>Is there any reason for the
>different sized tubes that come
>on rockets besides the motors
>that fit in them?Not really, except for the fact that models with different diameters of tubes are more interesting to look at. Also, if you've ever noticed real rockets, whether NASA or military, many have different diameters. The different sizes of tubes give you the ability to easily model these without having to "roll your own".
Not to mention the interesting looks you can give fantasy or sf-type designs. For instance, Estes used to sell a model called the Explorer Aquarius that looked like a sort of long distance deep-space probe. The main body tube was a BT-55 (1.325" diameter), and there were 18 (3 groups of 6) BT-20's (.736" dia) attached around it. These gave the appearance of being fuel tanks.
Another reason is for motor clustering, which is using two or more motors at the same time. Examples include the Estes Cobra (discontinued in the early 70's) which used 3 18mm motors (A, B, or C). The Cobra's main tube was a BT-60, which holds 3 BT-20 motor tubes in a triangular configuration. I have a semi-scratch-build two-stage Fat Boy that uses a 24mm (BT-50, D motor size) central motor and 3 18mm motors in the booster, and a single 24mm mount in the upper stage. Really nice flights to over 1200 feet.
Other cluster/main tube combos are: 4 13mm (BT-5, mini motors) will fit inside a BT-55, and 7 13mm (one in the middle and six around it) will fit - barely - inside a BT-60. There are others, some mixing motor sizes.
Anyway, I hope this helped answer your question. Have fun and experiment with your designs.
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