Efficient descent

In terms of propulsive efficiency, a slow descent like a gravity turn is poor, since the longer the descent takes, the greater a gravitational AV is added which the propulsion system must thrust against (hover is the limiting case). The most propul-sively efficient descent from orbit would be an impulsive burn that completely nulls the horizontal component of velocity at the beginning, a ballistic free-fall to the surface, and an impulsive burn just above the surface to kill the vertical velocity.

The real world often does not allow such an efficient strategy, which would require perfect knowledge of the horizontal velocity, and an infinite thrust capability. It also relies on perfect timing of the final braking burn - too soon, and the vehicle comes to a halt at significant altitude and falls; too late and it crashes without slowing.

As an example of the altitude-efficiency relationship, the AV capability of Surveyor 5's vernier engines was compromised by a helium pressurant leak during coast to the Moon. The descent sequence was modified en route by ground controllers, to fire the solid retro motor later (and therefore more efficiently) leaving less A V to be met by the verniers. However, this strategy resulted in retro burnout at an altitude of 1.3 km, instead of the originally planned 11km. There was clearly little margin for off-nominal retro performance or other errors.

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