P1: ZBU Final Pages Qu: 00, 00, 00, 00 Encyclopedia of Physical Science and Technology EN005C-201 June 15, 2001 20:23 Electric Propulsion Robert G. Jahn Edgar Y. Choueiri Princeton University I. Conceptual Organization and History of the Field II. Electrothermal Propulsion III. Electrostatic Propulsion IV. Electromagnetic Propulsion V. Systems Considerations VI. Applications GLOSSARY Resistojet Device that heats a propellant stream by pass- ing it through a resistively heated chamber before Arcjet Device that heats a propellant stream by passing the propellant is expanded through a downstream a high-current electrical arc through it, before the pro- nozzle. pellant is expanded through a downstream nozzle. Thrust Unbalanced internal force exerted on a rocket Hall effect Conduction of electric current perpendicular during expulsion of its propellant mass. to an applied electric field in a superimposed magnetic field. Inductive thruster Device that heats a propellant stream THE SCIENCE AND TECHNOLOGY of electric by means of an inductive discharge before the propel- propulsion (EP) encompass a broad variety of strate- lant is expanded through a downstream nozzle. gies for achieving very high exhaust velocities in order Ion thruster Device that accelerates propellant ions by to reduce the total propellant burden and corresponding an electrostatic field. launch mass of present and future space transportation Magnetoplasmadynamic thruster Device that acceler- systems. These techniques group broadly into three cat- ates a propellant plasma by an internal or external mag- egories: electrothermal propulsion, wherein the propel- netic field acting on an internal arc current. lant is electrically heated, then expanded thermodynami- Plasma Heavily ionized state of matter, usually gaseous, cally through a nozzle; electrostatic propulsion, wherein composed of ions, electrons, and neutral atoms or ionized propellant particles are accelerated through molecules, that has sufficient electrical conductivity to an electric field; and electromagnetic propulsion, wherein carry substantial current and to react to electric and current driven through a propellant plasma interacts magnetic body forces. with an internal or external magnetic field to provide a Encyclopedia of Physical Science and Technology, Third Edition, Volume 5 Copyright C 2002 by Academic Press. All rights of reproduction in any form reserved. 125 P1: ZBU Final Pages Encyclopedia of Physical Science and Technology EN005C-201 June 15, 2001 20:23 126 Electric Propulsion stream-wise body force. Such systems can produce a range υe and logarithmically on the amount of propellant mass of exhaust velocities and payload mass fractions an order expended: of magnitude higher than that of the most advanced chem- mo ical rockets, which can thereby enable or substantially υ = υe ln , (2) m enhance many attractive space missions. The attainable f thrust densities (thrust per unit exhaust area) of these sys- where mo and m f are the total spacecraft mass at the start tems are much lower, however, which predicates longer and completion of the acceleration period. Conversely, the flight times and more complex mission trajectories. In ad- deliverable mass fraction, m f /mo, is a negative exponen- dition, these systems require space-borne electric power tial in the scalar ratio υ/υe: supplies of low specific mass and high reliability, inter- m f = eυ/υe . (3) faced with suitable power processing equipment. Opti- m mization of EP systems thus involves multidimensional o trade-offs among mission objectives, propellant and power Inclusion of significant gravitational or drag forces on plant mass, trip time, internal and external environmental the flight of the spacecraft adds appropriate terms to Eq. (1) factors, and overall system reliability. An enduring inter- and considerably complicates its integration, but it is still national program of research and development of viable possible to retain relation (3), provided that υ is now electric thrusters has been in progress for several decades, regarded as a more generalized “characteristic velocity and over the past few years this has led to the increas- increment,” indicative of the energetic difficulty of the ing use of a number of EP systems on commercial and particular mission or maneuver. However represented, the governmental spacecraft. Meanwhile, yet more advanced salient point is simply that if the spacecraft is to deliver a EP concepts have matured to high credibility for future significant portion of its initial mass to its destination, the mission applications. rocket exhaust speed must be comparable to this charac- teristic velocity increment. Clearly, for missions of large υ, the burden of thrust generation must shift from high rates of ejection of propellant mass to high relative exhaust I. CONCEPTUAL ORGANIZATION velocities. Unfortunately, conventional chemical rockets, AND HISTORY OF THE FIELD whether liquid or solid, monopropellant or bipropellant, are fundamentally limited by their available combustion A. Motivation reaction energies and heat transfer tolerances to exhaust The stimulus for development of electrically driven speeds of a few thousand meters per second, whereas space propulsion systems is nothing less fundamen- many attractive space missions entail characteristic ve- tal than Newton’s laws of dynamics. Since a rocket- locity increments at least an order of magnitude higher. propelled spacecraft in free flight derives its only Thus, some fundamentally difierent concept for the accel- acceleration from discharge of propellant mass, its equa- eration of propellant mass that circumvents the intrinsic tion of motion follows directly from conservation of limitations of chemical thermodynamic expansion is re- the total momentum of the spacecraft and its exhaust quired. Into this breech step the family of electric propul- stream: sion possibilities. mυú = mú υ , (1) e B. Conceptual Subdivision where m is the mass of the spacecraft at any given time, So that propellant exhaust speeds in the range above υú its acceleration vector, υe the velocity vector of the ex- 10,000 m/sec desirable for interplanetary flight and other haust jet relative to the spacecraft, and mú the rate of change high-energy missions can be obtained, processes basically of spacecraft mass due to propellant-mass expulsion. The different from nozzled expansion of a chemically reacting product mú υe is called the thrust of the rocket, T, and for flow must be invoked. More intense forms of propellant most purposes can be treated as if it were an external force heating may be employed, provided that the walls of the applied to the spacecraft. Its integral over any given thrust- rocket chamber and nozzle are protected from excessive ing time is usually termed the impulse, I, and the ratio of heat transfer. Alternatively, the thermal expansion route the magnitude of T to the rate of expulsion of propellant may be bypassed completely by direct application of suit- in units of sea-level weight, mgú o, has historically been la- able body forces to accelerate the propellant stream. Either beled the specific impulse, Is = υe/go.Ifυeis constant of these options is most reasonably accomplished by elec- over a given period of thrust, the spacecraft achieves an trical means, which constitute the technology of electric increment in its velocity, υ, which depends linearly on propulsion. P1: ZBU Final Pages Encyclopedia of Physical Science and Technology EN005C-201 June 15, 2001 20:23 Electric Propulsion 127 Historically, conceptually, and pragmatically, this field necessity for impeccable reliability of both components has tended to subdivide into three categories: of the system over long periods of unattended operation in the space environment. 1. Electrothermal propulsion, wherein the propellant is heated by some electrical process, then expanded C. History of Effort through a suitable nozzle 2. Electrostatic propulsion, wherein the propellant is The attractiveness of EP for a broad variety of space trans- accelerated by direct application of electrostatic portation applications was recognized by the patriarch of forces to ionized particles modern rocketry, Robert H. Goddard, as early as 1906. His 3. Electromagnetic propulsion, wherein the propellant is Russian counterpart, Konstantin Tsiolkovskiy, proposed accelerated under the combined action of electric and similar concepts in 1911, as did the German Hermann magnetic fields Oberth in his classic book on spaceflight in 1929 and the British team of Shepherd and Cleaver in 1949. But the first Over their periods of development, each of these ap- systematic and tutorial assessment of EP systems should proaches has spawned its own array of technical special- be attributed to Ernst Stuhlinger, whose book Ion Propul- ties and subspecialties, its own balance sheet of advantages sion for Space Flight nicely summarizes his seminal stud- and limitations, and its own cadres of proponents and de- ies of the 1950s. tractors, but in serious assessment, each has validly qual- The rapid acceleration of the U.S. space ambitions in ified for particular niches of application, many of which the 1960s drove with it the first coordinated research and do not seriously overlap. Throughout the history of EP de- development programs explicitly addressing EP technol- velopment, the original subdivision of the field into elec- ogy. In its earliest phase, this efiort drew heavily on reser- trothermal, electrostatic, and electromagnetic systems has voirs of past experience