But that's not even the real problem, the real problem lays in the difficulty in scaling up the above mentioned rocket motor. Lets say we want to double the propellant load by doubling the motor length. Now you have twice the propellant flow rate through the nozzle, so the nozzle throat now has to have twice the surface area. Now the core of the propellant grain is smaller than the nozzle throat, so we have to increase the core size to prevent erosive burning. So now the grain has a larger core and less of the motor volume is occupied by propellant, so the grain needs to become longer and the motor gets heavier.
If you want/need 1,200 pounds of thrust for a short duration, a standard KN/SU propellant motor would be the way to go. In fact, for a booster of a two stage rocket KN/SU would be ideal. Don't worry, I've got plans...
In the end, what it boils down to, is that with a slower burning propellant you can maximize your propellant loading in a given motor. There is also some advantage to burning propellant at higher altitudes in that there is lower air density, of course you have the disadvantage of carrying the propellant to higher altitudes as well.
Now, back to the SBS-800 motor. It is to be the test bed for KN/XY propellant in large diameter propellant grains. The motor casing is 6061 T-6 aluminum, 3.5" diameter with .125" wall thickness, overall length of 24". The motor will have a steel nozzle and a 6061 T-6 upper bulkhead. Dual o-rings at each end will seal the closure and nozzle. The 3.25" inside diameter will leave room for an EPDM rubber insulating layer, and a 3" diameter grain cast into heavy paper liners.
I started working on grain casting before construction of the motor, as there were some problems to overcome with casting. The xylitol based propellant is very sticky, and I mean sticky. I read somewhere that it was being used as glue to manufacture plywood, and I believe it. It also has the tendency to soak through wax paper, or any paper for that matter. I suppose the smaller molecules of xylitol have something to do with that, and the fact that it's slow to set up doesn't help either. My first problem was finding a mandrel to make the casting tubes on. I finally found a wine bottle that was exactly 3" in diameter, and just long enough to make a 6" long tube.
I made an epoxy/fiberglass tube for the first casting tube. I thought it would be rigid and make for a good inhibitor. I ran into problems with the fiberglass tube as it softened with the heat of the melted propellant. I had to slide a PVC tube over the fiberglass tube to help it hold its shape. I was also concerned about the propellant adhering properly to the fiberglass, the heat deformation may result in spotty adhesion. So for the next grain I decided to use home made paper tubes. I bought some heavy brown wrapping paper, and cut a 7" long end off the entire roll in my power miter saw. I wrapped the paper around the bottle and applied a slightly thinned carpenters glue with a brush as I rolled the paper onto the bottle. The first tube I made used three wraps of paper. Once dried I removed it from the bottle, it was a nice tube. But seemed a little wimpy. So the next tube used six wraps of paper. Again, a nice perfectly cylindrical tube, but it was almost too thick. So I made the last tube from 5 wraps of paper.
Here is the second grain cast. The wood base is drilled to hold the 1" wood dowel as a coring tool. The paper casting tube drops into the circular groove cut into the base and the white disk is plastic sheet I cut out to fit inside the paper casting tube, the plastic disk doesn't stick to the propellant and seals the bottom of the casting tube. The wood dowel coring tool is given a single wrap of aluminum foil to allow its removal from the grain. As the grain hardens from the ends in, I remove the aluminum foil. If you try to remove the foil before the propellant is cured, you will pull off chunks of uncured propellant.
This seems to work fairly well, and I think the grains will cure faster with the paper casting tubes as they will breathe some. I've noticed that the propellant cures very slowly if it is sealed from contact with the air. It would be nice to be able to have the core open all at once, to allow for faster curing, but I haven't come up with a way to do that just yet.
Here's is the start of the nozzle turning on my new (new to me) lathe. Once the steel bar was chucked up, I faced the end to true it.
That chunk of steel is 3.5" diameter and weighs about 13 pounds. I'm starting by drilling out a 5/8" bore.
Here I've started boring the inside taper.
Here is the finished convergent taper, though not yet polished.
Now I'm starting to reduce the outside diameter.
Here's the nozzle getting close. I still need to turn the divergent outside diameter. It's under two pounds now, so I've already removed 11 pounds of metal.
Here is the SBS-800 casing in back, with the nozzle, forward closure, one of three grains, and a grain and nozzle of the T-4 motor for comparison. The T-4 nozzle is 1.6" in diameter.
Update:The SBS-800 has had its first test firing in Static Test 82. While the motor performed well, the propellant burned much more quickly than desired. I believe the KN/XY propellant has a pressure point where it burns much more quickly. I'm going to try running the motor with a lower and more neutral Kn in its next test.
Here's a picture of the motor after its first test firing. The retaining rivets are 3/16" steel, there are 12 rivets at both the nozzle and the forward closure.