Example direct descent Surveyor

Cruise attitude

Pre-retro manoeuvre 30 mins before touchdown to align main retro with flight path

Main retro start by altitude-marking radar which ejects from nozzle. Craft stabilized by vernier engines

Main retro start by altitude-marking radar which ejects from nozzle. Craft stabilized by vernier engines

Main retro burnout and ejection. Vernier engines control descent

Vernier engines shut down

Vernier engines shut down

Figure 5.1. Surveyor descent sequence.

was modest - their function was to control attitude (using gyros for attitude determination) during the retro-motor firing. One second after vernier ignition, the large solid rocket motor was fired at 76 km altitude as the vehicle descended at 2.7 km s~x, burning for about 40 s to leave the vehicle descending at just over 100ms~x. The motor, which comprised 655 kg of the 995 kg launch mass, was ejected 10 s later, at 40 km altitude. Subsequently the descent was controlled only by the vernier engines, using a four-beam Doppler radar altimeter and velocity sensor. (On Surveyor 1, one of the beams lost lock briefly, probably due to a spurious return from the retro motor casing as it fell away.)

The guidance law was essentially to perform a gravity turn, i.e. thrusting against the instantaneous velocity vector, with a constant deceleration of 0.9 lunar gravities. This law defines a parabola in range-velocity space, which was approximated in the lander guidance software as a set of straight-line segments.

Altitude marks were generated at 310 m and 4.5 m; the lander took 19 s to make that part of the descent. Thereafter, the vernier thrusters were turned off, and the spacecraft hit the ground 2 s later.

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