Something else to consider, when mixing grain types but with same geometry, put the faster burning propellant near the nozzle. For the same reason Tony said above. This is opposite what Loki does in their cocktail reloads. They put the slower red near the nozzle and the faster blue forward but the grain length and core size makes the difference there.
It's like it's rocket science or something
Ummmm,
I know Scott from a long time ago. It's science, extensive testing and maybe VooDoo. Before Scott took over Loki, he stopped by our group in Peoria and was teaching us on how to characterize mixes. It was just before Steve Eves Saturn V launch. He was just getting the hang of characterizing himself and I suspect Jeff Taylor was his mentor. Jeff had just hired him to help out with Loki. Maybe the sorcerer's apprentice?
Scott suggested we mix up some brown stuff of his own inkling he called "Baby Sh!t" but never worked with himself before. Well it was his load cell, equipment and electronics so why not? Local prefect (it was his shop) said, "Ok." So we went on with the mixing. Scott stayed the weekend at a motel and indoctrinated us to "Burnsim" which was out there back then. Things looked good on the computer.
He took us through weighing everything including the casting tubes (that was a trick!) to take into account the mass of everything involved with the testing motor. Had a special 2 grain, 38mm casing with a pressure tap that could record the internal motor pressure while burning (our prefect made on his lathe(s)). Would take a long time to explain it but essentially an oil column of low flashpoint oil transfers pressure to a transducer to record the internal pressure spike inside the motor while it's burning besides the external load cell that measures the thrust force on the casing. Dual channel recorder used. Do a bunch of "runs" and use mathematical goobledy goob to calculate the "a" and "n" coefficient of Saint Robert's Law a.k.a. Vieille's Law.
Well, everything was setup and when 5... 4... 3... 2...1 happened, the motor broke free of the downward facing restraint and smashed into the load cell. It was still restrained and didn't fly off in any direction so still safe but the initial impulse was pretty high and it destroyed the load cell. The load cell looked o.k. physically but electronically it was gone/dead. Scott didn't have a backup so we were out of luck of trying to pull off a successful characterization test. The test casing stood up to the test and was still usable but the loadcell was dead. (It was his loadcell. We paid for the instruction so I think he still made a profit even with dorking his equipment.)
In spite of this I still had great fun that weekend mixing and being around people who knew a lot more than me. That was a "blast". Wished I was in a position to recreate the excitement of those days.
Our Prefecture never did get involved characterizing propellant. The Math is out there but no one has ever written a step by step protocol that the Mathematically impaired could follow to pull it off. Scott gave us an instruction sheet but there were still a few holes.
Yeah I read "Experimental Composite Propellant". Things like computers, interfaces, load cells, motor restraining devices, calibrating load cells, getting cases with pressure taps, data reduction for "a" and "n" is totally lacking.
If that wasn't the case, there would be an absolute pile of propellant formulations with "a" and the "n" exponents published in the Research section of this forum so folks could use Burnsim to work with a propellant. Even then, if another group tried to characterize the same mix, they might get different numbers depending on the equipment they use. Rocket science you know.
Nonetheless, a good "a" and "n exponent" should give a decent starting point for an experimenter to start from.
Ahhhhhhhh, One little caution here. With a given propellant the "a" and "n exponent" can change with motor size. What "a" and "n" works with 38mm might not with 75mm or 98mm. I heard that fact stated in the early 2000's about Research propellants of which I think is true.
The key with some motor mixes is nozzle throat and core size of the grains. Generally it's the wider grains at the bottom and narrower at the top. Unless the same core size works with a given motor. Grains don't always have to be stepped. Sim........Sim......Sim.....Sim. Then live test several times in a very safe venue.
Go with erosive phenolic nozzles in AT stuff and hardware and all bets are off with Research propellant. Burnsim can't help one very much. One can start with the nozzle throat it recommends but may or may not be too large. Most of the time it's too large. Burnsim only works with a fixed, non-erosive, graphite like nozzle which makes sense. Nozzle erosion results in a myriad of factors that I believe are impossible to mathematically quantify. Would take too long to explain. (burnrate, pressure, velocity and heat are some of the factors.)
If Scott Kormeier has a mixed inverted core arrangement with the wider core at the top and a narrower one at the bottom, I assure you it was with extensive testing to make sure it would work in a commercial motor forsale to us. Plus if he is mixing propellant types in the grains then I know for certain he is testing like crazy to make sure he can get the propellant certified and out there for the "rank and file" to enjoy the effects. I don't believe there is any mathematical construct out there currently that can predict the outcomes with mixing propellant types in the same motor. Some of this has to be trial and error. That's fine as long as testing shows it can be done. If it was me, I wouldn't want a bunch of PO'd customers so I'd test a motor mix extensively before trying to release it.
Kurt Savegnago
