June 18 and 22, 2002 Meeting Notes
Joseph LaGrave (Saturday)
Two Inch Biprop
We made a functional mock-up in brass of the radiatively
cooled TZM chamber and tested it this week.
It has a fuel injector ring that clamps under the catalyst pack like our
one inch biprops, but instead of screwing directly into the chamber / nozzle
section, it bolts onto a dedicated clamp ring, which fits underneath a lip on
the chamber / nozzle. This gives more
even clamping pressure, reduces the machining operations on the parts that are
more likely to be consumed, and is recommended practice in the NASA SP on
radiatively cooled engines to reduce stress concentrations. The chamber/nozzle was a total of 3.8 long,
with a 1.9 ID, a 45 degree converging angle to a 0.5 throat, expanding at 16
degrees to a 0.9 exit for 3.24 expansion ratio, a decrease from our standard
4x expansion ratio because we planed on running only lower pressures.
We made Isp measurements, but because we dont have flow
meters on the fuel side, they are just pound seconds of thrust divided by
pounds of oxidizer. For peroxide, with
optimal O/F ratios of 7:1 or so, peak oxidizer-only Isp is about 15% higher
than true Isp, which will be found at a slightly higher O/F ratio. It was still a useful metric for us.
Because this was mostly intended to be a test for the
radiatively cooled TZM chamber, we didnt have a water jacket of any kind around
the chamber. For cooling, we mainfolded
together four machinists cooling spigots and arranged them to flood the
chamber with water while operating.
This did cause a lot of water to be kicked around in the exhaust jet,
which made some of the runs visually look rough, even when they were dead
smooth in the data.
The catalyst pack was the lightly-compressed test pack with
only 60 silver screens, which is a touch marginal for monoprop operation, but worked
fine for all the biprop runs. A 0.120 peroxide
jet was used for all tests, at a regulated tank pressure of 250 psi. There were indications that the jet should
have been a bit smaller when operating at 250 psi tank pressure, because there
was a slight increase in roughness when the regulator let the tank pressure
drop a few psi below 250. All the
biprop runs made between 40 and 50 pounds of thrust, taking around 12 seconds
each. A monoprop-only run gave a true
Isp of 96, which may be slightly low due to the marginal catalyst pack, but is
probably pretty close, because we only see a 115 Isp when we run at 600 psi
The first test was with kerosene and a 0.040 jet. It lit right up and ran perfectly smooth,
but was noticeably rich. We expected
this, as the smallest 0.018 jet on the one inch engine didnt seem lean yet.
We reduced to a 0.030 jet, and it ran a bit better, but the
indicated Isp was only 118, which means the true Isp was barely better than the
monoprop. We had seen better results
than that with the one inch motors, so we were obviously getting crappy
combustion. There were soot deposits on
one side of the nozzle, indicating that the fuel was not being mixed well. We are just injecting the kerosene directly
from the jet, so it is basically a single, high pressure stream that shoots
across beneath the catalyst pack and impacts the opposite wall. Scaling the engine makes this less and less
acceptable. We may be able to crutch it
with a much larger combustion volume, but we will probably have to make a
better injector for liquid fuels.
We decided to try the ethane again, which should have much
better combustion characteristics, due to gas/gas combustion. We only had one successful set of tests with
the ethane on the one inch engines, at 0.090 jet from 250 psi regulated ethane
pressure. Three other attempts to light
it failed for unknown reasons.
Our first test was a 0.120 jet with 350 psi on the ethane
regulator. It lit right up for a
perfectly smooth run with a 172 indicated Isp, and it was LOUD (noise is proportional
to the third power of exhaust velocity).
We repeated the test with similar results. We dont know conclusively why we had problems with it before,
but there were a number of potential second order changes: The temperature was at least ten degrees
warmer, which increases the ethane tank pressure, which helps the regulator
hold more constant. The catalyst pack
holder is now stainless, which retains a bit more heat. The combustion chamber has a slightly
Because that was about as much ethane as we could flow
through the existing setup, we replumbed everything for a higher flow
setup. We removed the regulator
altogether, so we would be running directly from the CGA 350 bottle connector,
through a 6 hose, to a pro-race solenoid.
This was a major change from the regulator to 3 hose to cheater
solenoid in the first tests.
Ethane specs: http://www.concoa.com/frames/technical/gases/ethane.htm
Without the regulator, the pressure will vary from 543 psi
at 70 degrees, to 708 psi at 90 degrees, above which it will scale like a normal
gas. It was over 90 degrees on our test
day, but the ethane tank wasnt that hot, and you could feel it being
noticeably cooled as we drew gas out of it.
It is worth noting that ethanes density is very poor, only 0.38
g/cm3 at 60 degrees, and worsening with increased temperature. This means that even with peroxides high
O/F ratio, the ethane tank would need to be 65% of the volume of the peroxide
tank, and capable of holding significantly more pressure, meaning the ethane tank
could easily mass more than the peroxide tank.
Another issue is that once all the liquid ethane has been vaporized, you
still have about a third of your total ethane mass as a pressurized gas, which
can only be extracted in blow down mode.
That isnt much of a hardship, because you want your thrust to tail off
towards the end of a space-shot burn, and you dont have the issues with
required injector pressure drop with gaseous injection.
Our first test with the new ethane plumbing retained the
0.120 jet on the ethane, and it did not light.
There was a noticeable hydrocarbon smell, so we were clearly flowing
much more ethane. We dropped the jets
down through 0.100, 0.080, and 0.070 with it getting closer and closer to
smooth combustion. The 0.080 and 0.070
runs would light, but run extremely harsh.
We thought there might be a fundamental problem with not regulating the self-pressurizing
ethane, but when we dropped to a 0.060 jet, it went back to perfectly smooth
combustion, with an indicated Isp of 172.
We dropped to 0.55, and got another perfectly smooth run,
but the Isp dropped slightly.
We increased to 0.65, and got another perfectly smooth run
with an indicated Isp of 184, a peak between the 0.60 and rough running 0.70.
Leaning it out a bit would probably give us a true Isp of
about 156, which is probably not too far off for a pressure ratio of only 12 or
We had basically all the data we wanted from this test
motor, so we decided to try a high pressure run to see if we could get an
indicated Isp over 200. We set the tank
pressure at 600 psi, which gave us a 280 psi chamber pressure on the small
motors. The chamber pressure would be
higher for the biprop, because the fuel is injected after the catalyst pack
pressure drop. We also expected to pick
up a little bit more Isp due to better combustion at the higher pressures.
It lit right up, and we saw our first ever overexpanded exhaust
plume, indicating that our chamber pressure was indeed quite a bit higher. Thrust was slightly over 100 pounds, and it
was running smoothly. After five
seconds of burning, a chunk of the nozzle came off. This wasnt all that exciting, just resulting in the thrust
vector spraying water in different directions.
I stopped both the peroxide and ethane immediately, then we checked out the
damage before running the rest of the peroxide through the engine in monoprop
We have decided to have our bar of TZM cut into two 6 long
chambers instead of three 4 long chambers, so we are sure to have plenty of
combustion volume. Our first oxidation
protection attempt will be a platinum plating, which requires an initial
flashing of nickel to adhere to molybdenum.
If that doesnt work out, we will try some form of silicide
coating. Molybdenum oxide is volatile,
so if the oxidation protection layer fails, the engine basically evaporates
http://media.armadilloaerospace.com/misc/burnThrough.mpg ( you can barely see the plume with the good
combustion and bright lighting )
Rocket Drawn Parachutes
We did a set of experiments with rocket drawn
parachutes. Our recovery system is
probably going to consist of both a rocket puller, and a backup piston ejection
system in case the rocket fails. We
test fired one of the commercial ballistic chutes for ultralights a while ago,
but we need an electrically actuated system instead of a pull-cord system, and
it will be good experience to put our own system together.
Our test rig consisted of a strip of aluminum with a lip
bent into the top end, and a wire cable attached to the bottom end. We attached various Aerotech solid rocket
motors to the top, abutted against the bent end, and we ran the wire cable down
to various parachutes we had around. We
dumped the ejection charges out of the single use motors we used, and we
greased the delay grain on the reloads so it wouldnt burn at all. We had two CATOs when we had the single use
motors attached to the aluminum strip with hose clamps, because the cases are
so fragile. We went to reloadable
motors, which have sturdier cases, and we just duct taped them to the aluminum
bar, which worked fine. For launching,
we just stuck the aluminum strip in a tube, bottle rocket style, with the
parachute lying loose to the side, connected to a long rope tied off to the
trailer. There was much cursing of the
damn copperhead igniters that I still had in my launch box. We will get some real igniters before we try
The first shot is of a D21 motor pulling a small rocketman parachute.
The second shot is of an F50 motor pulling a 16 diameter
military surplus parachute. When the
parachute came down on top of the motor, we burned a couple holes in it. Instead of using a metal strip to hold the
motor and cable, we will build a holder out of phenolic tubes, which will keep
any exposed surface from getting very hot.
The third shot is of a G64 trying to pull a very heavy duty
12 drogue chute. It failed.
Getting a proper parachute is now on the critical path for
the tube vehicle. I want to get a
pretty heavy duty chute of at least 20 diameter, preferably without too
terribly many shroud lines. We have the
ultralight chute if we need it, but it is rated for 900 pounds, which is three
and a half times the weight of the vehicle, so it would drift for a long time.
Tube Structure Tests
We finished all the assembly work on the tube vehicle. Total weight is 257 pounds dry, still
missing a nosecone and parachute.
The only thing that seems lacking right now is that lifting
the vehicle by the legs is giving some disturbing sounds from the bottom
bulkhead, and has bent the legs up a bit already. We have stainless backing plates on the top nut side of the U
bolts, but we will probably have to add backing plates between the leg bars and
the bulkhead on the bottom. I think we
will find that we now have round indentations in the plywood when we look at it
The vehicle is designed so that all the forces are
transmitted through the bottom bulkhead, which is backed up by a full-circumference
thrust ring above it. The parachute
attachment cords go through holes in the top bulkhead and centering rings, down
to the attach points on the leg U-bolts on the bottom bulkhead. The main engine and the landing shocks also
feed through the bottom bulkhead.
Our largest concern is that the tank takes all of its
acceleration loads through the rigid plumbing to the main engine. It is well placed with centering rings, but a
hard landing shock may push the top fitting into the engine. I am also a little concerned that there is
nothing to keep the tank from rotating, possibly loosening some of the
plumbing. A flange mount tank will be
welcome for future vehicles.
We did two sets of tests: dropping the vehicle onto the
shock cords, with it suspended so it wouldnt hit the ground, and dropping the
vehicle onto the ground to let the landing gear absorb the shock. The flight computer was running to log
accelerometer values, but it wasnt an ideal collection platform, because it
was only sending 20 packets a second. I
got instantaneous acceleration, and acceleration averaged over the sample
period, but it almost certainly missed the peak instantaneous accelerations.
The first shock cord drop test was not released very evenly,
so it was tilted when it pulled up taut.
This resulted in a significant jerk on one of the side axis, with a 5G
peak, as well as 4G on the vertical. It
had a few more 2G vertical bobs before settling down.
The second shock cord drop from a bit higher up had a tenth
of a second 6G vertical acceleration.
The third test showed an instantaneous 12G acceleration.
The shock cord drops were rather exciting, with a 250 pound,
metal finned vehicle bouncing around on the end of some ropes, but the landing
gear tests were pleasantly anti-climatic.
The vehicle just set down with a thud, with no noticeable rebound. The telemetry would show one frame of 4G
averaged acceleration, then a few minor oscillations. Im sure we missed the peak accelerations, but it makes sense
that the shock absorber decelerations will be much less than coming up with a
jerk on a Kevlar rope.
I need to make a high fidelity logging mode that keeps all
200 accelerometer samples a second, and we should try some of these again.
Next week we will see if the abuse has hurt any of the