Chapter 1
Introduction

Electric propulsion (EP) is an in‐space propulsion technology aimed at achieving thrust with high exhaust velocities, which results in a reduction in the amount of on‐board propellant required for a given space mission or space propulsion application compared to other conventional propulsion methods. Reduced propellant mass can significantly decrease the launch mass of a spacecraft or satellite, leading to lower costs from the use of smaller launch vehicles to deliver a desired mass into orbit or to a deep space target. Alternatively, the reduced propellant mass enabled using EP can be used to increase the delivered payload mass on a given launch vehicle.

In general, EP encompasses any propulsion technology in which electricity is used to increase the propellant exhaust velocity. There are many figures of merit for electric thrusters, but mission and application planners are primarily interested in thrust, propellant flow rate or specific impulse, and total efficiency in relating the performance of the thruster to the delivered mass and change in the spacecraft velocity (Δv) during thrusting periods. Although thrust is self‐explanatory, specific impulse (Isp) is defined as the mean effective propellant exhaust velocity divided by the gravitational acceleration constant g, which results in the unusual units of seconds. The total efficiency is the jet power produced by the thrust beam divided by the electrical power into the system. Naturally, spacecraft designers are then concerned with providing the electrical power that the thruster requires to produce a given thrust, as well as required propellant flow, and in dissipating the thermal power that the thruster generates as waste heat.

In this book, the fundamentals of EP are presented. There is an emphasis on ion and Hall thrusters that have emerged as leading EP technologies in terms of performance (thrust, Isp, and efficiency) and use in space applications, but other thrusters such as electromagnetic thrusters are also described. Ion and Hall thrusters operate in the power range of hundreds of watts up to tens of kilowatts with an Isp of thousands of seconds to tens of thousands of seconds, and produce thrust levels of a fraction of a Newton to over a Newton. Ion and Hall thrusters generally use heavy inert gases such as xenon or krypton as propellants, but other propellant materials, such as cesium and mercury, have been investigated in the past. Xenon is generally preferable because it is not hazardous to handle and process, it does not condense on spacecraft components that are above cryogenic temperatures, its large mass compared to other inert gases generates higher thrust for a given input power, and it is easily stored at densities of about 1 g/cm3 with low tank mass fractions. Krypton is also an emerging propellant that is cheaper than xenon and provides slightly higher Isp (due to its lower mass) than xenon for a given thruster electrical configuration. Electromagnetic thrusters can use nearly any propellant, but tend toward lower atomic mass to improve performance and Isp. However, this book will primarily focus on xenon as the propellant in ion and Hall thrusters, and performance with other propellants such as krypton can be examined using the information provided here. The 2nd Edition of this book provides updates on the progress made in hollow cathode technology and ion and Hall thrusters since the 1st Edition, and includes new chapters on magnetically shielded Hall thrusters, electromagnetic thrusters, and future thruster concepts.

1.1 Electric Propulsion Background

A detailed history of EP up to the 1950s was published by Choueiri [1], and information on developments in EP since then can be found in reference books [2] and on various internet sites [3].Briefly, EP was first conceived by Konstantin Tsiolkovsky [4] in Russia in 1903 and independently by Robert Goddard [5] in the United States in 1906 and Hermann Oberth in Germany [6] in 1929. Several EP concepts for some space applications were published in the literature by Shepherd and Cleaver in Britain in 1949. The first systematic analysis of EP systems was made by Ernst Stuhlinger [7] in his book Ion Propulsion for Space Flight published in 1964, and the physics of EP thrusters was first described comprehensively in the book by Robert Jahn [8] in 1968. The technology of early ion propulsion systems that used cesium and mercury propellants, along with the basics of low‐thrust mission design and trajectory analysis, was published by George Brewer [9] in 1970. Since that time, the basics of EP and some thruster characteristics have been described in several chapters of textbooks published in the US on spacecraft propulsion [10–13]. An extensive presentation of the principles and working processes of several electric thrusters was published in 1989 in a book by S. Grishin and L. Leskov [14] (in Russian). A survey of the early flight history of ion propulsion projects in the US was published by J. Sovey et al. [15] in 1999.

Significant EP research programs were established in the 1960s at NASA’s Glenn Research Center (GRC), Hughes Research Laboratories (HRL), NASA’s Jet Propulsion Laboratory (JPL), and at various institutes in Russia to develop this technology for satellite station keeping and deep space prime propulsion applications. The first experimental electric thrusters were launched into orbit in the early 1960s [15] by the United States (US) and by Russia. The US demonstrated the first extended operation of ion thrusters in orbit with the Space Electric Rocket Test (SERT‐II) mission [16] launched in 1970. The SERT‐II mercury ion thrusters were also the first to use a hollow cathode “plasma‐bridge neutralizer” to provide complete ion beam neutralization in space. Experimental test flights of ion thrusters and Hall thrusters continued from that time into the 1990s.

A detailed description of flight electric thrusters, with performance information and photos, is given in Chapter 12. Briefly, the first extensive application of EP was by Russia using Hall thrusters for station keeping on weather and communications satellites [17]. Since 1971, when the Soviets first flew a pair of SPT‐60s on the Meteor satellite, over 250 SPT Hall thrusters have been operated on dozens of satellites to date [18]. Japan launched the first ion thruster system intended for north–south station keeping on the communications satellite “Engineering Test Satellite (ETS) VI” in 1995 [19]. However, a launch vehicle failure did not permit station keeping by this system, but the ion thrusters were successfully operated in space. The first commercial use of ion thrusters in the United States started in 1997 with the launch of a Hughes Xenon Ion Propulsion System (XIPS) [20], and the first NASA deep space mission using the NSTAR ion thruster was launched in 1998 on Deep Space‐1 [21]. Since then, Hughes/Boeing launched their second generation 25‐cm XIPS ion thruster system [22] in 2000 for station keeping applications on the high power 702 communications satellite [23]. The Hughes/Boeing 702 spacecraft is the first satellite to use electric thrusters for all propulsion applications (orbit raising, station keeping, momentum management, and attitude control).

The Japanese Space Agency (JAXA) successfully used the μ10 ion thrusters to provide the prime propulsion for the Hayabusa asteroid sample return mission [24] launched in 2003, and an upgraded version of this microwave ion thruster [25] was used for prime propulsion on Hayabusa‐2 launched in 2014. The European Space Agency (ESA) used the Safran manufactured PPS‐1350‐G Hall thruster on its SMART‐1 mission to the moon [26] also in 2003.The Russians have been steadily launching communications satellites with Hall thrusters aboard since 1971, and will continue to use these devices in the future for station keeping applications [18]. The first commercial use of Hall thrusters by a US spacecraft manufacturer was in 2004 on Space Systems Loral's (SSL) MBSAT, which used the Fakel SPT‐100 [27]. ESA launched the QinetiQ/UK T5 ion thruster on the GOCE mission [28] in 2009. This was followed in 2010 with the launch of Aerojet’s BPT‐4000 Hall thruster [29] (now called the XR‐5) on the AF/LMC AEHF satellite. ESA launched the QinetiQ manufactured T6 ion thruster [30] on the BepiColombo mission to Mercury in 2018. SSL launched its first spacecraft that used the 4.5 kW Fakel SPT‐140 Hall thruster [31] in 2018 for orbit raising and station keeping. The first spacecraft that did not utilize any chemical propulsion (no kick‐stage thruster for orbit insertion or chemical thrusters for orbit maintenance) was the Boeing’s “all‐electric” 702SP satellite [32] launched in 2015. NASA/GRC’s NEXT ion thruster was flight demonstrated in 2021 on the DART mission [33]. Additional ion and Hall thruster launches are ongoing now for emerging LEO communications constellations, such as the thousands of SpaceX Starlink launches each with a Hall thruster used for orbit insertion and station keeping. This trend will continue going forward using various thrusters produced by commercial vendors, with Hall thrusters dominating the present market.

In the past 25 years, EP use in spacecrafts has grown steadily worldwide [34,...