Realistic trajectories

In practice, descent trajectories are intermediate between these extreme styles.

The usual approach is to null any significant horizontal and vertical motion above some nominal altitude. If a throttlable engine is used, then the vehicle is

Example - direct descent - Surveyor

programmed to descend at a constant rate to some other nominal altitude, say 2 m above the surface at which the velocity is nulled, and then the engines are shut off and the vehicle free-falls a short distance to the surface.

Note that in general, even if a non-throttlable engine is used at a constant thrust, the thrust-to-weight ratio, and therefore the acceleration, will change as the fuel mass of the lander reduces.

The Surveyor landers performed soft-landings on the Moon, in preparation for Apollo (Figure 5.1). The lander was turned to a predetermined orientation using Sun and star attitude references. A pulse radar altimeter generated an altitude reference mark at 100 km altitude. After a predetermined delay (8 s) throttlable 'vernier' engines were ignited; their velocity increment at this point

0 0

Post a comment