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September 25, 2001 Meeting Notes

September 25, 2001 Meeting Notes


In attendance:


John Carmack

Phil Eaton

Russ Blink

Bob Norwood


Lots of damage repair on the big lander systems going on. So far, it doesn’t 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 doesn’t 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 I’m not sure about building a new one by then.


The new electronics box would have several improved crash robustness features:


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 wouldn’t 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 didn’t 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 didn’t 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 travel.


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 isn’t 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 didn’t 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 aren’t 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 wouldn’t have helped last week’s 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 wouldn’t want to run a motor long like that, but it didn’t 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 on.


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 pilot’s 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 space.





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