Lack of 10.5mm motor availability is a good reason to build the sustainer for a project such as this with a 13mm sustainer. The scale properties would be slightly off, but no one would ever notice unless it was pointed out to them.

Our Internats models were constructed as a 'vertical cluster,' with the igniters for both the booster and the sustainer ignited simultaneously. The igniter for the sustainer lit a short 30mm-long bit of t-fuse that was inserted into the sustainer nozzle, giving a short delay before staging. The booster would be about 15 meters high at the point of staging. Also, a short piston was built into the nose of the V-2 to add some additional performance and help protect the nose of the booster.

It was simple, tricky, and effective, and in ten years of using that approach no other country ever figured out what we were doing.

Alternately, I am sure that a creative modeler could figure out how to stuff a timer and battery into a BT-80 nose cone. You may be able to steal some ideas from this thread.

James