One factor that can prevent drag separation is torque in the air frame causing friction at the point of separation. I've had this happen. Don't assume that a size reduction from the booster to the sustainer increases the likelihood of drag separation. A larger booster is also relatively heavier, which works against drag separation. Having said the above, it appears that your rocket should drag separate.
In general, this is done with motors that don't have a thrust ring. Some cases, such as the old AMW cases, had a spring-like thrust ring that could be removed. It may also be possible to remove the thrust ring. I explored this a few years ago with CTI motors (for 75 and 98mm). As I recall, this is a modification that would need to be done by the manufacturer in order to retain the status of a commercial motor.
That sleeve idea is very interesting. Did you then have to make the oversize tube yourself?
With CTI, just turn down the aft closures on the smaller motors. I believe the 75 and 98 can be removed. Yes, they will become research motors.
CTI 29mm and 54mm have factory flush aft closures available:
Your are correct regarding the 29mm. The tapered closure is the same diameter as the tube.
If I used this approach for what I have in mind, ideally the interstage coupler tube would be the only oversized tube. The flange on the aft closure would then be sandwiched between two regular-sized (98mm) motor mount tubes, and the thrust force from the booster would be transmitted up to the sustained through the flange. Is that the force path you end up with? Or does your oversized coupler tube butt up against another oversized tube in the the sustainer and provide a force path that doesn't run though the aft closure flange?
In the picture I posted above, both of the airframe tubes are oversized. Therefore, for that rocket, the airframes push against each other to transfer the forced. However, it is almost a certainty that I would also have had what I refer to as a faux nosecone recessed 5" or so down into the booster airframe. There would be a coupler ring glued into the airframe and the faux nosecone would sit on that ring. Then, the motor tube would sit on that faux nose cone, so its possible that some thrust gets transfered in that manner.
I don't know the circumstances where you were denied, but it is just fine to have electronics in the coupler. The pic shows several examples from my rockets. I personally don't like the idea of having the igniter in the coupler - I always do that from above - but separation charges or electronics for recovery of the booster are fine.
I’m working on a couple of egg timers, so I’ll have everything on one tiny board. My rocket was very much like what’s in your picture but made from cardboard for H to H flights. After I rearranged everything it got heavy and I used I to H. The tilt lockout is a great idea. I’ve clustered mixed media and had rockets go sideways. Don’t do it! I’m sure Murphy has something to do with the fact that rockets always seem to fly at the most expensive camper or into the crowd, they never seem to scoot out over the desert. I can see tilt lockout being a great safety feature.
