The grains were all trimmed to length using a power miter saw, so I needed to use PVC spacers between each grain. Each PVC spacer was made from 2" vacuum tubing and cut to about .125" long. I assembled this motor like I do my larger/longer 3.5" motors, that is, I laid the cut EPDM lining on a table top, then laid out each grain with a spacer between them on the lining, then wrapped the lining around the grains. I used regular transparent tape to hold the seam closed at several spots, then applied a continuous layer of heavy foil tape along the entire seam. This grain stack is then powdered on the outside with talcum powder, a little talcum powder is puffed into the casing and the grain stack inserted.
Number of Grains: 10
Propellant Diameter: 1.92"
Average Grain Length: 2.95"
Total Propellant Weight: 4.2712 pounds
Inhibitor Weight: .1389 (63 grams)
Internal Length: 31.625"
Throat Diameter: .625"
Kn: 391 starting, 425 max, 376 ending
I ended up with a somewhat lower Kn than I wanted, but once the nozzle is made there wasn't much I could do about
it other than make another nozzle. Although I've found the Kn can be somewhat lower in longer motors.
After postponing the test a day due to weather, and almost the second day as well, I managed to get out for a late evening test after the weather cleared. The motor had been assembled several days earlier, so all I had to do was load up the van and go for it. I did test my load cell on the test stand after the last test. The load cell is surrounded by a packing of silicone grease, then covered in a rubber boot, this grease layer and rubber boot protected the load cell from the fire during the last test. To my delight the load cell was dead on when a re-calibrated it after the fire.
Once set up, all clear...
5,4,3,2,1 and ignition!
Click Here for a short video of the test. 700K in wmv format.
The motor came to pressure instantly, throwing a tall column of white smoke with a nice roar. Late in the burn, you could hear the pressure start to tail off, then a couple of the empty casting tubes were harmlessly ejected.
Here is the thrust/time curve from the test.
Performance Data:
Total Impulse: 541.01 pound seconds/ 94% "K" class
Isp: 126.6
Burn Time: 3.6 (~3.4)
Burn Rate: .1597"/second
Peak Thrust: 226.98 pounds
Peak Chamber Pressure: ~525
This really was a nice burn, the Isp improved, as I had expected on a larger motor. The rapid thrust and regressive nature of the burn would make it well suited to a rather large rocket. The regressive thrust was expected, the long motor and a rather tight core to throat ratio is going to lead to some erosive burning. One could eliminate it for the most part by using a larger core diameter, at the expense of propellant weight. But I really don't see any reason to do that, as the motor performed well in all aspects.
Post firing inspection of the motor indicated no problems, but I did notice something interesting. The EPDM lining had burn through spots at several of the grain joint locations, but where the foil tape was used to close the EPDM seam, there were no burn throughs. I'm sure the burn throughs occurred late in the burn, as the casing was not damaged. But it does prove the usefulness of the foil tape, and as an extra precaution perhaps I should cover all the grain joint locations on the EPDM with a wrap of foil tape.
This motor is almost a baby "L" class motor. I can hardly wait to see it in a rocket. This motor should loft a 3.5" rocket to 7,000 to 8,000' with ease. I'll do another test of this motor, and I'll have to get some more supplies ordered before I offer this motor for sale. But look for it in the coming weeks.