September 25, 2001 Meeting Notes
Lots of damage repair on the big lander systems going
on. So far, it doesnt look like any of
the major components have actually been destroyed, just lots of brackets,
connectors, and cables between things.
I am wrestling with the decision to build a completely new
electronics box, or rebuild in the current one. I got the USB joystick working with the flight computer, but the
USB system seems to conflict with the ISA PCMCIA PC104 board we are using for
our wireless communication. We have
been planning to move to a separate wireless bridge instead of the 802.11b card
anyway, because we can then get higher power devices and we can shield the
entire PC104 stack when it doesnt have a radio on it. If I go to a new box, I can put all the
PC104 boards in a Versatainer, a really rugged and shock resistant
enclosure. I can definitely rebuild the
old box by this weekend, but Im not sure about building a new one by
The new electronics box would have several improved crash robustness
The shielded and supported PC104 stack. We have pulled boards apart during crashes
twice now, so it is clearly something we should fix, and hopefully we can reduce
our signal noise by shielding the noisy CPU bus.
Secured to the vehicle with broad ratchet straps, instead of
thumbscrews in weak brackets. If the
box had stayed on the vehicle, we wouldnt have broken all the connectors on it
during the crash.
Bob made an aluminum box corner bracket for me to securely
bolt all the fiber optic gyros to. We
have had the FOGs pull out of their angle brackets twice now, so it needs to be
fixed. It will also allow me to get all
of them case grounded together, where the previous arrangement didnt have one
axis properly grounded.
We are going to prepare to keep a drum of peroxide at Long
Range, because we may start going through ten gallons or so a week. I have ordered a drum spill containment
platform and some handling equipment, to be delivered directly to Long Range.
Ball Valve Throttle Tests
We did a series of tests with the ball valve open different
fractions on the test stand. The ball valve
has a pretty good sized vent hole in it on the inlet side, which probably helps
smooth the initial throttling curve a little over a pure ball valve, but it
still picks up real fast at the low openings.
The engine still had the 0.2 diameter orifice jet in it,
which limited the flow a lot more than the 0.5 ball valve, so we didnt expect
the upper valve range to make much difference.
All tests were at around 400 psi tank pressure, with one liter of
straight 90% concentration peroxide.
All the runs were smooth, and it looks to be fine for
throttling, but the useful range for us in really only from 25% to 50% ball
The performance difference between the 84% mix (4 parts 98%
and 1 part distilled water) we had been using and the 90% stock we are now
using was extremely pronounced in the large engine. The second test on September 15 was the same plumbing as the wide
open test today, but today it made nearly 100 pounds more thrust, which is the
cause of our crash last weekend. The
difference isnt normally so pronounced, but this engine is sized for an
unrestricted thrust of over 600 pounds, so at these jetted levels it is very
over expanded, which enhances changes in chamber conditions.
After doing these runs, I set up the flight computer to have
the ball valve chase the joystick throttle position, so we could see what it
feels like manually controlling it. We
dropped the turnaround delay for the motor drive from 100 milliseconds to 20
milliseconds, and we didnt smoke anything.
We loaded some peroxide and run it with the throttle on the test stand
without any problems.
The big engine is starting to channel now, so we are going
to have to mess with the catalyst pack this weekend. We are starting to suspect that our pack wear problems arent a
result of stripping, but a result of the pack getting crunched more and more
due to thermal expansion, and eventually opening up gaps at the sides. We can hear it rattle a bit in the big motor
now. We are going to try some experiments
with anti-channeling rings on or underneath our spreading plate.
Master Cutoff Valve
Before we intentionally fly a vehicle that would continue
flying after a power outage or computer crash, we plan on having an independent
micro controller and valve that can redundantly shut off peroxide flow to all engines.
This wouldnt have helped last weeks crash, which shut off
power just the way it was supposed to.
The idea is that if the attitude control goes away for any reason, it is
better to remove all thrust and fall, rather than continue thrusting in a
potentially wrong direction. This does
nothing for low-to-the-ground pilot safety, but will be important for later
high altitude, parachute recovery scenarios, and should hopefully make some
regulatory challenges easier.
Russ noticed that the motors can be driven with a higher
voltage to make them operate faster.
Our normally 0.8 second actuation time valves that are designed for 12v
operation will open nearly twice as fast at 24v. You probably wouldnt want to run a motor long like that, but it
didnt seem to have a problem 0.4 seconds at a time. We will probably do this for the master cutoff valve. Someone told me a while ago that the NOS
nitrous solenoids that we use are made with coils designed for 6v operation,
which is why they are very fast at 12v, and also why they get very hot if left
Russ is going to build a custom board for our master cutoff
computer using the embedded processors Long Range uses for their products. I had done a prototype with a basic stamp,
but it will be better to do a full-custom board incorporating the DC motor
drive circuit and the watchdog microcontroller. It will take one pair of lines running from the main computer
solid state relay board, it will have its own battery, and it will have two
wires going to the master cutoff DC motor.
On power up, it will close the valve.
If it sees a toggling watchdog signal every four milliseconds on its
communication wire, it will open the valve.
It will begin closing if it ever goes ten milliseconds or so without
seeing a change in the signal line.
The flight control program will normally not fire the
watchdog signal, so the master cutoff valve will be closed. When the pilots trigger is depressed, the
watchdog signal will begin, causing the master cutoff to open and feed the
attitude engines and the main throttle valve, which should still be closed
until throttled up. On trigger release,
the watchdog signal will stop, so the master valve will close simultaneously
with normal solenoid shutoff and main throttle closing.
We need a separate burst disk between the master cutoff and
the engine throttles, because peroxide can be trapped there. We may want a flight computer option to
crack open the main engine throttle while the master cutoff is closed, allowing
peroxide to dribble out into the engine instead of heating up in an enclosed