Vehicle Dynamics Considerations at Parabolic Velocities

Abstract

HE return velocity of re-entry vehicles has been increas­ ing in the past few years as more ambitious and sophisti­ cated space missions are proposed. Fig. 1 shows this trend and the relative magnitude of its associated problem areas. The difficult problem of re-entry has been joined by the problems of recovery and capture. These problem areas overlap and are not independent of each other; for example, it is of interest to note that the block labeled Parabolic Velocity Return Probe may be shifted to the right. This is equivalent to reducing the capture problem by allowing a multiple pass re-entry and increasing the recovery problem, since the final re-entry and landing point can be predicted only by accurate tracking throughout the entire multiple-pass re-entry process. The physical constraints that one must apply to the re­ entry process are markedly different for the instrumented probe and the manned vehicle. Additional restrictions of a biological nature must be included if the vehicle is manned. For this analysis, it was assumed that capture must be made in one pass and that the total aerodynamic load is restricted to 10 g's. The one-pass restriction on the manned vehicle eliminates the possibility that an into a highly eccentric elliptical orbit might occur which would complicate the heating and guidance problem and could carry the astro­ naut through the Van Allen radiation belt once again. Refs. 1, 2, and 12 consider the dynamics of a particle as it returns at parabolic velocity. A re-entry corridor has been defined, and problems associated with the undershoot and overshoot boundaries have been presented. This paper is concerned with the same problem, using three degrees of freedom instead of the previous two degrees of freedom analyses. The pitching moment equation has been added to study the effect of longitudinal vehicle dynamics on the trajectory. Fixed-stick dynamics of the re-entry vehicle are important because they represent the motions that the control system must be capable of regulating. If the dynamics are such that biological limitations are exceeded because of the oscillatory motion of the vehicle, the success of re-entry would be com­ pletely dependent on the autopilot. It is obvious that there are distinct reliability advantages if the fixed stick dynamics