The body tube makes up the main body of the rocket, as the name itself suggests. as a fuselage is to the aircraft, so is a body tube to the rocket. In launch vehicles, the body tube contains all the control systems as well as the main propulsion system. on the outer, as seen on the Space Shuttle, boosters or auxiliary propulsion systems are placed.
For model rockets, the setup differs. It contains the recovery system and engine mount inside it. Sometimes if the nose cone isn't hollow, or is very small to hold the payload, the body tube contains it. Avionics are also placed inside it.
The body tube has to be sturdy and stiff to withstand aerodynamic loads as well as protect the payload and the subsystems placed inside it. Also, it should withstand the force of burn-off as well as ejection charge.
There are many sturdy materials which can be used in fabrication of body tube. the standard ones are blue tube, cardboard, or kraft phenolic tubes. all these have high densities, and thus the rockets should be designed such that the weight of the body tube itself can be sustained.
