There is a limit to how much propellant can be crammed in an 18x70mm casing, which is why the best you might get out of one is a baby E (full D is more do-able).
I wonder how the market would react to longer 18mm motors, and whether they'd be a viable product. AT used to make 18mm full-E and baby-F motors, many moons ago. Of course there's the problem of shoving them in a low-power kit, most of which are designed to NOT take longer motors.
A solution that might not be prohibitively expensive would be to offer new, longer casings to fit the 18/20 closures (likewise for the 24/40 or 24/60). It *might* be possible to use currently-produced grains, just add a grain or two. Lots of testing would be needed; longer skinny motors, especially in small diameters, are problematic.
I wonder how the market would react to longer 18mm motors, and whether they'd be a viable product. AT used to make 18mm full-E and baby-F motors, many moons ago. Of course there's the problem of shoving them in a low-power kit, most of which are designed to NOT take longer motors.
A solution that might not be prohibitively expensive would be to offer new, longer casings to fit the 18/20 closures (likewise for the 24/40 or 24/60). It *might* be possible to use currently-produced grains, just add a grain or two. Lots of testing would be needed; longer skinny motors, especially in small diameters, are problematic.