November 8, 2003 notes
lbf Mixed Monoprop
Safety note: we found a crack in one of our 5 gallon HDPE
carboys that we use as a transfer container when loading peroxide from drums to
our run tanks. We always keep the
carboys on a containment palette, so there was no spillage, but a few hundred
ml of peroxide has probably leaked out into the containment sump when I noticed
the crack. We pumped the peroxide to a
new container and tossed the cracked one out.
I checked my records, and we had been using that particular carboy for a
bit over two years, which was probably a mistake. The first year of that was with 90% peroxide, which is known to
embrittle PE after extended exposure. We
had a bunch of new carboys, but we hadnt tossed the old ones out.
Our main task this week was getting more thrust out of the
large nozzle engines. Because we were
still getting the cold hole in the center of the chambers when the catalyst
wasnt packed so tight that it choked the flow down, we decided to try a pair
of catalyst blocks in a straight-through configuration with a spreading plate
at the top, instead of the three-pass plates on top and bottom. We had tried this exact configuration in the
past, but we had reason to believe that the addition of the perforated metal
flame holder under the three-pass pack was the important point that made
We put this new top chamber on the same bottom chamber we
built last week. It seemed like it was more
difficult to get preheated from a completely cold state than with the
three-pass top pack, but after it was warmed up it made an very smooth
run. The chamber walls and nozzle
heated up completely uniformly, and the thrust trace was very smooth, with less
than +/-5% thrust variance. After the
run, the catalyst was completely uniformly hot across the entire surface, with
no cold center. Unfortunately, it didnt
make much more thrust than the previous runs.
We repeated the test with identical results
289 lbf average thrust during the run with 230 psi tank
pressure, 342 lbf at 230 psi at line clear.
The thrust bump at the end of our test runs is due to having
a 12 10 hose going from the propellant tank to the engine. There is normally a fluid pressure drop when
the line is full, but as the last of the propellant starts to go through the
line, there is less fluid resistance, until the last of the propellant is just
being pushed through the valve, with no effective hose length. We often consider this line clear thrust
rating as what we would get on a vehicle with very short, high flow plumbing
from the tank to the engine. All
reported thrust numbers are also 10 15 lbf on the low side, because the load
cell calibration has a negative zero bias at the moment.
Not having to use the three-pass plates is a big fabrication
help. I was having a miserable time milling
down a ½ thick piece of 316 stainless plate for the big motor, and I wasnt
looking forward to making a ship-set of the small ones.
We cut open the hot chamber and removed the top section of
catalyst bale, leaving two separate sections with 330 grams of catalyst bale pressed
to 1.5 height each. The results were
another smooth run, but still hardly any increase in thrust after adjusting for
a slight difference in tank pressure:
327 lbf average during the run with 248 psi tank pressure,
375 lbf at 250 psi at line clear.
We made another run at higher pressure, which was even
smoother than the previous runs, with only +/-1.5% thrust variance.
507 lbf average with 393 psi, 555 lbf at 393 psi at line
This is actually good enough for us to fly the big vehicle
for initial tests, but still lower than we expected from the engine.
We still dont like working with the catalyst bale, because
it is difficult to build engines consistently with it, and it obviously
compresses down a whole lot under pressure, causing self-restriction. We wanted to try building a hot chamber with
only the foil monolith catalysts, which are consistent and have very low
pressure loss. We had tried that a lot
before, but without the cold section / flame holder arrangement. The 1 thick, 5.5 diameter 400 cell-per-square-inch
foil monoliths weigh 275 grams each, so two of them will by 16% less catalyst
than is in the current chamber, but will flow a lot more.
We were out of fresh monoliths, so we started scavenging
from previous experiments. We cut open
the welded-together top chambers with the two previous three-pass catalysts,
and we were pretty surprised to see that the outsides of the catalyst rolls
were pretty chewed up. The original
three-pass pack had been propane preheated with initial start by torching the
outside of the chamber, so that was understandable, but the second one had only
had cold propellant through it, so we had expected them to come out almost
perfect. It looks like the widely
varying temperatures in the different pass sections, combined with the plate
bars restricting radial expansion, result in the foil pores at the
intersections getting a lot more bent than unrestrained single pass catalysts.
We picked two monoliths from an old hot chamber that were
the best of the lot, but they were still domed a bit, and had ragged edges that
didnt fit very tightly. We separated
the two monoliths with a perforated plate with 3/16 square stock on each side,
in hopes that it would act as a flame holder section between the two blocks. We left about a 2 section for the glow plug
ignition chamber between the cold catalyst and the hot catalyst.
The assembly with used components was sort of ratty, but it
did turn out to work, and made a good deal more thrust than the old chamber:
448 lbf at 253 psi tank pressure, 537 lbf at 252 psi line
We made a longer run with similar results:
425 lbf at 248 psi tank pressure, 503 lbf at 237 psi line
This was a solid 30% more thrust than the old pack, but we
were seeing something odd the runs stayed perfectly clean, with no sign of
quenching, but things just werent getting as hot. The nozzle wasnt glowing as much, and the inside of the engine
after a run was only dull red instead of bright orange. Calculated Isp on a later run was also down,
working out to a measured 116, so even counting some lost to the warmup pulse
(not much on the hot engines), we were down 15% or so from the good runs.
Increasing pressure yielded similar results:
616 lbf at 392 psi tank pressure, 755 lbf at 391 psi line
A longer repeat test gave the same results.
We increase the pressure again, and it continued to run
clear with increasing thrust:
911 lbf at 618 psi tank pressure, but the nitrogen bottle
and regulator couldnt keep up with the flow, so the line clear had dropped to
927 lbf at 539 psi.
We increased again, and cleared 1000 lbf:
1010 lbf at 700 psi tank pressure, 1066 lbf at 672 psi line
The restrictions on this engine are probably in the plumbing
and spreading plate now, not the catalyst.
This all looks pretty good.
If we fill the big vehicle tank to 400 psi, we can get 3000 lbf max from
the four engines, and vehicle + 500 lb of propellant will be about 2000
lb. Giving some margin for differential
throttling, this will be a slow, dramatic liftoff, but it can go for 25
We hope the Isp comes back when we build fresh engines with
the new 600 cpsi catalysts that are arriving next week, and we are going to go
ahead and add a third catalyst to the hot section to help out, but even if we
dont get any better than 120 Isp, we can still do a lot of flight
testing. We will do another set of
engine development tests to optimize before building a ship-set of the 12
diameter engines for the X-Prize flights.
We will need our launch license before we can really fly the vehicle in