November 13 and November 17, 2001 Meeting Notes
On Tuesday, we were testing a couple minor modifications to
the flight control software that should make it a little easier to manage the
throttle. I reduced the turnaround
delay for the motor drive from 100 msec to 50 msec, and implemented a scaled
remapping of the throttle to ball valve position.
We did a quick hop for checkout, then loaded up for a full
length run. Shortly after liftoff, a
plume of peroxide started escaping from one of the engines. I set it down, and in the process of venting
of the rest of the peroxide, one of the other engines started doing it.
We knew pretty quickly what happened the four attitude
engines arent completely interchangeable with the tops, and two of them were
misaligned enough that we couldnt get one of the bolts in it. The O-ring had blown out at the point of the
We drilled the bolt holes in the tops out slightly oversize,
which allows them to fit in any orientation, and we also replaced the rubber
O-rings, which got very hard after firing, with high temperature silcone
Since we were taking the engines apart, this was also a
perfect time to test the chem.-etched injector spreading plates that ERPS had
made up for us. The micro-perf sheets
that we had been clamping at the tops of the motors was 7% open area, which was
quite a bit too much to provide enough pressure drop for smooth engine
operation. The ERPS plates are thicker,
and have far fewer holes in them.
We couldnt get test stand data because of a problem with my
new laptop, but we ran it once for a basic check, and it seemed to run well.
By Saturday, I had addressed the test stand issue, but Im
not real happy with it. Microsoft
changed the behavior of the serial DTR line with windows ME, where previously
it was off when not in use, but now it stays on from the time you boot. Our test stand electronics use the DTR line
to trigger the test valve, so as soon as it was hooked up, the valve would stay
open. In looking around online, several
other devices, like radio push-to-talk triggers, are also having problems with
this. Microsoft apparently has a fix
for it, but you need to sign up for a support contract to get it. For whatever reason, the couple fix programs
I downloaded online only temporarily cleared DTR for me, so I had to try and
work something out myself. I eventually
found an awful rain dance that does solve it run a little program that
clears DTR and spins in place, run the test stand program, which will fail with
a sharing violation trying to get the serial port, the ctrl-C break out of the
stub program, and DTR will stay off for the rest of the session. Blech.
We had a couple issues getting the runs working properly on
the test stand, but when we finally got it all worked out, the small engine
spreading plate turned out to be undersized enough to cut the output from 60
pounds to 30 pounds of thrust, and the big engine had a bit too much injector
area, because the thrust still tracked the pressure blowdown directly, instead
of falling off with the square root of pressure, as usually happens with
sufficient choking. However, both
engines are operating smoothly, so this seems to be a pretty good
combination. We were having some
solenoid trouble on the test stand, so the small engine thrust may not be
correct. In any case, we dont mind
losing thrust on the attitude engines, because we still have an excess of
The Doppler radar speed sensor that we had been waiting for
showed up on Friday, and I was all set to push really hard to try and get
auto-hover and auto-land operational for Saturday, but it turns out that the
sensor only returns the absolute value of speed, not including the
direction. That makes it useless for
our purposes, but I am hoping the company (GMH Engineering) can modify one for
We did a couple test flights, and everything worked fine
with the control changes, but it is still harder to smoothly fly and land with
the throttled central engine than when we were using the attitude engines for
throttling. I am hoping that computer
control will work all that out when we get a speed sensor.
We did have some link quality problems with the 802.11b link
between the laptop and the flight computer, so I moved a little closer outside
to get it to clear up. I have ordered
two full-watt units from Esteem , which should give us several miles of range,
even with omni antennas.
Bob brought the new dual tank frame over so we could adjusted
the seat position for proper balance.
Our new tanks should be here soon, so we will probably be flying this
vehicle relatively soon.
Mark Henry, the ex-propulsion lead for Beal Aerospace
stopped by on Saturday to talk with us, and we got a lot of good tips and
information from him. A few notes:
He favors a special silver plating process on stainless
screens, including the samarium nitrate coating that some literature had
reported as unnecessary. He said they
could get effectively unlimited catalyst life, and 40 msec startup times. Apparently the difference between pure
silver screens, which we found to not work very well, and the special plating
process is quite profound. We are going
to buy some packs from him to experiment with.
Anti-channel rings should be fairly thick, and made out of
the same material as the motor casing so that they expand with the case, rather
than the pack.
Silica-phenolic ablative nozzles were heavy, but very easy
to make and work with. Carbon-phenolic
nozzles lasted less than 20 seconds with peroxide engines.
He was fairly down on hypergolic miscible fuel peroxide
combinations, mentioning the sever motors that have been blown up in
experimenting with that.
If you are setting up a really large-scale concentrator, 90%
peroxide comes out to only about $0.50 a pound.