I finally got a chance to run the static test of the flight vehicle this weekend. From a vehicle perspective, it was very good and I'm ready to move forward with getting it ready to fly. However, from a test operations perspective, there were a couple of issues.
The only vehicle issue was a hold during the countdown sequence at T-2 seconds due to low igniter CDI current sense. I had not seen this issue in previous testing so the timing must have been marginal and apparently I just got unlucky this time. After pressurizing the tanks, the state machine enables power to the RCEXL CDI module, waits 100 ms, then checks to see if the current is at least 2/3 of the expected steady state value. The data shows it was at about 50% of the expected value when it was checked so it transitioned to the hold state. However, the next sensor data packet was just above the minimum so a longer delay would have worked. The root cause is a design issue of that portion of the state machine since the averaged data used for the CDI check is only sent at 10 Hz and it's very unlikely that it should ever have enough time to pass with only a 100 ms delay. The fix is to increase the delay before checking the CDI current. When the hold occurred, I manually turned the CDI on, verified the data showed the expected value, then issued a state machine jump back to the same state where it passed the check and continued the countdown. I had added the ability to manually jump to the state machine a couple of months ago so that paid off in this case.
Test operations wise, there were two issues. The first is that I had trouble getting a full LOX load, even with the modified sequence of closing the vent valve before closing the dewar. The tank should hold about 17 seconds of LOX but it ran out at around 12 seconds which caused the rest of the fuel to burn outside the chamber until I manually issued an abort. I built my test pit 4 feet below ground with berms on all sides so it was contained without issues. When loading the LOX tank, it seemed to take much longer than it did with the LN2 tests. I had ordered a low pressure dewar (25 psi) but when I picked it up and got home, I noticed they gave me a high pressure dewar (230 psi) and the pressure had already risen to 200 psi. I vented it for about 30 min and got the pressure down to around 40 psi but I wonder if the bulk temperature had already risen enough where an ambient pressure transfer would lose a significant portion of liquid. All the transfer hoses and the vehicle tank had new polyethylene cryo-rated foam so in theory that should have at least been better than before. I loaded LOX until liquid squirted out the vent, just like I usually do on past tests and it seemed full. The elapsed time between the completion of the LOX load and start of the test was less than 5 minutes. I did notice that the fuel line that passes close to the bottom corner of the LOX tank did not get as cold as it did during LN2/water testing (it froze during one of the tests last year). That could either be due to the better insulation or because LOX is somewhat warmer than LN2. I wasn't too worried since kerosene has a much lower freezing point than the water I was testing with.
The other test issue was with the helium pressure. I thought I had enough high pressure helium on hand but when I loaded the vehicle helium tank, I only got it up about 2080 psi where I was hoping to load it to at least 2500 psi. This wasn't a showstopper as it just resulted in a thrust dropoff during the run with it starting around 252 lbf and dropping off to around 183 lbf at the end of the run (based on chamber pressure). However, it could have been easily prevented by ordering another 3500 psi helium cylinder prior to the test.
Despite the minor issues above, many other things worked well including the flight computer, data recording, sensors, plumbing, custom cryo valves and the ground support equipment. This was my first vertical test and I was wondering how the blast deflector with the 24 x 24 x 2 inch concrete pavers would work out. There was a significant amount of erosion on the first paver as expected, some at the first joint where it turned to the second one, and essentially no erosion on the third one. I didn't know how far the plume would spread out but from the photos, it looked reasonable compared to other similar setups. The vertical support was two 8-foot 8020 strut sections connected together and they worked OK but the butt anchors were a little loose after I took it down. Next time I'll use a splice plate. The videos and still photos turned out OK except for some obscuration due to dirt kicked up by the plume spreading out on the deflector. After the LOX ran out, a piece of tape on the igniter line caught fire and the LOX fill tube insulation melted due to the extra fuel that burned outside the chamber. For future vertical static tests, I'll add some shielding to protect the components above the engine.
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