Test new book thread

The Rocketry Forum

Help Support The Rocketry Forum:


Well-Known Member
Oct 26, 2014
Reaction score
After finishing the grains, the grains were measured.

If 1.178” is the Dg, and the average grain length is 7.5”/4 = 1.875, the Bates ratio for this motor is: 1.178 : 1.875 : 0.5

Or: 1 : 1.59 : 0.42

The outer and inner diameters are in perfect Bates ratio, but the length is a bit short (1.59 instead of 1.81). In addition, the total grain length is 7.5 inches when we were shooting for 8 inches. The motor will be a bit soft and regressive – meaning it will be softer at the end of the burn than at the beginning. It will probably burn a bit longer than the 1 second. Instead of drilling the nozzle throat smaller than the target 0.23 inch diameter (0.21875” was the next size smaller) it was drilled 0.23”. Putting the as-built motor into Burnsim shows the following expected performance of the built motor:

Burnsim target 38mm 4 grain KNDX motor – low pressure test

I stored these grains for over 2 months. When tested, the motor had a brief (approx. ½ second) delay from the igniter burn to measurable thrust. The burn time was 0.87 seconds.