The supporting structure for systems capable of leaving the Earth and its atmosphere, performing a useful mission in space, sometimes returning to the Earth and sometimes landing on other bodies. Among the principal technologies that enter into the design of spacecraft structures are aerodynamics, aerothermodynamics, heat transfer, structural mechanics, structural dynamics, materials technology, and systems analysis. In applying these technologies to the structural design of a spacecraft, trade studies are made to arrive at a design which fulfills system requirements at a minimum weight with acceptable reliability and which is capable of being realized in a reasonable period of time.
The structural aspects of space flight can be divided into six broad regions or phases: (1) transportation, handling, and storage; (2) testing; (3) boosting; (4) Earth-orbiting flight; (5) reentry, landing, and recovery on the Earth; (6) interplanetary flight with orbiting of or landing on other planets. Each phase has its own structural design criteria requiring detailed consideration of heat, static loads, dynamic loads, rigidity, vacuum effects, radiation, meteoroids, acoustical loads, atmospheric pressure loads, foreign atmospheric composition, solar pressure, fabrication techniques, magnetic forces, sterilization requirements, accessibility for repair, and interrelation of one effect with the others. Heavy reliance is placed on computer-generated mathematical models and ground testing.
The basic spacecraft structural design considerations apply equally well to both crewed and crewless spacecraft. The degree of reliability of the design required is, however, much greater for crewed missions. Also, the spacecraft structures in the case of crewed missions must include life-support systems and reentry and recovery provisions. In the case of lunar or planetary missions where landing on and leaving the foreign body are required, additional provisions for propulsion, guidance, control, spacecraft sterilization, and life-support systems must be realized, and the structure must be designed to accommodate them.
Testing
To ensure that spacecraft structures will meet mission requirements criteria in general requires testing to levels above the expected environmental conditions by a specific value. The test level must be set to provide for variations in materials, manufacture, and anticipated loads. In cases where structures are required to perform dynamic functions repeatedly, life testing is required to ensure proper operation over a given number of cycles of operation. See also Inspection and testing.
Boost
The purpose of the boost phase is to lift the vehicle above the sensible atmosphere, to accelerate the vehicle to the velocity required, and to place the spacecraft at a point in space, heading in the direction required for the accomplishment of its mission. For space missions, the required velocities range from 26,000 ft/s (8 km/s) for nearly circular Earth orbits to 36,000 ft/s (11 km/s) for interplanetary missions. Achievement of these velocities requires boosters many times the size of the spacecraft itself. Generally, this boosting is accomplished by a chemically powered rocket propulsion system using liquid or solid propellants. Multiple stages are required to reach the velocities for space missions. Vertical takeoff requires a thrust or propulsive force that exceeds the weight of the complete flight system by approximately 30%. An example of a multiple-stage booster is the Delta launch vehicle used for the crewless missions (see illustration). The Delta II 7925 vehicle has the capability to place 4000 lb (1800 kg) into a geosynchronous transfer orbit. See also Interplanetary propulsion; Propellant; Rocket propulsion.
Exploded view of typical Delta II 7925 three-stage structure. (After Commercial Delta II Payload Planners Guide, McDonnell Douglas Commercial Delta Inc., 1990)
Space phase and design considerations
The space phase begins after the boost phase and continues until reentry. In this phase, the structures that were stowed for launch are deployed. Important design considerations include control system interaction, thermally induced stress, and minimization of jitter and creaks.
The spacecraft control system imparts inertial loads throughout the structure. In the zero gravity environment, every change in loading or orientation must be reacted through the structure.
Spacecraft structural design usually requires that part of the principal structure be a pressure vessel. Efficient pressure vessel design is therefore imperative. An important material property, especially in pressure vessel design, is notch sensitivity. Notch sensitivity refers to the material's brittleness under biaxial strain. This apparent brittleness contributed to premature failure of some early boosters. See also Pressure vessel.
Meteoric particles may have extremely high velocities relative to the spacecraft (up to 225,000 ft/s or 68 km/s). Orbital debris also include residual particles resulting from human space-flight activities. Collisions involving these bodies and a space station and other long-duration orbiting spacecraft are inevitable. The worst-case effects of such collisions include the degradation of performance and the penetration of pressure vessels, including high-pressure storage tanks and habitable crew modules. An essential parameter in the design of these structures is the mitigation of these effects. See also Meteor.
Radiation shielding is required for some vehicles, particularly those operating for extended times within the Earth's magnetically trapped radiation belts or during times of high sunspot activity. The shielding may be an integral part of the structure. Computer memories are particularly susceptible to radiation and cosmic-ray activity and must be shielded to survive. Effects of radiation on most metallic structures over periods of 10–20 years is not severe. The durability of composite structures in space is a major concern for long life. Based on available data, the synergistic effects of vacuum, heat, ultraviolet, and proton and electron radiation degrade the mechanical, physical, and optical properties of polymers.
Temperature extremes in the structure and the enclosed environment are controlled by several techniques. Passive control is accomplished by surface coatings and multilayer thermal blankets which control the radiation transfer from the spacecraft to space and vice versa. Because incident solar radiation varies inversely with the square of distance from the Sun, means of adjusting surface conditions are required for interplanetary missions. Heat generated by internal equipment or other sources must be considered in the heat balance design. Other techniques used to actively control spacecraft temperatures are thermal louvers and heat pipes. See also Heat pipe.
Thermal gradients must be considered in spacecraft design, especially when the spacecraft has one surface facing the Sun continuously. In some cases it is desirable to slowly rotate the spacecraft to eliminate such gradients.
Spacecraft structures usually are required to be lightweight and rigid, which results in the selection of high-modulus materials. Titanium and beryllium have low densities and relatively high modulus-density ratios. Alloys of these metals are relatively difficult to fabricate, and therefore their application is quite limited. The more common aluminum, magnesium, and stainless steel alloys are basic spacecraft structural materials. They are easy to fabricate, relatively inexpensive, and in general quite suitable for use in the space environment. Plastics are used in spacecraft structures when radio-frequency or magnetic isolation is required. They are also used in situations where some structural damping is desired.
The modern requirements for low weight, high strength, high stiffness, and low thermal expansion (for precision optical pointing) have prompted the use of composite materials for spacecraft structure. These materials consist of high-strength reinforcement fibers which are supported by a binder material referred to as the matrix. The fibers are typically made of glass, graphite, or carbon, and the matrix is an epoxy resin. See also Composite material.
Reentry phase
Although the atmospheric layer of the Earth is relatively thin, it is responsible for the reduction of vehicle velocity and the resulting deceleration loads, as well as for the severe heating experienced by reentering vehicles. A body entering the Earth's atmosphere possesses a large amount of energy. This energy must be dissipated in a manner which allows the reentering vehicle to survive. Most of the vehicle's original energy can be transformed into thermal energy in the air surrounding the vehicle, and only part of the original energy is retained in the vehicle as heat. The fraction that appears as heat in the vehicle depends upon the characteristics of the flow around the vehicle. In turn, the flow around the vehicle is a function of its geometry, attitude, velocity, and altitude. See also Atmospheric entry.
Spacecraft are seldom designed to reenter the Earth's atmosphere (the space shuttle being an exception), but may be designed to enter extraterrestrial atmospheres. In either case, the structural design is similar.
High-speed reentry causes extreme friction and heat buildup on spacecraft that must be dissipated by using high-temperature ceramic or ablative materials. The space shuttle is covered with special thermal insulating tiles that allow the structural elements to remain cool when the surface reaches 1200°F (650°C), and its leading edges are protected by a carbon-carbon reinforced material that can withstand temperatures as high as 2300°F (1260°C).
Satellites whose orbits decay into the Earth's upper atmosphere become flaming objects as they rapidly descend. Generally, most or all of the satellite is consumed before it reaches the surface, but there are exceptions such as the March 22, 2001, reentry of the Russian space station, Mir.
In crewed applications, vehicles employing aerodynamic lift during reentry have several advantages over zero-lift ballistic bodies: (1) The use of lift allows a more gradual descent, thus reducing the deceleration forces on both vehicle and occupants. (2) The vehicle's ability to glide and maneuver within the atmosphere gives it greater accuracy in either hitting a target or landing at a predetermined spot. (3) It can accommodate greater errors of guidance systems because for a given deceleration it can tolerate a greater range of entry angles. (4) Greater temperature control is afforded because aerodynamic lift may be varied to control altitude with velocity.
Structures
Erectable structures take many and varied shapes. They are sometimes relatively simple hinged booms, while on other occasions they become quite large and massive. Many more spacecraft structures are rigid than erectable or inflatable.
The space shuttle or Space Transportation System (STS) can carry 65,000 lb (30,000 kg) of cargo to and from low Earth orbit. See also Space shuttle.
The International Space Station (ISS) is a cooperative, 16-nation effort. It will include six laboratories and weigh a million pounds when assembled. See also Communications satellite; Meteorological satellites; Military satellites; Satellite navigation systems; Scientific satellites; Space station.
