Electric Propulsion Systems: An Introduction
Electric propulsion systems describe a wide variety of differing mechanics to propel a spacecraft. Electromagnetic, electrothermal, and electrostatic thrusters rely upon the supply of electric power for propulsion. The subsequent pages within the electric propulsion section contain specific information and technologies pertaining to the differing subsets of electric propulsion systems.
A Brief Overview of Electric Propulsion Technology
The basic operating principle behind the operation of an electric propulsion system is the creation, acceleration, and expulsion of charged particles, using primarily electricity to accomplish said functions. Electric thrusters have significant advantages over other types of propulsion systems, most notably in their extremely high specific impulse ranges. A contrasting drawback with the electric propulsion system is, however, the extremely low thrust ranges generated. In the following sections, each type of electric propulsion system is described and comparatively analyzed against other electric propulsion types.
Electrostatic Propulsion Systems
Figure 1: Visual representation of a gridded electrostatic ion thruster (Mysid, 2012).
Electrostatic propulsion thrusters rely upon electric fields for accelerating and expelling ions to produce thrust and propel the spacecraft (European Space Agency, 2004). The production of ions for acceleration is achieved through several differing means, including the conventional electron bombardment method or the electron cyclotron resonance method, which electrically charges atoms from an onboard fuel supply (NASA, 2004). This fuel supply is an inert gas, often xenon or krypton, which is injected into the ionization chamber then expelled for propulsion (NASA, 2004). Electrostatic thrusters provide relatively large Isp values, ranging from 1,000 to approximately 10,000, and also have relatively high efficiencies ranging between 55% - 98% (Jordan, 2000). While electrostatic thrusters produce minimal thrust relative to alternative propulsion methods, the various implementations of this technology provide advantages not offered by other propulsion methods.
Electromagnetic Propulsion Systems
Figure 4: Cross section of a Hall Effect thruster with an extended insulator channel (Jahn & Choueiri, 2002).
Electromagnetic propulsion systems expel charged plasma particles, similar to electrostatic thrusters; the temperature and density of plasma generated and expelled by electromagnetic thrusters are, however, considerably larger and produce significantly higher exhaust velocities (Jordan, 2000). The basic operating principle for all electromagnet thrusters is the production, acceleration, and expulsion of plasma through the use of powerful electromagnetic fields (Jordan, 2000). A conductive propellant, typically a high mass gas such as xenon, is injected into the primary chamber. Once inside, perpendicular electric and magnetic fields interact with the propellant, and electrons traversing across or trapped within said fields ionize the injected particles (Jahn & Choueiri, 2002). The horizontally located magnetic field accelerates the positively charged ions, and collisions from trapped electrons further contribute to the acceleratory effects (European Space Agency, 2004). Powerful magnetic fields allow for denser, higher temperature plasma to be accelerated and expelled, while maintaining high exhaust velocities and high mass efficiencies (Jahn & Choueiri, 2002). Several varieties of electromagnetic thrusters are comparatively analyzed on the electromagnetic propulsion systems page, and an overall recommendation is concluded indicating the thruster best meeting the stated mission constraints.
Electrothermal Propulsion Systems
Figure 10: A cross section of a resistojet thruster (Jordan, 2000).
Electrothermal thrusters differ from both electromagnetic and electrostatic propulsion systems due to their operational design; electromagnetic and electrostatic systems propel charged ions through the use of electric and magnetic fields, while electrothermal systems heat the propellant, and rely upon thermal dynamics to propel the system (Jordan, 2000). In typical operation, a propellant is electrically heated, which increases the pressure and expands the gas, forcing the energized mass out of the nozzle and providing thrust to the spacecraft (Jahn & Choueiri, 2002). There are three types of electrothermal propulsion systems; arcjet, resistojet, and inductively or radiatively heated systems (European Space Agency, 2004). Each of these three propulsion systems is considered viable, and each is discussed in the succeeding paragraphs on the electrothermal propulsion systems page.
Electric Propulsion Systems: Conclusion
Of the three primary electric propulsion system types, electrostatic, electromagnetic, and electrothermal, specific thruster systems and methods were discussed, and their validity assessed. For the electrostatic propulsion system type, the Hall Effect thruster is considered the most viable technology; in the succeeding category, the electromagnetic MPD thruster is recommended; finally, the VASIMR propulsion system is recommended over the various electrothermal propulsion types. The VASIMR system, which has an extremely high Isp range and comparatively moderate thrust and efficiency levels, is a technology with significant relative advantages. Higher performance levels than Hall Effect thrusters, and comparable performance with the MPD thruster without corroding electrodes, the VASIMR system is the primarily recommended electric propulsion technology for the stated requirements. While developmentally adequate, its current stage of deployment is significantly behind the other two systems; however, the VASIMR system’s overall performance indicates an extremely viable technology at a relatively high stage of development. A more in depth summarization of the various technologies is found in the preceding sections, and specific values for thrust, Isp, and energy conversion efficiency are found in Table 1. In summation, the VASIMR technology is recommended as the most viable electric propulsion system, with a partial recommendation to the MPD thruster system.