June 25, July 2 and 6, 2002 Meeting Notes
Tube Vehicle Work
We added metal reinforcement plates to the bottom bulkhead
to keep the fin attachment tubes from pushing into the plywood as much. This helps, but it still complains a bit
when carried around by the fins, so we have been trying to move it other ways
when possible. Joseph can carry it from
the parachute harness with his bobcat, and we can get a dolly under the engine
bell and walk it around, but neither is really ideal. We should probably design vehicles so they can be moved with a
I havent been real fond of the fins in general. When (not if) we break the fins off of this
vehicle, we will probably rebuild with a flare bottom as the stabilization
mechanism. Flares give more drag for a
given amount of stabilization, but it should be more robust.
Deakins Metal Spinning http://www.metalspinning.com/ is
getting a quote for spinning the nose cone / upper fairing assembly for the
vehicle out of 0.050 aluminum.
We have decided to use the big 30 parachute for this
vehicle, which will hopefully bring it down soft enough to not break
anything. It is designed for a 900
pound load, and we are going to be under 300 pounds, but we want a landing soft
enough that the vehicle doesnt sustain damage, not just soft enough that it is
survivable for an light plane pilot with a crunched airplane. We are going to use H motors to deploy the
For hover and auto-land testing, we cant just have the
altimeter strapped inside the tube, looking down through the bottom, because it
doesnt get a reliable signal until it is five feet away from the ground. For testing, we strapped the altimeter to
the top of the tube. One incident of
note was that when I noticed the altimeter signal acting much noisier than
usual, it turned out to be due to a loose strap blowing under the optics.
In preparing for a hover test, we balanced the vehicle as
well as we could by blocking it up under the engine bell and tipping it each
way. We would up adding about 10 pounds
of lead ballast at the top to center the CG over the engine.
We were all set to do a captive hover test on the vehicle,
when the computer flaked out. This had
happened a couple times recently, so we stopped work to see if we could get a
definitive answer to the problem. I had
recently changed the electronics box back from the tall stack of PC104 modules
in a VersaTainer to our older SBC with only two modules stacked on top of it. I had several reasons for this: the PC104 CPU had started losing its
configuration data when left alone for a while, the SBC was slightly faster
(133mhz vs 100mhz) which may keep it from being maxed out and dropping the
occasional sensor sample, and the SBC has four serial ports on board, which we
need to add the GPS along with the IMU and altimeter. The drawback to this computer is that it twiddles the parallel
port bits during the boot cycle, which causes solenoids to fire if attached to
a vehicle. We may want to add a
separate switch for actuator power, which we only turn on after the computer
When it first falked during tests last Tuesday, it acted
like the battery was too low, continuously resetting but not getting into the
boot cycle. Because of another issue
with the configuration of the new A/D board on the system, I didnt have the
battery voltage logged to check. That
weekend, it happened again, and I knew the battery was almost fully charged,
and it in fact came back to life after I opened it up and prodded it a little. My theory at the time was that one of the
not-very-well-tied-down serial cables might have shorted something out and put
it into a reset condition. I made a
nice little bracket to mount all the DB9 and DB25 connectors on, which I was
hoping would solve the problem.
When it happened again, we finally nailed down the real
problem this computer draws more power, and we are jumping over the two amps
that our 5v power supply can provide.
It may boot, but once it heats up a bit, it drops the voltage and it resets. When we used this computer in previous
electronics boxes, we had a different power supply. We considered cobbling something together so we could still do
our hover test, but we decided to just let Russ make a new board with a much
higher margin power supply.
Once we do some hover tests, we plan on flying this to
parachute deployment altitudes. The
Washington AST offices attempt to help with our local FAA waiver seems to have
failed, so we are more or less resigned to doing our first mid altitude flight
test at the Oklahoma Spaceport, even though it is a six hour drive. We should be doing this next month. The goals for the first test series will
just be to fly light propellant loads to depletion, so we can see what the aero
loads look like to the computer. The
only thing of note will be that it will be an actively guided launch, so it
will fly without a launch rail of any kind.
We have all sorts of ways of determining altitude for
parachute deployment GPS, laser altimeter, integrated accelerometers, and
barometric (I have a sensor, but I have never integrated it). However, the current plan is to use a manual
telemetry triggered deployment command, and only have a backup timer deployment
that fires if telemetry is lost for some reason.
Later flight tests will involve leaving a propellant
reserve, and reengaging the engine for stabilization and impact attenuation
before landing. We will need to cut
away the parachute to keep it from tipping the vehicle over after landing, and
if there is any significant drift, it may need to cut away even before landing
to allow it to kill horizontal velocity.
We dont have any form of ground speed sensor that will allow us to do
that automatically yet, because the GPS update rate is too low. It may require a blended GPS/inertial
system, although some form of radar may work better.
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Two Inch Biprop Work
I built a new fuel injector ring that has a complete annular
manifold around the chamber, and a small 0.005 gap between the fuel ring and
the combustion chamber that feeds from the manifold. This should give an almost perfect distribution of gaseous
ethane, but will probably be too low of a velocity for kerosene
atomization. It will also cool the
injector ring better.
We were planning on testing an immersion cooled chamber,
where we just have the entire thing submerged in a bucket of water. This definitely doesnt work for high heat
load engines, but we are curious if it may work with low pressure peroxide
biprops, which run cooler than most rocket engines. We made the chamber out of aluminum for good thermal spreading,
and we want to try hard-anodizing the inside to reduce heat transfer into the
metal, as the Agena upper stage engine did.
We tried a new aluminum alloy for this: 2011. Russ reported that he could easily take 0.125 cuts on this with
excellent chip control, and he turned out the complete chamber in an hour and a
half. In general, we use 7068, the
strongest aluminum, when possible, but it is only available in bar stock. 7075 is the next strongest, and can be had
in more forms, including plate. If we
dont need maximum strength, and we have to cut a lot of metal, 2011 looks
good. If we try a regeneratively cooled
2 motor, it will probably be made from 2011.
We only use 6061 when we need a specific form, like the thick wall
structural tubes that our fins mount on.
Before trying the immersion cooling, we wanted to get
parameters for a clean run on the horizontal test stand with water jet cooling,
because we had both a new injector, a longer combustion chamber, and I had
slightly jetted down the peroxide from 0.120 to 0.110. I had realized that the 0.120 jet that we
had been using on the test stand with the 2 cat packs was only appropriate
when we were making 100 lbf of monoprop thrust, which is a lot more than we are
flowing during the biprop tests.
The biprop runs that we were calculating Isp from were
extremely smooth, but when the ethane mixture was richened, it would go into
extreme roughness. At the time, I
thought it was due to either the ethane evaporation process or poor combustion,
but in retrospect, it was probably due to the peroxide flow getting feedback
when the flow rate dropped on biprop ignition.
Peroxide flow decreases at least 30% when it goes into biprop mode. The indicated (oxidizer only) Isp was
increasing up to the point where it went rough, so we likely did not find the
peak in our tests, and we were probably running lean and hot.
The initial drop to 0.110 was way too tentative, and we did
still experience rough biprop burning.
I dropped it to 0.080, and the times we did get ignition were all
smooth. Now that I have gone back and
referred to my earlier calculations at the April 20th 2002 update,
it looks like the peroxde jet should be 0.060 to get a 22 psi pressure drop
across the jet. When we have pressure
drops like this, we reliably get smooth runs.
In many cases, we still get smooth runs even without any effective
jetting (like the smooth biprop runs with a 0.120 jet) due to the catalyst pack
providing damping, but we cant count on it.
We had a lot of solenoid problems that basically invalidated
almost all the testing we accomplished.
There were three distinct issues conspiring against us:
Some times, there wasnt enough current to open both peroxide
and fuel solenoids together. This was
likely more the fault of the wiring than the battery, but we hooked two
batteries together to get enough push to reliably open both. We have had enough battery issues that I am
going to put a high current 12v power supply on the trailer for running the
solenoids, instead of relying on batteries.
A 50 amp auto battery charger will probably work well.
The pushbuttons started to fail on us. They were from Radio Shack, and really werent
up to the 8.8 amp draw of the solenoids.
We finally tossed out that box and moved to one that we use for manually
firing vehicle engines, but I am ordering a big button box designed for heavy
machinery that will positively make this problem go away.
We had bizarre flow issues, where even monoprop runs would give
different (but still smooth) thrust levels on different button pushes. We changed out the solenoid, and when it was
taken apart, we found there was a small piece of hard white plastic stuck at
the valve seat. We had seen this exact
problem a long time ago, and we had thought it was a fragment from one of the
manual ball valves that had started leaking.
We have a completely different plumbing setup now, but one of the
spring-return ball valves on the fill manifold has been leaking a bit, so I
wouldnt be surprised if that is the problem.
The problem is likely due to some peroxide wicking into a tiny area in
the valve seal, then decomposing and cracking it, but it might also be due to excessive
vacuum during loading, because the valves are only rated to 29 Hg, which is a
97% vacuum. Does anyone have any
experience with vacuum failure modes of ball valves?
So, with all these issues, almost none of the data is likely
worth much, but here are the runs we did on Saturday, all at 250 psi regulated
0.110 peroxide, 0.055 ethane (no regulator, high 80s
temperature): did not light
0.110 peroxide, 0.044 ethane: lit briefly right at
peroxide depletion, so we assume too lean
0.110 peroxide, 0.070 ethane: lit briefly after several
0.110 peroxide, 0.080 ethane: lit, but ran very rough. I let it go, and it melted a hole in the
aluminum chamber where it wasnt getting much cooling water.
Russ made another chamber quickly, and I cut the peroxide
jet down more to avoid roughness.
0.080 peroxide, 0.080 ethane: lit briefly after several
0.080 peroxide, 0.070 ethane: lit after several tries and
0.080 peroxide, 0.060 ethane: did not light
0.080 peroxide, 0.060 ethane, mess with electronics: good
smooth burn after a couple tries, but an odd discrete step in the biprop
0.080 peroxide, 0.060 ethane: did not light
0.080 peroxide, 0.070 ethane: did not light
We were at the last of our peroxide drum, so for our final
run we tried a 0.030 kerosene jet. The
solenoid was obviously plugged up again, but it still lit. Kerosene lights far, far easier than Ethane,
but we have yet to demonstrate a high Isp run with it.
I will have a high amp power supply and good pushbuttons for
our next test, but we need to find a good inline filter for our loading
system. We need an all-stainless (ideally
316, 304 acceptable) liquid filter that wont trap liquid, and can stand up to
at least 600 psi (ideally 1000) during loading. McMaster doesnt seem to have what I want, so I am open to
suggestions from anyone. I may buy
something low pressure that we can use for our biprop tests, but we will need a
high pressure one for our vehicle tests.
The machining of our TZM radiatively cooled chambers will be
finished next week, which we will send them off for two different oxidation
protection coatings. We are going to
have Dallas Precious Metals platinum plate one of the chambers, and http://www.hitemco.com/ put a silicide
coating on the other one. The goal is
to be able to do 80 second biprop burns multiple times.
We are considering making a heat soak chamber for the 5.5
engines, which will allow us to get some experience with the vertical test
stand, and give an intermediate step before the 12 engine. We do hope to have a fully functional
3000-5000lbf reusable biprop by the end of the year. If it is regeneratively cooled, we will be able to run higher
pressures, but a radiatively cooled engine will probably be limited to around
150 psi Pc to keep the heat loads acceptable.