The technological and commercial expansion of electric propulsion

Acta Astronautica 159 (2019) 213–227

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Review article

The technological and commercial expansion of electric propulsion a,∗

b

c

d

e

Dan Lev , Roger M. Myers , Kristina M. Lemmer , Jonathan Kolbeck , Hiroyuki Koizumi , Kurt Polzinf

T

a

Rafael – Advanced Defense Systems, Haifa, 3102102, Israel R Myers Consulting, LLC, Woodinville, WA, USA Western Michigan University, Kalamazoo, MI, 49008, USA d George Washington University, Washington D.C., USA e The University of Tokyo, Bunkyo-ku, Tokyo, 113-8654, Japan f NASA Marshall Space Flight Center, Huntsville, AL, 35812, USA b c

ABSTRACT

The use of Electric Propulsion (EP) on satellites for commercial, defense, and space science missions has been increasing in recent decades, from the first successful operation in 1964 aboard the Zond-2 spacecraft to the present day. This paper provides an overview of the technological and commercial development of EP systems that have been deployed. A review of the early years of EP application ends in 1980, when the first geostationary commercial satellite using EP, Intelsat-V, was launched. Beyond 1980, all EP-based spacecraft deployment data through 2018 are presented, divided by spacecraft type: GEO-synchronous satellite, LEO satellites, deep-space missions and small satellites. To date, a total of 587 spacecraft have been launched with some variant of electric propulsion. During the 1960s and 1970s, all 48 spacecraft using EP were government missions, with the US and USSR leading in the development, production, and flight of these systems. These first platforms included a variety of pulsed plasma thrusters, resistojets, arcjets, ion thrusters and Hall thrusters. The number of GEO satellites with electric propulsion systems has increased significantly since 1981, from an average of less than 5 satellites per year during the 1980s to over 15 in recent years. The corresponding annual fraction of EP based GEO satellite launches, compared to all GEO satellite launches, has increased from 20% during the 1980s to over 40% in recent years. For LEO applications, a gradual increase in the utilization of EP has been realized. Of the 167 EP-based LEO platforms deployed, resistojets were the most prolific legacy thruster type (124 S/C) with Hall thrusters gaining traction in recent years (25 S/C), appearing on 19 of 45 satellite missions in the past decade. Of all EP-based LEO missions, approximately half served as testbeds for new technologies. Through 2018, eight deep space spacecraft with EP have been launched, with the US, Japan, and the European Union leading these efforts. Small satellites are also benefiting from this technology, with 24 EP-based small satellites launched to date. Nearly half of these were launched between 2016 and 2018, demonstrating accelerated growth and a large potential for the future of this spacecraft class.

1. Introduction N the hundred years since electric propulsion (EP) was originally conceived, it has been developed by an increasing number of research and industrial entities worldwide [1]. To date, a myriad of technological subclasses of EP exist [2,3], each at a different Technological Readiness Level (TRL) [4], from basic notions of particle acceleration techniques to space proven applications. The literature includes a variety of review papers on the use and proliferation of EP technologies. Choueiri [1] presented a thorough overview of the evolution of EP technology from first theoretical conception to the deployment of the first spacecraft to use EP. Mazouffre [2] focused on the scientific and technological aspects of EP while surveying both existing technologies and emerging propulsion variants, still under development or qualification, which have yet to be deployed in space. Other EP review papers addressed flight history of specific types of EP technologies such as electromagnetic thrusters [5], ion

∗

thrusters [6], resistojets [7] or Pulsed Plasma Thrusters [8,9]. Although these publications were thorough and exhaustive they focused on one particular thruster technology, and some were published over 20 years ago. Recent review papers focused on the utilization of EP for specific spacecraft applications such as LEO satellites [10] or small spacecraft [11]. However, no publication directly addressed EP for deep space missions or EP technology trends for GEO satellite platforms, on-board most EP systems operated to date. Lastly, Martinez-Sanchez [12] published a complete overview of EP technology and flight history. However, this article was published over 20 years ago and therefore could not include some of the recent and important trends in the use of EP aboard commercial platforms. In this paper, we review the expansion process of EP from the first missions, on-board the sub-orbital American Scout rocket missions [6] and Soviet spacecraft ‘Zond 2’ [13], thru the year 2018. The review includes only EP technologies on-board spacecraft, of which almost all obtained flight heritage. To complete this review we focus on four

Corresponding author. E-mail address: [email protected] (D. Lev).

https://doi.org/10.1016/j.actaastro.2019.03.058 Received 30 November 2018; Received in revised form 23 February 2019; Accepted 18 March 2019 Available online 21 March 2019 0094-5765/ © 2019 IAA. Published by Elsevier Ltd. All rights reserved.

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particular spacecraft niches: (1) communication satellites in GEO, (2) satellites in LEO, (3) deep space missions, and (4) small satellite platforms under 50 kg. For each niche, we present statistics showing the chronological increase in the number of satellites carrying EP systems, identify technology trends in using existing EP for each application and discuss the world-wide growth in EP system suppliers. 2. Early years of EP applications: 1962–1980 During the first two decades following the first launch of an EP system in 1962 [6] and first successful in-space operation of an EP system in 1964, most development efforts have been invested in maturing four main types of EP technologies – ion thruster, Pulsed Plasma Thrusters (PPT), resistojets, and Hall thrusters. The principal drivers supporting the research, development, and ultimately qualification of each of these technologies were government entities; either space agencies or different branches of the military. Most of the early missions were technology demonstrators and served purely scientific or technological purposes such as the Scout, Vela, Space Electric Rocket Test (SERT), Zond, Lincoln Experimental Satellite (LES), and ‘Meteor’ missions [1,14]. The lead players capable of developing and implementing EP technologies were the large geo-political entities, namely the United States and the Soviet Union; and to a lesser extent, Japan and Europe. The first truly successful operational uses of electric propulsion were for spacecraft attitude control on the Russian Zond-2 in 1964 [13] (Fig. 1), which used pulsed plasma thrusters, and by the United States’ Vela satellites in 1965, which used resistojets [15]. The first American use of PPTs was on the American LES satellites [5],ý [16], which in 1968 started using pulsed plasma thrusters for fine ephemeris control to achieve a “drag-free” orbit. One of the PPTs aboard LES-6 operated for over 8900 h [17]; thereby proving the ability to operate EP systems for prolonged periods of time. In 1971, the USSR launched the first satellite carrying two Hall thrusters of the SPT-60 type, aboard the Meteor #18 satellite [16]. Subsequently, five consecutive Hall thruster-based missions were conducted aboard the Meteor-Priroda satellite platform by the year 1981 [18]. In parallel with the above ground-breaking missions, over 20 experimental missions were conducted by the United States and the

Fig. 2. Illustration of Intelsat-V 2 - the first commercial satellite to use electric propulsion.

former USSR, in which a variety of electric thrusters were tested. These were the Soviet gas-fed ion thrusters aboard the Yantar series [19]; the potassium-fed and air-fed magnetoplasmadynamic (MPD) thrusters aboard the Kren and Kust series [20]; and the American cesium-fed and mercury-fed ion thrusters aboard the SERT, program 661A, ATS, and SEPS satellites series [6]. Although most of these experimental missions were executed during short ballistic suborbital trajectories the knowledge and experience gained laid the groundwork for future electric propulsion development. Thanks to their ability to leverage small chemical thruster technology, resistojets were extensively developed during this time [7,21]. Consequently, resistojet technology matured and eventually, driven by its simplicity, benign failure modes and relative ease of operation, was adopted by the commercial sector. In late 1980, Ford Aerospace's communications satellite Intelsat-V 2 was launched and successfully operated using resistojet thrusters for GEO orbit NSSK [22] (Fig. 2). While not the first communications satellite to use EP for stationkeeping, it marks the point in time when EP technology using a chemical monopropellant (hydrazine) could compete with high performance chemical bipropellant propulsion at an equal or lower cost. This attribute opened the gates to an increased use of resistojets, and a growing appetite for the development of higher performance electric thrusters. Following the first launch of the Intelsat V series of GEO satellites, which used high power resistojets for North-South StationKeeping (NSSK), followed rapidly by their use on RCA's Satcom1 [23]. The use of high power resistojets continues to this day on the original Iridium constellation (77 spacecraft), a couple dozen LEO satellites and many geosynchronous satellites (over 20 to date) [15]. In parallel to the commercial use of resistojets, considerable research and development, as well as several demonstration missions, continued with the objective of increasing the capabilities and reducing the cost of space missions. It was acknowledged that the use of these propulsion devices would thrive with the commercialization of an increasing number of satellites [12] due to the increased financial incentives in the commercial marketplace. All EP-based satellites launched in the years 1962–1980 are listed in Table 1. Following Intelsat-V, there was a steady increase in the number of satellite integrators implementing EP-based propulsion systems on their satellite platforms. Over time, EP technology has made great technological and commercial progress. The main driver for the impressive advancement is the slow and steady commercialization of the space industry led by a constant demand for an increasing number of communication satellites. In parallel, EP technology demonstrated the capability to perform a variety of functions and is increasingly used in almost all space applications, from tiny CubeSats, through Earth observation satellites in LEO, to remote deep space missions [24]. 3. GEO communication satellites

Fig. 1. Picture of Zond-2 - the first spacecraft to successfully use electric propulsion.

Commercial communication satellites in GEO have undergone fast 214

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Table 1 Spacecraft with electric propulsion launched in the years 1962–1980. Satellite Name

Launch Year

Type

Prop.

Orbit Type

Power per Thruster [Watt]

Isp [s]

Thrust [mN]

Manufac.

Country

Ref.

Scout (code A) SERT-1

1962 1964

Ion Ion

SubOrb. SubOrb.

8.9 28 5.6 8.9 2 8.9 89 89 89 89 89 8.5 187 187 10 18 89 89 18

EOS LeRC Hughes EOS

USA USSR USA USA USA USA USA USA USA USA USA USSR USA USA USA USA USSR USA

[6] [8] [6] [25] [25] [25] [25] [25] [6] [25] [25] [26] [25] [25] [25] [25] [26] [6]

LES-6

1968

Ion PPT Ion Resist. Resist. Resist. Resist. Resist. Ion Resist. Resist. Ion Resist. Resist. Resist. Resist. Ion Ion Resist. PPT

7400 4900 8050 7400 410 7400 230 230 230 230 230 5100 123 123 4000 150 132 132 150 14,000 6700 150 300

[6] [6]

1964 1964 1964 1965 1965 ≥1965 ≥1965 ≥1965 1965 1965 1965 1966 1967 1967 1967 1967 1968 1968

770 1400 600 770 100 770 30 30 30 30 30 400 92 92 500 3.6 30 30 3.6 500 20 3.6 10

USA USA

Scout (code B) Zond-2 Scout (code C) GGSE-2 SolRAD-8 Navy Satellite 1 Navy Satellite 2 Navy Satellite 3 SNAPSHOT Vela-5 Vela-6 Yantar 1 ATS-2 Advanced Vela 7 Advanced Vela 8 ATS-3 Yantar 2 ATS-4

Cs Hg Cs Cs Teflon Cs NH3 NH3 NH3 NH3 NH3 Cs N2 N2 Ar NH3 NH3 NH3 NH3 N2 Cs NH3 Teflon

USA

[9]

Yantar 3 Advanced Vela 9 Advanced Vela 10 ATS-5

1969 1969 1969 1969

500 30 30 20 3.6 850

14,000 132 132 6700 150 4200

USSR USA USA USA

[26] [25] [25] [6]

1970

Air NH3 NH3 Cs NH3 Hg

LEO LEO LEO GEO

SERT-II

Ion Resist. Resist. Ion Resist. Ion

USA

[6]

Advanced Vela 11 Advanced Vela 12 Yantar 4 Navy Satellite 1 Navy Satellite 2 Navy Satellite 3 Navy Satellite 4 Sol-Rad 10 Meteor 1-10 TIP-1 ATS-6 SMS

1970 1970 1971 ≥1971 ≥1971 ≥1971 ≥1971 1971 1971 1972 1974 1974

Resist. Resist. Ion Resist. Resist. Resist. Resist. Resist. HET PPT Ion PPT

NH3 NH3 Air NH3 NH3 NH3 NH3 NH3 Xe Teflon Cs Teflon

LEO LEO LEO

30 30 500 3 3 3 3 10? 500 30 150

132 132 14,000 235 235 235 235 235 800 850 2500 400

89 89

USA USA USSR USA USA USA USA USA USSR USA USA USA

[25] [25] [26] [25] [25] [25] [25] [25] [27] [9] [6] [9]

Meteor 1-19 Kren-1 (Kosmos-728) TIP-2 Kren-2 (Kosmos-760) Meteor 1-25 TIP-3 Meteor 1-28 Kosmos 1066 (Astrofizika) Scatha (P78-2) MS-T4 Tansei-4 Intelsat-V 2

1974 1975 1975 1975 1976 1976 1977 1978 1979 1980 1980

HET MPD PPT MPD HET PPT HET HET Ion MPD Resist.

Xe K Teflon K Xe Teflon Xe Xe Xe NH3 N2H4

LEO

500 3000 30 3000 350 30 350 350 30–45 30 350

800 3000 850 3000 1100 850 1100 1100 350

20

USSR USSR USA USSR USSR USA USSR USSR USA Japan USA

[27] [27], [28] [9] [27], [28] [18] [9] [18] [18] [6] [29] [12]

SubOrb. LEO SubOrb. LEO LEO

LEO GEO GEO LEO GEO LEO LEO GEO LEO GEO LEO

LEO

LEO LEO LEO GEO GEO

LEO LEO LEO LEO LEO Molnya LEO GEO

growth in both revenueý [30] and number of satellites carrying EP the past four decades. The increasing demand for telecommunication services, whether commercial or military, has served as an incentive for the maturation of space-proven 1–5 kW electric thrusters. Traditionally, EP is used to perform station-keeping maneuvers in order to maintain the spacecraft in its designated slot in the GEO belt. Most GEO satellites carrying EP systems incorporated electric thrusters solely for northsouth-east-west station-keeping maneuvers. The propulsion systems consumed between 1.5 kW and 4.5 kW of electrical power, depending on the type of electric thruster. The first EP technology used on-board GEO satellites was the resistojets on the Intelsat-V satellite series in late 1980, as previously described in section ýII. These resistojets consumed 500 W and delivered a specific impulse of 300 s. Although the specific impulse was only a modest 50% higher than that delivered by traditional hydrazine-based

300

0.089 18 0.14 89 89 0.089 18 28

44–356 44–356 44–356 44–356 44–356 20 4.5

EOS GE GE GE GE GE EOS TRW TRW TsAGI AVCO TRW TRW AVCO TsAGI EOS AVCO Fairchild MIT Lincoln TsAGI TRW TRW EOS AVCO NASA LeRC Westinghouse TRW TRW TsAGI AVCO AVCO AVCO AVCO AVCO Fakel EOS Fairchild MIT Lincoln Fakel KeRC

20

KeRC Fakel

20 20 0.14 0.24 0.45

Fakel Fakel Hughes ISAS/MELCO Ford/TRW

mono-propellant thrusters (∼200 sý [31]), resistojets provided bipropellant thruster performance with a monopropellant system, and this benefit, when combined with their ease of integration and benign failure modes, provided satellite manufacturers with significant advantages. Over the subsequent 12 years, resistojets were increasingly used in GEO satellites, and were the only EP technology used for commercial satellites. In parallel to resistojets, Hall thrusters, namely the Russian SPT-70, were used by the Soviet Union aboard the Kosmos and Luchý [18] satellites. However, because these developments were happening behind the Iron Curtain, they were not disseminated to the West and there was no Western interest at that time in Hall thrusters. The breakthrough in the widespread commercial application of EP occurred in 1993 when arcjets were employed for the first time onboard the Telstar-401 communications satelliteý [32]. The use of arcjet 215

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technology, using the same hydrazine propellant already used for chemical monopropellants and resistojets, presented yet another leap in spacecraft propulsion performance that allowed for a significant reduction of the spacecraft mass, or alternatively, for an increase in the payload mass. The use of arcjets for NSSK also demonstrated the crucial link between the selection of in-space propulsion technology and launcher requirements: the large propellant mass reduction enabled by arcjets resulted in a significant reduction in launcher costs by enabling launches from Cape Canaveral (Florida at 28.5° N) to compete with launches from Kourou (French Guiana at 5.2° N). The introduction of arcjets, with specific impulses of over 500 s, changed the commercial satellite marketplace and drove the broad, world-wide adoption of EP technology. The commercial success of the Telstar-401 mission removed major barriers preventing private and commercial satellite operators from harnessing EP technology. Additionally, the technical advantage demonstrated by the first platforms to use arcjets led to a dynamic competition between satellite primes. Following the first use of arcjets for GEO satellites, other satellite primes were forced to identify propulsion technologies with comparable, or better performance to compete with the capabilities enabled by arcjets. This competition led to the first commercial use of Hall thrusters in 1994 (Gals-1) and ion thrusters in 1997 (PAS-5), as well as the subsequent Western adoption of Hall thrusters. The adoption of Hall thrusters is notable as that technology was developed in the USSR and as a flight-ready technology it only became available to the West after the end of the Cold War. From the mid-90s to approximately 2010 the two most prevalent EP technologies for GEO satellites were arcjets and ion thrusters. In particular, two satellite platforms led this trend – Lockheed Martin's A2100ý [33] and Boeing's 702ý [34] platforms. Starting in the early 2000s Hall thruster technology was adopted by an increasing number of Western primes (Alcatel Space in 2002, Space Systems Loral and Astrium in 2004). Once a particular satellite manufacturer had invested in integrating a particular EP technology there was a strong incentive to stay with that technology to avoid repeating the integration investment. This has led to a steady increase in the use of Hall thruster technology for in-space propulsion, a trend that continues to this day. Throughout the entire length of the investigated period, resistojets were still in use, thanks to their low price and relative ease of integration and operation. They were used extensively on the Orbital Sciences Corporation (OSC) GEOStar-2 platformý [35]. Despite years of successful use of EP for station-keeping, satellite primes and operators hesitated to use EP for significant fractions of orbit raising maneuvers due to the perceived elevated risk and financial penalties resulting from the long trip times to GEO. Additionally, the benefits of EP were not always fully realized even for the NSSK maneuvers because some satellite operators still required a three-year chemical propulsion contingency once on orbit. Three missions played a key role in reducing the perceived risk of EP for orbit raising. In 2000 Boeing began using their 25-cm XIPs system to

augment orbit transfer by performing the final orbit circularization of their 702 satellites on some spacecraftý [34]. In 2001 Artemisý [36] used RIT-10 ion thrusters for a major fraction of the GTO to GEO transfer to compensate for a malfunction in the launch vehicle's chemical upper stage. The Artemis ion thrusters were operated over 18 months to reach GEO. Finally, Smart-1ý [37], was launched in 2003 as a secondary payload to GTO, along with two commercial communication satellites. Over the course of 13 months Smart-1 reached the Moon using a single 1.5 kW Hall thruster for propulsion. The thruster operated at maximum power of 1.2 kW due to power constraints on the small spacecraft. Although not a GEO satellite, SMART-1 was the first spacecraft to complete a full transfer from GTO to GEO – and beyond – using nothing but electric propulsioný [38]. The combination of these three missions with larger EP orbit raising roles, the successful experience with station-keeping, and the development of higher power Hall thrusters led the US Air Force and Lockheed Martin to design the Hall thruster-powered AEHF GEO spacecraft. The satellite incorporates EP for orbit raising, with an onboard bipropellant system designed to initially boost the spacecraft perigee over the van Allen belts. A set of 4.5 kW Hall thrusters operating two at a time provided most of the orbit raising from GTO to GEO, an approach described conceptually in Ref. ý [39]. This orbit raising plan was complicated when AEHF Space Vehicle 1 (SV1), launched in 2010ý [40], experienced a failure of the on-board bipropellant system, forcing the use of the Hall thrusters for much more of the orbit raising than plannedý [41]. Over the course of a year the Hall thrusters were successfully used to reach GEO without compromising mission life. This was followed by the launch of AEHF SV2 in 2012ý [42] and SV3 in 2013ý [43], both which performed nominally with the Hall thrusters boosting the spacecraft to GEO over 3 months following the initial perigee raising using the bipropellant system. In 2014, SPT-100 Hall thrusters were used aboard the Express-AM5 and Express-AM6 satellites to perform a planned 2000 km orbit-raising maneuver to reach their final orbit which started from a Proton-M launcher super-synchronous insertion orbitý [44]. These EP orbitraising efforts culminated in early 2015 when Boeing delivered the first all-electric satellites carrying XIPS-25 ion thrustersý [45]. These were the first satellites ever launched without on-board chemical propulsion, and thanks to the great propellant mass savings afforded by the high specific impulse propulsion system, for the first time two GEO communication satellites could be stacked and launched aboard a low-cost launch vehicle like the Falcon-9 ý [46] (note that duel-launches were performed in the past with the Ariane-V launch vehicleý [47]). In 2016, a second pair of all-electric satellites was successfully launched, and they continue to operate to this dayý [48]. Fig. 3 presents the number of EP-based GEO satellites launched in the years 1981–2018. A 3-year moving average was used since the average variance in the order-to-launch turnaround time for GEO satellites is approximately three yearsý [49]ý [50]. Electric thruster subclasses are also presented for each year. The presented values

Fig. 3. Number of EP-based GEO satellites launched in the years 1981–2018 (3-year moving average), divided into electric thruster subclasses. 216

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include all GEO satellites that incorporated EP systems, including both technology demonstration and experimental satellites, which are as important as the commercial satellites because they often serve as precursors to future commercial or military GEO missions. The figure shows a dramatic increase in the number of EP-based satellites launched from 1993 to 2000 followed by a slight decrease before resuming a gradual upward trend through 2016. The rapid increase in the early years (1993–2001) was thanks to several families of satellites (AMC, Echostar, and NSS) utilizing Lockheed Martin's A2100 platformý [33]ý [51] launched in late 1990s and early 2000s and demonstrates the transformational impact of arcjet systems. Following the increase in arcjet flights is the increase in ion thruster systems driven by an increase in Boeing's use of this technology. The increase at the end of the investigated period (the years 2012–2018) is most likely due to the increased confidence in EP technology and the slow entrance of newly developed propulsion systems into the GEO satellite market, such as the Chinese ion thruster (LIPS-200ý [52]ý [53]), European Hall thruster (PPS-1350ý [54]ý [55]), and the American 5 kW Hall thruster (XR-5ý [56]). Specifically, the year 2018 saw an impressive use of 5 kW Hall thrusters as 8 of the 12 GEO satellites using Hall thrusters that year incorporated either the SPT-140 or the XR-5 high power Hall thruster systems. Overall, four subclasses of electric thrusters were used for GEO station keeping and later for orbit raising maneuvers – resistojets, arcjets, ion thrusters, and Hall thrusters. The technology selection for a specific mission was based on assessment of its performance, integration requirements, cost, and risk. Resistojets and arcjets were chosen because of their unique combination of performance, ease of integration, benign failure modes, and lower cost, which reassured the satellite operators and enabled their use. On the other hand, ion thrusters and Hall thrusters were chosen for their specific impulse and lifetime, which enabled a significant reduction in spacecraft mass. Of the four, resistojet technology was the most mature at the beginning of the investigated period, making resistojets the most prolific by the year 2000. To date, 85 GEO satellites incorporated resistojets, of which 34 satellites carry Aerojet Rocketdyne's MR-501B (EHT) and 36 have Aerojet Rocketdyne's MR-502 (IMPEHT) resistojets. Additionally, nearly all the 62 GEO satellites harnessing arcjet technology used Aerojet Rocketdyne's MR-510 aboard Lockheed Martin's A2100 satellite platformý [57]. The first operational use of arcjets for GEO satellites by Japan also used the MR-510 on the DRTS satellitesý [40]. Competition with arcjet technology drove the acceptance of the higher specific impulse ion thruster technology. Hughes, later acquired by Boeing and most recently by L-3 Communications, served as the leading ion thruster developer with the XIPS-13 and XIPS-25 thruster systems that were used on Boeing's 601 and 702 satellite platformsý [34],ý [58]. Of the 63 GEO satellites incorporating ion thrusters in the investigated period (1981–2018), 35 satellites used the XIPS-25¸ 22 used the XIPS-13 and 6 satellites used thrusters developed and produced by other entities (Chinese, European or Japanese RF ion thrusters). Hall thruster technology migrated to the West during the mid to late 90's and in the past 20 years has been increasingly used by Russian, American, and European companies. Hall thruster technology is attractive owing to its high specific impulse compared to arcjet technology, as well as its high thrust-to-power ratio (albeit with lower propellant mass savings) and its lower cost relative to ion thrusters. The success of utilizing Hall thrusters aboard GEO satellites made that technology the most used form of electric propulsion for GEO satellites by the year 2018. Due to the strong usage of Hall thruster technology in the past decade, in Fig. 4 we present a breakdown of the number of satellites using Hall thrusters subdivided by manufacturer and thruster model. As shown in the figure, Fakel's SPT-100 1.5 kW Hall thrusterý [59],ý [60] has dominated the Hall thruster market, deployed on 84 of 130

Fig. 4. Number of GEO satellites using Hall thrusters in the years 1981–2018, by Hall thruster type and manufacturer (Ekspress-A4 carried both SPT-100 and KM-5 thrusters. Stentor carried both SPT-100 and PPS-1350 thrusters).

satellites. However, in recent years, new American, European, Chinese, and Russian Hall thrusters penetrated the GEO satellite propulsion marketý [24]. Most of the newer Hall thrusters were introduced after 2010, an indication of the industrial proliferation of Hall thruster technology for GEO satellite platforms. It is interesting to note that for each type of the four electric propulsion technologies there is almost always one thruster model that dominates that particular technology's market. The most dominant electric thruster models, for each one of the four technologies used onboard GEO satellites, are presented in Table 2. Fig. 5 presents a three-year running average of the fraction of EPbased satellites launched to GEO relative to the overall number of satellites launched to GEO every year. Additionally, a linear trend line was added to illustrate the growth rate of the EP-based GEO satellite launch fraction. The figure shows a clear increase in the fraction of GEO satellites that launch with an EP system, from 10% in 1981 to nearly 50% in 2018. Since 1998 EP-based GEO satellites have accounted for more than 30% of the GEO satellite market. The figure also shows two distinctive peaks around the years 2000 and 2006. The earlier peak is due to the expanded use of arcjets and ion thrusters on Lockheed Martin's A2100 and Boeing's 702 platforms, respectively. The second peak is an outcome of the increased usage of Hall thrusters by American and European primes during these years. Starting from 2008 there is a generally steady increase in the fraction of EP-based satellites launched to GEO. Based on this trend from the past decade it can be assumed that the relative number EP-based GEO satellites will grow above 50% of the yearly number of satellites launched to GEO sometime in the near future. Apart from the number of EP-based GEO satellites, the progress in the usage of EP technology can be demonstrated by examining the EP system's specific impulse. Specific impulse may serve as a metric of the advancement of electric thrusters because it reflects the fuel-efficiency capability of an electric thruster, which increases the attractiveness of the particular technology. It should be noted that besides specific impulse, thrust level, system price, and operational simplicity are also important factors when choosing between the different EP thruster variants. We focus here on specific impulse because it is the clearest way to distinguish the impact of EP systems. Fig. 6 presents a three-year running average of the average specific impulse for all GEO satellites with EP launched in that year. The 217

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Table 2 List of the dominant GEO electric thruster models. Thruster Technology

Thruster Name

Manufacturer

Year of first space operation

Number of satellites/Total number of satellites (per technology)

Power per Thruster [kW]

Specific Impulse [s]

Resistojet

MR-502 (IMPEHT) & MR501B (EHT) MR-510 XIPS-25 SPT-100

Aerojet Rocketdyne

1981

70/85

0.9

300

Aerojet Rocketdyne L-3 Communications EDB Fakel

1993 1997 1994

61/62 35/63 84/130

2 2/4.3 1.5

600 3500 1600

Arcjet Ion Thruster Hall Thruster

maneuvers – short periodic orbit maintenance activations, long duration high impulse orbit raising operation, low thrust attitude control, continuous operation for drag compensation, and end-of-life disposal to name a few. Each mission requires a specific propulsion system tailored to meet mission maneuverability needs, and these diverse needs result in a much greater range of options of propulsion systems for LEO satellites relative to GEO communication satellites. The earliest successful operational use of electric propulsion on a LEO satellite was on the TIP/ Nova small satellites, which used pulsed plasma thrusters for fine orbit control starting in 1981ý [5],ý [16]. However, developments in other satellite systems and small hydrazine thrusters reduced the relative benefits delivered by early electric propulsion systems and delayed further application. More recently, as EP technology has matured and LEO mission propulsive requirements have increased, use of EP systems on LEO satellites has been revisited. A key constraint imposed on LEO satellite by EP systems is limited power. Unlike GEO comsats, LEO satellites are small, lower power platforms that typically have commensurately small, lower power payloads. Since these smaller LEO platforms are only capable of generating hundreds of watts, due to the limited size of their solar panels and only partial exposure to the sun arising from longer fractions of each orbit spent in the Earth's shadow, they can usually only accommodate low power electric propulsion systems requiring no more than a few hundred wattsý [65]. Given this propulsion system framework, seven technology subclasses of electric thrusters have been used in LEO since 1981: (1) Electrospray thrusters, (2) resistojets, (3) arcjets, (4) hollow cathode thrusters, (5) PPTs, (6) ion thrusters and (7) Hall thrusters. These seven electric thruster technologies have been incorporated onboard a total of 167 LEO satellites weighing over 50 kg. The 50 kg lower limit was taken to distinguish these LEO platforms from the small satellite platforms that commonly have electrical power limitations due to their small size. Fig. 7 shows the number of LEO satellites incorporating electric propulsion systems divided into electric propulsion technology subclasses. The figure shows that before 1997 only a few LEO missions used electric propulsion. These were mainly the NOVA navigation satellites

average is calculated by summing of the specific impulse for each EPpowered spacecraft launched in that year and dividing that sum by the total number of EP-powered spacecraft for the same year. The figure shows that the average specific impulse in the first decade, since 1981, is approximately 500 s. This is mainly due to the extensive use of resistojets (300 s) with a relatively low number of Hall thrusters (1500 s). The entrance of arcjet (600 s) and ion thruster (2400 s) technologies into the propulsion market, in the mid and late 90s, along with a decrease in GEO satellites carrying resistojets, resulted in an increase in the average specific impulse. The average value of specific impulse, in these years, is over 1000 s. Lastly, the extensive use of Hall thrusters for GEO satellites during the last decade drove up the average specific impulse to approximately 1500 s. In recent years, both Lockheed Martin and Orbital ATK modified their EP satellite platforms from using arcjets and resistojets to accomodating the XR-5 5 kW Hall thruster aboard the Modernized A2100 [61,62] and GEOStar-3 [63] satellite platforms, respectively. As a consequence, both arcjets and resistojets are expected to decrease further, and the shift to high-specific impulse electric thrusters will take place. It is expected that the average specific impulse will keep increasing slightly to over 1500 s and maintain a constant value. In particular, the trend indicates that Hall thruster technology is expected to surpass the other EP technologies in terms of the number of GEO satellites. Overall, during the years 1981–2018 a total of 340 GEO satellites incorporating EP systems were launched. The most used variant to date is the Hall thruster, used on approximately 38% of all EP-powered satellites, followed by resistojets (25%), ion thrusters (19%) and arcjets (18%). 4. Low earth orbit (LEO) satellites LEO satellite platforms are designed to perform a variety of missions such as Earth observation, atmospheric monitoring, low-latency communication to Earth, or purely scientific missionsý [64]. To do so, the propulsion system may be required to perform many different

Fig. 5. Relative fraction of the number of EP-based GEO satellites launched in a given year relative to the total number of GEO satellites launched in the years 1981–2018 (3-year moving average). 218

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Fig. 6. Yearly-averaged specific impulse of thrusters launched each year aboard GEO satellites in the years 1981–2018 (3-year moving average).

by the United States, the twin Soviet experimental ‘Plazma-A’ satellites carrying Hall thrustersý [1], the Japanese ETS-III satellitesý [66], and the European EURECA experimental satelliteý [67], both carrying ion thrusters. In this period, the main space agencies or military branches of the dominant geo-political powers at the time, namely the United States, Soviet Union, Japan, and Europe, used LEO missions to test new electric propulsion technologies. No commercial LEO missions were conducted during this period. In 1997 the first satellites of the Iridium communication satellite constellation were launched. Each one of the Iridium satellites incorporated one MR-501 resistojetý [40], using them to perform orbit trim and deorbit maneuvers. In total, 95 Iridium satellites incorporating resistojets were launched. Between 1995 and 2000, three arcjet-based technology demonstration flight missions were conducted. These missions were used to test new arcjet technologies, but none of the tested arcjets evolved into commercial products for use in LEO. It is interesting to note the Space Flyer Unit (SFU) that is considered by many an MPD thruster mission due to the relatively high discharge current it employs, and was operated in 1995 and retrieved by the space shuttle in 1996ý [68]. Despite their lower performance relative to arcjets, resistojets are perceived as a simpler and more cost-effective EP system for LEO mission. To that end, resistojets were developed by Surrey Space Technology Ltd (SSTL) and in 1999 UoSat-12 was launched, carrying the first nitrous-oxide resistojetý [69]. Despite its relatively low performance (specific impulse of 127 s) the resistojet on-board UoSat-12 opened the door to over 20 successive resistojet-powered commercial LEO satellite missionsý [70]. Fig. 7 also shows that only four missions carrying ion thrusters were launched to LEO orbits in the past 25 years. The first satellite was

Shijian-9A, employing the LIPS-200, a Chinese 1 kW ion thruster operated in LEOý [52] as a flight-test in preparation for future missions aboard GEO platformsý [71]. This satellite, launched in 2012, carried both an ion and a Hall thruster. The second thruster was the QinetiQ T5 Kaufman-type ion thrusterý [72], which was chosen for the GOCE LEO mission where high specific impulse and low-thrust throttling capability were needed to reduce propellant mass and allow for accurate, continuous drag compensationý [73]. The third ion thruster system was the MIPS, designed by the University of Tokyo and operated aboard the Hodoyoshi-4 mini-satellite ý [74–76]. Finally, the fourth thruster was the Japanese Kaufman-type ion thruster used to perform drag compensation for Slats. This thruster was a low-power version of the Kiku-8 ion thruster and was developed by JAXA and MELCOý [78],ý [79]. All four ion thruster missions were technology demonstrators and none of these technologies have been used to date on commercial satellites. While many EP based LEO missions included demonstrations of arcjets, PPTs, ion thrusters and Hall thruster systems, the Hall thrusters have become more dominant in recent years as mission heritage was gained and newer players, such as South Korea and Israel, developed their own indigenous versions of electric thrustersý [80–83]. Fig. 8 shows the distribution of the number of LEO missions utilizing the different thruster technology subclasses between the years 1981–2018 (top) and zoomed in on only the past ten years (2008–2018) (bottom). It is evident that resistojet technology is the most prolific and powers 74% of all EP based LEO missions, followed by Hall thrusters (15%). It is also evident that in the past decade Hall thrusters have become more prominent powering over 40% of all EP based LEO satellites for this time period. Excluding Iridium, the relatively low number of LEO satellites

Fig. 7. Number of LEO satellites incorporating electric propulsion systems by year and by thruster technology subclass. Total number of LEO satellite incorporating EP is 167. 219

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countries with space-proven electric propulsion hardware used in LEO are the United States, Russia, Japan, the EU, China, the UK, South Korea, and Israel. To assess the types of technologies suitable for commercial LEO missions we divide all EP based LEO missions into ‘Tech Demo’ and ‘Operational’ missions. ‘Tech Demo’ missions are missions dedicated to the testing of new propulsion system types (e.g. ShinJian-9A), new satellite platforms (e.g. STSAT-3) or which use electric thrusters as payload (e.g. Venμs). ‘Operational’ missions are missions in which the electric propulsion system is an integral part of the satellite and serves as the primary means of propulsion. ‘Operational’ missions usually involve either commercial platforms (e.g. Iridium), observation/navigation (e.g. Deimos-2), or scientific government-owned satellites (e.g. GOCE). In addition, in this assessment, satellite constellations are counted as one mission since all satellites combined are essential for the execution of that mission. For example, all of the five ‘RapidEye’ observation satellites, launched in 2008, are necessary to maintain the required frequency of imaging of areas-of-interest, to perform the missioný [84]. For this reason ‘RapidEye’ is counted as one mission. In the same manner Iridium, Nova, and DMC-3 constellations are regarded as one mission for each constellation. Using this approach a total of 64 EP based LEO missions were launched in the years 1981–2018. It is noteworthy that many of the EP based LEO satellites were technology demonstration missions, as opposed to operational missions, and that the lower cost of developing and launching a LEO satellite was a prime enabler of these flight demonstration missions. Fig. 9 shows the mission type distribution (‘Tech Demo’ vs. ‘Operational’) as well as the distribution of electric propulsion technology subclasses for each mission type. It can be seen in the figure that about half of all EP based LEO missions are of the ‘Tech Demo’ type (35 of 64) – attesting to the common use of LEO platforms as testbeds for new technologies. Additionally, note that LEO technology demonstration satellites flew for many electric propulsion subclasses. Many of the ‘tech demo’ missions incorporating Hall thrusters served as predecessors for subsequent ‘operational’ type missions. In addition, several ‘operational’ type missions relied on established low power (< 1 kW) electric thrusters such as the MR-501 resistojet, SSTL's low power resistojet, Fakel's SPT-type low power Hall thrusters, or the South Korean Hall Effect Propulsion System (HEPS). The majority of ‘operational’ type EP based LEO missions incorporate resistojets, most likely owing to their simplicity and low cost. However, in missions where high maneuverability is required, as in the case of DubaiSat-2ý [85], Hall thrusters are utilized because of their relatively high performance. Rounding out the list, one of the 29 ‘operational’ missions used PPTs (Vela). The advantage of using a high specific impulse electric propulsion system is usually not a significant factor when choosing between EP and a chemical propulsion system since the ΔV required for LEO platforms is

Fig. 8. Distribution of the electric propulsion technology subclasses, for spacecraft in LEO, in the years 1981–2018 (top) and 2009–2018 (bottom). The cross-hatched segment represents the Shijian-9A satellite (2012) that incorporated both an ion and a Hall thruster.

utilizing electric thrusters is mainly due to the lack of commercial incentives for those missions relative to GEO missions. The lack of incentives and commercial interest has also prevented the industry from developing and proving suitable low power technology that could meet LEO satellite maneuverability demands. This has kept low power thruster development in the hands of various governments that invested in technologies that could meet Earth observation needs. To date, the

Fig. 9. Breakdown of electric propulsion technology subclasses for ‘Tech Demo’ and ‘Operational’ mission types for spacecraft in LEO. 220

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often relatively low. Several factors may change today's picture and could increase the use of electric propulsion on LEO satellites:

increasing number of EP-based deep space missions, continuing their leadership in this area. In addition to enabling spacecraft to reach a given destination, a high Isp EP system also offers the advantage of a high degree of spacecraft maneuverability once the spacecraft is ‘on station’ at a destination. This has proven very useful in the Dawn mission [97,98], for example, where the spacecraft entered orbit around Vesta and propulsively reduced its altitude below 200 km to obtain high-resolution lowaltitude images and measurements of the surface. Once it was finished at Vesta, the Dawn spacecraft still had enough ΔV capability to depart Vesta and subsequently enter orbit around Ceres, where it again reduced orbital altitude to obtain high-resolution low-altitude data. Another instance where use of a high Isp EP system afforded a high degree of spacecraft maneuverability was the Hayabusa-1 sample return mission to the asteroid Itokawa [93,94]. Not only was a large ΔV increment required to reach the target asteroid and later return the spacecraft to Earth, but the maneuverability afforded by the propulsion system was required to complete the mission. The high system Isp allowed first for the study of the asteroid's physical characteristics upon arrival, land and retrieve a sample. After a failure of the chemical system during the asteroid encounter, the EP system was used to successfully complete the spacecraft's journey back to Earth. The benefits of electric propulsion for high impulse missions continue to entice space agencies to harness the technologies and develop new ones. Several near-term missions have been proposed, such as DESTINY+ [103,104] (high-Isp version of the μ-10 ion engine), Psyche [105] (SPT-140 Hall thruster [106]), Power and Propulsion Element [107] (newly developed HERMeS Hall thruster [108]) and DART [109,110] (NEXT-C ion thruster), which if successful could enable a greatly expanded use of electric propulsion for deep space missions. Most of the proposed missions require the imparting of significant ΔV to the spacecraft to reach the given destination. These missions, much like the past missions listed in Table 3, require the high Isp levels offered by EP systems to reach the proposed destinations and many of the missions could not be executed using a lower Isp chemical propulsion system.

1) Growing commercial incentives – In the past three years there has been a fast-growing commercial market for using LEO telecommunication satellite constellations to enable low-latency internet access to large parts of the worldý [86]. These satellites, which must be positioned above an altitude of 1000 kmý [87], require performing an energetically-costly orbit raising injection maneuver. The associated ΔV required to perform the maneuver makes high specific impulse electric thrusters an attractive option for these satellite platforms, which is why companies such as SpaceX and OneWeb are developing their own EP-based platformsý [88],ý [89]. 2) Increased use of secondary launch opportunities – For many cases, even though launching as a secondary payload reduces the cost of launch, the satellite is usually not placed in the desired operational orbit, requiring the use of a higher performance on-board propulsion system for final orbit placement. 3) Longer mission lifetime – The average LEO satellite lifetime is growing due to technological advances. Longer LEO missions will require additional propellant, making high specific impulse and long-life electric thrusters an attractive option. 4) Deorbit requirement – To reduce the amount of space debris, many countries require that each LEO satellite be equipped with propulsion to enable the spacecraft to maneuver into a disposal orbit. The deorbit requirement increases the overall required ΔV for the satellite platform, again making electric thrusters more attractive for LEO missions. 5) New maneuvers – Low thrust high specific impulse capability allows for a variety of maneuvers that have not been performed in the past. Altitude change, plane change, and phase change [90] (to enable satellite servicing or re-positioning), drag compensation, and full attitude control (replacing the reaction wheels) are just a few of these maneuvers, and plans to perform these maneuvers open the door for the use of EP on future LEO missions.

6. Small spacecraft under 50 kg

5. Deep space spacecraft

Mini-satellites, Microsatellitesý [111], and CubeSatsý [11] are a rapidly growing niche in the space industry. Between 2010 and 2018, over 200 small satellites (those less than 50 kg) have been launched. Moreover, it is estimated that in the next three years, this figure will increase to over 250 spacecraft per yearý [112]. It should be noted that due to the nature of the small satellite market, which is dominated by many small private companies or academic institutions, information on flight systems is usually limited or not available. Specifically, thruster manufacturers are not given access to flight data because many spacecraft users do not want to reveal details of their flights. For this reason the information presented in this section includes less information on propulsion system capabilities, such as thrust or specific impulse, and more extensive analysis on particular EP technologies suited for small satellites and their applications in space. The first small satellite launched with an electric propulsion system launched in 2006, and as of 2018 only 25 incorporate electric thrusters. This is due to the fact that most small satellite designs are quite minimalistic, and they are usually used by academic institutions or space start-up companies to conduct very low cost flight experiments. In many cases, the mission design does not require or include a propulsion system. Additionally, most of the propulsion devices available to CubeSats are costly compared to the cost of the satellites themselves, placing them out of range for most academic institutions. Furthermore, CubeSats launched from the ISS have very strict requirements that limit the incorporation of propulsion systemsý [113]. Despite these barriers, the rapid increase in the number of small satellites, the desire to use these satellites to perform new and more demanding mission that require propulsion, and the requirement to dispose of satellites in LEO at

Electric propulsion is an attractive choice for high total impulse deep space missions thanks to its high specific impulse compared with other types of flight propulsion systems. However, this potential is only beginning to be realized, so the analysis presented in this section is based on fewer flight missions and is limited relative to the analysis presented above. Ion thrusters, with their high specific impulse, have been the most common type of electric thruster used for deep spaceý [91]. To date, six of the eight EP-equipped deep space spacecraft use ion thrusters for primary propulsion. Depending upon the mission requirements, the electrical power consumed by electric thrusters powering deep space spacecraft has varied from several watts (electrospray thrusters on Lisa Pathfinder) to 2.5 kW (NSTAR thrusters on Dawn). Thrusters on these missions have spanned a total impulse range from 2 × 103 N-s to over 2 × 107 N-s. Fig. 10 and Table 3 list the eight deep space missions to date that have used electric propulsion technologies. The low number of flights is mainly due to the high risk and cost associated with deep space missions, which has led spacecraft designers to choose well-established chemical technologies with extensive in-flight operation history in lieu of EP systems. However, the table illustrates that, over time, confidence in electric propulsion technologies has grown, leading to an increasing number of deep space spacecraft using electric propulsion systems. The United States, Japan, and the EU are the present leaders to date with respect to deep space missions that utilize EP systems for primary propulsion. It is expected that, in the future, these nations will fly an 221

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Fig. 10. Deep space missions utilizing electric propulsion in the years 1981–2018. Table 3 List of deep space missions using EP launched to date. Mission Name

Destination

Launch Year

Prop. Sys. wet (dry) mass, kg

EP Technology

Power, kW

Total Impulse, N-s

Deep Space 1 [92] Hayabusa-1 [93,94] SMART-1 [95,96] Dawn [97,98]

1998 2003 2003 2007

129.5 (48) 125 (59) 113 (31) 554 (129)

NSTAR (Ion Thruster) μ-10 (Ion Thruster) PPS-1350 (HT) NSTAR (Ion Thruster)

0.48–1.94 0.25–0.35 0.65–1.41 0.52–2.57

Hayabusa-2 [94] PROCYON [74,76,77]

Asteroid Asteroid Moon Protoplanets Vesta & Ceres Asteroid Asteroid

2014 2014

μ-10 (Ion Thruster) I-COUPS (Ion Thruster)

0.31–0.42 0.038

Lisa Pathfinder [99] BepiColombo [100–102]

L-1 Mercury

2015 2018

127 (61) 9.5 (7) (with cold gas system) – 1696 (581) (with bi-prop chemical system and EP system dry mass)

2.73 × 106 (qualified) 1 × 106 (space proven) 1.2 × 106 (space proven) 1.23 × 107 (planned) > 2 × 107 (space proven) 1.2 × 106 (space proven) 2 × 103 (qualified)

(Electrospray) T-6 (Ion Thruster)

0.015 2.5–4.6

the end of a mission has increased the need to integrate some means of propulsion into these spacecraft. Propulsion systems for small satellites must address the following requirements:

– 1.15 × 107 (planned) > 1.15 × 107 (test and extrapolation)

could cause satellite components to overheatý [118]. 5) Low power – One of the major limitations for electric thrusters onboard small satellites is the low available electrical power due to the limited solar panel surface area of the small spacecraft. The typical value of the available electrical power depends on the size of the spacecraft and ranges between 5 W on a 1U CubeSat to several hundred watts on a 50-kg-class satellite. On a typical 3U CubeSat, power is limited to around 10 W unless the satellite has deployable solar panels, which can significantly increase available surface area for solar cellsý [119]. This of course comes at the cost of increased mass, cost, complexity, and increased drag. 6) Cost-Effectiveness – Most CubeSat designers are either academic institutions or small organizations only capable of designing and flying small satellites. These entities have small budgets and may be willing to compromise on reliability to drive down procurement costs. Additionally, the low cost of CubeSats puts a limit on the total cost of the EP system, which cannot be larger than the overall cost of the satellite. For these reasons electric thrusters for CubeSats should have a simple design and be assembled by low-cost components; eventually lowering the end price of the propulsion system. Furthermore, many missions simply do not require a propulsion systemý [112], mainly because attitude control can be accomplished through other means, such as magnetorquers or reaction wheelsý [120].

1) Volume and Mass – Small satellites are volume-limited and subject to a stringent propellant mass requirement, and for higher ΔV missions this increases the incentive for high specific impulse propulsion systems. Depending on the size of the spacecraft, the available volume for the propulsion system ranges from less than ¼ Uý [114] up to several Uý [115], but to-date small satellites have not yet flown with a propulsion system larger than 1U.1 The volume restriction implies that the propulsion system's wet mass can only be up to a few kilograms. 2) ΔV – The ΔV requirement is determined by the nature of the mission. Typical values range widely because small satellite mass and mission requirements can both vary significantly. Typical values range from 2 m/s for satellite de-tumble and attitude adjustmentsý [116] to several km/s for orbit changes and deep space missionsý [11]. 3) Low Electromagnetic Interference – Due to the compactness of small satellites, all their subsystems are within close proximity to each other. Any electrostatic or electromagnetic interference caused by the electric thrusters may harm on-board electronic components, hindering the mission. While electromagnetic interference has not yet been an issue in small satellites, manufacturers often attempt to mitigate potential problems by shielding the propulsion system from the rest of the spacecraftý [117]. 4) Thermal control – Another issue associated with the compactness of small satellites is thermal control. Many electric thrusters are sources of large thermal loads which, if not properly dissipated,

The list of small satellites launched with electric thrusters as of 2018 is presented in Table 4. The first small satellite to incorporate an electric propulsion (ION) was launched in 2006 and used a Vacuum Arc Thruster (VAT)ý [116],ý [121]. The satellite was not tested in orbit due to a failure of the Dnepr launch vehicle, which led to the loss of the mission. Following the ION CubeSat mission that used vacuum arc thrusters, three other types of very low power electric thrusters have been used on small spacecraft; Micro Pulsed Plasma Thrusters (μPPTs), electrospray thrusters and resistojets. Through 2018, 24 small spacecraft employing an electric propulsion system have been launched. The four types of

1 A “U” is a unit of measurement for CubeSats, representing one 10 cm × 10 cm x 10 cm cube. Thus, a “3U” satellite consists of three 10 cm cubes yielding total spacecraft dimensions of 30 cm × 10 cm x 10 cm.

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Table 4 List of all small spacecraft (under 50 kg) using electric propulsion that were launched in the years 2006–2018. Name

User Type

Launch Mass, kg

Year of Launch

Thruster Type

Thruster Manufacturer

Application

UWE-4 Hawk C Hawk B Hawk A Irvine-2 Irvine-1 Aerocube-12B TurboDove CANYVAL-X Tom

Academic Commercial Commercial Commercial Academic Academic Technological Commercial Academic

1 13 13 13 1 1 4 2.7

2018 2018 2018 2018 2018 2018 2018 2018 2018

Electrospray Resistojet Resistojet Resistojet Electrospray Electrospray Electrospray Electrospray VAT

TU Dresden Deep Space Industries Deep Space Industries Deep Space Industries Accion Systems Accion Systems MIT Enpulsion George Washington University

Pegasus (QB50 AT03)

Academic

2

2017

PPT

AOBA-VELOX 3 AeroCube 8D AeroCube 8C Horyu 4 (AEGIS)

Academic Technological Technological Academic

2 3 3 10

2016 2016 2016 2016

PPT Electrospray Electrospray VAT

University of Applied Sciences Wiener Neustadt Nanyang University MIT MIT Kyushu Institute of Technology (KIT)

Tech demo Formation Flight Formation Flight Formation Flight Tech demo Tech demo Tech demo Orbit Change Formation Flight Orbit Maintenance Tech demo

BRICsat-P

Academic

1.9

2015

VAT

George Washington University

AeroCube 8B AeroCube 8A WREN CUSat top

Technological Technological Commercial Academic

3 3 0.25 25

2015 2015 2013 2013

Electrospray Electrospray PPT PPT

MIT MIT StaDoKo Cornell University

CUSat bottom

Academic

25

2013

PPT

Cornell University

STRaND-1 PROITERES FalconSat-III Illinois Observing Nanosatellite (ION)

Academic Academic Military Academic

3.5 15 50 2

2012 2012 2007 2006

PPT PPT PPT VAT

SSTL Osaka Institute of Technology (OIT) Busek University of Illinois

Orbit Maintenance Tech demo Tech demo Attitude Control Orbit Maintenance Attitude Control Orbit Change Tech demo Tech demo Tech demo Attitude Control Orbit Maintenance Attitude Control Orbit Maintenance Attitude Control Tech demo Attitude Control Launch Failure

and do not require a neutralizer. VATs use the cathode as propellant which is ablated during each discharge. In addition to the VAT that was on the ION mission, The George Washington University has developed the Micro-Cathode Arc Thruster (μCAT), based on the VAT technology. The μCAT was flown on the BRICSat-P CubeSat and is planned to fly again aboard the BRICSat-2 mission currently scheduled for launch in 2019. Kyushu University has also developed its own VAT that has flown aboard the Horyu-4 nano-satellite. VATs were used for de-tumbling, attitude control, and orbit maintenance. Pulsed Plasma Thrusters rely on a discharge between two electrodes to ablate a non-conductive propellant. PTFE is the most common propellant used for PPTs and is the only propellant type to have been used in space to date, although other materials have been proposed, such as PFPE, water, or airý [127–129]. One of the main advantages of PPTs is the ability to scale them to the required satellite dimensions. Thanks to this ability PPTs have been employed both on mini-satellites (FalconSat-III in 2007ý [130]) and even pico-satellites (Wren in 2013ý [131]). For both VATs and PPTs, the amount of thrust developed is dependent on the amount of propellant ablated during each pulse and the repetition rate of the discharge. Both thruster types deliver impulse bits that are on the order of tens of microNewton-seconds. Completing maneuvers that require significant impulse requires the use of either several pulsed thrusters or a small number of thrusters operating at high pulse frequencies, both of which are difficult to achieve on small satellites that are limited in power and in their ability to dissipate the heat produced by the propulsion system. Electrospray thrusters for small satellites make use of a strong electric field applied to a conductive liquid to extract and accelerate small charged droplets, ions, or mixtures of both from a propellant reservoir. The main benefit of electrospray systems is their compactness, which is achieved by: (1) the lack of required propellant pressurization in a relatively simple propellant management system that uses liquids with low vapor pressures (ionic liquids) and capillary forces to feed the liquid propellant to the thruster, (2) the fact that no

Fig. 11. Percentage of small satellites (under 50 kg), divided into micro-electric thruster subclasses, launched in the years 2006–2018.

electric thrusters (VAT, μPPT, electrospray and resistojet) flown on small spacecraft have been under development for over a decade alongside other micro-electric propulsion devicesý [122–124] that have not yet been used in space. The distribution of the four electric thruster technologies used in space is presented in Fig. 11. While the sample size is small, it can be seen that of the four electric thruster technologies, to date the μPPT and electrospray thrusters were used on more than two thirds of the missions, while VATs and resistojet thrusters were used aboard only four and three spacecraft, respectively. Since all micro-electric propulsion technologies are still in the relatively early phases of technological maturityý [76] and possess similar propulsive attributes, it is impossible at this time to predict which will become the industry leader. Vacuum arc thrustersý [125] and pulsed plasma thrustersý [126] are similar technologies that create a quasi-neutral plasma discharge 223

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insertions at both Vesta and Ceres. The Japanese mission Hayabusa-1 demonstrated how a versatile EP system can be used for propelling a satellite back to Earth to complete a sample return mission. Since 1981, 340 GEO satellites with EP technologies have been launched, with over 57% of them employing either an ion thruster or a Hall thruster, replacing the well-established resistojets and arcjets due to their higher specific impulse and proven reliability. The increasing trust in electric propulsion is reflected in the fraction of GEO satellites employing EP systems; from an average of 20% of all launches during the 80s to over 40% in recent years. Additionally, the increased use of ion thrusters and Hall thrusters raised the average specific impulse of EP systems launched each year from about 500 s in the 1990s to over 1500 s today. There has been a slow increase in the use of EP on LEO satellites; from fewer than 10 satellites in the 1980s to 45 LEO satellites launched in the past decade (2009–2018). The exception to this trend was the Iridium satellite constellation that consisted of 95 satellites, all employing resistojets; demonstrating the reliability of LEO EP systems and its utility for large satellite constellations. We noted that even with its lower performance, resistojet technology is employed the most on EPbased LEO platforms, but that Hall thrusters have been increasingly used in recent years. We showed that more than half (35 of 64) of the EP-based LEO missions were technology demonstration missions, serving as testbeds for new technologies, including new electric thrusters. In the past two decades EP was used in eight deep space spacecraft, with most harnessing the advantages of high specific impulse propellant-saving ion thruster technology. The high maneuverability achieved with EP enabled these deep space spacecraft to perform sophisticated asteroid rendezvous and sample return missions that would have been impossible using chemical propulsion. We observe strong growth in the small satellite sector, where CubeSat missions requiring propulsion are now routinely proposed. This overall growth in the sector and the propulsive capabilities offered by micro-electric propulsion systems will enable the performance of more complex missions by even the smallest satellite designers, such as universities and other small organizations. Additionally, having an EP system on board can allow small spacecraft to continuously compensate for drag, keeping the satellite in orbit for a longer period and making it easier to justify the initial spacecraft investment. As of 2018, the four EP micro-propulsion technologies used on small satellites were electrospray thrusters, vacuum arc thrusters, micro pulsed plasma thrusters, and resistojets. Additional thruster technologies for these platforms are under rapid development. Due to the fast expansion of the small satellite market and the increasing utility of propulsion in the performance of these missions, we foresee significant growth in the small satellite micro-electric propulsion systems. Overall, since the first launch of an electric propulsion system in 1962, a variety of subclasses have permeated into all sectors of

ionization volume is needed to produce charged particles, and (3) the suitability of these devices to be integrated in small packages using micromachining/MEMS fabrication techniques. The power required and the thrust available are dependent on the number of emitter tipsý [132]. As a result, electrospray propulsion systems are scalable for small satellite applications, and they are suitable for all types of small spacecraft missions. In recent years, MIT and FOTEC have developed a number of laboratory thrusters and characterized their performanceý [133]. Flight versions of these thrusters have been integrated into AeroCube8 and TurboDove satellites for technology demonstrationsý [134],ý [135]. Finally, a new type of micro-thruster technology has been under development for several years – the Radio Frequency Thruster ý [136]ý [137]. This EP technology is scheduled to fly in the near future ý [138]. It is interesting to note that more than half (14 of 24) of the EPbased small satellites were launched in the years 2016–2018, implying the potential for strong near-term growth in small satellites flights that use micro-electric propulsion systems. 7. Conclusion The last five and a half decades have shown an increased trust in electric propulsion systems from both satellite manufacturers and operators. There has been a steady increase in the number of satellites launched with electric propulsion systems, to a total of 587 spacecraft. The first two decades, the 1960s and 1970s, saw the first utilization of electric propulsion aboard a total 48 spacecraft. These missions, flown by governments and primarily demonstrating propulsion technologies, incorporated resistojets, pulsed plasma thrusters, ion thrusters and Hall thrusters. In late 1980 the first commercial satellite to use EP, IntelsatV, was launched to GEO orbit. While this launch marked the start of commercial EP usage, the introduction of higher-performing arcjets in 1993 resulted in EP gaining world-wide acceptance and increased market share due to impact on launcher competitions. Since 1981 the number of EP-based satellites, summarized in Fig. 12 for every year from 1981 to 2018, has seen a gradual, steady increase. Building on the NSSK applications, missions such as NASA's Deep Space 1 in 1998, the European Space Agency's SMART-1 mission and JAXA's Hayabusa-1 asteroid mission in 2003, AEHF SV1 in 2010, and the first GEO dual-satellite launch using Boeing's 702SP fully electric bus in 2015 have shown the ability of EP systems to successfully achieve primary propulsion tasks such as orbit transfers, despite the increased time the low thrust EP systems takes to reach the final orbit. EP systems perform a given mission using significantly less propellant relative to lower specific impulse chemical systems, or they allow longer satellite missions for the same propellant mass. Electric propulsion has enabled missions that were unthinkable with conventional propulsion systems, such as NASA's Dawn mission, which completed successful orbit

Fig. 12. Number of EP-based spacecraft launched in the years 1981–2018, divided into mission type. 224

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spacecraft missions. Thanks to its performance advantages relative to other flight propulsion systems, electric propulsion is widely used and its share of the satellite propulsion market is expected to further expand in the years to come.

[23] [24]

8. Dedication

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This paper is dedicated to David C. Byers (1939–2018), whose technical and programmatic leadership was critical to the broad acceptance of electric propulsion. His understanding of the commercial incentives for the use of new technology, the proper role of government research, development and demonstration missions, his technical acumen, and his passionate support for the entire electric propulsion community will be sorely missed in the years to come.

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Acknowledgments

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The authors thank Daniel Narviv (IAI), Leonid Appel (Rafael), Andrew Hoskins (Aerojet Rocketdyne), and Tony Schönherr (ESA) for their advice, comments, and help in shaping this paper.

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