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July 31 and August 4, 2001 Meeting Notes

July 31 and August 4, 2001 Meeting Notes


In attendance:


John Carmack

Phil Eaton

Russ Blink (Tuesday)

Bob Norwood (Saturday)


Attitude Engine Tests




We test fired all four new motors today. None of them leaked, and they all produced the desired thrust, but we do have variable amounts of roughness in each of them.




The first engine was quite rough the entire time. The second started out very smooth, but abruptly got rough in the last quarter of the run. The third engine started mildly rough, and slightly increased in roughness as it ran. The fourth engine also increased in roughness during the run, and was slightly worse overall.


We should still be able to use them as-is, but it would be nice to get them all consistently smooth. We have seen a couple runs start out extremely smooth, then deteriorate and never regain the smoothness on future runs. We believe that it is related to compression and settling issues with the catalyst packs, but there could be other factors involved, like stripping silver from the top foam discs. The second batch of foam that we had plated was done a little thinner, which did make it easier to cut, but seems to allow it to strip off more.


We have a couple directions to try for improvement:


Compress the catalyst packs more. We can probably also remove some of the discs, and add a spacer under the retaining plate. A hose clamp will probably be about the right size.


Increase the injector pressure drop by either blocking off some of the middle of the etched metal injector plate, or adding a metering jet before the engine.


We will probably lose some thrust one way or another.


Another idea that we had considered before, that I am looking into again, is trying to find a non-compressible catalyst material, like a sintered silver disc, or one of the ceramic materials with a baked on permanganate.



Big Tube

We bought a section of 48” diameter sonotube to let us start prototyping some things for big vehicles down the road. It is never going to leave the ground, but it will let us try out a bunch of things without cutting up some expensive filament wound tube.


A 48” tube is bigger in person than I was expecting…


Phil in the tube for scale:




Big Engine


Russ is patiently cutting down a 100+ pound bar of brass into our 600+ pound thrust main lifting engine for the manned vehicle.


Bob had the quarter inch perforated plate for the catalyst retaining plate cut at a water jet shop.


Phil found a good local distributor of Fike Burst disks and assemblies, which we will be using one the big vehicle.


Six foot braided hose is long enough for the manned vehicle, so I have ordered four assemblies from McMaster. Delivery is estimated at nine days, so they probably won’t be here by next weekend.



Inclinometer Flight Test


So far, we have always been manually leveling the lander before each liftoff, because if it was uneven, the “zero position” would have it slowly acceleration in one direction. I have added support for using the 3-axis accelerometer as an inclinometer to initialize the angles before liftoff.


It actually worked better than I expected – as I smoothly increased the throttle, it leveled itself out before it even left the ground.




However, we are still experiencing an intermittent problem with the electronics box. During testing before the flight, we had a voltage drop on the non-solenoid batteries, which showed only six volts. The computer operates fine at that voltage, but the gyros start rolling whenever it drops below nine volts. The batteries were freshly charged, so it shouldn’t have been anywhere near that. After opening up the box and rebooting, the voltage was back up to 12.4v where it should be.


The computer cut out during the test, which is a completely separate behavior from an undervoltage. We curtailed the rest of our tests until I could try and find something wrong with the electronics.


When we pulled everything apart and opened the box up, the computer seemed to boot normally, but I had already unplugged the laptop and airport, so I didn’t completely check it out. One thing that occurred to me as a possibility is that the computer might have been fine, and we might have lost the network link. I had the AirPort base station sitting out in the hot Texas sun for fifteen minutes or so, and it might have cooked a bit. The flight computer explicitly cuts of all engines and exits if it loses network connectivity with the laptop. We need a way to distinguish this from a complete crash, perhaps by having blinking lights on the electronics box. Version 3 of the electronics box is going to have an LCD display for the pilot, so we may just wait for that.


I was unable to repeat either problem but when I took things apart more fully at home, I found two “possibles”. When we went to the dual battery power supply last week, we didn’t have any 14 gauge wire on hand for the battery terminals, so we had one connection that had a blue crimp terminal on a doubled up 18 gauge wire. I couldn’t get it to make anything misbehave by pulling on it, but it was visibly wiggling a lot more than the others. I will replace this with the proper wire gauge.


The other thing was that when I pulled the PC104 stack out to work on some more testing of the A/D board, the main power lines to the computer pulled out of their screw terminals. They might have been loose. I made longer leads and secured them as tightly as possible.


Another issue is that we left the bigger 3.4 Ah battery to run the solenoids, and we only had a 1.2 Ah battery for the rest of the electronics. We normally draw about 1.4 amps, so it is only good for about 45 minutes of run time at best, and that might be a heavier draw than it really likes. I have ordered three more of the 3.4 Ah batteries, so we can replace the smaller electronics battery and have a complete spare backup set.


I am working on rewiring the electronics box to more securely bolt down the power distribution terminals, which also could have conceivably bumped into something.


I did some more testing to try and reduce the noise we are getting on our A/D board. If I pull the PC104 stack out of the box, run it from a bench power supply, and hook a batter directly into the ribbon cable end, bypassing the breakout board and all the wire runs, then I get a noise range of only 10 units (on a 16 bit A/D), which would be decent. The chip docs say you should only get a variance of 5 units, but 10 would still be a huge improvement.


I wired up a new connector for one of the FOGs and wired it directly into the A/D board, and it saw a variance of around 130, which is only a tiny bit better than we see in the normal flight computer configuration. That’s only nine stable bits, which sucks.


If I hook the same FOG up to my 12 bit dataq, I get only LSB noise, the same as I get with a battery connected to it. One LSB noise on a 12 bit A/D should not show up as more than 32 units on a 16 bit A/D, and likely less. My only current guess to explain the differences in the battery and gyro behavior is that the 12 bit dataq, which can only be sampled at 240 hz, has some degree of internal filtering going on, and there is high frequency noise on the gyro signals. The breakout board has holes for adding resistors and capacitors to create simple filters on each channel, which we may experiment with.





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