August 13, 20, and 24 2002 Meeting Notes
Last weekend was Quakecon, so we didnt have an Armadillo
We finally got the seated lander back in flying trim. On Tuesday, we did all our plumbing and
engine checks, then did a very conservative liftoff. The computer box was not leveled properly, so it held an angle a
few degrees off vertical, and skimmed along the ground.
Next Tuesday, we leveled the computer, and finally got the
laser altimeter running off of the main vehicle power supply instead of a separate
battery, which required adding a new 6v regulator to our power supply
board. We now have +5v for the computer
and some sensors, +15v for the Crossbow iniertial unit, +6v for the laser
altimeter, and unregulated +12v for some other sensors.
We loaded up for a short flight, and it lifted off and set
down fine, then I lifted it up a bit higher, but when it came back down the
second time, the computer reset, leaving one engine firing , causing it to flip
over and land shark a bit. We broke a
gauge, a hose, and a valve, but nothing structural was hurt. We had perfect telemetry until the second landing,
with the laser altimeter data looking good enough to use for auto-throttle.
On investigating, one of the makeshift terminal lugs on the
power supply board had vibrated loose enough to rattle, which was almost
certainly the cause of the problem.
When the power supply board was built, we made do with soldering some
crimp logs on the board to act as sideways screw terminals, but this shows that
was a Bad Idea. Russ made some somewhat
better terminal connector posts for us out of standoffs, and we routed things
to save a couple connectors on the critical path, but we really need to build a
brand new power supply board with good screw terminals, and we should take the
opportunity to power both of the available +5v / gnd connectors on the computer
with separate connectors to give us a level of redundancy against connector
We were ready to do another flight on Saturday, probably
with the laser altimeter controlling the throttle, but it turned out that we
were at the bottom of this drum of peroxide, so we didnt get a chance.
On Tuesday, we tried our last bet for sealing the TZM
chamber with materials on hand, a high temperature silca gasketing material. It didnt work much better, but when I
re-faced the injector, I found that it was noticeably warped, so we may not
have given the gasket material a fair shot.
The radiatively cooled engine running at temperature showed
up several weaknesses that we are addressing:
The brass clamp ring was obviously deforming after hot
runs. We expected this, so we have
moved to stainless steel clamp rings. I
started to make one on my Sherline mill, but I managed to strip something on
the Z feed while plunging an end mill into a stainless plate, so I had Dyna
Turn run off a few for us. It is going
to be very nice when we finally get our big mill installed
The 316 socket head cap screws were suffering. This wasnt as expected, but they were
definitely stretching, and getting generally screwed up to the point that a
couple of them had to be cut off to separate the chamber. System 22 www.system22.com
was able to find me a small lot of A286 alloy fasteners in the size we need
(10-32 2), and get them to me on short notice. These are rated for use to 1200 F, and retain strength quite a
bit higher, so they should be fine until they start glowing bright red. Interestingly, the matching flange nuts are
silver plated to reduce galling. To go
beyond this level of fastener to inconel would involve long lead times and
large lots, so we hope these do the job.
At about $8 each, they are already getting a bit expensive.
We have yet to get a good seal between the injector and the
TZM chamber. We have probably been
facing three separate issues on this:
the silicide coating is not terribly smooth, the bolts and clamp ring
were probably easing up as the runs got longer, and by the end of our
experiments, the actual injector was warped, so sealing wasnt possible. We have gently sanded the top flange a
little smoother, which should be ok for the silicide coating, which should have
penetrated 0.003 into the metal, as well as built up 0.003 above it. We may give it another try with the silica
gasket material now that we have smoothed it, and have the high temp bolts and
clamp ring, but I have also ordered the high tech solution a few gas
pressurized metal O-rings from www.helicoflex.com
, which should be here any day now. They
are $50 each for 5, and they are not supposed to be reused. Interestingly, for sealing onto rougher
surfaces, they are silver plated (no peroxide leaking past that!), because
silver is the most malleable coating they can put on it. If higher temperature service is needed,
they move to nickel plating, but it halves the roughness that can be tolerated
on the surface.
The warping of the injector may require us to make it out of
stainless, but our next injector is going to have a slightly different design
that may let it stay closer to the peroxide temperature instead of the chamber
I also broke down and finally bought a pair of cavitating
venturis from Fox Valve www.foxvalve.com
so we can see if that brings our runs back to the extreme smoothness that we
have seen on some other engines, as opposed to the 7% or se roughness level
that we are currently seeing. Their
prices are pretty expensive, around $600 for a fancy tube fitting, but we will
finally get to see if they are worthwhile over the simple tuning jets we are
30 Second Regen Run
We tested a 2 diameter regeneratively cooled biprop today
made out of aluminum, with very good results.
The inside of the chamber was hardcoat anodized to act as a layer of
insulation, but it was mostly just plain old aluminum.
We started off with full blast water running from the hose
through the cooling jacket. This worked
perfectly as expected, because it should certainly cool better than the water
flood cooling that we had used on the other aluminum chamber.
0.080 peroxide jet
0.040 kerosene jet
250 psi regulated pressure
steady 47 lbf
14 second run.
We then cut down the water flow rate until it was measured
to be what the peroxide flow would be during operation. This was doubly conservative, because we
chose a slightly lower flow rate, and the water was operating at atmospheric
pressure, while the peroxide would be at at least 200 psi, which would hold off
After eight seconds of burning, the water exhaust turned to
steam, and I stopped the kerosene flow.
It quickly cooled back down under the remainder of the monoprop run.
We richened the mixture by moving to 0.050 kerosene. This started boiling after 9 seconds.
We richened the mixture again by moving to 0.060
kerosene. This did not boil the water
This is VERY rich, but nice and safe for the time being.
We then plumbed it up in actual regenerative cooling
mode. For our first run, we started at
a lower 200 psi, which dropped the thrust to 35lbf, but it ran fine the entire
run. In hindsight, lowering the
pressure on a regen engine actually increases the heat load into the coolant, because
heat transfer scales with the 0.8 power of chamber pressure, so higher pressure
causes steeper thermal gradients, but less total heat transfer per unit of
We then increased the pressure to 250 psi, and made another
perfect run at 48 lbf. Right after the
run, we checked the inlet tube with an IR thermometer, and found it to be 223
We then loaded two liters of peroxide for a long run, which
also went flawlessly for 30 seconds. We
measured our kerosene flow on this run, and found out just how rich we were
actually running we burned 1.7 liters of kerosene with the 2.0 liters of
peroxide, for an O:F of 2.2:1, about three times as rich as optimal. Our true Isp was also quite low, only 151 s. Going to a leaner jet would get us another
10% or so, but we still need to improve our injector to get the rest of the
performance we want. We will be happy
when we have a measured 200 Isp.
With regenerative cooling, we are not limited to the lower
combustion chamber pressures that we would be with radiative cooling, so we
decided to switch out our fiberglass kerosene tank for a tank that could handle
higher pressures. Back when we first
started the biprop testing, we were using a nitrous tank for the fuel, but the
single ended tank was a big hassle to fill and vent. We had tapped another port into the bottom of the tank, but we
changed over to the fiberglass tank before we ever used it. Today, we swapped back to this tank, but we
didnt bother to plumb the sightglass in yet.
The regulator we had running the constant pressure feed
would only get us to 380 psi, and it dropped off during the run, but it worked
fine, making 67 lbf for 10 seconds.
However, on termination we had a serious kerosene leak past the fuel
solenoid. We shut off the bottle valve,
but it was still leaking, so we depressurized and pulled everything apart. We drained everything out, and found out
that we had never washed this tank out after it had been tapped, so there were
lots of aluminum chips throughout the system, which wedged the solenoid open
and scratched the bottle valve seat. We
flushed everything out, dumped out the engine, and set things back up for
We were just about out of our first five gallons of kerosene,
which showed us two more things. The
five gallon pail of kerosene from Home Depot actually had some dirt or other
crud at the bottom of it. We have been
working under the notion that we dont need to worry about contamination on the
kerosene side, but we should probably start filtering to make sure there isnt
anything that can plug a solenoid or jet getting through. The other thing we noticed was that the new
kerosene we bought, a different brand, had a noticeably different odor. Kerosene is such a loosely defined term that
we should probably buy a drum of Jet-A or some other more specific spec for our
We reduced the fuel jet size to 0.050 for the next run to
see if that would still run stably with regenerative cooling.
We started it up, and
Our very first engine explosion, just as the catalyst pack
reached operating temperature. Everyone
was safe on the other side of the concrete wall, watching it on the video
monitor. We recovered all of the pieces,
some of which were pretty impressive looking.
Obviously the kerosene leak had allowed kerosene to saturate
part of the catalyst pack. We had
dumped the engine out, but it had been sitting there on its side, with the
venturi injector funneling the kerosene down to the screens for quite a while,
and surface tension probably held quite a bit in there. Phil had even said that we should do a water
run to clean out the engine, which might have saved it. Or it might not have, with water not being a
very good kerosene cleaner. What we
need to do if we ever suspect kerosene in the catalyst pack is to take it apart
and put it in the ultrasonic cleaner with alcohol, then let it completely dry
The explosion happened in the middle of the catalyst pack,
and rather surprisingly, it didnt even damage the load cell or regen chamber,
because almost all of the force was expended radially. The test stand was rather mangled, with the
center bar pushed down to the trailer, and we got peroxide all over everything,
but we cleaned it all up and built a new vertical test stand by the end of the
night. We had been planning on doing
this before testing a hypergolic engine, but if we had done it earlier, we
probably wouldnt have had this problem today.
Test biprop engines vertically!
All things considered, we are quite happy with the results
today. We will continue testing the
regen chamber with better injectors, leaner mixtures, and possibly higher
pressures, hoping to push the Isp up to 200 or so, and we are laying out what
we need to build a regen chamber for the six inch cat packs, which should be
good for up to 1000 lbf, which is a valid engine for a single-man space shot
vehicle in a differentially throttled cluster of four.
Regen engines have a couple minor drawbacks: a pressure drop
through the cooling jacket, and potentially trapped peroxide in the jacket when
you close the throttle. We may add a
nitrogen purge to the cooling jacket for that reason. The upsides are that you wont radiatively cook your vehicle, you
can run high pressures, and, probably most significantly, it was dirt cheap to
make. The TZM chamber started with a
$1200 piece of bar stock (to make two chambers), required $1200 worth of specialist
machining (again, for two chambers), then required a $3000 (for a single
engine!) silicide coating, and looks like it will require $100 worth of high
end components every time you take it apart and reassemble it. Each step also took a couple weeks to get
done. The regen chamber required under
$100 worth of aluminum and parts from McMaster, which arrived in two days, Russ
did the machining in a couple days, and the hard coating was $60, the minimum
order, done in two days.
We may have some difficulties melting out the throat on the
big engine with a simple cooling jacket, which may require going to a slightly
more complicated chamber design, but we are going to try the simplest approach
We need to get some high flow nitrogen regulators before
testing a bigger biprop(at the 100 acre site!). At a minimum, we need to be able to expel about one gallon a
second at 250 psi, but being able to expel two gallons a second at 500 psi
would also be nice. That is at least
8cfm of pressurized volume, or 133 cfm of standard pressure volume. We could guarantee a source pressure of 1500
psi, but we would probably have to swap bottles each run at that pressure. Anyone have any suggestions? Anyone know what the max flow rate out of a
standard nitrogen bottle valve is?
(water cooled, 30 second regen, high pressure, kaboom)