Design

Perhaps the first point to note regarding the tripod-shaped Surveyors is that they were delivered directly from the launcher, rather than separated from a carrier vehicle (such as a cruise stage, flyby craft, orbiter or descent stage). The Moon's proximity and lack of atmosphere make this architecture a possible solution, and indeed it was also used by the Soviet Ye-8 series. Its use for missions to worlds beyond the Moon has in practice been precluded, for example by requirements for both an entry shield and mid-course correction capability, the need for relay communications, and often the presence anyway of an orbital element to the mission. (Such a 'lander-only' mission architecture may also be appropriate for some minor-body missions, however.)

The Surveyor spacecraft employed a distinctive open structure of tubular aluminium, onto which the spacecraft subsystems and payload were mounted. The landing gear comprised three hinged landing legs with shock-absorbers and hinged footpads, backed up by three crushable blocks mounted on the underside. The footpads and crushable blocks used aluminium honeycomb to ensure damping of the landing loads. Electronic equipment was housed in two thermally controlled compartments attached to the spaceframe.

A vertical mast carried the solar array and a planar high-gain antenna, both of which were articulated. Two deployable low-gain antennas were also incorporated. Radio communications operated in the S-band, and the transmitters could feed either low power (100 mW) or high power (10 W) to any of the three antennas.

Reading an account of the history of the project (Koppes, 1982), one can speculate that the open structure may have arisen as a result of the prime contractor's highly granular division of the project, into about a hundred 'units' or 'control items', rather than the now conventional set of major subsystems (structure, thermal, power, communications, propulsion, etc.). While this may have limited the scope for design optimisation, there was ample scope for flexibility from one mission to the next. Variations occurred in both payload instrumentation and engineering subsystems.

Propulsion for braking and descent was provided by a main retro-rocket using solid propellant (a Thiokol TE-364), complemented by a three-nozzle throttlable vernier propulsion system using liquid bipropellant (monomethyl hydrazine hydrate and MONO-10 oxidiser). Two radar systems were used, the first to initiate firing of the main retro motor, the second forming part of a closed-loop control system with the vernier engines in the final stages of descent. The first radar was mounted in the nozzle of the retro motor and, its job having been done, was jettisoned before the retro fired. The retro motor was nominally planned to burn out after -40 s at around 10 km altitude, after which it too was jettisoned and the vernier motors took over the braking for the final part of the descent. Attitude measurement was done using Sun and Canopus sensors, and gyroscopes. Attitude control was achieved using a nitrogen cold-gas system. See Chapter 5 for more detail on the descent phase.

Power was provided by a 0.855 m2 solar array, which generated up to 85 W, and silver-zinc rechargeable batteries. On Surveyor 1-4 an additional, 'auxiliary' primary battery was installed to ensure operation until shortly after landing.

Thermal control was achieved by a mixture of passive and active control. Passive control was achieved by a combination of white paint, high IR-emittance thermal finish and polished aluminium, while the electronics compartments were equipped with insulating blankets, conductive heat paths, thermal switches and electric heaters.

The landing sites were equatorial on the near-side (as for Apollo), with the exception of the more scientifically-driven Surveyor 7, which touched down in the southern highlands to sample contrasting terrain. The landing accuracy with respect to the intended target location ranged from 2.4 km (Surveyor 7) to 28.8 km (Surveyor 5). Landings generally occurred shortly after local sunrise, allowing the maximum period of time for surface operations before sunset.

The three main payload experiments were the TV camera (10.6 kg), soil mechanics surface sampler (SMSS, 9.2 kg) and alpha-scattering instrument (12.4 kg), although only the camera was carried on all seven missons. Additional experiments and engineering sensors included strain gauges and temperature sensors distributed throughout the spacecraft, mirrors, magnets and photometric targets for the camera. A descent camera was carried, but not used, on Surveyors 1 and 2 only. Its mounting position was used instead for the SMSS on Surveyor 3.

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