Cluster the booster.
Main is still midline C6-0 ducted straight to the sustainer.
Secondary is a BT-5 motor tube with 13 motor, hidden “behind” the main (yeah, it will ruin the perfect symmetry, and you may need to run it a bit into the sustainer motor mount to give you more length). But at least from one side view it will be invisible, and if you paint it white it should be pretty well camouflaged in the other angles,
With a central C6 and auxiliary A10-3T (you want a delay for this to slow down before the A10-3T deploys the chute) you go from 8.82 to 10.82 total NS, but 2 more important factors
Nice initial power spike off the pad to get the stack moving
A recovery system for the booster. You pack a chute in the BT-5, as I said, you can use a longer tube than the current booster section and nest it into the sustainer motor mount. It just slides out at separation. Nose cone optional but perhaps advisable, chute might slide out from inertia when the booster kicks off the sustainer
In my experience (this is my most common technique for long gap black powered staging, since the long booster can’t successfully tumble), the slight asymmetric thrust from the off center auxiliary has negligible effect on flight path.
View combined data and thrust curves for multiple motors - Stage 1: 1 x Estes A10T- Stage 1: 1 x Estes C6
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