Check out the gap staging on the new Boosted Bertha. It has vent holes in the booster’s centering rings to vent to the rear.

You can download the instructions from the Estes site from the Boosted Bertha page.

I built a Double Mini Max a few years ago that uses gap staging. I used the method of venting at the top of the booster, thru several small holes. They actually are drilled thru the coupler where it is glued into the booster. Staging an A10 to A10 looks almost instantaneous. The gap is maybe 1”, and I used a lengthened motor tube in booster to help direct the blast from booster motor into upper stage nozzle.

__________________
Lee Reep
NAR 55948

Projects: Semroc Saturn 1B, Ken Foss Designs Mini Satellite Interceptor
In the Paint Shop: Nothing! Too cold!
Launch-Ready: Farside-X, Maxi Honest John, Super Scamp