June 21, 2003 Notes
Big Engine Mounts
We debated the exact engine mounting strategy for the big vehicle
for some time, but we settled on a simple solution that wastes a bit of weight,
but works out conveniently. The initial
test flights will be with 5.5 diameter engines with 2 diameter throats, but
the final engines will be much larger.
We built mounting plates that clamp to the bottom of the tank manway
flange on the inside, and are supported by two ¾ thick studs on the
outside. To attach to the tank, we cut
¾ coupling nuts at an angle, and welded them to 3 by 3 squares of perforated
metal. These squares will be bonded to
the tank with epoxy / chopped glass fiber.
The engines will be canted with angled shims above the mounting plate,
just like the smaller vehicle.
I learned some lessons machining the mounting plates. Since I had to make four of them (and will
probably have to make more when we crash the vehicle
), I wanted to let the CNC
program run completely automated, which meant no tool changes on our mill. I was a little surprised when the 0.5 and
0.75 diameter holes bored with a 3/8 end mill came out distinctly non-circular. Tool deflection was a bigger issue than I
expected none of the previous parts I had made actually required tight
tolerances like that. I wound up having
to cut the holes in two passes and at a very slow rate, but in the end it did
work out exactly how I wanted, where I could just clamp a plate on the mill,
start the program, and come back to a finished plate. I am going to buy a bunch pf short, stubby end mills to minimize
deflection for hole milling in the future, but bringing the spindle down to
right over the plate does make work piece clamping more difficult.
More 50% Firings
We learned a lot, but we dont have a solid solution yet.
We received another 2 thick, 6 square block of impregnated
ceramic catalyst from CPI, as well as a 4 thick block, and an uncoated 2
thick block. It was fairly easy to
rough-cut the block to cylindrical shape with a band saw, then get it to final
shape on a bench grinder. We saved the
scraps for various tests.
We got a new supply of nitric acid for catalyst cleaning, so
we tried to clean the catalyst we had been using for the earlier tests. Even after significant soaks at fairly high
concentrations, it never regained the activity that the brand new catalyst displayed. At the time, we thought there might be some poisoning
that the nitric wasnt removing, but based on later tests, we now think that
there is a degree of mechanical stripping of active catalyst.
We found a local supplier of 50% concentration food grade
peroxide, and purchased a drum. In drum
quantities, food grade was $1/lb, compared to $0.50/lb for technical
grade. We are still looking for a
supplier of semiconductor grade at 50% concentration. I finally got around to ordering a TDS (total dissolved solids)
meter for testing peroxide, so I will report some numbers next week.
To test the poisoning of the catalysts, we took two chips of
the catalyst and let them each soak in 100 ml of 50% peroxide, one technical
grade, the other food grade. They both
caused lots of bubbling, but the food grade sample cooked all the peroxide off
faster. We then placed them in another
100 ml each, and the difference grew more pronounced. On initial immersion, the temperature of the food grade sample
was higher, and when we checked on it again an hour later, the food grade
sample was reduced to completely still water, while the technical grade sample
was still slowly foaming away with some peroxide left to decompose. We removed the samples and did eye dropper
tests of fresh food grade peroxide on both, which showed the one that had been
immersed in technical grade to be noticeably slower to react. They both still catalyze immediately, but
the technical grade sample isnt nearly as vigorous. We should have contrasted it with a completely fresh chip of
catalyst, but we didnt think about it at the time.
We fabricated a big combustion chamber to sit between the
catalyst and the nozzle, in hopes that extra volume would get a more consistent
performance, and also to give us an extra half inch to put inert screens
between the spreading plate and the catalyst.
After all of our tests, we are now of the opinion that post-catalyst
combustion volume is of no value with this combination. In the few references we have seen to 50% peroxide
work, it is mentioned that the reaction is not self-sustaining without the
presence of a catalyst, which would also argue against the value of
We added 16 inert stainless screens above the catalyst to
fill out the space. All of our test
firings were very smooth today, even with no explicit metering orifice to
provide an injector pressure drop, which is visible as the thrust curves
dropping linearly with tank pressure.
This was a large difference from the incredible chuffing that we had on
our first successful tests, but we arent sure if it is just because of the
higher flow rates, or because of the inert screens at the top of the pack.
We had some crud on the top flange, so we had a leak there
on most of our tests. It may be a
permanent gouge, so we will probably use a copper gasket for the next test.
These firings were much louder than normal monoprop firings
in the 100 200 lbf range, but the Isp only calculated out to be in the 80-100
range, which is a little puzzling. There may be a core of high velocity gasses,
with a jacket of unreacted, low velocity gasses bringing down the Isp. In theory, the Isp should be around 150 even
at very low chamber pressure. We arent
expecting any good Isp until the burn stays cloud-free the entire time, but the
noise level was unexpected for the low performance.
We did runs at both 200psi and 400psi tank pressure, with
basically similar results. For long
runs, we could get it to start with a nice clear exhaust plume, but after a few
seconds it would cloud up.
Interestingly, the thrust continued to follow the tank pressure, without
any inflection as the exhaust clouded.
We have seen behavior like that before on 90% monoprop motors without
any metering jets that had catalyst deterioration, as the Isp drops, the flow
increases to compensate for it.
One supposition was that as the engine heated up, the
expansion allowed more flow to bypass around the side of the catalyst. We removed two of the screens at the top and
added a Teflon encapsulated silicone O-ring.
We barely put any crush pressure on it, because we were unsure of the
strength of the ceramic catalyst. The
O-ring didnt improve the engine behavior at all, and when we opened it up, the
catalyst web had crushed some under compression, shearing off vertical fragments. If we need to seal against the catalyst, we
will probably have to cast a solid ceramic outer layer around the porous
catalyst to give it enough strength to compress the O-ring. I was a little surprised that we didnt melt
the O-ring at all with our torch preheating before the run. This implies that the top of the pack isnt
getting all that hot, even when we make the bottom of the pack glow red hot.
We tried adjusting the methanol content to be notably
richer, but that run was much worse, with no flame at all, and lots of wet
Someone mentioned that the food grade peroxide drum had some
fine print on it that said 40% to 60% concentration under the big 50%
marking, so we decided to test the density to see if we might be working with
notably weaker peroxide. The food grade
peroxide came out to 1.15 g/cc, and the technical grade came out to 1.17 g/cc. We should have explicitly measured the
temperature, but it was probably about 85 degrees F, or 30 degrees C. The book value for 50% peroxide at 25 C is 1.1914
g/cc, and the value for 40% is 1.1487 g/cc.
The food grade peroxide is only about 41% concentration, while the
technical grade is about 46%. Someone
probably got a raise for making it company policy to dilute to the lower limit
We are certainly going to check each drum we
buy in the future, because a 60% mixture could be dangerous, as well as skewing
the O/F ratio.
Density table from p 199 of Hydrogen Peroxide by Schlumb,
Satterfield, and Wentworth:
Wt % H2O2 0C 25C
0 0.9998 0.9971
5 1.0193 1.0145
10 1.0393 1.0324
15 1.0598 1.0507
20 1.0804 1.0694
25 1.1014 1.0885
30 1.1226 1.1081
35 1.1441 1.1282
40 1.1661 1.1487
45 1.1883 1.1698
50 1.2110 1.1914
55 1.2342 1.2137
60 1.2579 1.2364
65 1.2822 1.2592
70 1.3071 1.2839
75 1.3326 1.3086
80 1.3589 1.3339
85 1.3858 1.3600
90 1.4136 1.3867
95 1.4421 1.4142
100 1.4709 1.4425
Since the peroxide wasnt as concentrated as expected, all
of our mixture ratios were running on the rich side, which hasnt helped, but
probably isnt the core of our problem.
The fact that the runs can start clean, then go cloudy implies that the
catalyst pack may need to be at a higher temperature than the steady state
reaction temperature, and only functions initially due to the torch heating. A longer pack may or may not help with this,
we have seen very odd behavior with 90% peroxide and silver screen catalysts,
where a pack of a certain length would never catalyze completely even on tiny
amounts of flow, but adding another quarter inch of catalyst all of a sudden
allowed huge amounts to catalyze perfectly.
We can also experiment with higher volume propane torches for
preheating, and possibly a flow-through the top heating arrangement that would
give much more thorough heating.
Another thing we noticed on disassembly was that the new
catalyst was less active than the first pack (that we had cleaned with nitric
acid), so we may still be experiencing some poisoning from the food grade
peroxide. We do believe that there is
some degree of mechanical stripping of the catalyst going on as well as a
degree of poisoning. The fresh catalyst
probably has some poorly bonded bits of catalyst that offer high surface area
initially, but are rapidly washed away under high pressure liquid flow, leaving
the strongly bonded catalyst with less total surface area. We could test this by doing running five
gallons of distilled water through a new pack instead of peroxide, and seeing
if the pack then has reduced activity compared to its corner scraps.
See if nitric acid cleaning the second pack brings it back
to the activity level of the first pack that was cleaned. If true, it means that the food grade
peroxide is still poisoning to some degree.
Make a test run with proper O/F ratio for our 40% peroxide
to see if that behaves any better.
Fabricate a chamber that will let us test with the 4 thick
Investigate the rolled metal foil catalysts.
Investigate more thorough pre-heating options.