The elements of design of a spacecraft are governed by its mission and operational requirements. Thus the wide panorama of different flight projects undertaken by the United States and the Soviet Union has resulted in widely varying spacecraft designs. The spacecraft's destination, the tasks that are to be performed, the duration of the space fright, the physical conditions to which die spacecraft will be exposed, and whether or not it will be manned are all basic considerations in spacecraft design.
Axiomatic in the design of a spacecraft is the continual compromise between the most desirable and the feasible, within budgetary and time-for-completion requirements. From the beginning of design concept through engineering development and test, low final weight is a prime concern for economic reasons. Large launch vehicles are costly, and no economic way has yet been found to recover space boosters. For information on launch vehicles, see the article rockets.
Many factors must be considered simultaneously in designing a spacecraft. The stringency of the requirements calls for a team of technical specialists, systems engineers, and administrators that work closely together. These teams, with their unique industrial, academic, and governmental backgrounds, utilize the experience of past space programs in design concepts.
Unmanned Satellites and Space Probes. An earth satellite may serve as a communications relay, monitor the earth's weather and map its natural resources, or detect nuclear explosions. Each of these missions imposes special requirements for on-board sensors and instrumentation, altitude control, rocket propulsion, electric power supply, transmitters and receivers, computers, inertial guidance reference, and, in some cases, provisions for landing and recovery.
Most satellites and space probes are not designed for return to earth and recovery. Therefore they do not have to withstand the high-g loads of reentry, nor do they require heat shielding. In orbit, in the weightless condition, the spacecraft may appear flimsy by terrestrial standards. Extensible antennas may stretch out hundreds of feet. Large panels of solar cells may unfold to absorb sunlight, converting the radiation to electric power. Storage batteries provide a ready power supply for short-term, heavy power requirements and for those periods when the spacecraft may be in the earths shadow.