|astronautix.com||Skip lunar reentry trajectories studied for Apollo|
The Mission Analysis Branch (MAB) of MSC's Flight Operations Division studied the phenomenon of a spacecraft's "skip" when reentering the earth's atmosphere from lunar trajectories and how that skip relates to landing accuracies.
When an Apollo CM encounters the earth's atmosphere (this study used 91,440 meters (300,000 feet) as the practical altitude), the vehicle bounces or "skips" back above the atmosphere. From this point, the spacecraft follows a ballistic trajectory until it re-encounters the atmosphere. During this skip portion of reentry, there is no control of the vehicle's flight trajectory. The length of this skip is, therefore, determined by the angle and speed at the start of this ballistic trajectory. The distance of the skip in turn determines the spacecraft's landing area. Variations in both speed and angle at the start of the skip thus are directly related to landing accuracy, but the effect of these variations is felt much more in shallow than in steep trajectories. In light of these factors, MAB recommended that, for Apollo flights, the skip phase of reentry be made at the steepest practicable angle consistent with maximum allowable acceleration forces. References: 16 .