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August 18, 2001 Meeting Notes

August 18, 2001 Meeting Notes


In attendance:


John Carmack

Phil Eaton

Russ Blink



I have added dust caps to all the AMP CPC connectors on the electronics box. They aren’t waterproof, but it is still an improvement over having open pin holes to the inside of the box. We did have once case a while ago where hosing down the engines got a few drops of water inside the box.


I have tested a 0-15 psi absolute pressure sensor for barometric altitude determination. This will be integrated into the electronics box to supplement GPS before we do a high altitude launch. Notably, this sensor is 1/10th the price of most other Omega pressure transducers, because it is used in automotive manifold applications.


We now have a delivery account set up with a local nitrogen distributor, because we were up to refilling our tanks every week. We reworked the fill cart some more so that it can handle the largest high pressure bottles. One bottle is still only good for a bit more than two fillings of the big lander tank to 600 psi. Once we get the main lifting engine in, we will probably reduce the pressure a fair amount, and loading more than a gallon at a time will also reduce nitrogen use, so we will probably get three flight tests per nitrogen bottle change.


I wanted to cut off a 2’ section of the sonotube so we could lay it flat for evaluating some things. I started with a cutoff grinder, but that made a smoky mess of the cardboard tube, and didn’t cut very well. We got a little jig saw from home depot, and that went through it pretty quickly.


Phil was working most of the day on drilling and tapping all the flange holes for the 700 pound thrust motor. We didn’t quite have enough time to fire it, but we should on Tuesday. We will run it initially with restricted plumbing, so it is only expected to make 150 pounds of force. When Russ finishes the 1/2" tank manifold, we will water test it flat out with the big valves to see if we rupture the spreading plate, then we will take it out to our test range for a flat out run.


Big Lander Hops


Analysis of the last test hops showed that the control law gain that was appropriate for the small lander was too high for the big one. While the total thrust to weight is the same, the polar moment is relatively smaller on the big one, and the lever arms are longer. The high gain was making it far overshoot the desired rate when making corrections, which caused it to far overshoot again on the way back. It flew, but it was very lurchy.


The first hop today was with the gain dropped from 0.025 to 0.01, with four liters of peroxide at 500 psi.


It couldn’t get stably into the air with the gain set this low and the offset CG, and it tipped over again on landing.


We decided to add the outrigger legs to the vehicle to avoid tipping it again. That added 40 more pounds to the vehicle, bringing it up to 175 pounds dry. We didn’t add the cross braces, which would have tacked another 40 pounds on it. It looks like the extension legs are strong enough for the attitude engines, but a very hard landing might bend something without the braces.


I left the gain at 0.01 for the next hop, because the engines had moved much farther out, increasing their torque. We filled the tank to 600 psi to compensate for the additional mass. It got into the air, but it started rotating almost immediately. It turned out that we had canted the engines the wrong way. Anything that can be backwards, will be. We drew arrows on the legs to hopefully prevent this from happening again.


There was still a fair amount of overshoot at this gain level with this engine configuration, but it was less than at 0.025 with the short legs.


For the third hop, we corrected the roll angles and I dropped the gain slightly to 0.008.


The engines started clouding up again on this run. They still made effectively their full thrust, because I was able to lift it off the ground, but I set it back down quickly because my view was getting covered up. This gain level was probably a bit too low, because the lander had a very lazy correction rate. I think I will leave it at 0.01 for our next set of tests.




We still aren’t sure why these engine cat packs aren’t lasting very long. The cat packs in the small lander have run dozens of times without problems, but the big ones seem to be wearing out after four or five runs. The foam was plated slightly differently on our current batch, which is the leading guess. The foam in the old packs was plated so thick that the foam would crack when bent, while it is more flexible on the current foam. The other difference is that the old packs were highly compressed, nearly two to one. It would be nice to get rid of the questions regarding plating. We still have pure silver screens we could use, and I have made another inquiry to a different company about pure silver porous materials.


We need to increase our minimum propellant load to at least six liters for each hop to get a reasonably long hover time. Once we get a lifting engine in the center so we can tolerate more pressure drop, we should be able to load up to 16 liters or so. With a pilot, that will still only be about 15 seconds of flight. We are probably going to get a longer tank to extend that.


I am working on a completely different flight control algorithm that treats the solenoids as binary devices, instead of using PWM to simulate a proportional valve. This won’t really have a gain parameter, and should be adaptable to a wider range of vehicles, but it will rely on very low latency in the sensing and actuation. We will probably test this out first on the little lander to save peroxide during the tests.






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