Latest News - 11/4/2009

Wow, lots to talk about - haven't tested the new motor yet but it's getting much closer. One thing that has changed is the tanks on the test stand. I proof tested the stainless steel fire extinguisher tanks and all was fine but when I was cleaning them out in preparation for the cold water tests, I noticed about a one inch long shard of metal sticking up inside near where the bottom fitting was welded on. I'm guessing the hole didn't get deburred before the fitting got welded on. I managed to break off and extract the shard but then I started looking more closely at the inside of the tank and that's when I started getting nervous. The various weld seams of the tank are just lap joints and there is a little bit of an edge sticking out along the whole length of each weld. I could imagine all sorts of debris getting caught in those overlapping joints that would be difficult to clean because the lap joint geometry changes slightly as it is pressurized. That in conjunction with a general worry about fatigue life of the tank led me to change to a different tank. I previously had good luck with the E-sized aluminum oxygen cylinders so I decided to go with something similar. I found that a 15 lb CO2 cylinder would give me about the same internal volume (2.6 gal) as the 2.5 gal fire extinguishers and it is rated to 1800 psi. The thick bottom makes it easy to drill and tap a hole for a threaded fitting. This particular cylinder I'm using is the B15 model from Catalina Cylinders but I got them from BeverageFactory.com for about $90 each. The only real downside of using a thicker tank on the test stand is it will take a lot longer to chill down the LOX tank because it weighs 16 lb empty. I've posted a CAD drawing of a fitting for the CO2 cylinder on the Test Stand page.

The new purge system seems to work properly. Each propellant operates separately with its own solenoid valve and check valve. The check valve is adjusted so as the pressure in the feed line to the engine drops more than 40 psi below the tank pressure, the purge gas (helium, same as the tank) will blow out the propellant line and *hopefully* allow me to run back to back tests without having to take everything apart and clean it. When I checked out the solenoid valves on the test stand for the first time, I ran into an interesting problem. Whenever I would actuate the fuel purge solenoid, all the solenoids on the test stand would de-energize. I assumed it was some sort of software problem like writing a whole word instead of a bit but I couldn't find anything obvious. Then, I noticed that the indicator on my screen for the 24V "power good" went out whenever I energized that particular solenoid. It turns out that solenoid had an internal short to case on the pin I happened to pick for the positive side of the power supply. I took it apart and never actually found anything obvious but the problem was occurring 100% of the time and now I'm not seeing it at all anymore. I went ahead and swapped the solenoid wires internally just to be safe as a short of the negative terminal to ground won't cause any adverse effects. I also noticed the other solenoid valve (LOX side) didn't seem to be putting out as much flow as the fuel solenoid valve. When I energized the coil, it didn't make the same strong click as the other one. I took it apart, cleaned it, and reassembled it with some help from Circle Seal for the adjustments. They emailed me a service and test procedure for the SV series valves so if anybody needs it, just let me know. Both of these valves came from eBay and they looked to be in nearly new condition when I received them. In any event, both of them seem to be working properly now.

I re-calibrated the load cell up to 140 lb - that's all the weights I had. Plus, since these aren't slotted weights that you would typically use in this application, I had to lift the entire amount each time I added more weight on and my poor back wouldn't take much more. I was curious to see how much static friction the linear slide had since that was a concern and it was so much less than the cable/pulley friction that I didn't even notice it. However, I did measure the load cell during one of the cold water tests and the flex hoses generated about 2 lb of force on the motor when they were pressurized.

I've completed the cold water flow vs. pressure tests went but I ran into another interesting problem. I was seeing strange zero shifts on the fuel pressure transducer about about 10 psi. I initially suspected a grounding problem because whenever I would bump the test stand, the pressure would jump all around. It turned out to be a bad crimp in one of the excitation pins for that particular transducer. I've been using that connector for several years so I guess it's better to fail now than during a test. I'm debating whether to go and inspect all the crimped connections on the test stand and maybe put a dab of solder on the connector just to be safe.

The test stand control program also got a makeover this summer. I never was really happy with the sequencer and I wanted to set it up to do a real countdown so I completely rewrote the section that handles the digital I/O and at the same time, ported it from NI-DAQ to NI-DAQmx. There were also some issues with the IRIG card that were bugging me. When I did my first uncooled tests in 2004, I was using a borrowed CompactPCI IRIG card that went into the same chassis as the DAQ card. For all the subsequent tests, I had to use my eBay special PCI IRIG card that goes into the PC a couple hundred feet away from the MIO card. Normally the two boards would share a 10 MHz reference clock in addition to a start trigger but I've been just connecting the start trigger assuming that the reference clocks were close enough. Well, it turns that the DAQ master clock (NI PXI-6030E) has a clock accuracy of +/- 0.01 % while the bc635PCI IRIG card specs its drift at 2 ms/hr. In actual use, I was seeing about 33 ms of skew over 700 seconds which is about 2 video frames. The whole purpose of using the IRIG card and video overlay is to allow correlation of the data with the video and if there is a large skew, then there's really no point. So, I'm going to run the 10 MHz reference clock from the output of the IRIG card to the PXI chassis CLK10 in connector on the back. I went this route instead of connecting directly to the DAQ card because if the clock goes away for some reason, the PXI chassis will automatically switch back to its internal clock and the DAQ card won't just freeze as it would if the reference clock disappears during acquisition. Finally, I changed the output module to write direct to CSV instead of the binary LabVIEW WDT format. By the way, I really hate the way LabVIEW tries to be clever and automatically display times in the local time zone. In general, I'm tired of fighting with LabVIEW and I'm close to just ditching it and rewriting the whole thing in C/C++. Maybe it's just my lack of experience with it but some things that should be simple just seem to be hard to do with LabVIEW sometimes.

I also spent some time playing around with an ADIS16350. This is a triple axis gyroscope/accelerometer inertial sensing system that I may put in the flight vehicle for basic pitch and roll guidance. I didn't get very far with it - just got it to talk to a 68HC11 eval board I was using.

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