January 29 and February 2, 2002 Meeting Notes
Joseph LaGrave (Saturday)
We did a lot of engine test firings this week with the 1
diameter by 2 long catalyst pack motor.
1: Initial pack length of 1.5 (two 0.25 spacers),
6/64" jet, low pressure. Cloudy result, but smooth thrust.
2: Reduce jet to 5/64", same result, although clearing
a little towards the end.
3: Replace one 0.25" spacer with more screens.
4: Double pressure. Thrust doubles.
5: Go back to 6/64" jet. Little increase in
6: Increase jet to 8/64". Little increase in
7: Double pressure again. Thrust doubles again.
8: Long run at high pressure. Stays level and smooth
the entire time.
All screens were 20 mesh, alternating stainless steel and
pure silver. We bought pre-cut
stainless screens from McMaster, which makes life much easier. We also found that using a soft plastic
underneath the silver screens when we are punching them works much better than
the hard polycarbonate we were using before.
We were not using a spreading plate at the top of the pack
at all, just relying on the screens to spread the peroxide and the jet in the
fitting leading to the engine to meter the flow. This may not scale to larger motors, but it worked very well on
These were probably the smoothest runs we have ever seen,
but I am rather mystified with some of the results. Increasing the
pressure seems to give a directly proportional increase in thrust, but the
thrust stays constant across the entire run, even on the last run where the
ullage pressure decayed nearly 100psi. The thrust didn't even vary with
the square root of ullage pressure, as we have seen before on tightly
The only thing I can think of is some interaction with the
solenoid, so we will do future tests with the big ball valves.
While it was curious that the first clear run produced
exactly the same thrust and smoothness as the two previous cloudy runs, it did
run for half again as long on the same amount of peroxide. That is
probably a pretty good failure mode for an engine -- when it clouds up, your
Isp drops from about 115 to 75.
It was interesting that the initial runs were cloudy with
only 9 pounds of thrust, but after we got it running smoothly by adding less
than 20% more screens, it was able to cleanly make 40 pounds of thrust at high
pressure. Two possibilities are
suggested, both with some supporting evidence:
The screens take a while to get activated. The end of the second run did start clearing
up, which might have made the next run clear without adding any more
screens. The tests on Saturday also seemed
to warm up a lot quicker on the used pack.
The fresh silver packs could gush raw peroxide a couple times before
warming, but even after we had flushed the pack with water, the last run on
Saturday was hot after only a short pulse.
Some of that may be due to the different valve, but even a cracked open 1/2"
ball valve probably flows a lot more than the solenoid.
A large part of the activity takes place towards the end of
the pack at this size. When we took the
motor apart on Saturday, the last quarter inch of screens had been thermally
shrunk, and much of the pack above that looked relatively untouched.
We did a lot of work on the test stand in preparation for
biprop testing. The polycarbonate
shield is now bolted to the stand around the engine, and there is a second,
taller post so we can strap both tanks to the stand at proper heights. All plumbing that is used in the fuel side
of things is brass, which we never use with peroxide, so we wont accidentally
mix them up. The intention is to start
a normal monoprop run with the computer, and manually pulse open the fuel
solenoid when it is running clear. We would
only be doing this for a few seconds at a time, which the engine should be able
to take in a heat-sink mode, because it has 0.6 of brass all around the
chamber, and more around the nozzle.
We used gasoline today, but we are intending to use gaseous ethane
as soon as we get a bottle of it.
Working with a liquid fuel and another pressurization system is a pretty
big hassle. A gaseous fuel that wont
puddle and self pressurizes will be a big advantage.
We replaced the solenoid with the ball valve for the tests
today, and did most of our tests at low pressure, in preparation for biprop
tests. After the great runs on Tuesday,
we didnt get a single clear run today.
We did try pulsing some fuel into cloudy peroxide runs, but it didnt
light. When there is un-decomposed
peroxide coming out of the engine, that means the chamber temperature is below
the vaporization point, which is well below the autoignition temperature of the
1: 200 psi, 8/64 jet.
Rough run, never cleared up. We
were expecting the rough run, because the lack of significant changes in output
with jet sizes on Tuesday implied that the solenoid was doing the metering, and
that the jet was much larger than needed to flow the given results.
2: change to 5/64 jet.
Still cloudy, but started smooth for a few seconds before turning rough. We are confused. Roughness at end possibly due to tank swirl that only shows up
with ball valve?
3: close up a jet (with a hammer) so that it is even smaller
than the 5/64 jet. Less thrust, still
4: I forgot what we tried here. Same results as 3.
5: higher pressure (400 psi). More power and still smooth, but still cloudy.
We took the engine apart to see if there was anything
noticeable inside. The bottom 15 or so
discs came out easily. We suspected
that the motor might have stretched on the hot end like the little one we fired
a couple weeks ago, but the dimensions were still correct. That means that screens had crunched
themselves against the motor due to different expansion ratios. We pressed out the rest of the screens, and found
that the anti-channel rings were no longer a tight fit either.
6: Reversed pack,
putting the anti-channel rings and normal dimensioned screens at the end. It ran cleaner, but still not clear, and it
was mildly rough. The roughness wasnt
too surprising, because we discarded the first five screens that were mashed up
by the process of pressing the pack out of the engine, so the new pack probably
wasnt compressed as much as it should have been.
Our current theory is that the final long run on Tuesday ruined
the fit on the anti-channel rings and the screens at the end of the pack, so
all the runs today were channeling some peroxide around the outside.
We are going to try using gapless retaining rings as
anti-channel rings next week, probably putting them through the entire pack,
every ten screens. They will probably
lose their spring after firing, but they should still be good fits.