Once upon a time, in a galaxy far, far, away...
...there was BT-1, BT-2 and BT-3.
BT-1 started out life with an ID of 0.75" and 0.84" OD in 1960, then it changed to 0.765" ID x 0.845" OD in 1961.
BT-2 had an ID of 0.72" (1960).
BT-3 had an ID of 0.725" and an OD of 0.765" (1961).
In 1963, all of Estes Industries' parts were recoded according to a new numbering scheme:
BT-2 eventually became BT-20.
BT-3 eventually became BT-30.
Because BT-1 was bigger than BT-30, it was given a new name: BT-40.
And the rest is history...
MarkII
(McMahon)You are correct, sir. (/McMahon)
But also, by that time they had the mylar tube for the Streak, which was same ID as bt-2/bt-20 but smaller OD. And so they called it BT-10.
The BT-40 was discontinued eventually (since it was just remaining stock from Model Missiles Inc.)
Next to be introduced were BT-50 and BT-60.
Then BT-5 (because was smaller than BT-10) and BT-55 (because it was inbetween 50 and 60).
Then BT-70 (for the GT3/and the Sprite and the Starlight rings, and later for the Little Joe II and Saturn 1B service module).
Then BT-52 (for upper section of the Thor-Agena, since it was between 50 and 55)
Then BT-65 (for the payload section of the original Scrambler, as it was between 60 and 70)
Then a few new tubes for the Saturn 1B:
BT-3 (for the escape tower motor)
BT-51 (for the tank tubes, as it was between 50 and 52),
BT-100 (for the SIV-B) and the slightly larger BT-101 (for the engine skirt)
BT-63 (for the core where the tank tubes were mounted... it was later used in the Saturn V for the removable motor mount.)
Then the new tubes for the Saturn V in 1969:
BT-80 (for the SIV-B)
BT-58 (for the Service Module, as it was between 55 and 60)
They used that slate of tubes in a bunch of different rockets.
I think that pretty much did it, as new tubes after that just got a computer SKU number.
Then they started incorporating the Centuri tubes in some rockets like the Space Shuttle, the Astrocam, and the Challenger-II. I think the Space Shuttle tank tube (Centuri ST-20) was called BT-77. Don't remember what the ST-8 SRB tubes were called (BT-48 maybe?) The Centuri ST-13 parts were relabeled BT-56 (since it was slightly larger than BT-55).
Then new parts started appearing with the E2X line that eventually used a numbering based more-or-less on the part diameter. Hence you have the BT-1090 tubes used in the E2X kits the BT-1835 in the Terrier, and the BT-200 tube in the SpaceShipOne.
And then of course there were the North Coast tubes that were labeled BT-3.00, BT-4.00 and so forth.