Tuesday, November 9, 2010
Moonburner Motors Get Bent
First time I've tried to upload a vid clip.
Hope it works OK.
When I first started experimenting with making my own sugar motors, I made several decisions under their own merits which eventually lead to an epiphany. When combined, these factors
created an interesting moonburn synergy.
First, I chose to use 15/16" diameter [24mm] phenolic cases that are thicker walled, than Aerotech, being 3/4"id instead of 7/8"id. To simplify the nozzle issue, I elected to buy Aerotech nozzles instead of producing my own from scratch, though this requires that I turn down the nozzle OD to fit my cases. Like most other experimental motor makers, I still wanted to maximize the propellant fraction. I decided to pour directly in the cases, including the convergent frustrum. No casting tubes or liners required. After only a couple false steps, I tried silicone tubing as my core spindles, which works great when casting sugar propellant. The silicone spindles are longer than the motor case, and extend all the way from the nozzle to beyond the top end. A rod or dowel, as the case may be, the same diameter as the nozzle throat, but loose inside the silicone, extends through, acting as an alignment guide and throat plug. After curing, the rod is drawn out, then with a steady tug, the silicone stretches, losing contact with the core wall, and pops right out pretty as you please.
Though I started with core burners, a major goal all along was to make moonburners since I couldn't buy any commercial ones for a long time, and then when I could, only in 'J' and above.
With the old Aerotech D-G moonburners, the fuel was precast and had a drilled port to one side that you would have to blindly hunt for with a piece of 'S' bent Thermalite. Even for
someone experienced with these motors, it was tough to install, tough on the Thermalite and chuffed too often at the best of times.
Epiphany; My flexible silicone spindles need not be straight. I attached the silicone tubing to the nozzle throat with a short plug, then 'S' turned the tubing over to the case wall where
it's held in place with a suitably fashioned wire clip. This creates a smooth pathway for the ignitor during installation AND expulsion, even when using Copperheads. This 'S' turn also creates a short area of coreburn configuration near the nozzle, producing a higher takeoff spike before it settles into full moonburn mode. Perfect for a medium size 'D'-'E' bird.
Another goal of making motors was that my wife [now ex] and I had always been into clustering, especially air-starts. Estes 'D's are great for this but keep costing more and more.
I was already enamored with the idea of focused thrust [long before Flis-Kits] and wanted a way to add that feature to already existing rockets with air-start clusters. The curvy silicone core spindles work even better with the angled nozzles. Now there's only one shallow bend instead of an 'S' curve. This also reduces the coreburn section, making the motor more of a true moonburner. Since classic moons ramp up and down more gently than other motors, they are less prone to affecting the flightpath if multiple ignitions are uneven, or lacking. Focused thrust, of course, further reduces possible flightpath disturbances. I made a 10 degree angled nozzle holder for my lathe, so that I could modify the Aerotech nozzles before gluing them into the cases.
Angled lathe tooling with unmodified nozzle
installed, and a used angle nozzle motor.