The Surveyor Spacecraft and Systems by Hamish Lindsay

The Surveyor Spacecraft and Systems

by Hamish Lindsay.

The Surveyor Spacecraft and Systems.

Spacecraft structure.
The basic Surveyor spacecraft structure consisted of a tripod of thin-walled aluminium tubing and interconnecting braces providing mounting surfaces and attachments for the power, communications, propulsion, flight control, and payload systems. A central mast extended about one meter above the apex of the tripod. Three hinged landing legs were attached to the lower corners of the structure. The legs held shock absorbers, crushable, honeycomb aluminium blocks, and the deployment locking mechanism and terminated in footpads with crushable bottoms. The three footpads extended out 4.3 meters from the centre of the Surveyor. The spacecraft was about 3 meters tall. The legs folded to fit into a nose shroud for launch.

Electronic Equipment.
The electronic equipment, requiring temperature control throughout the mission, was mounted on a thermal tray that distributed the heat throughout its compartment. An insulating blanket of 75 sheets of aluminised mylar was sandwiched between each compartment’s inner shell and the outer protective cover.

Compartment A contained two radio receivers, two transmitters, the main battery, battery charge regulator and the main power switch, maintained an internal temperature between 4.4°C and 51.7°C.

Compartment B housing the central command decoder, boost regulator, central signal processor, signal processing auxiliary, engineering signal processor and low bit rate auxiliary maintained an internal temperature between -17.8°C and 51.7°C.

Thermal Control.
Thermal control was achieved by a combination of white paint, high IR-emittance thermal finish and a polished aluminium underside. Two thermally controlled compartments, equipped with super insulating blankets, conductive heat paths, thermal switches and small electric heaters, were mounted on the spacecraft structure.

Power Generation.
The 0.855 square metre solar panel, made up of 3,960 cells, was the spacecraft’s primary power source during flight and during operations in the lunar day. It could deliver a maximum power of 90 watts. On command from Earth it could track the Sun to within a few degrees. On the Moon’s surface it was designed to deliver a minimum of 77 watts of power at a temperature of 60°C and a minimum of 57 watts at a temperature of 115°C.

The main power reservoir was a 14-cell rechargeable silver-zinc battery. Fully charged, the battery provided 3,800 watt/hours. A non-rechargeable silver-zinc auxiliary battery installed in a sealed magnesium container provided a back–up for both the main battery and the solar panel. Peak unregulated power capability was limited to 1,000 watts by the main and auxiliary batteries. The regulated power of 29 volts was provided by a boost regulator.

Communications.
Communications were achieved using a movable large planar array high gain antenna mounted near the top of the central mast to transmit television images, two omnidirectional conical antennas mounted on the ends of folding booms for uplink and downlink, two receivers and two transmitters providing 100mWatts or 10 watts of power.

The system operated in the S-Band with an uplink frequency of 2113 MHz and a down link frequency of 2295 MHz.

Television System.
The Surveyor I spacecraft carried two vidicon television cameras, but only used one. The main camera was mounted nearly vertically, at an angle of 16° to the vertical axis of the spacecraft, pointing at a movable mirror. The mirror could swivel 360° and tilt from 65° below the horizontal plane where it reflected a portion of a landing leg to 40° above the horizon. The camera could be focussed by Earth commands from a distance of 1.2 metres to infinity. Its iris setting, which controlled the amount of light entering the camera, could adjust automatically to the light level, or could be commanded from Earth. The camera had a variable focal length lens, which could be adjusted to either a narrow angle 100 mm focal length lens with a 6.4 x 6.4 field of view, or a 25mm wide-angle lens with a 25.4 x 25.4 field of view.

A focal plane shutter provided an exposure time of 150 milliseconds. The shutter could also be commanded open for an indefinite period of time. A sensing device coupled to the shutter kept it from opening if the light level was too intense. A too-high light level could occur from changes in the area of coverage by the camera, a change in the angle of mirror in the lens aperture, or by changes in Sun angle. The same sensor controlled the automatic iris setting. The sensor device could be overridden by ground command.

The camera system could provide either 200 or 600 line pictures. The latter required that the high-gain directional antenna and the high power level of the transmitter were both operational. The 600-line mode provided a picture each 3.6 seconds and the 200-line mode every 6l.8 seconds. The 200 line picture required 20 seconds for a complete video transmission using a bandwidth of 1.2 kHz, while the 600 line picture required 1 second using a 220 kHz bandwidth.

A filter wheel could be commanded to one of four positions, providing clear, coloured or polarising filters. The first and second Surveyors did carry a downward-looking approach television camera mounted on the lower frame of the spacecraft. However this camera was not used during the mission and was not included in the later missions.

On Earth the images were displayed on slow scan monitors coated with a long persistency phosphor, selected to match the maximum frame rate.

Navigation and Propulsion System.
A Sun sensor, Canopus tracker and rate gyros on three axes provided attitude knowledge. Propulsion and attitude control were provided by cold-gas (nitrogen) attitude control jets during cruise phases, three throttleable vernier rocket engines during powered phases, including the landing, and the solid-propellant retrorocket engine during terminal descent. The retrorocket was a spherical steel case mounted in the bottom centre of the spacecraft. The vernier engines used monomethyl hydrazine hydrate fuel and MON-10 (90% N2O2, 10% NO) oxidiser. Each thrust chamber could produce 130 N to 460 N of thrust on command - one engine could swivel for roll control. The fuel was stored in spherical tanks mounted to the tripod structure. For the landing sequence, an altitude marking radar initiated the firing of the main retrorocket for primary braking. After firing was complete, the retrorocket and radar were jettisoned and the Doppler and altimeter radars were activated. These provided information to the autopilot, which controlled the vernier propulsion system to touchdown.

Individual Mission Instrumentation.

Surveyor 1.

No instrumentation was carried specifically for scientific experiments, but considerable scientific information was obtained.

Surveyor 1 carried two television cameras – one mounted on the bottom of the frame for approach photography, which was not used due to radio bandwidth limitations, and the survey television camera. Over 100 engineering sensors were on board. Surveyor 1 had a mass of 995.2 kg at launch and 294.3 kg at landing after depletion of propellants and jettison of the altitude marking radar and main retro-rocket casing.

Surveyor 3.

Surveyor 3 was similar in design to Surveyors 1 and 2 but had several changes in the payload. It carried a vidicon tube survey television camera with 25mm and 100mm focal length lenses, soil mechanics experiments, and devices to measure temperature and radar reflectivity as on the earlier missions, but the TV camera had an extended glare hood.

A surface sampler, consisting of a 12 cm long by 5 cm wide sharpened scoop mounted on a 1.5 metre pantograph arm, replaced the approach television camera. It was mounted below the television camera and was designed to dig, scrape and trench the lunar surface while being observed by the television camera so the soil characteristics could be determined. It could also be dropped onto the lunar surface under force provided by gravity and a spring. Two flat auxiliary mirrors were attached to the frame to provide the camera with a view of the ground beneath the engines and one of the footpads.

Surveyor 3 had a mass of 1026 kg at launch and 296 kg at landing.

Surveyor 5.

The instrumentation for Surveyor 5 was similar to that of the previous Surveyors and included the survey television camera and numerous engineering sensors. An alpha-scattering instrument was installed in place of the surface sampler, and a small bar magnet attached to one footpad was included to detect the presence of magnetic material in the lunar soil. Convex auxiliary mirrors were attached to the frame to allow viewing of the surface below the spacecraft.

Surveyor 5 had a mass of 1006 kg at launch and 303 kg at landing.

Surveyor 6.

With a payload virtually identical to that of Surveyor 5, this spacecraft carried a television survey camera, a small bar magnet attached to one footpad to detect magnetic material, an alpha-scattering instrument to study surface composition, and convex auxiliary mirrors mounted on the frame to view the surface under the spacecraft, as well as the necessary engineering equipment. The main differences were that Surveyor 6 had polarising filters on the TV camera, a different type of glare hood, and had 3 auxiliary mirrors instead of 2.

Surveyor 6 had a mass of 1006 kg at launch and 299.6 kg on landing.

Surveyor 7.

Surveyor 7 was similar in design to Surveyor 6, but the payload was the most extensive flown during the Surveyor program. It carried a television camera with 25 and 100 mm focal length lenses, polarising filters, an alpha-scattering instrument, a surface sampler similar to that flown on Surveyor 3, bar magnets on two footpads, two horseshoe magnets on the surface scoop, and auxiliary mirrors. Of the auxiliary mirrors, three were used to observe areas below the spacecraft, one to provide stereoscopic views of the surface sampler area, and seven to show lunar material deposited on the spacecraft. It also carried over 100 items to monitor engineering aspects of spacecraft performance.

Surveyor 7 had a mass of 1039 kg at launch and 306 kg at landing.

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