CONCEPT OF ELECTRIC PROPULSION REALIZATION FOR HIGH POWER SPACE TUG

Popular at the beginning of the Space Age, ambitious projects aimed at Moon, Mars, and other space objects exploration, have returned with new technology and design level. High power space tug with electric propulsion system (EPS) is mainly considered as a transport vehicle for such missions. Modern high power space tugs projects as well as their spacecraft (SC) power and propulsion systems are reviewed in the paper. The main technologies and design solutions needed for high-power EPS realization are considered.


INTRODUCTION
At present, there is a growing interest in radically new space transport vehicle development.High power space tug with EPS is mainly considered as a transport vehicle for implementation of ambitious deep space missions as well as for near-Earth application.
Due to high speci¦c impulse values, electric propulsion application for SC stationkeeping, SC orbit insertion, interplanetary and deep space missions, etc. provides signi¦cant fuel saving in comparison with conventional chemical propulsion.So, number of SC with EPS is constantly increasing.
Electric propulsion as a space technology was born in 1960s.During past decades, EPS technology was rapidly progressing.New kinds of electric thrusters appeared and a lot of §ight and laboratory EPS models were developed, manufactured, and tested.
Laboratory models of di¨erent types of high-power thrusters were intensively researched in 1960s1970s for providing near-Earth and interplanetary perspective missions [1,2] but they did not ¦nd practical application and remained at laboratory level of development.Lack of onboard power needed for electric propulsion supplying was the main reason.
In the last decade of the XX century (19902000), main activity was focused on commercial and science electric propulsion application with single thruster PROGRESS IN PROPULSION PHYSICS of kilowatt power level and available onboard power level up to 10-kilowatt level [3,4].Modern commercial SC onboard power level is about 20 kW [5] and single thruster power is up to 5 kW.Spacecraft onboard available power level and power of §ight electric propulsion are kept constantly increasing.Moreover, 260-kilowatt SC power level is reachable now, it has been demonstrated at the International Space Station (ISS) [4].Thus, taking into account demonstrated onboard power level and renewal of high power thruster development activity [614], there is a good background of the high-power space tugs development.

PROJECTS AND MISSIONS
There are a number of modern projects aimed at solar [4,15] or nuclear [1618] electric propulsion tugs development.The power level of considered tugs lies in the range from hundreds of kilowatts up to tens of megawatts.
Collaborative group of Russian enterprises, headed by the Russian Federal Space Agency (Roscosmos) and the State Atomic Energy Corporation ¤Rosatom,¥ is currently working on an innovative project to create a transport power module (TPM) based on megawatt-class nuclear power propulsion system (NPPS) [18,19].
National Aeronautics and Space Administration (NASA, USA) also shows interest in high-power space tug development.NASA£s initial plans provide for stepwise SC onboard power growth (30 kW 90 kW 250 kW) up to multimegawatt level in 2026 [20].One can suppose that space agencies of other countries have such plans or may even have current activities too.
Such projects implementation will allow providing new challenging near-Earth and deep space missions which are hard to realize in other way.
These missions are given below [21]: heavy payload transfer to geostationary orbit; removal of out-of-operation satellites and space debris from near-Earth orbits; Earth protection from asteroid and cometary hazard;

Moon exploration program;
Mars manned mission; deep space missions; etc.
Projects mentioned therein di¨er by missions and technical parameters but their analysis allows one to identify common features; general tendencies in a space tug development and technical problem have to be studied and resolved.

Prometheus Project
The Prometheus Project was an element of the NASA Prometheus Nuclear Systems and Technology Theme.The Project was to develop a Deep Space Vehicle (DSV) for outer solar system robotic exploration missions that would combine a safe, reliable, Space Nuclear Reactor with electric propulsion [16].The nuclear reactor, the power conversion system (PCS), and the propulsion system were referred to as a nuclear electric propulsion (NEP) system.
Main NEP technologies are: primary power source ¡ Nuclear Reactor with Radiation Shield; PCS based on gas-turbine Brayton cycle; and electric propulsion system based on Ion Thrusters (ITs) and Hall E¨ect Thrusters (HETs).
The Reactor Module, at the forward end of the DSV (Fig. 1), comprises a high-temperature gas-cooled reactor directly coupled with redundant Brayton turboalternators for power conversion, producing about 200 kW of electrical power.Aft of the reactor is the Radiation Shield Segment, which provides a conical shadow of reactor radiation attenuation to the remainder of the DSV.Control and monitoring for the reactor is provided by the Reactor Instrumentation and Main propulsion is provided by Ion and Hall thrusters mounted on two deployable thruster pods, making up the Electric Propulsion Segment of the SC Module.An SC docking adapter (Docking Segment) is also included in the SC Module to support early on-orbit operations and docking with the interplanetary transfer stages.The docking adapter provides power, communications, and attitude control functions for the DSV in the postlaunch phases through deployment and commissioning.
The EPS includes ITs and HETs mounted on two pods.Each pod contains four ITs, three large HETs for thrust augmentation, and six small HETs for attitude control.The power and Xenon fuel feeds are controlled internally in the EPS by eight IT Power Processing Units (PPUs) and Xenon Feed Controls (XFCs), respectively, six large HET PPUs and XFCs, and six small HET PPUs and XFCs.Attitude Articulation and Control System (AACS) will control the electric propulsion valve drive electronics.
Unfortunately, the Prometheus Project was directed to not proceed into the next phase [16].NASA reevaluated its priorities in light of available funding.The Agency nuclear initiatives were postponed to a large extent, and work within the nuclear systems program was reprioritized.Nuclear electric propulsion was given the third priority behind nuclear surface power and nuclear thermal propulsion.

High Power Solar Electric Propulsion Freighter
The use of Solar Electric Propulsion (SEP) can provide signi¦cant bene¦ts for the human exploration of near-Earth asteroids.These bene¦ts include substantial cost savings ¡ represented by a signi¦cant reduction in the mass required to be lifted to low Earth orbit ¡ and increased mission §exibility [4,22].The key technology required for the SEP vehicle is the development of an autonomously deployable solar array with approximately 800 m 2 of solar cells.For such large high-power solar arrays, mass reducing is provided by operating the array at high voltage.A peak-power voltage of 300 V was assumed in the vehicle mass estimates.High-power HETs, with an input power of approximately 40 kW, that provide a speci¦c impulse of 2,000 s and can process over 5,000 kg of Xenon are also required.Direct-drive systems, in which the HETs are operated directly from a high-voltage solar array, are projected to provide signi¦cant mass savings, substantially simplify the thermal control subsystem, and facilitate the development of the direct-drive PPU (DDU).Conceptual design of a 300-kilowatt SEP tug is given in Fig. 2.
Solar electric propulsion design power level is a little bit higher than the ISS one (268 kW).At the same time, SEP solar array design e©ciency (33%) significantly exceeds the ISS one (11%) [20] and it will provide considerable reduction The development of a PPU with the characteristics required for the 300kilowatt SEP Freighter ¡ 43-kilowatt input power; 250-to 350-volt input voltage; 95 percent e©ciency; and a mass of ∼ 80 kg ¡ will certainly be challenging.A high-voltage solar array, with a nominal peak-power output voltage of around 300 V, provides a substantial mass reduction for the SEP vehicle relative to a 100-volt array.A high-power Hall thruster operating at a speci¦c impulse of around 2,000 s requires an anode voltage of around 300 V; therefore, it is natural to utilize direct-drive approach in which the Hall thrusters are operated directly from the high-voltage solar array with a minimum of power processing electronics in between.
The SEP main technologies are: primary power source ¡

Mars-2033 Project
The Mars-2033 NASA project architecture includes a cargo transfer vehicle (CTV) with NEP system providing 5 MW of electrical power and a crewed Mars transfer vehicle (MTV) with NEP system with two reactors providing a total of 10-megawatt electrical power [23].Both vehicles use a low-thrust, high-e©ciency (5000-second speci¦c impulse) magnetoplasmadynamic (MPD) system to conduct a spiral-out of the Earth gravity well, a low-thrust heliocentric trajectory, and a spiral-in at Mars with arrival in 2033.
The MTV uses two reactors, each providing 5 MWe, and a total of four Brayton power conversion units.There are two thruster arms with four 2.5 MWe MPD thrusters (two operational, two spare) on each arm.Each thruster arm has a radiator to reject heat from the PPUs.The total planform area of the PPU radiators is 136.7 m 2 (273.4 m 2 e¨ective radiating area).Six liquid-hydrogen (LH 2 ) tanks that are 7.6 m in diameter and 19 m long occupy the middle truss section of the vehicle.The main radiator is comprised of two sections of doublesided §at panels attached to the center truss structure on either side of the propellant tanks due to center-of-gravity requirements.The total planform area of the main radiator is 2722 m 2 (5444 m 2 e¨ective radiating area).The MTV is 182 m long and must be assembled in orbit.The con¦guration of the MTV is shown in Fig. 3.
The EPS of CTV and MTV are based on MPD thrusters with hydrogen as a propellant.The MPD thruster operates at 2.5 MWe at a constant I sp of 5,000 s with a thruster lifetime of 7,500 h.
The MTV uses four operating thrusters for a total power level of 10 MWe and has 4 nonoperating spares for redundancy.Likewise, the CTV uses two operating thrusters at a total power level of 5 MWe and has two nonoperating spares for redundancy.The TPM main characteristics are: one nuclear reactor of 3.5-megawatt thermal power level; four closed gas-turbine Brayton-cycle units for thermal power conversion and generation of 1 MWe; PMAD: managing available onboard power ∼ 1 MWe; primary electric propulsion: high-power ITs of total power ∼ 1 íWe with I sp ∼ 7000 s; and electric propulsion for TPM attitude control: ITs with I sp ∼ 7000 s.

APPROACH AND ISSUES OF ELECTRIC PROPULSION SYSTEM REALIZATION FOR HIGH-POWER SPACE TUG
Mentioned above space tugs are intended for distinct missions.So, they have power propulsion systems (PPS) based on di¨erent power source types and electric propulsion types (Table 1).
Power source and conversion types selection issues were particularly considered in the previously mentioned projects and, as it was shown, there are some speci¦c features of their combined operation as well as features of interaction with main electric load ¡ EPS [24].These features should be taken into account while developing PPS operation algorithm.However, EPS and PMAD integration and interference features have not yet been investigated in detail; so, in this paper, some EPS and PMAD design aspects that should be taken into consideration are presented.
In spite of signi¦cant distinctions of abovementioned projects, there is a set of common technical solutions related to EPS and PMAD design: EPS is based on combining several simultaneously operating thrusters into one assembly referred to as ¤cluster¥ [11]; all considered types of electric propulsion thrusters (IT, HET, and MPD) have cathodes ¡ sources of electrons.These cathodes provide thruster operation and have important function ¡ to neutralize ion §uxes generated by thrusters; application of so-called ¤direct drive¥ concept is proposed.It provides signi¦cant cables and transforming electronics mass savings, since the discharge nominal voltage needed for electric propulsion thrusters is supplied directly and there is no need of any intermediate transformation; and number of simultaneously operating thrusters can vary in accordance with space tug operation algorithm.Electric propulsion and power management and distribution systems can have di¨erent architecture options that should be considered while analyzing their integration issues.These architecture options and issues are given below.

High-Power Electric Propulsion and Power Management and Distribution Systems Architecture
Electric propulsion system includes thruster, PPU and propellant feed system (PFS).The architecture of typical §ight quali¦ed EPS which are under utilization is the following: each thruster has its own PPU and PFS elements.
Multithruster EPS can be designed with the help of two distinct approaches: (1) integration of several independent EPS; and (2) creation of cluster assembly with several simultaneously operating thrusters, common PPU, and PFS.In the second case, it can be considered as a single multichannel thruster.

Figure 5 Independent (a) and common (b) EPS architectures
Two ultimate cases of EPS architecture are given in Fig. 5.
(1) independent EPS architecture (see Fig. 5a) includes a set of independent thruster modules.Each module consists of single thruster, cathode unit, PPU, and PFS.Propellant storage tanks and onboard power system could be common; and (2) common EPS architecture (see Fig. 5b) is divided into functionally independent subsystems.A subsystem includes several thrusters, cathode unit, PPU, and PFS.Thus, single PPU, single PFS, and single cathode unit could provide operation of a number of thrusters.
Common EPS architecture allows getting signi¦cant EPS mass pro¦t, it also provides the reduction of EPS parts nomenclature and total cost.In addition, the cluster can be optimized for providing required reliability and lifetime using minimal number of thrusters.So, cluster is a quite universal technology and it can be used for the majority of electric propulsion thruster£s types.
However, there are some issues that should be taken into account while developing the cluster design.As it was shown before [2529], several thrusters can operate with common cathode.From the physical point of view, there are no factors limiting the number of thrusters operating with one common cathode.However, when considering EPS transient modes and operation algorithm, such factors come into play.
As it was mentioned above, EPS should provide the possibility of operation of any intermediate number of thrusters from 1 to N , where N is the maximal number of simultaneously operating EPS thrusters.The existing §ight cathode design allows the emission current to be regulated within a factor of 5 to 10 (see, for example, [30]) without signi¦cant reduction of cathode e©ciency and lifetime.The value of cathode emission current should be equal to the value of thruster discharge current or to the value of summary discharge currents of several operating thrusters.Hence, it appears that the number of thrusters operating with common cathode is limited and determined by the regulation range of cathode emission current.So, for cathode technology, the state-of-theart number of operating thrusters will be 5 to 10, it will provide the possibility to switch on any number of thrusters from 1 to N .
Another EPS cluster architecture feature relates to the possibility of negative interference of thrusters.Electric propulsion thrusters use electric and magnet ¦elds; while operating, they generate high power electromagnetic and thermal radiation.Therefore, there is a danger of negative thrusters interference though Hall-and ion-based clusters tests have shown [28,3133] that negative interference can be successfully minimized or even entirely eliminated.
Thrusters interference minimization approach consists in optimization of distance between thrusters and in selecting the optimal thrusters assembly con¦guration.On the one hand, thrusters can be situated at the far distance from each other, but it leads to increasing cluster overall dimensions.On the other hand, in case of thrusters compact arrangement (for example, cluster based on BUSEK BHT-600 Hall thrusters, see [33]), single thruster magnet system switching oc ould lead to negative e¨ects on magnet systems and operating modes of other thrusters.
As for PMAD, there are the following architecture options: centralized, channelized, or hybrid [34].Centralized architecture is characterized by the presence of one (central) distribution node.Advantages of the centralized architecture are high e©ciency and low mass of the PMAD components.There are several independent nodes in channelized architecture.Advantages of channelized architecture are good fault tolerance and reliability as well as low cabling mass.The most attractive architecture is the hybrid one, since it combines the advantages of the others: modularity; recon¦guration possibility ¡ replacement of any thrusters cluster from any PMAD module (cross ties); highest fault tolerance and reliability as well as good cabling mass and e©ciency.

Issues of Direct-Drive Concept Realization
Even without the additional challenges of developing and qualifying new parts, PPUs are expensive and time-consuming to develop.There is the possibility to simplify PPU design and to reduce its mass signi¦cantly.At the direct-drive concept realization, the operating voltage of the thrusters is matched to the turbo-alternators or solar array output voltage, eliminating the need for high power direct current (DC) to DC discharge converters.
Electric propulsion thrusters, however, are subject to large-amplitude discharge current oscillations; so, a direct drive system would consist primarily of ¦ltering, switches to isolate thrusters, and low-power auxiliary supplies for cathodes and magnets.If direct drive can be successfully developed, it would result in a signi¦cantly higher-e©ciency system and signi¦cantly less waste heat.It

PROGRESS IN PROPULSION PHYSICS
could signi¦cantly reduce PPU mass with additional savings in structural and thermal management, and array mass.Direct drive systems are potentially much easier to develop than the conventional PPUs, reducing development cost, time, and risk.Direct drive has been studied at power levels up to 1 kW but successful startup and stable operation at high power with multiple thrusters have been demonstrated [22].Issues of DDU realization are also discussed in [35,36].Advantages (without color) and drawbacks (grey color) of DDU-based SEP in comparison with conventional PPU-based are given in Table 2 [34].One of the issues that needs to be solved for DDU realization is the discharge current distribution between several simultaneously operating and galvanically connected cathodes.Cathodes discharge voltage values cannot be absolutely identical, and even a little di¨erence of voltage values can cause signi¦cant emission current value changing.Therefore, in the case of cathodes parallel connection, some cathodes would be overloaded and others, inversely, would be underloaded.Cathodes parallel operation can be provided by special means (see, for example, [25]); however, it will require EPS design complication and additional ground testing.

Typical Requirements to High-Power Electric Propulsion System Main Components and Testing Issues
There are special programs aimed at high-power EPS components development [37] (high-power thrusters with high current cathodes [38], gimbals, PPUs, propellant management systems, storage systems, etc.).The most probable thruster candidates for high-power EPS realization are the same as for abovementioned projects (see section 2): HETs [6,9,13,3942], ITs [7,8,43], and MPD thrusters [10,14,44].Summarizing reference data mentioned above, typical requirements to EPS components can be obtained: 2050-kilowatt thruster with thrust and speci¦c impulse regulation (multimode thruster).The speci¦c mass of thruster is 12 kg/kW; cathodes with discharge currents up to hundreds amperes and with the possibility to regulate current values in a wide range; PPU speci¦c mass less than 1.8 kg/kW of components, e©ciency > 95% at the maximum §ight allowable operation temperature of 60 • C. In case of DDU concept realization, the speci¦c mass will be signi¦cantly less and the e©ciency will be higher; the propellant storage system with a low tankage fraction and reduced propellant residuals; the propellant management system with precise control of the propellant §ow rate; and low-mass thruster gimbal.While considering high-power EPS realization, one more important issue should be mentioned.There are practical limitations for high-power EPS complete cycle of ground testing [45].The main technical problem is to simulate the environment in which the EPS would operate in space.The existing facilities allow providing a necessary level of vacuum for only single high-power thruster testing.Modernization of existing facilities or building the new one is very expensive; so, it is proposed to use §ight demo missions or to create special space platforms to provide §ight testing of newly developed high-power EPS key components.
Actual high-power space tug development projects have been analyzed from the point of view of applied technologies.Perspective design solutions for space tug of EPS realization have been considered.It is shown that high-power EPS design should be determined taking into account the interference between EPS and PMAD.
The EPS cluster architecture with direct-drive joint application is the most perspective design direction.However, to realize these concepts, a detailed research aimed at better understanding of operating thrusters interference as well as thrusters interaction within EPS + PMAD assembly should be carried out.During this investigation, EPS and PMAD architectures should be determined, the issue of discharge current distribution between several simultaneously operating and galvanically connected cathodes should be resolved, and negative thrusters interference should be minimized.

Figure 1
Figure 1 Prometheus project DSV

Figure 2
Figure 2 Three-hundred-kilowatt SEP tug.Dimensions are in meters

Figure 3
Figure 3 Mars transfer vehicle

Figure 4
Figure 4 Transport power module based on a megawatt-class NPPS

Table 1
Characteristics of the considered high-power tugs * Lesser value corresponds to DDU option.