I've been working on a BT-60/Bt-70 version with slightly different fin shapes that I call "the Troll". It flew nicely on D12-0/D12-7 and C11-0 and C11-7 combinations. On the 6th flight it failed to stage, forcing the BNC-80BB halfway down the sustainer body tube!
Engines were not right up to each other, but the gap was less than 1"

On "the Troll" 2.0 both engines will be closer and both will use engine hooks. The 1.0 used an engine block in the bottom of the booster to prevent engine kickout. this seemed to cause a bit of a venturi effect with a resulting loss of thrust.

The maiden flight was in Lafayette with the local TRA boys back in January. Staged beautifully, and the booster went into a glide-nicer than anything I ever got from any Black Widow booster I ever flew! Unfortunately it glided right into the side of the club's trailer, with parts going everywhere! Seems the epoxy I used didn't like the 16 degree F. weather.

__________________
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Meat Loaf