II. The angular motion of a freely falling rocket
The motion of a rocket with its propellant exhausted and above the heights where aerodynamic forces can be used to control its motion, can be considered as that of a rigid body in free flight, subjected to small perturbations by weak aerodynamic forces. This permits the separate consideration of the motion of the centre of mass of the rocket along an approximately ‘free fall’ trajectory and the rotation of the rocket about its centre of mass. The rotational motion of free rigid bodies is well known and may be readily visualized by means of Poinsot’s construction (Corben & Stehle 1960). This analysis may be applied to the motion of a rocket with an accuracy which depends on the smallness of the residual aerodynamic forces and the time interval over which the ‘free fall’ approximation is applied. The Skylark rocket vehicle is a long axisymmetric body of approximately uniform mass per unit length. The momental ellipsoid of such a body is a long ellipsoid of revolution with its major axis along the spin axis of the rocket. In this case, the angular motion will consist only of roll and regular precession. In the early stages of the flight the rocket is given some spin motion by aerodynamic forces on the fins. The angle between the geometrical axis of the rocket and the angular momentum vector is small and can change only slowly because of the aerodynamic forces which are important during the initial stages of the flight. The rate of precession of the rocket axis is much smaller than the rate of spin. In these circumstances, the angular motion will be as shown in figure 11 and can be regarded as roll about the vehicle axis OV with angular velocity ω and precession of this axis about an invariant direction OC with angular velocity Ω. The semi-angle, COV = ρ , of the precession cone is given by cos p = I L / I T ω / Ω , where I L and I T are the moments of inertia about longitudinal and transverse axes passing through the centre of mass.