Uber Tuber - 3 Stage Tuber with Clustered First Stage Booster
Having flown the 3-stage Tuber variant on a D12-D12-D12 combo in late 2004, I knew it needed something different.  As it was, it crawled off the rod, lumbering into the sky.  By the time the third stage lit, it was nearly horizontal.  It needed a better first stage thrust curve; something like a D30 or E30 would be just right.  But since no one makes one (in black powder), the next best thing is a clustered booser (clooster?).

Having successfully clustered boosters on my Thridget and Midget70 birds, I knew just what to do here.

Adding three 18mm outboards to the first stage solves the problem, and does so without needing an FSI E60 :)
 

 
 
Here's a look at the aft end of the first stage booster.  The three outboards were built and finished first, then glued onto the booster.  Tape was used to mask off unfinished strips where the glue was applied.  On the booster, the paint was sanded down to the bare wood to accept the glue.

The three motor hooks are clearly visible here.

 
 
The forward ends of the outboards are beveled to reduce drag and to improve looks.  Drag looks ugly, right?

The vent holes are just visible here.  The beveled plug is made of balsa, and also functions as the engine block.  The plugs were bored axially then radially.  Any ejection gases will pass forward through the plug then radially out the vent hole.

 
 
In this shot, the burn string attach points are visible.  Piano wire was bent into two U-shaped pieces, then epoxied through the aft centering ring into the anular space between the motor tube and airframe.

The burn string is critical.  The center motor lights the next stage, so the rocket must be held on the pad in case is doesn't light.  Otherwise, things will get ugly.