On TRF, Cavecentral wondered about the origin of the Estes BT-56 tube size.
"What is the difference between BT-55 and BT-56? Even BMS doesn't have BT-56. I doubt I'll need one unless I find a cone or tube that is slightly off an not know why."
The BT-56 is a carry over from Centuri, their ST-13 body tube.
The Estes BT-55 is 1.325" diameter, the Centuri ST-13 was 1.34" diameter.
After the Centuri line was dropped, Estes kept the tube size to use the (Centuri) Enerjet 1340 nose cone and fin can. Check out the Enerjet 1340 - CLICK HERE
That nose cone, fin can and BT-56 (ST-13) was used more recently in the Eliminator model and a few other easy to assemble Estes rockets.
The Estes tube size system never made sense to me.
The BT-60 was named because three BT-20 engine tubes fit inside it . . . 20 X 3 = 60.
The rest of the tube size designations seem arbitrary.
On the other hand, the Centuri tube names make perfect sense.
The ST-7 was close to .7" diameter.
The ST-10 was right around 1" outside diameter.
The ST-13 was around 1.3" diameter.
According to Woody's Workshop, Estes no longer lists the BT-56 tubing for sale on their website. You can still get the BT-56 tubing through other vendors.