October 22 and 26, 2002 Meeting Notes
We hydrotested the 60 gallon Structural Composite tank http://www.structural.com/base_pages/composite.htm
. The tank is rated for use at 150 psi,
with a 600 psi burst. Russ made tight
fitting aluminum closures for both ends, but that is still the expected failure
point, because the threads are directly into their reinforced polyethylene
liner. We filled it completely up with
water and took it incrementally to 325 psi.
It wad some disturbing snap-crackle sounds when it was being pressurized
up the first time, but after that it didnt complain at all.
These tanks weigh a bit under one pound per gallon of
capacity, and they were very cheap -- $350 each in this size.
We fired our first engine on the new high capacity vertical
test stand, and we ran into some problems.
The new engine is a 5.5 diameter cat pack, like our existing main
engines, but it has a 2 diameter throat with a 1 radius, as opposed to our
previous 1.75 throats with a fairly sharp bend. Even after a number of warmup pulses the engine would never run clean. We let it sit until Saturday, hoping that
the now-wetted pack would season, but it didnt get any better at all. We recompressed the pack, adding screens and
spacers until it was at the maximum 5000 psi gauge reading (about 10,000 pounds
total force, or about 450 psi on the pack a little light, but thats all the
gauge goes to) while we tightened the bolts down. That got a little better, but not much. We thought it was clearing up at the end, so we went ahead and
moved the engine to the tube, but the flight tests were still poorly catalyzed.
We initially suspected that the larger throat motor was
having trouble building backpressure, but we are now fairly convinced that it
is the cold weather. It was down to 60F
this week, quite a lot colder than anything we have run for the better part of
a year. Thinking back, we had a lot of
problems last winter as well. The clincher was that the attitude engines,
which had fired perfectly last week, took a very long time to clear up
today. We are considering warming our
peroxide, and possibly designing pre-heating motors that are sort of
I made two changes to the flight computer software:
There is now a -detach option that lets the software
continue operating without a controlling terminal. So far, all of our flights have been initiated by just telneting
to the flight computer over 802.11b, and launching the flight software from the
command line. I have been concerned
that if we ever do lose telemetry link, that the TCP connection for the telnet
session could block the software. The
actual telemetry and command/control is done with UDP, but there are some
informational messages which get sent to stdout. With the detach option, I can close the telnet session
completely, and leave all communication as UDP.
I added full inertial velocity and position tracking. The Crossbow inertial unit I am using doesnt
have enough accuracy in the accelerometers to use this for hovering purposes
(the vehicle position starts drifting almost immediately), but it will be good
enough to back up the GPS for apogee determination and some recovery purposes
for higher flights.
We finished everything necessary to hover test the tube -- carrying
hooks for moving it around, a bulkhead fill port, final engine installation,
and Russ put some paint on it. We pulled
it out and tethered it down for hover tests, but the main engine was still not
catalyzing well enough to lift the vehicle, just scoot it around a little on
the ground. We were only running 300
psi, but the vehicle only weighs 290 pounds, and even the old engines with the
smaller, sharp throat made more thrust than that.
We are loading peroxide in a different way for this vehicle,
because the tank liner cant take a vacuum.
We load the peroxide into our normal test stand tank, then pressure feed
it into the tank through a vent tube on the bottom. This works fine, although we are definitely noticing the huge
nitrogen consumption of the big 60 gallon tank (which we are only loading 5
gallons at a time into right now).
Assuming we get the engine running cleanly with net thrust,
we only have a few things left before flying it to altitude: finish vehicle parachute
straps, parachute coupler sanding, and multiple parachute launch tests. Our local FAA office continues to ignore us
on our waiver application, so it looks like we will probably have to fly it in