Powered reascent

Scientific return from a lander may be substantially enhanced if it can make surface measurements at more than one location. In principle, any vehicle that can make a soft landing can make a takeoff. This was performed on Surveyor 6, which after 177 h on the lunar surface, reignited its engines for 2.5 s, lifting it to 3 m high and translating 3 m to one side, allowing stereoscopic imaging and study of the original footpad imprints.

The most efficient horizontal transfer trajectory (in the absence of drag) is an impulsive-ballistic one, where a maximum thrust burn puts the vehicle on a ballistic trajectory, and a second burn close to impact brakes the motion. On a sufficiently large body (or short trajectory) where the 'flat earth' approximation applies, this trajectory is parabolic and the velocity impulse AV (applied at 45° to the horizontal) relates to the horizontal distance travelled D, as taking a time H2(AV/g). The maximum altitude attained is just half the range. Clearly, an equivalent impulse must be applied on landing to bring the vehicle to rest.

Ascent may be required instead to reach orbit or a hyperbolic escape trajectory, to return samples to Earth for analysis. In these cases most of the same considerations apply as to powered descent, although issues such as the stor-ability of propellant may come into play. Further, note that the algebra of rocket staging is such that the specific impulse performance of the last stage is strongly leveraged - an important factor in Mars sample return considerations. However, the AV to return from the Moon is essentially that of lunar escape velocity: the 2.7 km s~ can be provided by a single stage using storable propellants, as in the 520 kg UDMH/nitric acid ascent stage used by Luna 16 to return a 39 kg entry capsule to Earth.

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