I fear I am reaching the upper limits on motor size using a standard mix of KNO3 and sugar. The brittle nature of the propellant is really giving me problems. Combined with a rapid burn rate, it makes designing large motors difficult to say the least. Now, I could move on to KNO3/ Dextrose. But it's an expensive form of sugar and only slightly reduces the problems indicated. I would still like to find a cold cast form of sugar propellant using KNO3, sugar, and polyester resin. So I went back to some of my old tests, and decided to try some variations. With my digital data acquisition and the strand burner, I can come up with more useful data than I did on the early tests.
I weighed the KNO3, sugar and sulfur into a small bowl and mixed and ground them together with a spoon for several minutes. I weighed the 20% PR (polyester resin) into a small portion cup and mixed in 4 drops of hardener. Once the hardener was mixed in I added the PR to the dry mix. After several minutes of mixing, the mixture was crumbly, but would compress into a shape easily. I tamped several 2" straw sections into the propellant for strand burn tests, with the remainder being compressed by hand into the bottom of two portion cups.
After 24 hours the propellant is not curing. In fact, for the last 10 hours it has been under a desk light to add a little heat to it. It seams like it may be starting to set up, but it's way too slow. I'm not sure what the problem is, I may have just used to little of the curing agent. Or, perhaps something in the mixture is preventing it form curing.
I did an open air burn of about a 1" long x 3/8" diameter strand of not yet cured propellant. It burned smoothly and slowly, about 14 seconds for one inch. It was the best burn I've gotten from PR yet. Perhaps there is some hope!
I increased the PR for this batch, and reduced the sugar in an attempt to make the propellant a little easier to work with. It didn't help the mixing much, nor did it seem to improve the castabiltiy. But it may make it stronger. I also increased the amount of curing agent, I used 80 drops which was about 4 grams in a 650 gram batch of propellant. I'm a little optimistic here, I went ahead and cast 2 grains for the T-4 motor while I was at it. I will do strand burn tests before I test it in a motor.
Test X3-3 62% KNO3/ 22% PR/ 16% Sugar
This test is to see if the sulfur is causing the PR not to cure, or cure very slowly. The first two tests have been 3 and 2 days since they were cast, and the propellant is not curing.
I used granular KNO3 and powdered sugar, mixed by shaking for several minutes. The batch total weight was 100 grams. After mixing the KNO3 and sugar, the PR was added, then 20 drops of hardener. The propellant was mixed for about 4 or 5 minutes with a fork, then cast into straws, strands, a paper and a plastic cup.
This batch cured quickly, within an hour. That pretty much confirms my suspicion that the sulfur was inhibiting the cure. The strands and grains from X3-1&2 never did set up completely. They ended up a soft but brittle play dough consistency. I suppose I could try expoxy as a fuel/binder, but I don't like the expense of epoxy.