It was designed to use FSI's 21mm motors, but I converted it to use standard 18mm motors.  The tube is just a tad bigger than BT-55, probably BT-56/ST13 equivalent.  I used BT-55 centering rings with a couple wraps of masking tape.

It's only had a few flights, but managed to pick up some dings, hence the black sustainer and yellow booster fins.  While I haven't officially retired it, I tend not to fly it much since it's such a rare bird.