In considering the motion of the rocket, at each instant of time only the state of those material particles which a t that instant are within the control surface passing through the exterior surface of the body of the rocket and the exit section of the nozzle shall be included. In order t o obtain the equations of motion of the rocket, the following procedure is used. An arbitrary but fixed instant of time is considered. A fictitious solid body is denoted by S with mass m, which would be obtained if the rocket at the instant t solidified and ceased giving off particles. The solid body S will not be homogeneous; in some of its parts, it will have the density of a metal and in other parts the density of a gas, and so forth. It shall be assumed that the fictitious solid body S is invariably fixed to the body of the rocket and from the instant t onwards (instant of solidification) moves together with the rocket. The momentum of the body S shall be denoted by Q. | Equations Of Motion Of A Rocket by Nasa Nasa Technical Reports Server (Ntrs), Paperback | Indigo Chapters