The ground conditions have been terrible, a lot of melting snow, so there's a layer of mud over frozen ground. I had wanted to fill the flight N2O tank at the test site, but I decided not to drag the big K tank on site, and to just fill the flight tank (static tank) and take it with me. That will also help keep the tank warm, and the nitrous temperature up. I had chilled the flight tank on an unheated porch overnight, and kept the supply tank warm. This temperature differential will be the driving force needed to transfer the nitrous from one tank to the other without a pump. The flight tank was placed on a scale and filled with exactly 5 pounds of N2O. The flight tank was then allowed to warm to room temperature before we left for the test.
I weighed the engine with fuel and supply line prior to leaving for the test location, the engine weighed in at 9.4 pounds.
The temperature was in the lower 40's, not cold by this winters standards, but cool enough I left the flight tank inside until the last minute. John met me at my place, and we headed out to Joel's house where we would perform the test.
John making final preparations for the test.
Here's the tank mounted on the rail, you can see the power supply/amp for the load cell mounted to the tower in the lower left corner of the picture.
This is the load cell and combustion chamber. Notice the supply hose fittings are touching the case of the load cell.
John had one camera, Joel had the other for a different camera angle. I was at the launch controller and had the pull line to start the nitrous flow. After securing all of Joel's pets... all was set. I opened the N2O tank valve and took my position at the launch controller.
5,4,3,2,1 and ignition! The igniter fired and the 40 gram APCP starter grain started burning. I gave a quick pull on the nitrous valve line, and whoosh!
Click here for a video of the two camera angles combined. 2 minutes 10 seconds, 11.1 MB
Here is the moment of ignition.
1/20 of a second later.
Well into the burn now.
From another camera angle.
After the test we admired the damage to Joel's driveway!
It's too bad the video just can't reproduce the sound better. This really was an impressive display, I guess you just have to be there!
This is the thrust/time curve from the test.
The problem with my data is that the oxidizer supply line was pressing against the side of the load cell. I think it had two effects on the data, first it may have lowered the initial recorded thrust, then, with the the line sort of wedged in place it didn't take the pressure off the load cell as soon as it should have. I think that's the reason we see about 15 seconds of thrust and it should have been about 10 seconds. I knew going into this test I didn't have my load cell configured the best, so the data is pretty much useless for calculating the motor performance. But judging by the sights, sound and amount of earth moved, I'd say the motor performed very well. This first test was more of a proof of concept test, I'll work on a better system for data logging on the next test.
A post test inspection of the motor showed no damage to anything. The epoxy/aluminum fuel grain was 98% consumed, with only a small sliver remaining near the injector. The burn also progressed well into the PVC, with the center and lower PVC almost completely consumed. This resulted in some excessive heating of the combustion chamber in the central area. Not enough to cause permanent damage, just some slight heat discoloration.
The large flight tank only filled with 5 pounds of N2O caused a fairly long tail off as the gaseous N2O burned off. This may have led to the burn through of the PVC pipe after thrust terminated.
This test utilized 3) .125" diameter injectors and a .75" diameter nozzle throat. This results in a throat to injector area ratio of 12 to 1. Other than one nice "pop" at about 3.5 seconds into the burn, it seemed pretty stable. That pop is clearly visible as a flash if you watch the video frame by frame. I'm really not sure what caused that, I suppose it could have been a momentary nozzle blockage by a piece of fuel.
All in all, I'm very pleased with the results of this test. I'm looking forward to many more hybrid tests in the coming months, followed by some flights.