Propulsion Subsystems full report
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09-04-2010, 10:35 PM
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Spacecraft are provided with sets of propulsive devices so they can maintain three-axis stability, control spin, execute maneuvers, and make minor adjustments in trajectory. The more powerful devices are usually called engines, and they may provide a force of several hundred newtons. These may be used to provide the large torques necessary to maintain stability during a solid rocket motor burn, or they may be the only rockets used for orbit insertion.
Smaller devices, generating between less than 1 N and 10 N, are typically used to provide the delta-V for interplanetary trajectory correction maneuvers, orbit trim maneuvers, reaction wheel desaturation maneuvers, or routine three-axis
stabilization or spin control.
Many of the activities of propulsion subsystems are routinely initiated by AACS. Some or all may be directly controlled by or through CDS.
This photo shows one of the Magellan spacecraft's four rocket engine modules, that were mounted on struts not unlike Cassini's, pictured below. Each module has two 445-N engines, one gold-colored 22-N thruster, and three gold colored 1-N thrusters. The 445-N engines were aimed aft for large midflight course corrections and orbit-trim corrections, and for controlling the spacecraft while its solid rocket moror burned during Venus orbit insertion. The 22-N thrusters kept the spacecraft from rolling during those same maneuvers. The 1-N thrusters were used for momentum wheel desaturation and other small maneuvers
Other components of propulsion subsystems include propellant tanks, plumbing systems with electrically or pyrotechnically operated valves, and helium tanks to supply pressurization for the propellant. Some propulsion subsystems, such as Galileo's, use hypergolic propellants--two compounds stored separately which ignite spontaneously upon being mixed in the engines or thrusters. Other spacecraft use hydrazine, which decomposes explosively when brought into contact with an electrically heated metallic catalyst within the engines or thrusters. Cassini, whose propulsion system is illustrated below, uses both hypergolics for its main engines and hydrazine monopropellant for its thrusters.
The Deep Space 1 spacecraft was a pioneer in the use of ion-electric propulsion in interplanetary space. With their high specific impulse (due to high nozzle exit velocities), ion engines can permit spacecraft to achieve the high velocities required for interplanetary or interstellar flight.
DS-1 ION ENGINE.
An ion engine functions by taking a gas such as xenon, and ionizing it (removing electrons from the atoms) to make it responsive to electric and magnetic fields. Then the ions are accelerated to extremely high velocity using electric fields and ejected from the engine. Electrical power comes from arrays of photovoltaic cells converting sunlight, so the technology is also called solar-electric propulsion. The act of ejecting mass at extremely high speed provides the classical action for which the reaction is spacecraft acceleration in the opposite direction. The much higher exhaust speed of the ions compared to chemical rocket exhaust is the main factor in the engine's higher performance.
The ion engine also emits electrons, not to take advantage of accelerating their tiny mass, but to avoid building a negative electric charge on the spacecraft and causing the positively charged ion clouds to follow it
1. Processes and Materials of Manufacture by R.A. LINDBERG
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