was a pale flame visible in the low pressure runs, but the higher
pressure runs had completely clear exhausts, just like a 90% monoprop engine
working perfectly. From the noise, it
was obvious that exhaust velocities were higher than pure monoprop.
0.120 metering jet restriction 154
psi tank pressure, 60 lbf
Change restriction to 1/8 NPT 162
psi, 125 lbf
Ten second long run repeat of
Increase tank pressure to 362
psi, 245 lbf
Change restriction to ¼ NPT 168
psi, 163 lbf
Long run, increase tank pressure,
long run 386 psi, 340 lbf
I measured Isp on the last run fairly
accurately as 140 s. The best we ever
saw on 90% hydrogen peroxide monoprop was 125 s, at a tank pressure over twice
as high. We did see just at 200 s with one
of our small biprop engines, even at a lower tank pressure, but the simplicity
of the mixed monoprop makes it a clear winner for our current purposes. The mixed monoprop Isp will increase when we
get 50% peroxide that is actually 50% instead of 40%, if we put the mixture
ratio at stoichemetric, and when we run it with high flowing plumbing.
We are really psyched about these
results. The odds are looking very good
that this will be the propulsion system for the X-Prize vehicle. Cheaper, higher performance, and no
availability problems. Big wins.
The next step is to continue
opening up the flow rates to see when this amount of catalyst finally gives
out, then set up for some very long duration runs to make sure the engines or
catalyst dont give out at thermal equilibrium. We still have a small ¼ ball
valve and 10 of hose on this setup, so there is a large feed system loss that
we can make go away by moving to a ½ valve and a shorter hose. If it is still running clear with that, we
will swap the small nozzle for one of the 2 throat nozzles, which should be
good for over 1000 lbf. We will
probably be using the denser 400 ppi catalyst roll on future engines, which
will give us even more potential. If we
can get by with only 2 x 1 thick rolls of 400 ppi catalyst, we can finally
fire the 12 diameter catalyst / 4 throat engine, which may see 5000 lbf. If we have to fabricate a 1 catalyst extension
for that big engine, we might consider a different design.