My max successful gap stage is 51 inches (done twice). I have done 72 inches on a test stand but was unsuccessfully in flight. I routinely and successfully do 18 and 36 inches.
https://www.rocketryforum.com/threads/record-for-black-powder-gap-staging-51-inches.146385/
IMO vents are necessary for All gap staging for two reasons.
1. You must have a free pathway for the white hot gas to flow to expose the nozzle opening. Because the nozzle is a “cul de sac”, you can’t get flow THROUGH the nozzle, you have to get hot photons close to the nozzle as they have no problem penetrating the “air dam” of the nozzle. Here’s a great NARAM presentation which appears to debunk Stine’s particle theory in his book.
2. Zero delay motor “blow through” (there is no clay cap, just a solid propellant slug) seems weaker than the typical standard Estes “shotgun” ejection charge, but it does have some pressure, and particularly for short gaps (and anything where the motors are not taped is essentially a short gap) IMO needs a vent to prevent the sustainer from “blowing off” before it ignites.
if you are going minimum diameter to minimum diameter, the vent needs to come out the side of the rocket. If you are going LESS than minimum diameter, you can put the holes in the stuffer tube or chimney or whatever you call it, these should be placed just below the base of the sustainer motor casing. but you will need holes somehere in the outer portion of the booster to let the gas out. HERE you can be creative, for cosmetic purposes you cam put holes in the BOOSTER centering rings and duct the gas out the rear, so no holes in the side of the rocket.
For me, biggest challenge in long gap staging is booster recovery, long boosters do not tumble well. They tend to return ballistic if no other means of recovery added. My favorite solution is a cluster booster with a small short delay motor “pod” on the side with a nose cone and chute. Almost ANY delay is short enough for BP powder, as the zero delay will burn and ignite sustainer well before even the shortest delay auxilliary motor. You WANT some delay, as you’d like the booster to slow a bit before deploying a chute. I usually find an A8-3 nice, it has a big nozzle, so I can easily get an E-match in there (I like E-matches for clusters, they are fairly reliable and they fit in As and Ds, Unfortunately, the cheap ones don’t fit into B and C motors, at least the ones I have don’t.) An unexpected benefit of the A motor, if the A auxilliary lights but the main doesn’t, the rocket barely makes it off the pad and the sustainer CANNOT light since main booster is source of sustainer ignition. I have seen this once, it is entertaining, a bit embarrassing, but very safe as the rocket has no power to go anywhere and just falls to the ground next to the pad.
While I generally do well with the new(well, not so new anymore) pyrogen free Estes starters for individual motors, I don’t have as much success with them for clusters.
hope you get two straight and vertical trails and a short recovery walk for booster and sustainer