(aerospace engineering) The use of rocket engines to accelerate space vehicles.

A system that provides control of location and attitude of spacecraft by using rocket engines to generate motion. Spacecraft propulsion systems come in various forms depending on the specific mission requirements. Each exhibits considerable variation in such parameters as thrust, specific impulse, propellant mass and type, pressurization schemes, cost, and materials. All of these variables must be considered in deciding which propulsion system is best suited to a given mission. Typical spacecraft applications include communications satellites, science and technology spacecraft, and Earth-monitoring missions such as weather satellites. Orbital environments range from low-Earth to geosynchronous to interplanetary. See also Astronautical engineering; Rocket propulsion; Satellite (spacecraft); Space flight; Space probe; Specific impulse.

The two fundamental variables that define the design of spacecraft propulsion systems are the total velocity change to be imparted to the spacecraft for translational purposes, and the impulse necessary to counteract the various external torques imposed on the spacecraft body. From these, the required quantity of a given propellant combination can be specified. Propellant accounts for almost 60% of the lift-off mass of a communications satellite.

The specific impulse has a significant effect on the total propellant load that a spacecraft must carry to perform its assigned mission. Since a massive satellite must be boosted into space by the use of expensive launch vehicles, such as the space shuttle and Ariane, significant cost savings may be gained if smaller, less expensive launch vehicles may be used. The size of the required launch vehicle is directly proportional to the mass of the pay-load. Since most of the other components that make up spacecraft are relatively fixed in weight, it is critical to utilize propellant combinations that maximize specific impulse.

For modern spacecraft the choices are either bipropellants, which utilize a liquid oxidizer and a separate liquid fuel; solid propellants, which consist of oxidizer and fuel mixed together; or monopropellants, which are liquid fuels that are easily dissociated by a catalyst into hot, gaseous reaction products. High specific impulse is offered by bipropellants, followed by solid propellants and monopropellants.

Spacecraft attitude control schemes play an important role in defining the detailed characteristics of spacecraft propulsion systems. Essentially, there are three methods for stabilizing a spacecraft: three-axis control, spin control, and gravity gradient. In three-axis systems the body axes are inertially stabilized with reference to the Sun and stars, and utilize rocket engines for control in all six degrees of freedom. Spin-stabilized spacecraft use the inertial properties of a gyroscope to permanently align one of the axes by rotating a major portion of the spacecraft body about this axis. This approach significantly reduces the number of thrusters needed for control. Gravity gradient control is a nonactive technique that relies on the Earth's tidal forces to permanently point a preferred body axis toward the Earth's center. See also Gyroscope; Inertial guidance system; Spacecraft structure.

Translation of a spacecraft, independent of its control technique, requires thrusters aligned parallel to the desired transla-tional axis. Usually, all three axes require translational capability. Combining the two requirements for attitude control and translation results in the minimum number of rocket engines required to perform the mission. These are supplemented with additional thrusters to allow for failures without degrading the performance of the propulsion system. Simplistically, it would be reasonable to assume that the propellant-engine combination with the highest specific impulse would be the preferable choice. However, the ultimate requirement is the lowest possible mass for the entire propulsion system. The complexity of the system is greatly influenced by, and is roughly proportional to, the specific impulse, since bipropellants require more tanks, valves, and so forth than either solid systems or monopropellant systems. This is primarily due to the differences in density between liquids and solids, and the fact that bipropellants require high-pressure gas sources to expel the fluid from the tanks and into the rocket engine chamber. For communications satellites in the lift-off weight range of 3000 lbm (1360 kg), the trade-off between specific impulse and system mass dictates the use of a solid rocket motor for the main-orbit circularizing burn and a monopropellant propulsion system for on-orbit attitude control and translation. Spacecraft launch masses above about 5000 lbm (2268 kg) require the use of all-bipropellant systems.