Jason (Blackshire on YORF) came up with a great two-part 13mm engine mount adapter.
This adapter breaks in the middle. The engine slides in, nozzle down, in the lower half against a lower engine block. The upper half slides over the exposed top of the engine. In the upper top half of the adapter there is a longer engine block. Both halves of the adapter are taped together. The adapter and 13mm engine are slid into a standard 18mm engine mount.
I've used mine on occasion to fit a 13mm engine in an 18mm mount for the schoolyard launches.
Blackshire's conclusions are simple and valid:
Three 18mm A8-3 engines for $7.99 OR
Four 13mm A10-3t engines for $7.99
The thrust curves on these different diameter engines are pretty close.
The A10 has a higher initial peak and burns slightly longer.
The A8 has a lower initial peak, slightly higher sustain and shorter burn time.
The A10 thrust curve is HERE
The A8 thrust curve is HERE
I wasn't using the 13mm adapter as often as I should have. I figured the extra weight of the adapter with the engine installed would weigh far more than an 18mm A8-3 would. Boy, I was wrong!
An A10-3t engine in the 13mm adapter weighed .42 oz.
The A8-3 engine (alone) weighed .56 oz!
I guess I'll make a few more adapters and save some money while I'm at it.
Note: Jason sent me a message at YORF and added a few new pieces to the 13mm adapter history:
Thank you! I had never compared the weights of the two. Tim Van Milligan is partly responsible for the design. My original design called for peeling off the outer layers of the centering rings so that the adapter would fit inside a yellow engine spacer tube (to give the adapter the same shape as an 18 mm engine, mainly for its aerodynamics). Tim simplified (and no doubt lightened) it to create the variant that appeared in the "Peak of Flight" newsletter.