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a craft designed for flight into or in space—for example, launch vehicles (space rockets) and artificial satellites of the earth and other heavenly bodies.
The term “spacecraft” is general and encompasses various types of such craft, including vehicles using a nonreaction principle of motion (for example, a solar sail). Launch vehicles are used to develop the required velocity for space flight. They may be divided into two primary groups: (1) near-earth orbital spacecraft, which move in geocentric orbits and do not leave the earth’s field of gravitational influence, and (2) interplanetary spacecraft, which leave the earth’s sphere of influence and enter that of the sun, the planets, or their natural satellites. In addition, a distinction is made between unmanned spacecraft (unmanned satellites of the earth, the moon, Mars, and the sun; unmanned interplanetary probes) and manned spacecraft (orbital spacecraft, interplanetary spacecraft, and manned orbital stations). A large number of these types of spacecraft have already been developed; interplanetary spacecraft for flight to and landing on other planets, as well as reusable shuttle craft, are now under development.
A spacecraft’s flight is divided into the following phases: insertion, in which the required velocity in a given direction is imparted to the spacecraft; orbital, during which the spacecraft’s motion takes place for the most part through inertia, according to the laws of celestial mechanics; and landing. In a number of cases spacecraft are equipped with rocket motors, which make possible changes (corrections) in trajectory during the orbital phase and deceleration of the spacecraft during landing. For modern spacecraft using chemical rocket engines, the powered phase (orbital insertion, correction, and braking) is considerably shorter than the orbital phases.
Rockets are the only available means for accomplishing space flights. A rocket’s maximum velocity depends on the velocity of the reaction jet, which is determined by the type of fuel and the efficiency of the motor, and on the ratio of the mass of the fuel to the total (initial) mass of the rocket (that is, the efficiency of the design of the rocket), as well as on the mass of the payload. The speed of the reaction jet of a rocket using modern chemical fuels is 3,000-4,500 m/sec; however, even a well-designed single-stage rocket is not capable of developing the velocity necessary for space flight (about 8 km/sec). Therefore, it is common practice to use multistage rockets from which sections (fuel tanks and motors) separate as the fuel is burned.
The rockets mainly used in space flight (the launch vehicles) have two to four stages. The configurations of such rockets vary widely; their common distinctive feature is a relatively small structural mass (together with the power plant it normally does not exceed 10-12 percent of the mass of the fuel). The provision of high rigidity and durability in such a design is a complex engineering problem. A rocket performs under very great static and dynamic loads; therefore, maximum use of the strength of its materials and maximum efficiency in the design of individual components are needed, despite the considerable size of the rocket as a whole. The equipment of a rocket includes a number of systems and components for flight control, staging, fuel-tank supercharging, and fuel feed control. The power plants of space rockets usually consist of several motors, whose operation is synchronized.
The control system of a rocket provides for flight along a predetermined trajectory, stabilization of the rocket relative to its center of mass, control of the engine (thrust control, ignition, and shutdown), and the initiation of staging commands. The control system is a complex group of instruments and components (gyroscopic, electronic, electromechanical, and so on) and, in a number of cases, includes an on-board computer. Space rockets are one of the most significant achievements of modern science and technology; their development requires a high level of technological advancement in many fields, among them metallurgy, chemistry, radio electronics, and computer technology.
A distinctive feature of most spacecraft is their ability to operate independently in space for long periods. In many respects (for example, the laws governing their motion; the thermal conditions on board the craft), spacecraft are similar to independent heavenly bodies on which the necessary conditions for the existence of humans and the functioning of equipment have been created. Many spacecraft have systems for internal temperature control, for power supply for on-board equipment and in-flight maneuvers, and for radio communications with earth. The necessary living and working conditions are provided in the sealed cabin of manned spacecraft—provision is made for regeneration of air, with temperature and humidity control, and for food and water supply. The solution of problems of life support on manned orbital stations and interplanetary spacecraft is especially complex.
Many spacecraft are equipped with attitude control systems. A spacecraft is usually oriented to perform specific functions (scientific observation, radio communication, illumination of solar batteries, and so on). The accuracy of orientation of a spacecraft may vary from 10°-15° to as little as a few seconds of arc, depending on the task. Trajectory changes (corrections, spacecraft maneuvers, or braking before atmospheric reentry or descent to the surface of the earth or other planet) are needed for any fairly complex flight configuration. Therefore, all manned spacecraft and most unmanned craft are equipped with an inflight maneuvering system and on-board rocket engines.
The maintenance of the required temperature on board a spacecraft is a special problem. In contrast to conditions on the earth’s surface, in space only radiative heat transfer occurs between individual bodies; a spacecraft is affected by external heat fluxes—radiation from the sun, the earth, or another nearby planet—which are usually of variable intensity (for example, when the spacecraft enters the earth’s shadow, or as a result of flight at varying distances from the sun). In turn, a spacecraft must radiate into space a certain amount of heat; the amount depends on the level of absorption of external heat fluxes and internal heat generation. Spacecraft usually have a radiation surface (part of their outer hull or a separate radiant heat exchanger) designed specifically to radiate the craft’s own internal heat while absorbing little external heat. The spacecraft’s thermal balance (its temperature) is controlled by varying the heat input to the radiation surface as well as the radiation of the surface itself (for example, by means of special louvers). As a result of weightlessness, the heat processes on board the spacecraft are characterized by the absence of convective heat exchange; therefore, the control of internal thermal conditions is one of the functions of the temperature control system.
The problem of supplying power to the on-board equipment is dealt with in several different ways: (1) by using solar batteries to convert solar radiation into electric power (this is the power supply method that is most widely used on modern spacecraft; it provides a service life of up to several years for the equipment); (2) through the installation of fuel cells, a power source having a high level of power generation per unit of mass, which produce electric power through electrochemical processes taking place between two active agents, such as oxygen and hydrogen (the water thus produced may be used in the life-support systems of manned spacecraft); (3) through the use of on-board nuclear power plants with reactors and radioisotope batteries. Chemical power sources (storage batteries) are used only on spacecraft with a short equipment service life (1-3 weeks) or as buffer batteries in power-supply systems (for example, in combination with solar batteries).
The flight of both manned and unmanned spacecraft would not be possible without radio communication and the transmission of telemetry and television data to earth, the receipt of radio commands, periodic trajectory measurements, and telephone and telegraph communication with the cosmonauts. These functions are performed by on-board radio systems and ground-based instrumentation control centers.
One of the most complex problems of space flight is the descent of a spacecraft to the surface of the earth or another planet, when the spacecraft’s velocity must be reduced to zero at the moment of touchdown. Two braking methods are possible: the use of braking reactive force or the aerodynamic forces acting on the spacecraft in the atmosphere. In the first method the spacecraft or part of it (the descent capsule) must be equipped with a retrorocket and a considerable fuel reserve; therefore, powered descent is used only for landing on heavenly bodies lacking an atmosphere, such as the moon. Descent by aerodynamic braking is more advantageous in terms of mass and is the primary descent method used by spacecraft for earth reentry. During descent along a ballistic trajectory the deceleration load reaches 8–10 g; descent along a gliding trajectory, on which lift as well as drag works to slow the spacecraft, makes it possible to reduce the g-load by a factor of 1.5-2.0.
Intense aerodynamic heating of the descent capsule takes place during the atmospheric reentry phase. Therefore, the spacecraft is provided with a heat shield made of ceramic or organic materials with great heat resistance and low heat conductivity. At the end of the descent trajectory, at an altitude of several kilometers, the spacecraft’s velocity has been reduced to 150–250 m/sec. A parachute system is normally used for further velocity reduction before touchdown. A soft landing system, which reduces touchdown velocity virtually to zero, has been successfully used on the Voskhod and Soyuz spacecraft.
A spacecraft’s structure is characterized by a number of features connected with the specific properties of outer space—high vacuum and the presence of meteorites, intensive radiation, and weightlessness. Under vacuum conditions, the nature of friction changes, and “cold welding” arises, which requires the selection of appropriate materials for mechanisms and the sealing of some units. The action of very small meteorites on a spacecraft’s surface during a prolonged flight can bring about a change in the optical characteristics of windows, some instruments, radiation surfaces, and solar batteries, which necessitates special coatings and special treatment of the spacecraft’s surface. The probability of meteor puncture of the outer hull of a modern spacecraft’s pressurized compartments is small. Provision must be made for protection of large manned spacecraft and orbital stations intended for prolonged flights.
Cosmic radiation (charged particle streams present in the earth’s radiation belt and during solar flares) can affect solar batteries, parts made of organic compounds, and other spacecraft components; therefore, in a number of cases, protective coatings must be applied. Special measures are taken to protect cosmonauts from outbursts of cosmic radiation.
A high degree of reliability is essential for all types of spacecraft, particularly manned craft. Such reliability is provided by measures taken at all stages of spacecraft development and pre-flight preparation, including an increase in the reliability of structural components and equipment, strict quality control during all stages of manufacture, thorough adjustment of system and components through simulation of actual flight conditions, and integrated preflight tests. Spacecraft reliability is increased by double and triple redundancy of some instruments and components, as well as by the use of automatic troubleshooting circuits, with subsequent replacement of failed components.
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K. D. BUSHUEV