For the line drawing in the catalog's Countdown Special that wasn't a Cobra, I went with the Li'l Augie. It's called a Ducted Thrust Augmentation experiment, with two stages.
The outer 1" tube surrounds the inner BT30 tube, drawing air into it, like a combustion chamber. Maybe that helps it fly higher, I'm not sure. Actually, I used BT50 for the outer tube, since that was close to the mailing tube used for the original Li'l Augie, and was in the parts list for the Countdown Special.
I am sure that it's also a 2 stage rocket. The booster stage is just an engine pushing up against the 2nd stage engine. At burnout, the entire first stage engine is expelled, but nothing else. The augmentation tube also serves as the surround for the 1st stage engine.
As you can guess, that puts the 2nd stage engine at least 3 inches up into the augmentor tube. Which, if you've tried before, likely brought you into familiarity with the
Krushnic Effect . (Not the Kalashnikov Effect which the people of
Mozambique are familiar with.) To break the Krushnic effect, the air gets pulled into the vents at the top of the augmentor tube, allowing full thrust from the second engine.
Well, that's the theory. I've not launched mine yet. But, I did build it. As you'll note in the photo, the rockets of this era were still built with shock cords laced through the body tube. Still no 3-fold mount in these rockets. Or Kevlar cord, for some reason.