A few days back I posted this picture from the 1973 Estes catalog - What's wrong with it?
If the inside and outside diameters are correct, the tube thickness should be .04" instead of 0.021.
From RichsRockets :
No, because inside and outside diameters differ by TWICE the tube thickness... think about it.
I remember seeing this ad as a kid and wondering what parallel winding meant and why was a line drawn around the center of the tube. Still doesn't make sense to me.
The illustration in the 1973 catalog looked like two smaller length tubes joined by an internal coupler. Parallel wound BT-30 tubes had a wrap joint that ran down the length of the tube.
The inside and outside tube dimensions don't add up!
Outside Diameter .765"
Inside Diameter .725
BT-30 tubes were thicker walled, more like a BT-55 or BT-60, but not that thick!
The listed wall thickness is listed as .021". That's probably close to the truth.
I would guess the printed inside diameter was incorrect.
TRIVIA: I read somewhere the original BT-30 sized tubes were made from file folder manila material. Gleda Estes hand rolled them on the kitchen floor.