I finally got around to looking at the data from the static test from back in April. The motor was mostly comprised of hardware from a previous motor that I lengthened to make room for another propellant segment. That results in the initial Kn rising from 340 to 425. The other big difference was that I added 1% RIO to the propellant. I also changed the closure method from snap-rings to a bolt-ring closure designed for the increased chamber pressure. The propellant grain consisted of 5 segments in a bates grain configuration weighing 9.247 lbs with a port/throat ratio of 1.5625. The propellant was mixed in a ratio of 65/35/01 (KNO3/Sorbitol/RIO) and cast into 3" mailing tubes that have an ID of 3" and an OD of 3.125". The casing insulation consisted of four layers of rosin paper. The casing and bulkhead are fabricated of 6061-t6 aluminum. The nozzle is fabricated from 12L14 "leaded steel" and had a throat diameter of 0.64" and an expansion ratio of 12.
The total impulse was 1213 LB-Sec (5395 N-Sec.) making it a small M-class motor. I was also pleased to find that the calculated ISP was 131. Slightly higher than I expected given the fact that I didn't spend a lot of time premixing the propellant ingredients and used the KNO3 as received.
There was no measurable throat erosion post firing and the nozzle appears to ready for another firing. When I took the motor apart post test I found that the insulation was severely breached at the bulkhead end. I think that the higher chamber pressure created a much harsher environment for the insulation than I was expecting. The insulation breach resulted in a small blister in the casing about .125" in diameter and about .005" high. I'm happy to replace the casing since it was one of the most out of round 6061 tubes that I have ever purchased. The casing thickness provided for a fairly significant safety margin, so I'm considering pushing the design a bit by using a .065" walled casing which will allow the insulation to be more than doubled and should make for a very lightweight and powerful motor. Overall, I'm pretty pleased with the results.
4 comments:
Nice one!
Great motor work... you have come up with some good conclusions also.
I think you can, and should try for a thinner casing - all too often we in hobby-amateur rocketry wind up making things a bit too heavy in the motor department.
Better to get a thinner case and bulk up the insulation to protect it.
the casing should be light-weight but the insulator should also be doubled so that it would avoid erosion or overheating.
what happened to that PVC pipe?
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