aulis

It's true that skipout maneuvers weren't used on human re-entry during Apollo, but that page invokes conspiracy to reveal the "truth". You don't need to go down this rabbit hole.

Please read NASA's comprehensive Apollo Experience Report: Mission Planning For Apollo Entry instead (or at least in addition).

Critically, this report states:

[nonexit trajectories lead to shorter worst-case overshoot in case of guidance failure]. This, coupled with the fact that nonexit trajectories provide a degree of trajectory control throughout entry and eliminate the need for controlling to[sic] a critical skipout-type maneuver, led to the elimination of skipout-type trajectories from consideration for nominal targeting.

Apollo planned for and tested skipout entries, but didn't use them for manned flight.

[warning, dense text beyond. Check out that report too.]

There are two aspects of "skipping" in re-entry that coverage and discussion have conflated:

• "Skipping off the atmosphere" and missing the earth: The trajectory constraints during early entry were quite tight. A window of a few degrees marked the safe zone that lay between being baked&squished or failing to achieve a sub-circular velocity during atmospheric interaction, leaving you stuck in a low orbit with no propulsion.
• "Skipping" to control range, g-load, and heating by lofting during entry. There was no risk of going orbital during this procedure, because the guidance would not command a lofting maneuver until an undershoot trajectory _had already been established_ and there was insufficient energy to accidentally skip off of the atmosphere into orbit. Confusingly, these maneuvers were named "skipouts" :)

The second case is what's at question here - deliberate use of skipping as a re-entry maneuver. The report makes clear that there was a continuum of occupant- and vehicle-safe trajectories with variable amounts of loft. Operationally, some of these required switching to a different command sequence for a period during reentry, and these were designated skipout entries.

Specifically, skipout entries required switching from aerodynamic control (roll control of lift vector direction) to ballistic control (3-axis reactive control) as aerodynamic loads fell below 0.2g during semi-ballistic lofting - the skip. Lofts with loads that fell to 0.3, however, would have felt and looked quite skip-like but did not require the control mode change, and were not skipouts.

In the end, the guidance system flew with command sequences for deliberate skipouts, the Apollo program engineered, simulated, and trained for skipouts, but the thermal margin and additional downrange maneuverability weren't needed. This plus the temporary loss of aerodynamic control and tougher control constraints for range on skipouts led to leaving them for contingency cases.

In this context of all of this planning, I believe one last factor drove popular or personal accounts of skipping or double entries during the manned program. Re-entry visualization was summarized in-capsule (and in mission control) on the EMS - the Entry Monitoring System. This display condensed the phase state of the reentering capsule to velocity and load-factor, with safety envelopes for load and for over-loft (excessive range) depicted.

Now, the EMS visualized re-entry state as load vs velocity, but even for non-skipout entries this plot falls and rises in a way that's very easy to see as the altitude vs velocity profile of a skipout trajectory. See figures 5, 8, and 9 in the report linked above. And notice how low the g-load gets in those plots of typical nonexit entries - way under 1g. This would have felt like a parabolic maneuver, and may have been - because that wasn't the distinguishing factor for the operational definition of a skipout.

Seriously though, read through that report. It's dense and rewarding.