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Hover tests

February 14, 2005 notes

February 14, 2005 notes


Hover Tests


Our first attempt at a hanging hover test showed that we did indeed suffer from the two problems we thought might show up: the engine ran rough above 60% throttle, and the GPS lost lock due to vibrations. Interestingly, Elon Musk at Space-X had mentioned to me that they reproduced the exact same GPS vibration issue we saw. He was also able to pass on some relevant advice: bonded rubber isolators really don't work very well to reduce vibration, but wire rope isolators do a good job. The same line of wire rope isolators that we used to use as landing gear scales all the way down to tiny ones for isolating electronics components in these situations. I ordered some of the small isolators, but unfortunately our electronics are hanging vertically, so the compact wire rope isolators were supporting the GPS in shear, which didn't work out well. We wound up going with our previous solution of wrapping the GPS board in foam and sticking it to the board.


We already had the bleed solenoid plumbed up to the vehicle to address the roughness by venting ullage nitrogen into the top of the engine, so I changed the code to automatically enable the solenoid whenever the throttle exceeded 40%.


Russ finished the isolated voltage signal circuitry, which took us a day to get working properly. I now have logged voltage levels from the main computer batter, the actuator battery, and the master cutoff battery, even though they are all independently isolated. This is very nice. You can see clear voltage drops when the bleed solenoid actuates on the main battery, and when the jet vanes start moving back and forth on the actuator batteries. I tried to buy a wattmizer solenoid from Snap Tite to replace the 8 amp nitrous solenoid we are using for the bleed valve, but our distributor said that Snap Tite now has a 100 unit minimum order for that configuration. WTF?


We got the repaired (again) differential pressure transducer back from Omega, so we spent a full day working with that. One of the techs said that we blew out the amplifier by hooking the negative output of the four wire sensor to ground and just reading the positive output on a single ended channel, so I rewired everything to read the two output channels independently and subtract them in software. One thing that tripped us up is that the sensor (PX82) seems to be sensitive to common vacuum, even though it is insensitive to common pressure. I'm not sure exactly how that can be, but it messed up our results until we figured it out. The other thing that was causing grief was that the combination of our A/D board (Diamond MM32) and this sensor gave different readings if it was read only once a second versus if it was read 20+ times a second. This caused me fits, because my "-testADC" and "-testLevel" options were only reading and printing once a second, and the values I was getting for the sensor were fluctuating as if one of the lines were floating. I only clued in to the issue because I looked at the sensor graph from the normal flight control loop, which runs at 180 hz, and was perfectly smooth.


With the combination of using a faster A/D sampling rate and always venting the loading vacuum, it looked like we were getting clean and repeatable numbers as we loaded two drums of water into the tank, checking every hundred pounds. Unfortunately, the next day we met, the zero point had shifted significantly on the sensor. We calibrated again with two drums of water, and the deltas still looked consistent, but the zero shifted AGAIN after we power cycled everything later that day. I ordered a second sensor for backup, which should arrive in a week or so. We will swap it out and see if this particular unit is just some sort of lemon.


A couple times during our testing we froze the loading check valve open during the nitrogen pressurization. This is a pretty big problem, because propellant can then just backflow out, filling the pressurization line and possibly going back into the nitrogen tanks if we allowed the pressure to equalize. We could alternatively pressurize on the dry side, but we were concerned that the venturi effect might harm the sensitive differential pressure transducer (now that I have looked at the spec sheet, we shouldn't have worried). As a solution, we extended the pressurization hose with 40' more line and stuffed the line into a drum filled with water so the cold nitrogen can heat up some before pushing into the check valve. In the three loadings we have done since then, we haven't frozen the check valve.


We also tested the pop-safety relief valve on the tank. It is a big 1" valve that is intended to be able to vent faster than we could possibly pressurize the tank from our nitrogen tanks, so there is no chance of us bursting the tank if we accidentally, say, had all six bottles on the six pack open instead of just one at a time. When we bench tested it we were disappointed to find that it popped at 250 psi instead of the 300 psi I had ordered. On the tank, we found another characteristic -- when it pops at 250 it won't close again until the tank drops all the way to 200 psi. We are going to have to be careful aiming for 240 psi or so for our boosted hops. I have ordered another one of these to see if we get something closer to 300 psi.


It was raining again on Saturday, and while we have been doing hover tests in light rain, we are probably pushing our luck, because our connectors are not fully sealed. We decided to reschedule for Sunday, which turned out to be a perfect day for testing.




The first hover test was done on the blow-out stands and went flawlessly, with a rapid warmup and a perfect hover for 15 seconds before descent. All telemetry was perfect, the engine ran smooth with the bleed valve engaged, and the GPS precision didn't change at all, so we set up to do a ground liftoff with more flight head room.




The warmup was again rapid, but just as it lifted off the GPS precision dropped significantly, causing an immediate auto-land. The GPS didn't totally lose it, but the PDOP went from 170 to 700, which isn't a good level to be at. We didn't want to vent out the entire drum of propellant through the main engine while the vehicle was sitting on the ground, because the backwash would heat up too many components and the tank base, so we attached our little waste-catalyzer engine to the liquid drain valve to dump everything out. We had clearly improved the GPS resistance a lot, so we opened up the electronics and repacked the GPS with another layer of softer foam around the initial layer we had used.


We set up for the third run, and had a problem. The engine just would not clear up, no matter what I did. The thermocouple was reading plenty hot but we have found that with a 12" diameter engine, there can be large variances across the face. All of our normal tricks with throttling up for a big slug, then putting it back at the idle level failed to get it to clear up or build significant chamber pressure. The engine had just run clean an hour previously, and nothing had really changed. Our current theory is that because we used the burn-off engine to vent the tank, the engine didn't get its usual nitrogen vent cooling. The engine would have heat soaked a lot back from the hot pack into the cold pack, raising it well above its operating temperature of 100 C. We know there are issues when combustion starts in the cold pack instead of at the flameholder, and that may have been the case here. We were running low on battery power, so on Tuesday we will try a repeat of the ground liftoff test with a cold start of the engine. If the engine is actually dead after only five firings, we will be extremely disappointed.


The big lesson from these tests is that bleeding ullage nitrogen into the engine dome seems to have killed the high throttle chugging roughness dead, which is a Very Good Thing. In our course of trying every damn thing we could think of to fix the problem on the test stand, John Carr was the one that convinced we to try gas bubbling, and I was honestly quite surprised that it turned out to be the miracle cure. Russ has mentioned that since then he has found some references to helium injection being used to cure some chugging problems in the Apollo program, so I guess it isn't such an oddball idea after all.


We also configured a pair of high cracking pressure relief valves to replace the check valves on our LOX test engine purge plumbing. We have seen enough normal check valves fail in the last few years that I had paranoid worries about backflow of both LOX and methanol into our purge system. Doug Jones at XCOR had mentioned that relief valves were much more reliable than low pressure check valves, so we set a couple up for our engine. It was a good thing we explicitly tested them, because the particular brand I got actually had a reverse-pressure bleed hole, so it didn't function like a check valve. We soldered the bleed hole closed, and they seem to work perfectly now.




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