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Propellant Weight: 50.123 pounds
Average Propellant Grain Length: 7.9"
Propellant Diameter: 5"
Core Diameter: 1.77"
Web Thickness: 1.615"
Throat Diameter: 1.25"
Core to Throat Area Ratio: 2.0051
Density: 0.0616 pounds per cubic inch
Kn: 382, 430, 382
Assembled Motor Weight: 79.6 pounds
Test day was finally here. I drove out to our farm pond, and proceeded to dig a hole inside the earth dike area. I dug the hole a little over 3 feet deep, then added about 2 inches of sand in the bottom of the hole. Once the motor was in the hole , I shoveled some dirt back in to retain the motor. Then I went about setting up the data acquisition hardware, video cameras and the fire control circuit. I positioned the launch controller on the opposite end of the pond, on the outside of the dike, and in a shallow ditch.
Cameras were recording, data was recording, all clear. The night before I had been pretty excited about today's test, not that I wasn't still excited, but no butterflies today. Just a sense of, "I've done all I can do, now just push the button."
5,4,3,2,1 and ignition!
There was a slight pause, I heard the motor chuff once. Then it came to life.
The motor shortly after ignition.
The van was 100' from the motor, too close for me, so I couldn't see the data coming in or the motor itself. But I really didn't need to, the roar was deafening, even from my distant position. Then the cloud of white smoke shooting high into the air. I was a little too close to the tree you can see in the picture, even 30 feet up or so, the tree branches swayed in the exhaust of the motor and leaves and small branches blew off the tree.
The motor burned, and it burned. I wasn't really counting the time, but it seemed to burn forever. Then I could hear the motor pressure tailing off. Then silence, I walked up the dike to take a look. The motor was still burning a black smoke, the same as I have seem with all these KN/ER burns. The EPDM liner smoldering no doubt. Once the smoke stopped, I checked on the motor itself. Everything seemed to be fine, no blow by at the nozzle, no apparent heat damage.
Click Here for Camera 1 video, wide angle. (Divx file 2.17 MB)
Click Here for Camera 2 video, close up. (Divx file 2.32 MB)
If you don't have Divx, you really need to get. Click Here to go the the Divx web site.
This is the chamber pressure/time curve from the test.
I used a 5,000 psi transducer to be on the safe side, so the resolution isn't the greatest. As you can see from the graph, it took nearly two seconds for the motor to reach peak thrust. I ran the Kn lower on this motor than I should have, but I like to test a new design at more conservative Kn's just to be on the safe side.
Now for the Numbers:
Burn Time: 11.25 seconds
Burn Rate: .1435" per second
Peak Chamber Pressure: 548.39 psi
Peak Thrust: ~974.87 pounds
Total Impulse: 6,315.46 pound seconds
Isp: 126 seconds
Total impulse, peak thrust and Isp were calculated using a nozzle thrust coefficient of 1.45. That's an estimate as there's no way to know for sure what the Cf was unless I had recorded the thrust as well. But that should be really close as the motor burned very close to predicted values.
The Isp of 126 seconds is somewhat lower than previous tests, but still ballpark for what one would expect. Running this motor at a slightly higher Kn would shorten the burn time, and bring the Isp back up to 130+ I'm sure. I left the motor in the ground for the time being, I wanted to let it cool down before attempting to remove it. And I wanted to get the data analyzed as quickly as possible.
This is the motor after the test. You can see the transducer pressure line to the left.
Here we are looking into the nozzle. I think most of the slag that forms on the nozzle happens at the tail end of the burn. The picture clearly shows no problems at the nozzle end.
Forward closure to the left, nozzle to the right. Again, the forward closure end gets a heavy accumulation of residue. As the burn winds down, the chamber pressure drops to a level where the propellant barely burns, causing it to drop to the forward closure (as the motor is tested upside down). Most of the residue appears to be burnt EPDM rubber.
Here is the casing liner and the casting/inhibitor tubes. The nozzle end is to the right. The liner burned through between the fourth and fifth, and fifth and sixth grains, (from nozzle). That's backwards from most motors. Again, due to propellant smoldering after the chamber pressure drops to a point where it will hardly burn.
The motor casing shows some heat discoloration where the liner burned through. But doesn't appear to have damaged the casing at all. As it stands, the motor is in fine shape, even the nozzle held up well.
I added up the cost of a reload for this motor the other day, the propellant is about $78, another $11 for the casting tubes, and about $5 more for the EPDM liner. All told, under $100 for an "O" class reload. Not bad, I could afford to fly that once in a while. Speaking of which, I've had people asking me what kind of a rocket I plan to fly this motor in. Frankly, I hadn't given it a lot of thought. I wanted to see how things went with the static test first. I did of course, run a few simulations for fun. This motor could put a pretty big rocket up over 20,000'. I'll have to see if I can get a wavier for that kind of altitude, maybe next Spring. Or maybe I'll get started on that "P" motor.
Another successful test of the erythritol propellant. This propellant continues to perform as expected even in a "largish" motor.