Last weekend, I finally had a chance to test some of my Putative Purple v1.0 propellant.
My first impression upon examining the cast propellant was that it was rather soft, as though it was a slightly incomplete cure. This could be due to residual moisture in the chemicals due to the humidity in the air; when I work on v1.1 of the propellant, I’ll want to up the curative quantity to compensate.
I cut several 29mm grains to 1.75″ long, which is the same as the standard length of Aerotech’s H128 motor grains. I drilled 1/4″ cores in three of the grains (by hand, holding the drill bit in my fingers), and then burned off the cored scraps in a stainless steel bowl in the middle of my driveway to assess the color at 1atm pressure.
Success! The flame color was a very pinkish purple.
Buoyed by this initial success, I decided to do a burn test in a motor casing.
Due to the soft nature of this propellant batch, I worried about runaway Kn increases due to erosive burning. So, I set up a Dr. Rocket 29/180 casing with the phenolic nozzle from an Aerotech H128 reload (0.181″ throat), and only two of the three grains. I plugged the forward closure to avoid needing to worry about a delay grain. I put tape over the forward end of the topmost grain, and then inserted the two grains and a grain-length piece of empty casting tube into the liner. (This is the same technique Aerotech uses for their D & E reloads in the 29/40-120 motor casing.) The motor was assembled otherwise according to the usual Aerotech instructions.
I pounded a dowel into the ground in the back yard to make a motor-sized hole in the dirt, then inserted the motor, nozzle-up. In the event of a casing failure, this should be somewhat safer than an open-air motor test, as it’s likely to contain (or at least significantly slow) any pieces that are thrown out horizontally, and only allow pieces to be thrown straight up in the air.
The burn was fast — similar to the burn rate of Aerotech Blue Thunder propellant. The flame color was mostly blue, with a reddish tint visible mostly around the edges. The flame was quite clear, as well, making it even more difficult to view the color.
All in all, the test was a success, and pointed out a number of potential changes to make in the next version. I’ll probably save the rest of the cast propellant for the fire pit at ROCstock, as I’m not too happy with the hardness or the flame color when burned under pressure.
Tags: chemicals, propellant, Putative Purple, testing