here is one source: http://www.ninfinger.org/rockets/body_tubes.html

Yep, BT-63 was the "core" tube for the eight BT-51 "tank" tubes in the Estes Saturn IB.

Since it slip-fitted over BT-60, it was used in the K-36 Saturn V kit as part of the removable engine mount, that allowed using either a 3-18mm motor cluster, or a single 24mm motor.

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