Bogazici University Space Technologies Laboratory - Department of Mechanical Engineering
Satellites and spacecrafts moving in an orbit or in interplanetary space use propulsion systems that use the principle of conservation of momentum to provide thrust. Typical in-space propulsion systems convert the chemical energy stored in the on-board propellant into kinetic energy. Propulsion systems that use alternative sources of energy (batteries, solar panels, radioactive sources of energy, nuclear reactors, etc.) are being developed as an alternative to the standard space propulsion systems. Since in these alternative propulsion systems, the energy carried by the vehicle is not limited to the energy stored in the propellants chemical bonds, these types of propulsion systems can provide higher levels of impulse for the same amount of propellant or can accomplish certain missions with less amount of propellant. There are numerous ways of using - External energy sources for propulsion. In some of these systems the propellant is ionized and plasma is obtained, this plasma is then accelerated with the help of electromagnetic forces and expelled from the spacecraft at high velocities to produce the desired thrust. These thrusters, also called plasma rockets, are being considered for use in earth orbiting satellites of various types (communication, meteorology, military, intelligence, etc.) as they provide possibilities for the extension of satellite life as well as the reduction of amount of fuel required for its lifetime. Additionally, the development and use of these thrusters will be significant for the realization of certain manned and/or unmanned interplanetary missions.

Among the various types of plasma thrusters developed over the last few decades, Hall effect thrusters and ion engines are the most studied ones. These two types of plasma thrusters have also been deployed as a test-bed or for actual use in various satellites and spacecrafts. Since with the use of - External power supply, in comparison with the chemical thrusters, the average exhaust velocity of the particles thrown out of the plasma thrusters is significantly higher, the specific impulse of these plasma thrusters is considerably greater than the chemical thrusters specific impulse. The maximum specific impulse (Isp) for chemical thrusters is 450 seconds (hydrogen-oxygen combustion), while the specific impulse for a typical Hall thruster is 1600 seconds, and for a typical ion engine is 3000 seconds. For ion thrusters, it is possible to reach much higher specific impulse values (i.e., 6000-7000 seconds).

In ion engines, the propellant (neutral gas), that is accelerated and ejected out of the thruster at high momentum, is first ionized by being stripped off an electron and a gas that has positive ions along with neutral atoms and free electrons (called plasma) is obtained in a section of the thruster called the ionization chamber. The generated plasma ions are then ejected out of the thruster at high velocities, using the high electric field created between a pair of grids, to create the desired thrust. There are three main types of ion engines, differentiated by the main mechanism for ionization of the propellant gas. The first type is called the electron-bombardment ion engine (or Kaufman type ion engine). In this type of ion engines an internal cathode provides the electrons for the ionization of the propellant. In the second kind, called radio frequency (RF) ion engine, an RF antenna is used to deliver RF waves. In this type of ion engines, the ionization is provided by the energy carried by the RF waves in to the discharge chamber. In the third kind, called microwave ion engine, a microwave antenna is used to deliver microwaves into the discharge chamber that are used for the ionization of the propellant gas. Although today the ion thruster research is conducted in many different countries, historically electron-bombardment ion engines are developed in the United States, radio-frequency ion engines are developed in Germany, and the micro-wave ion engines are developed in Japan.

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