For this test, I wanted to start putting together the ground support equipment that would be needed to launch a rocket with a hybrid engine. So this test used several new pieces of equipment: A line cutting device for cutting a nylon fill line, a one way valve to prevent loss of N2O after cutting the line, a remote control valve for filling the flight tank, a spring activated N2O valve on the engine and a new N2O cylinder with a permanent vent hole.
I also made a new data cable that will allow me to monitor the data from the launch control table, now, all functions from filling of the flight tank to ignition can be performed from one chair. While this greatly improves the ease of firing the engine and safety, it does make for a lot more work setting up for a test.
Here's a graphic showing the fuel grain configuration.
Pre Firing Numbers:
Asphalt Fuel Grain Weight: 7.7 pounds w/inhibitor cap
Lower Polyester/HDPE Weight: 2.1
Total Fuel Grain Weight: 9.8 pounds
After everything was connected, I safed the test area and opened the supply bottle valve. Back at the control table I started the fill process by opening the remote fill valve. I didn't take more than a minute or minute and a half and the test tank started venting white vapor, indicating liquid N2O had filled to the height of the vent hole. Upon sight of the vapor, I turned the remote fill valve to the off position.
Here you can see the pop of the tube cutter in action. Cutting of the nylon fill line was next in the simulated launch process. While this wasn't necessary for a static test, it's pretty important to cut the fill line before a flight! The flight tank has a one way valve to prevent the N2O from venting through the fill system.
Next in the process is the pre-heater grain ignition. The pre-heater grain was about 10 grams of APCP strands and a standard igniter.
After allowing the pre-heater grain to burn for about 5 seconds, I opened the engine N2O valve via a spring released with a PIRM2 device.
Here is a picture of the spring release/PIRM2 on the engine.
Click Here for a video of the test, two camera angles plus a camera on the engine valve. A couple of minutes of me explaining the control system as well. About 4 1/2 minutes, 22 MB.
Post Firing Numbers:
Asphalt Fuel Grain Weight: 5.1 pounds w/inhibitor cap (2.6 lb. burned)
Lower Polyester/HDPE Weight: 1.3 (.8 lb. burned)
Total Fuel Grain Weight: 6.4 pounds
Fuel Consumed: 3.4 pounds
N2O Tank Volume: .4005 cubic feet
N2O Temperature: 75 degrees F.
N2O Density: 43 pounds per cubic foot
N2O Load: 17.22 pounds
Burn Time: 11.06 seconds
Oxidizer to Fuel Ratio: 83.51:16.49
Unfortunately I did not collect data from this test. I had the data recording on my PC, but when I opened the remote
fill valve, one of the guys thought the nylon fill line had popped off. So I closed the fill valve and shut down
the data recording, after I checked out the fill line I found no problems, the line had just jumped as the N2O
flow started. When I returned to the control table I had already checked off the "start recording" box
on my check list and didn't think to start it again. So no performance data, which is disappointing but not the
end of the world. It was by far the most impressive test of the HR5 engine to date. Which it should have been considering
it was the highest loading of N2O to date.
Looking at the fuel to oxidizer ratio, I really hit the nail on the head with this fuel grain configuration. The optimum for a N2O/asphalt burn is about 83:17 and this test ran at about 83.5:16.5. The new inhibitor cap worked great, with no appreciable ablation to speak of. This cap should easily be able to handle this engine burning twice the N2O load. Looking at the regression rate of the rest of the fuel grain is promising as well. There were a couple of depressions in the asphalt, that may burn through to the liner on a longer burn, but I think it would have to be a significantly longer burn. One other minor issue I noticed was on the polyester/HDPE portion of the fuel grain, I had measured incorrectly and had to glue a 1" length to the grain. At that glue joint there was fairly deep erosion, so if I have to do that again, I need to do a better job of gluing the two sections together to avoid any gaps.
In the end I was thrilled with the results of this test. Everything performed exactly as planned and brings me one step closer to a launch.
Here is a picture of the completed fuel grain. To the left is the polyester/HDPE grain, I used a cardboard tube to form the core, of course I left it in place to burn out. On the right is the asphalt grain, there was a small step in the bottom of the grain from the casting stand, so I filled that step with polyester resin to level it with the end of the casting tube.
This is the other side of the asphalt grain showing the epoxy/graphite/fiberglass "cap". I used some 3" thin walled PVC to form the core, again leaving it in place to burn out. As it turns out, very little of the PVC even burned out. It would seem running the fuel grain all the way to the forward bulkhead my be enough to prevent the forward "burn through" issue I've seen.
Here is the fuel grain complete just prior to installing the forward bulkhead. You can see the transition from the "cap", to the asphalt and then opening up to the polyester/HDPE grain. You can also see a little bit of aluminum foil still attached to the asphalt.
Here's an interesting picture of the asphalt fuel grain after the test. I broke this grain in two at the casting tube seam, which would be just about exactly in half. Notice how there are some waves and indentations in the fuel. I suppose minor variations in the injector holes, as well as testing in a vertical position may lead to some of the irregular burning.