This is a result of having low TWR on the orbital insertion stage, typically a characteristic of hydrolox. If you watch ULA streams, you'll see the DCSS & Centaur both do this as well. Basically, the problem is with such low thrust, you can't accelerate horizontally to orbital velocity fast enough; you'll fall back to earth before you get to orbit. As a result, the ascent profile is overlofted, throwing you above your target orbit & then you fall back down to it by the time you reach insertion. It's just to buy enough burn time without hitting the atmosphere again.

However, since you're curving around the earth as you go, some of that excess velocity turns into a minute orthogonal component that would make the orbit elliptical. The pitch down is to apply an opposing orthogonal component & return the orbit to circular by the time of insertion.

Yes, it's inefficient. But it's still (a very close approximation of) the most efficient way to get to space. It has to be inefficient because a low TWR is efficient--the theoretical ideal for any kind of vacuum orbital maneuver are infinite TWR infinitesimal burns seperated by coasts; super low TW hydrolox is far from this ideal.

Still worth using, because the combustion efficiency gains from using hydrolox more than offset the guidance losses caused by the low thrust.

This is a result of having low TWR on the orbital insertion stage, typically a characteristic of hydrolox. If you watch ULA streams, you'll see the DCSS & Centaur both do this as well. Basically, the problem is with such low thrust, you can't accelerate horizontally to orbital velocity fast enough; you'll fall back to earth before you get to orbit. As a result, the ascent profile is overlofted, throwing you above your target orbit & then you fall back down to it by the time you reach insertion. It's just to buy enough burn time without hitting the atmosphere again.

However, since you're curving around the earth as you go, some of that excess velocity turns into a minute orthogonal component that would make the orbit elliptical. The pitch down is to apply an opposing orthogonal component & return the orbit to circular by the time of insertion.

Yes, it's inefficient. But it's still (a very close approximation of; PEG isn't perfect) the most efficient way to get to space. It has to be inefficient because a low TWR is inefficient--the theoretical ideal for any kind of vacuum orbital maneuver are infinite TWR infinitesimal burns seperated by coasts; super low TW hydrolox is far from this ideal.

Still worth using, because the combustion efficiency gains from using hydrolox more than offset the guidance losses caused by the low thrust.