Edward
In a black powder gun there is very little difference in the performance of any of these powders. The reason is that the powders are tightly confined by the wadded and rammed ball.
In a rocket there is a huge difference since the powder is generally not confined. BP in any grain size (FFFFg is the finest powder, FFFg less fine, FFg even coarser) are all the same chemical mixture of potassium nitrate, charcoal and sulfur. BP burns much faster than any of the BP substitutes, so when you simply replace BP with Pyrodex, Triple 7, etc. your rocket fizzles and crashes.
The reason is that the chemicals in Pyrodex and Triple 7 burn much slower than BP. A few grains ignite and simply blow away the bulk of the powder grains before they get hot enough to ignite. Finer grain powders burn faster because they have more surface area per unit weight. The finer grained BP substitutes have unconfined burning rates about 10 x lower than FFFFg BP, but the burning rate is proportional to pressure, so confining the powder at several hundred PSI gets it to burn at the same rate as unconfined BP.
The folks who have successfully used Pyrodex {and this means the finer grained P (pistol) verses the coarser grained R (rifle)} or Triple 7 confine it in a relatively strong container (a centrifuge tube, etc.) and packed with a wadding to eliminate free space. The container opening has several wraps of electrical tape or some other barrier material that will not rupture until several hundred PSI pressure is developed within the container. What ever you confine the substitute power, make sure the container doesn't fragment into a lot of high velocity particles.
Bob Krech
