BASIC CONSIDERATIONS IN SPACECRAFT DESIGN

Spacecraft is a general term that includes sounding rockets, artificial satellites, and space probes. They are considered separately from the rocket-powered space launch vehicle, which gives escape velocity to the craft.

A space probe is a spacecraft that is launched at higher than Earth orbital velocity and escapes the Earth's gravitational attraction.

Space probes may be classed as lunar, planetary, or deep-space. Other classifications of spacecraft are manned or unmanned, active or passive. A passive satellite transmits no radio signals. It may be tracked optically or with radar, and radio communications signals may be “bounced” off its surface. Active satellites send out radio signals to make tracking easier and to transmit data from their instruments to ground stations or other craft.

One other general differentiation of satellites is by function: scientific or applications. A scientific satellite carries instruments to obtain scientific data on magnetic fields, space radiation, the Sun or other stars, etc. Applications satellites have utilitarian tasks; examples are Earth survey, communications, and navigation satellites.

Spacecraft thus differ greatly in size, shape, complexity, and purpose. Because more than 5,000 spacecraft have been launched since 1957, it is convenient to group them into program families—e.g., the Soviet Sputnik, Vostok, Soyuz, and Venera; and the U.S. Explorer, Intelsat, Apollo, Voyager, and Space Shuttle.Lightness of weight and functional reliability are primary features of spacecraft design. Depending upon their mission, spacecraft may spend minutes, days, months, or years in the environment of space. Mission functions must be performed while exposed to high vacuum, extreme variations in temperature, and radiation.

There are nine general categories of subsystems found on most spacecraft. They are (1) power supply; (2) on-board propulsion; (3) communications; (4) attitude control (i.e., maintaining a spacecraft's orientation toward a specific direction and pointing precisely at selected targets); (5) environmental control (e.g., regulation of temperature and pressure and removal of toxic substances); (6) guidance and velocity control; (7) computer and auxiliary hardware; (8) structure (skeleton framework of the spacecraft that physically supports all other subsystems); and (9) engineering instruments that monitor the status of the spacecraft.

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