The old 18mm Cox D8-3 "baby D" has been out of production for FAR too long and actually fills a needed niche that no other motor currently fills. It was GREAT in the Mars Lander, Estes 1284 Space Shuttle, and other large/heavy 18mm rockets. It was only like 13n-sec, but that's quite a bit over a 9n-sec C motor and often made the difference between a good flight and barely flying. I do not know if that motor is possible nowadays in a standard 18mm length casing as apparently BP has gotten weaker over the years. Why BP is weaker has not been satisfactorily ever explained to me. KNO3 and Sulfur should not change as long as same grade is always used. Possibly it has something to do with charcoal quality.

The Aerotech composite SU 18mm D10 is close but is a whole lot more impulse than what may be wanted or needed.

__________________
When in doubt, WHACK the GAS and DITCH the brake !!!

Yes, there is such a thing as NORMAL, if you have to ask what is "NORMAL" , you probably aren't !
Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY.
ALL systems are GO for MAYHEM, CHAOS, TURMOIL, FIASCOS, and HAVOC !