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Tank testing, Engine problems, Software changes, Tube tests

October 22 and 26, 2002 Meeting Notes

October 22 and 26, 2002 Meeting Notes


Tank Testing


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 didn’t 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.



Engine Problems


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 didn’t 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 that’s 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 regeneratively heated.


Software Changes


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 doesn’t 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.



Tube Test


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 can’t 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 Oklahoma.




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