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In this answer the following graph is displayed:

Reentry trajectory

It shows two slight increases in altitude (after 1st peak g and at the end of the blackout). For me, it looks like zoom climbs (trading kinetic energy (velocity) for potential energy (altitude)) made to slow down the space craft, but I cannot find any evidence.

What is the purpose of these climbs? Is it to slow down the spacecraft? Is it a common maneuver used during reentry (i.e. is the Apollo spacecraft the only one performing such maneuver)?

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Certainly trading kinetic energy for potential energy is an inevitable part of the process. In general this is important in controlling heating and g-forces. A fully ballistic re-entry would have only one (high) max g-force. Attempting to "pull-up" to achieve this is pretty universal. Its worth noting that there is nothing special about an ever-so-slight inflection of the trajectory, its all a spectrum.

That said, its unlikely that bobbing is ever the truly optimal solution, but the causes to do so range from: adding scope for altering trajectory, to being conservative on risking dipping too deep too soon and having too high peak g-force or heating.

As the actual trajectory is a result of all these competing factors, and the (in-)accuracy, of implementing them its unlikely you'll get a short and accurate answer for exactly "why" a particular path was taken.

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    $\begingroup$ This is pretty generic- the question is more specific to that flight. All of those factors couldve contributed but theres no source linked here that states definitively that they did. Possibly a NASA writeup on the trajectory would be published context that could definitively answer OP. $\endgroup$ – Magic Octopus Urn Jul 19 at 21:51
  • $\begingroup$ @MagicOctopusUrn: it seems a generic question to me. the actual question bit is: "What is the purpose of these climbs? Is it to slow down the spacecraft? Is it a common maneuver used during reentry (i.e. is the Apollo spacecraft the only one performing such maneuver)?" $\endgroup$ – ANone Jul 22 at 14:02

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