All of the outboard mounts are vented, rather than carried through for ejection; I used a BT-20 coupler in each mount as a thrust block, and drilled into each coupler from the outside of the airframe, then mounted a short length of 1/4" O.D. aluminum tube as a vent , glued into the hole. The outboard mounts are intended for use with booster motors, though it appears I'll have to friction fit those motors, as they ejected at burnout the first time I flew the rocket with outboard tubes loaded.
With the six fins, this rocket has a more rearward CP than the stock Fat Boy; in fact, according to VCP, it's stable with a D12-7 core and 3xC6-0 outboards with only one ounce of nose weight. I haven't had a chance to reliably confirm this in flight; the only semi-successful cluster launch so far suffered a late ignition that made the rocket fishtail alll over the place before stabilizing in cruise missile mode. The damage from that flight was repaired, and the next flight saw a CATO of the core D motor, but with iminimal damage to the rocket (shock cord torn out and core motor latch broken at the forward hook bend). It'll fly on a cluster again, soon -- and then I'll know for sure how well Barrowman calculations work on such a stubby rocket, and have an idea whether it's safe to add one more mount to allow flight with a D12 + 4xC6 cluster.
I'm also using this model to experiment with using Jet-X wick to airstart the outboards, but the D CATO was on the first such experiment, and ejection of the D motor casing extinguished all three Jet-X wicks -- fortunately, as the rocket would surely have been unsafe if it had launched on one or more outboards without the nose cone, nose weight, and recovery system. When I have results to report, they'll appear in the regular flight log pages.
The Fat Fat Boy was moved to the retired, destroyed, and lost roster after massive damage by a roman candle CATO of a Quest C6-0 booster motor (promptly replaced by Quest, I might add). The forward centering ring was burned and torn loose from the tube, the motor mount in which the CATO occurred was broken and thrust through the body between two fins, and a hole broken in the TTW tab on one of the adjacent fins; there was also extensive burn damage to the body tube in the recovery bay, apparently because the burning propellant ejected by the CATO came to rest within.
Well, in the immortal words of Commander Montgomery Scott, UFP, "Damage control is easy. It's readin' Klingon that's hard!"
The Fat Fat Boy is now reborn as Readin' Klingon -- the damage motor mount tube has been replaced, a new centering ring glued into place atop the burned one, a coupler (which had to be slit and overlapped slightly to fit the no-longer-smooth interior of the body tube) installed to strenghten and seal the recovery bay, and the remaining gap from the recovery bay to the outside (where the new centering ring and coupler don't quite meet) filled with paper towl soaked in yellow glue. It's very, very ugly -- looking like it's been through a phaser battle, which is what inspired the rechristening -- but it's flightworthy. Look for more reports of clustering experiments using Jet-X wick to airstart outboard motors. B)
Unfortunately, on the test flight to verify the repairs, the rocket
flew very well indeed -- and landed in one of the rocket-eating poplars
that border the field at 60 Acres Park in Redmond, WA. So far as
I know, the nose cone and parachutes are still up there, though the trees
have leafed out, making it hard to tell for certain. I've since built
another Fat Fat Boy.