Non-ballistic atmospheric entry is a class of atmospheric entry trajectories that follow a non-ballistic trajectory by employing aerodynamic lift in the high upper atmosphere. It includes trajectories such as skip and glide.[1][2]
Skip is a flight trajectory where the spacecraft goes in and out the atmosphere. Glide is a flight trajectory where the spacecraft stays in the atmosphere for a sustained flight period of time.[1] In most examples, a skip reentry roughly doubles the range of suborbital spaceplanes and reentry vehicles over the purely ballistic trajectory. In others, a series of skips allows the range to be further extended.
Non-ballistic atmospheric entry was first seriously studied as a way to extend the range of ballistic missiles, but was not used operationally in this form as conventional missiles with extended range were introduced. The underlying aerodynamic concepts have been used to produce maneuverable reentry vehicles (MARV), to increase the accuracy of some missiles like the Pershing II. More recently, the concepts have been used to produce hypersonic glide vehicles (HGV) to avoid interception as in the case of the Avangard. The range-extension is used as a way to allow flights at lower altitudes, helping avoid radar detection for a longer time compared to a higher ballistic path.
The concept has also been used to extend the reentry time for vehicles returning to Earth from the Moon, which would otherwise have to shed a large amount of velocity in a short time and thereby suffer very high heating rates. The Apollo Command Module also used what is essentially a skip re-entry, as did the Soviet Zond and Chinese Chang'e 5-T1.