For a few months I’ve been making and testing 3” long, core burning rocket motors in both 3/8” ID and ½” ID. The motors are hammer rammed on a mandrel, use parabolic converging angle shaped water putty nozzles with a 1/8” throat and core diameter. The tubes are made from 3” wide (76mm), fiber reinforced, water activated gummed packaging tape and wound on a custom made tube roller. The tubes are beautifully tight and have a consistent ID and OD that only varies by +/- 0.005”. For the purposes of this thread, let’s stick with the 3/8” ID motors to keep it simple.
My fuel is standard Teleflite mix of 63%KnO3, 27% powdered sugar and 10% dusting sulfur. The fuel was always milled for at least 2 hours at 60 rpm in a poor excuse for a tumbler, consisting of a thin walled plastic bottle on a threaded rod sticking out the chuck of an old drill. After milling the fuel still had some granularity to it and would make a hissing sound (granules) if you shook the tub that the fuel was kept in.
I tested the motors on a stand which uses a bar type load cell and an Arduino to send data to my laptop for recording. My build process is nailed down and I can crank out consistent motors whose thrust curves overlap with very little deviation. When testing, I use igniters that have their head touching the very top of the core. With a 2-3/16” core, the motors run for 1 second (+/- 0.05 sec), average 230gr of thrust (Isp = 2.3) and have a max thrust of 450gr. These motors contain 5 gr of fuel and have never blown out a nozzle.
New batch results –
This past weekend I finished and tested my new ball mill. It is well overbuilt, using a bottle made from 3” PVC which rolls on a double axel cradle at 63 rpm. To test it, I made a batch of fuel (100gr total volume as usual) and for some reason I added about 3% of finely hammered charcoal. I milled it for 2 hours and the fuel came out like talc. I’ve never made anything so finely ground before.
Following my usual motor building process, I built a few motors for testing the new fuel. All three of them blew the nozzles out!! I don’t mean from erosion, I mean all of the water putty was blown out with a POW, as well as a few layers of the paper tube. The blowouts all happened within 0.3 seconds of ignition and at a consistent 2000gr of thrust range.
I started widening the nozzle and cores and finally, at 5/32” for both the core and nozzle, I can get motors that run without blowing out. These motors now have a burn time of 0.50 seconds, have an average of 490gr of thrust (Isp = 2.4) and have a max thrust of 1350gr! So, slightly more overall impulse power, but all in half the burn time.
I tried an end burning type motor (using 1/8” throat and core) with a small 1” starter core. However, that only gave good initial launch thrust which quickly turned into very little thrust once the motor burned past the core.
From the look of it, what I have made is fuel that is VERY sensitive to internal burn pressure. By this, I mean that the graph of the Pressure vs. Burn Rate curve has a sharp knee in it making it difficult to balance throat/core diameter to core length as compared to my previous fuel.
Here is what I would like to know and understand…
- Is this result simply due to finally ball milling the fuel to a more fine state?
- Was it the addition of the charcoal that boosted the fuel and caused the pressure sensitivity?
- Is it both?
- Would further enlarging the nozzle and core bring down the max thrust and allow the motors to run longer? This means making new tooling.
- What would be the most effective way to proceed with the goal of building motors that burn for 1 to 1.5 sec and have respectable thrust?
Any help here would be greatly appreciated.