Here is an image of the cap, before assembly I also coated the inside with high temperature RTV.
I loaded the oxidizer tank prior to leaving for the test, due to the hot weather I wanted to do the filling at a controlled temperature. I got a fill of 12.4 pounds of N2O, somewhat less than I had expected, but still pretty consistent with the two previous tests. Ryland and his father made the trip from Oregon via Minnesota, after filling the cylinder, we headed out to Joel's acreage to perform the test.
Notice the wet, white towel draped over the N2O tank to keep it cool. No falling over of the oxidizer tank this time, I had anchored it to the frame. After setting up and starting the data recording, Ryland pushed the button to ignite the pre heater grain, which was simply two strands of APCP between a standard igniter. After a few seconds of pre heater burn time, I pulled the line to open the oxidizer valve.
From another camera angle.
Click Here for a short video with two camera angles.
No anomalies were noticed during the burn. After the burn I approached the engine and did a quick check of the
casing temperature with a bare hand. The area just below the nozzle was quite warm, warmer than I had hoped for,
but not sizzling hot either. The fuel grain area was just barely warm, and the nozzle area about the same as the
forward bulkhead area of the casing.
This is a graph showing oxidizer pressure, pounds of thrust and chamber pressure.
N2O Load: 12.4 pounds
Fuel Burned: 2.0 pounds
Burn Time: 9.8 seconds
Peak Thrust: 655.2 pounds
Peak Chamber Pressure: 346.85 psi
Total Impulse: 2,449.51 pound seconds
N2O only Isp: 197.54
Isp: 170.1
Oxidizer to Fuel Ratio: 86.11 to 13.89
The performance is down even slightly from the previous test, due mostly I'm sure, to a slightly lower oxidizer load. While the overall Isp is slightly better than what I had with the asphalt fuel, it's starting to look like an optimized N2O/asphalt propellant may give me better performance in the long run. Of course, that assumes I shorten the fuel grain on the asphalt. While that should improve the O/F ratio, and give me even better performance, it might also cost some performance by reducing the chamber length and mixing time. The best scenario might be to decrease the asphalt fuel grain length, and add a post mixing chamber to combustion chamber. If I try that, I'll have to come up with a better insulation scheme chamber wall.
Speaking of chamber insulation. Here's a picture of the upper fuel grain after removing the injector plate/bulkhead. You can see some erosion of the outer edge of the fuel grain at about the 7 o'clock position. This grain was pretty much perfectly cored, so it seems there is always going to be an area of gas rotation up there that is going to erode the outer grain surface. I suppose some minor irregularity in the surface starts the process, but it's always in just one small location. In this case, the PVC/EPDM/HTRTV protected it long enough to prevent a burn through. But it was getting close, the last layer of protection, the EPDM layer was burned through in about a dime sized area.
Interesting to note is that the oxidizer to fuel ratio was very close to optimum, yet the Isp suffered. Perhaps I need to recalibrate the load cell, or perhaps I'm ejecting unburned propellant out the nozzle. Despite the fact I don't like working with asphalt, I may have to try it again in a shorter fuel grain. Since I have an extra casing that's slightly shorter anyway... On a positive note, the nozzle is holding up fine, and still shows no signs of throat erosion or cracking. In fact, the nozzle is looking better all the time. The combustion residue carbon seems to be filling in any opening in the graphite grain structure, giving me a hard smooth surface.
I had considered opening up the injectors slightly, that should increase chamber pressure and improve performance some. But I think I'll hold off on that now. I'd like to get more data from changes in the fuel grain, before I start changing other parameters that would make it difficult to be certain what change caused what change in a static test.