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for a video of the test including three camera angles.
Here's the moment of ignition.
At ignition, the thrust pushed the trailer ahead about a foot. Knocking over the support leg for the oxidizer tank allowing the tank to drop on the engine. The support leg is supposed to be attached to the trailer, but I ran out time in the shop to finish it. I anticipated the trailer moving ahead, and had the support leg angled to accommodate the movement. But someone else repositioned the support before the test and I forgot to reset it.
You can see a couple of changes in the flame during the burn. My guess is the nitrous oxide sloshed back and forth a couple of times, momentarily starving the engine of oxidizer. Again, the cameras didn't do a good job of picking up the mach diamonds. With the naked eye there were quite a brilliant series of mach diamonds from the nozzle to the tail of the flame.
This was a "just for fun" camera angle. The camera was set back about 150' and slightly to the side, but was nearly knocked over by the exhaust anyway. If nothing else, the video is worth watching just for this angle.
Here is the moment of the casing burn through. The burn through occurred at 6.173 seconds into the burn. Lucky it didn't happen sooner, at this point the engine was running in the blow down gas phase of the burn.
N2O Load: 13.0 pounds
Fuel: 57.1% HDPE/42.9% Polyester Resin
Fuel Burned: 2.4 pounds
Burn Time: 6.963 seconds
Total Impulse: 2,703 pound seconds
N2O only Isp: 207.9 seconds
Isp: 175.5 seconds
Oxidizer/Ratio: 84.4% N2O/15.6 Fuel
You can see a few minor irregularities between the 2.75 and 4.25 second mark. I think as the oxidizer started to run low, the liquid was still sloshing and caused the slight drop in chamber pressure. At about the 6.4 second mark you see the drop in pressure caused by the burn through. My load cell wasn't functioning for this test. I haven't tested it yet to find the problem...
Here's the burn through area before disassembling the engine.
After the forward bulkhead was removed.
Despite the hole in the casing, the nozzle and forward bulkhead came out with just a fine coat of soot and in great condition.
Here's a look at the fuel grain from the nozzle end. No problems there at all.
Here's a shot using the cameras "Night Shot" feature. You can see the dimpled surface the HDPE pellets create. Which is the effect I was hoping for.
I'm not even going to go into all the little things that I did wrong on this test, but I made a big mistake by not heat protecting the liner at the upper bulkhead area. I was going to coat that area with a thick layer of RTV, as I had done on the first test of this engine. But in my haste I forgot to do it, when I remembered it, the engine was already assembled and I opted not to open it up and do it. From the first test, I thought that area stayed so cool I wouldn't have any problems. Obviously I was wrong. Another problem I had was in the fuel grain casting. The port was about 1/4" off center because I couldn't get my top centering ring over the Duralar covered coring rod. Of course, the thin area of the fuel grain is where the burn through occurred. I'm not saying it wouldn't have happened anyway, but it looks to me like the oxidizer doesn't just shoot down the fuel port, but some circular motion is involved with the outer layer of gaseous oxidizer I think.
The good news is that the only thing that was damaged was the casing, even then, I could cut the casing down a few inches and still use it again. I learned I need more thermal protection at the upper bulkhead, easy enough to do. I was shooting for an optimum oxidizer to fuel ratio of 85/15. This test ran at 84.4/15.6, I don't think I could get much closer than that! Even with the burn through late in the run, the Isp improved over the N2O/asphalt test. The HDPE/polyester fuel worked great, much easier to cast, and a lot easier to clean up than asphalt. With the oxidizer to fuel ratio nearly dead on, the burn is also much cleaner than the smoky asphalt, of course, I could reduce the length of the asphalt fuel grain to get a better ratio there too. But since I'm looking at much larger oxidizer load, and longer burn times, I'm going to need the added web thickness and low regression rate of this type of fuel.
This is still very much a learning process for me with hybrids, but I'm starting to "dial in" the parameters I'll need to launch a good sized rocket. I think I can run a little higher chamber pressure, so for my next test I think I'll open up the injectors just a tad. If all goes well after that, I'll open up the injectors even more, as well as increasing the throat to give me the thrust required for a launch.