|Launch & Static Tests|
I don't think the slight softness of the propellant will be a problem in a motor this size. In fact, it may be beneficial in preventing grain cracks. However, it could be a problem with a very large grain and high g forces associated with a rocket launch. I suspect the propellant will continue to harden with time, becoming more brittle in the process.
Grains: 1.51" diameter, .625" core
1) 2.442" 111.1 grams
2) 2.528" 116.4 grams
3) 2.453" 110.0 grams
4) 2.446" 112.4 grams
Total Grain Length: 9.869"
Total Grain Weight: .9821 pounds
Propellant Density: .06533 lb/cu. in.
Kn: 226 initial, 248 max (symmetric curve)
Nozzle Throat: .42"
Here is the thrust/time curve from the test.
Here are the details:
Peak thrust: 62.909 pounds
Burn Time: 2.534 seconds
Total Impulse: 111.12 pound seconds
Estimated Chamber Pressure: ~400 psi
Average Burn Rate: .1746"/second
Well, that was interesting. This first test was very conservative on the Kn, I suspected I'd have to run the Kn closer to 300 to get a chamber pressure in the 1,000 psi range. With the very low chamber pressure the Isp of 113 isn't too bad. I used a regular igniter and no ignition aid on the grains, and the motor lit very quickly with little time to thrust. I do think an ignition aid would help, and will give it a try in the future.
The 2.5 second burn time is great, just what I was looking for. Of course, I need to get the chamber pressure up and that will no doubt decrease the burn time. Still, I'm very pleased with the results of this first test. The thrust curve very closely matches the Kn curve of the grain geometry.
One thing I noticed on some of the sorbitol tests others have done. The curve seems to stay closer to predictions when the chamber pressure is lower, and the curve tends to be very peaked at higher chamber pressures. That's just an observation I made, with nothing to back it up. But I wonder if that effect will show up in my tests with XY.
More tests of course are in order.
A couple more notes after the test. When I went to clean up the motor after the test. I did notice a little more residue on the nozzle than I have seen with standard KNSU. The motor and nozzle were in fine condition with no blow by or erosion. The o-ring on the nozzle showed a lot more heat damage, the o-ring was basically melted to the nozzle. I guess that's to be expected with the longer burn time. Another interesting point. I used one layer of 20# paper and a single layer of aluminum tape as an outside grain inhibitor, both the paper and the foil tape were largely intact, as opposed to mostly gone when burning standard sugar propellant. The last thing I noticed was some deposits of fluffy black residue in the motor. I'm sure a lot these can be attributed to the low chamber pressure. It remains to be seen if these effects persist when the chamber pressure is increased.