Recent Developments in High Power Electric Propulsion: Outcomes of HiPER Project Activities

63rd International Astronautical Congress, Naples, Italy. Copyright ©2012 by the International Astronautical Federation. All rights reserved.

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RECENT DEVELOPMENTS IN HIGH POWER ELECTRIC PROPULSION: OUTCOMES OF HIPER PROJECT ACTIVITIES

Tommaso Misuri

Alta SpA, via A. Gherardesca 5, 56121 Ospedaletto, Pisa, Italy Contact email: [email protected]

Giovanni Cesaretti, Riccardo Albertoni and Mariano Andrenucci

Alta SpA, via A. Gherardesca 5, 56121 Ospedaletto, Pisa, Italy HiPER is a 3-year, European financed programme focused on the study of high power electric propulsion

systems and their possible applications in future space missions. Electric propulsion is going to play a major role in the future of space exploration, but to serve this purpose both thrusters and power generation system have to be further developed in order to reach a high maturity level also in such power ranges (tens of kW). The goal of HiPER was very ambitious, since it not only addressed technologies related to electric propulsion, but also attempted to consider major technological efforts in the framework of social and political scenarios, both internal to Europe and with respect to non-European partners. The main strategic objective of the HiPER project has been to initiate technological and programmatic consolidation in the development of innovative electric propulsion technologies, and of the related power generation technologies, so that be possible to readily fulfil future European space transportation needs. The project ended in March 2012 and all the main outputs emerging from the 3-year long activity are recalled in the present paper.

I. INTRODUCTION

Nowadays electric space propulsion is a valid alternative to chemical propulsion for many space applications, sometimes even the only viable option. The principal advantage of electric thrusters is their extremely high specific impulse, which allows for important mass savings (in terms of propellant mass).

HiPER represented the first joint effort of a large group of specialized partners aimed at defining the future development of electric space propulsion and at pointing out a series of possible mission scenarios where high power electric propulsion is going to play a key role [1,2]. Involving twenty partners from six European Countries, the project had the ambitious goal of addressing all the main technological issues, attempting at the same time to consider any major technological effort in the framework of social and political scenarios, both internal to Europe and with respect to non-European partners.

The whole HiPER project has been divided into five main technical parts, each of them fully dedicated to a different topic. The first two technical sections (dealt with in paragraph I and II) are respectively devoted to mission analysis and to electric power generation. Section III is instead focused on the three different thruster concepts (Hall thruster, Ion engine and Magnetoplasmadynamic thruster), that have been selected, analysed and further developed in the framework of HiPER programme.

II. TARGET MISSIONS

Selected scenarios try to harmonize the long-term plans by the main space agencies, with the sensitivity and the prediction capability of experienced space executives, and with the visionary inputs by small companies which are at the leading edge of space propulsion innovation. HiPER Mission Analysis team (Alta, Astos Solutions, CNES, DdeB, KopooS Consulting and Space Enterprise Partnerships) was devoted to define near and long term mission and transportation scenarios which could specially benefit from high power EP.

- Orbit transfer in the Earth-Moon system:

lessons learned from ISS in terms of routine module assembly and refuelling are directly applicable to the design, development, operations and management of future exploration missions. Larger payloads could be shipped by means of high power EP from LEO to GEO, EML1 and LLO at expense of longer transfer times with respect to conventional propulsion systems.

- Robotic Mars Sample Return (MSR):

considered as a key mission by all space agencies and a priority for Europe, MSR can represent an important milestone on an international roadmap leading to a human mission to Mars. It can also represent a very good candidate to exploit the benefit of EP (in combination with conventional propulsion) within a reasonable time-scale. In addition to the high scientific


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value deriving by the analysis of the samples (composition, assessment of evidence of prebiotic processes/past life, analysis of geological processes, etc..), it is recognized that MSR also would have a big impact on the public opinion, just the way it happened when Apollo samples returned from the Moon.

- NEOs exploration, exploitation and risk

mitigation: after some scientific/sample return missions to NEOs to search for new resources, the exploitation of NEOs can potentially lead to commercial utilization of new resources either on Earth and in-space. The capability to deflect threatening Earth-crossing asteroids can be also developed and thus contribute to increase mankind safety.

- Mars and its moons (Deimos and Phobos) fly-

bys and science: Mars and its moons fly-bys with crew and cargo missions to coordinate with or control robots on Mars surface.

- Outer solar system and beyond robotic

exploration and science: scientific/robotic missions to outer planets of the solar system. In particular, missions to Jupiter’s and Saturn’s moons to search for evidence of life.

In the following figure all the mission scenarios

considered along HiPER project are summarized and compared to a recently released Global Exploration Roadmap (in September 2011 by ISECG, International Space Exploration Coordination Group) to show the many similarities between the two intended programs.

Figure 1: Comparison between HiPER selected mission scenarios and NASA Global Exploration Roadmap

III. IN SPACE POWER GENERATION

A crucial issue when deciding to use a high power electric propulsion device to propel a spacecraft is how to generate the necessary amount of electric power in space.

Two are the existing technologies that have been investigated: solar power (the only feasible option in a short-term scenario) and nuclear power (an option that is expected to be fundamental in the mid-long term future).

Solar Power

Along these three years of intense activity, several topics have been investigated by HiPER SEP Team (Selex Galileo, Fraunhofer ISE, Politecnico di Milano and Aero Sekur) with the purpose of making a step forward in solar power generation technology [3]: light concentration systems (up to 10 times the nominal flux), an inflatable substrate and a flexible electrical network for solar array structural design, prototyping and verification. To assemble a first prototype, solar cells have been integrated on top of a flexible structure in combination with the concentrating lenses, using supports made of an innovative “smart" material (Shape Memory Alloy) for the lens deployment mechanism (see Figure 2).


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Figure 2: Lens and mechanism design (left) and

prototype (right) Power management and Distribution (PMAD) is

another critical aspect of the whole system. A direct drive approach has been selected as the most suitable solution (see Figure 3) and an architecture based on solar DC voltage level of about 300V and a current level of about 800A has been extensively simulated.

Figure 3: Block diagram of ADDA architecture

The solar cells which have been used in the frame of

bread boarding activity of HiPER are multi-junction (e.g. Ge/GaAs/InGaP lattice matched solar cells); their peak in efficiency is designed to be reached at concentrating factors of about 30X, but they will be also suitable to work at levels around 6 to 8 times the AM0 intensity. The light concentrator is a Fresnel lens. A lens with an axial symmetry will focus sunlight onto a straight line, centered over the active area underneath. A deployable support will position the lens over the solar cell line as in the following figure. Each lens operates coupled to a string of 10 solar cells, and it is realized by using space grade silicone or an optical grade plastic (e.g. polycarbonate). The Fresnel lens is comprised of precisely formed ridges which refract the light from a 10 cm aperture down to a strip of light focused in the middle of a 1 cm wide solar cell, to leave margin for pointing error.

Figure 4: Solar array breadboard

About the expected performance, with this

technology it is possible to unfold a group of flexible solar arrays capable of providing a power of 250kW (around the Earth) with a power density of 300W per square meter and with a specific power of at least 100W per Kg @BOL.

Nuclear Power

HiPER NEP team (Space Enterprise Partnerships, Rolls Royce and Acta) investigated nuclear fission electrical power generation for the exploration of the outer solar system [4,5]. Mission analysis identified a range of applications (one way journeys to Uranus, return missions to the Jovian and Saturn planetary systems, multiple infrastructure delivery missions) for which the nuclear ‘space tug’ concept was seen as the best fit solution.

The three main pillars of the investigation were the core reactor, the shield and the power conversion system. Besides, the ‘space tug’ had to be compatible with an Ariane 5 ECA launch, the main constraints being the fairing dimensions.

Both, Indirect liquid metal cooled and Direct gas cooled reactors, turned out to be capable of generating 200 kWe over the 10 year lifetime or even longer if required. Initial investigation showed Brayton cycle power conversion to have the most competitive specific mass for the 200 kWe power range. Reactor mass for


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the system working with liquid metal Indirect Brayton cycle reactor is only 25% of the the Direct Brayton system (476 compared to 2017 kg), although, in case of indirect cycle, the mass of a heat exchanger to transfer the thermal energy from the liquid metal to the cycle operating gas is to be added.

About radiation shielding, a 14° shadow layered shielding Concept Design (tungsten for gamma and a combination of Beryllium (Be), Lithium Hydride (LiH) and Boron Carbide (B4C) for neutron shielding) was shaped to match the flux distribution profile (flux of gamma and neutrons are quickly decreasing moving off the centerline of the reactor). The materials have been selected and arranged to optimize performance for radiation shielding, structural and thermal resilience as shown in Figure 5.

Brayton Cycle is considerably more efficient than a thermionic or thermoelectric design, with cycle efficiencies of 17-19% compared to 5% for the latter. Specifically, the indirect cycle was the more efficient as a result of a reheat loop but it is mechanically more complex.

Figure 5: Radiation Shield Structure

About the specific mass of the power generation

system, for a power level of 200kW 27 kg/kWe was used for the mission analysis update. This value increases to 35 kg/kWe when power management & distribution, battery, propulsion systems, without propellant, and supporting structure are included. Calculations showed that at 100 kWe, specific mass increases to 39 kg/kWe while for 2 MWe it can be reduced to~14kg/kWe.

To/From Radiato

~

To/From Core Lithium Circuit

Recuperator

Turbine

2x Comp.

Free Power Turbine

Outer Pressure

Vessel (lengthened to accommodate

2 x IHX)

IHX

IHX Preheater

Radiator

Alternator

Recuperator

Alternator

FPT Module

Gas Generator

30

30

40

90

Φ 50 cm

IHX Reheater

IHX Preheater

15

15

Figure 6: Indirect Cycle Power Conversion

Last but not the least, when talking about nuclear

generators safety issue is always the most debated. Here (NEP generators to be activated and used in space) even in the case of a catastrophic failure on the launch pad or at low altitude, the danger of a radio-active pollution is nil. Natural Uranium has a very low radio-active capability. It becomes dangerous only when nuclear fission process is activated, but that will only happen from LEO to outer space.

IV. HIGH POWER ELECTRIC PROPULSION DEVELOPMENT

Electric thruster development is the core of HiPER

programme, since everything else orbits around that. Having reliable and efficient high power thrusters on flight in the near future is an ambitious but achievable target. Three main types of electric thrusters have been selected as potentially successful candidates for space missions like the ones previously described: Hall thrusters, Gridded Ion Engines and Magneto-plasmadynamic thrusters.

Hall Thrusters

Hall thruster is the most mature technology among the ones here listed. They have a vast flight heritage and no failures reported when operated in space. Therefore, the aim under HiPER programme was to directly start developing a first high-power prototype, operating at a power level of 20kW.

Ordinarily operating at power levels of about 1kW, an increase of their power range of an order of magnitude makes Hall thrusters first-rate candidates for interplanetary missions and orbit raising tasks.

Six partners from the industry and scientific organizations gathered to address the different technological challenges related to high power HETs: Alta (Italy), IPPLM (Poland), CNRS (France), Tecnalia (Spain), Onera (France) and Snecma, Safran Group (France).


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Table 1 PPS-20k ML Operating Point

Measured Performances

Design Point

Thrust 1000 mN 1050 mN 1020 mN Isp 2400 s 2700 s 2500 s

Discharge voltage

400 V 500 V 450 V

Xe mass flow rate

43 mg/s 41 mg/s 42 mg/s

Total power 19.3 kW 22.4 kW 20.5 kW Thruster

efficiency 60 % 60 % 60 %

A monolithic HET architecture was proposed for the

20 kW thruster. In order to offer more thermal margin, a splitting of the neutral injection and anode was proposed. The magnetic field of the HET was generated by coils and a ferromagnetic circuit. In order to produce an azimuthally homogeneous field, eight coils were implemented in the external circuit and a single annular coil was adopted for the internal circuit. Concerning the cathode, alternative technologies were studied like RF cathodes. Nevertheless, due to costs and time limitation, it was decided to implement a state-of-the art hollow cathode.

Following the preliminary design, last year a 20 kW laboratory model has been manufactured, assembled and tested in a suitable vacuum facility [6]. The thruster was then tested at power levels ranging from 5 to 23.5 kW, characterizing 30 different operating points (measurements of thrust, thruster efficiency and Isp). The 20 kW thruster could be successfully operated and reached the expected performance (see Figure 7), in accordance with the given specifications.

Figure 7: 20kW HET firing at low power (left) and at

the nominal operating power (right)

Gridded Ion Engines Like HETs, also Gridded Ion Engines are a flight proven technology. Like HETs, they have always

been operated in space at power levels of the order of 1-2 kW (or lower).

Extending their operation to higher power levels seems to be slightly more complicated than for HETs and for such a reason a new architecture, exploiting three or even four grids instead of two for accelerating ions, has been investigated under HiPER framework by the GIE team (University of Southampton and Alta).

A DS3G (Dual Stage 3 Grids) is a gridded ion engine where instead of using a single ion optics stage to extract and accelerate an ion beam, two stages are used to separate the extraction and acceleration process. The main effect of this separation is to increase the thrust density with respect to conventional GIEs (up to 12N/m2 at an Isp f 10,000s have been simulated). This results in a compact thruster that can process tens of kW of power maintaining high efficiencies.

Plasma

3rd Grid Accel Grid

1st Grid Screen grid

2nd Grid Ext. Grid

d1

Extraction gap

d2

Acceleration gap

V1 V2

V3 V4 ∆Vext

∆Vacc

V

0

4th Grid Decel Grid

r1 r2 r3 r4

Figure 8: Scheme of a dual stage GIE

DS3G behavior has been analyzed using analytical

models available in the literature and numerical simulations performed with the FFX code [7]. The DS3G was found to be able to deliver lower beam divergences (about 40% less) than the Reference-GIE and a much higher thrust density.

The DS3G also suffered from lower erosion rates than the Ref-GIE and consequently was able to provide a longer lifetime. Besides, the DS3G seems to be able to work over a wide range of Isp level with a single ion optics design by only changing the applied voltages on the thruster grids. However, the lifetimes obtained for this “variable Isp” condition are poor due to an insufficient focusing of the ions created by charge exchange collisions


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Table 2

DS3G Calculated Performance vs. Ref-GIE

DS3G Ref-GIE

Thrust 0.4 0.18 Specific

impulse 10,000 s 4,500 s

Thrust density 12.7 N/m2 5.7 N/m2

Beam current 3A 3A

Power 25 kW 6 kW

lifetime 45,000 h 37,000 h

In addition to a thorough theoretical investigation of

the DS3G architecture, an extensive test campaign on cathodes has been carried out to assess their capability of successfully withstanding high currents for a sufficiently long time, without showing sensible performance degradation [8].

The main goal of the experimental campaign was to gather more information about the functioning of the cathode and the influence of the orifice size over the cathode insert temperature distribution. The cathode was tested with three different orifice sizes (3, 5 and 8 mm), at various current levels (ranging from 25 to 180 A) and with various xenon mass flow rates for a total of 130 h of which 30 were at the 180A discharge current level.

Figure 9. High current HC ready to be assembled (the

white leads are the thermocouples terminals). The lifetime of the cathode has then been estimated

on the basis of three different criteria, using the measured insert temperature profiles. It has been calculated that an increase in the inner cathode radius from 5 to 6 mm and a proportional increase in orifice diameter would be sufficient to achieve 17,000 hours of lifetime at 180A.

MPD Thrusters

When very high power levels are involved (hundreds of kW or even MW), MPD thrusters come into play as potential competitors of HETs and GIEs, thanks to their

higher thrust density which allows for a more compact propulsion system. The problem which always ruled out this kind of electric thruster from being chosen for a real mission, is that it shows an unstable behaviour for high values of the operating current (widely known as onset-problem), while it shows a poor efficiency at low power levels. The target throughout HiPER project was to carry out a wide experimental campaign on MPD thrusters coupled to a theoretical investigation of their performance and of plasma behaviour inside their accelerating channel. The aim was to check if an MPD thruster can be designed to operate in continuous mode at a power level of at least 100 kW with an efficiency higher than 30%. This would make it competitive, maybe even a winning choice, with respect to any other propulsion device for future missions entailing power levels of several hundred kW.

Two prototype thrusters have been manufactured and tested: one by Alta, operating up to 250 kW in short pulses up to 1s, another one by IRS, to verify the behaviour of the thruster and thruster components when operating in continuous mode.

Alta’s prototype has been design after the Russian

“Ageyev-type” high-current plasma accelerator with a central multichannel hollow cathode and a coaxial flared anode. The applied magnetic field was provided by a 430 turns coaxial solenoid capable of generating a magnetic field strength up tp 130 mT with a current of about 60 A. The thruster has been operated in a power range between 20 and 250 kW [9].

Figure 10: MPD thruster firing in Alta’s IV10 facil ity

A maximum thrust efficiency of about 30% was obtained at 200 kW for an applied field of 120 mT and a mass flow rate of 60 mg/s. At 100 kW, for the same mass flow rate and magnetic field, the thruster reached a thrust efficiency slightly higher than 20% and a specific impulse of about 2500 s (see Figure 11).

From a technological point of view, the experimental campaign highlighted that the mechanical stability of the multi-channel configuration is questionable for unsteady applications giving non substantial benefits in terms of terminal voltage and lifetime with respect to conventional solid rod cathodes.


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However, no signs of anode erosion or spotty current patterns were found during the test campaign.

Figure 11: Alta 100-kW class AF-MPD thruster figures of merit

Extended tests have been performed on pure-

tungsten hollow cathodes to assess the cathode erosion process during steady-state operations [10]. A reduced-scale multichannel hollow cathode was tested for 100 hours at 150 A and argon mass flow rate of 4.5 mg/s.

The maximum measured temperature on the external surface was of about 2800 K. It was found that the start-up phase is characterized by localized arc attachment on the cathode surface where the temperatures at the arc root are often higher than the tungsten melting point (about 3400°C) leading to erosion rates as much as 3-4 orders of magnitude higher than that seen in steady-state operation (~1 ng/C). Moreover, the erosion pattern was localized alongside the widest gaps between the rods. As a consequence, a more regular accommodation of the rods inside the main channel is expected to reduce the erosion rate.

Figure 12: Multichannel Hollow Cathode during operation

in Alta’s IV4 facility

Design of European AF-MPD Thruster As regards MPD thrusters, the main output of

HiPER program has been the preliminary design of a steady-state, radiation-cooled, applied-field thruster conceived to include the most promising design options and material selections suggested so far by the main research institutes with the aim of performance and lifetime enhancement (see Figure 13). The thruster consists of a central W1La2O3 cathode with several lanthanum hexaboride discs for a total emitting surface of 125 cm2 and a concentric W5Re conical anode. Carbon-carbon radiators or Na/K heat pipes can be used to reduce the anode temperature below 2500 K. The magnetic field can be provided by either permanent magnets or by a high temperature superconducting coil (YBaCuO). Both solutions need to be insulated from the anode thermal radiation by a cooled screen and/or radiation shields (Tantalum or TZM foils). High-purity argon (grade 5.0) has been chosen as propellant.

Figure 13: Schematic of the European AF-MPD thruster

(magnet not shown)


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V. HIPER ROADMAP

HiPER project has shown that the adoption of high

power electric propulsion systems represents a turning point for enabling a whole new class of missions, which otherwise would be impractical by means other propulsion devices.

Here, a roadmap has been sketched, considering the current development status of thrusters and power generation technologies and taking into account all the open issues that have to be tackled in the next years, in order to pave the way for a successful and widespread usage of high power electric propulsion in space.

For what concerns the thrusters, a substantial

amount of work has been carried out in the last three years with the aim of assessing and improving the existing electric propulsion technologies that are considered suitable for being used at high power levels (>20kW).

Among them, Hall thrusters technology is presently the most mature one. Such thrusters have already proven their effectiveness in several space missions, although they have always been designed to operate at lower power levels w.r.t. ones considered in HiPER framework (say <2kW). However, Hall thrusters are more easily scalable to higher power levels than other propulsion devices and they improve their efficiency in a non-negligible way when the operating power level goes up. A 20kW prototype has been designed, manufactured and extensively tested in HiPER program, showing an excellent performance. If used in cluster configuration of two, four or eight units, it is possible to reach total power levels comparable to the ones assumed for the previously mentioned space missions (with the notable advantage of having multiple thruster units onboard, which mitigates the risks connected to a malfunction of one of them).

High power gridded ion engines also have a high level of maturity and, like Hall thrusters, at lower power levels GIEs have been widely employed in many space missions. In HiPER project the main task was to focus the attention on a couple of key issues: the cathode development and the definition of an optimal thruster configuration for high power operation (so called Dual Stage 3 Grids). The main strengths of gridded ion engines are the very high specific impulse (a feature that can specially suits the needs of some class of missions) and their extremely long lifetime (which is estimated to be of the order of one hundred thousand hours according to the simulations and the experiments carried out up to now; even if this prediction turned out to be excessively optimistic, the lifetime is expected to be fully satisfying anyway).

The last kind of electric thruster suitable for high power operation is the Magneto-plasma-dynamic thruster. An intensive experimental activity has been carried out on MPD thrusters under HiPER project, in order to assess their performance and to make a step forward in understanding the intrinsic problems (linked to plasma discharge stability) that have always plagued this class of thrusters, severely limiting their efficiency. MPD thrusters are expected to the be the most effective devices when operating at very high power levels, i.e. greater than 100 kW (up to a few MW). Actually, they are the only electric propulsion thrusters capable of processing so much power in a compact size, this resulting in a much higher thrust density (N per m2 of exhaust section area) with respect to Hall thrusters and gridded ion engines. For this reason, going on with a thorough investigation of their behavior is crucial to further improve their performance and to enable their use for future missions. Of course, considering their current Technology Readiness Level and the required power level for a single thruster unit, MPD thrusters will probably play a significant role for space exploration only in the mid-long term.

On the side of the power generation technologies,

two are the viable options that have been investigated: solar power and nuclear power.

Obtaining the necessary power from solar arrays, is the most consolidated solution for present spacecraft. Solar energy is readily available in space and the technology to convert it into electric energy is mature and reliable. In the short term, solar power is the preferred choice. Of course there are two main drawbacks. First, if plenty of power is necessary (say hundreds of kW) the size of the solar panels grows larger and larger and handling them becomes truly challenging. Second, if the target mission is heading towards the outer planets, solar energy decreases fast with the increasing distance from the sun. Using solar energy beyond Mars, especially for feeding high power devices, is not advisable. Under HiPER program, newly designed solar cells equipped with light concentrators have been developed and assembled in an array as a prototype technology demonstrator. It has been shown that, relying on this technology, it is possible to unfold a group of flexible solar arrays capable of providing a power of 250kW around the Earth with a power density of 300W per square meter and with a specific power of about 100W per kg.

As stated above, if even higher power levels are required for a mission or if the spacecraft is operating far from the sun (further than the Earth), solar energy is not a satisfying solution. In such cases, the other option comes into play: nuclear power generation. By using nuclear power generators it is possible to generate a large amount of power, which can continuously feed all


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the electric systems. The concept of ‘space tug’ has been chosen as winning candidate for future missions entailing a nuclear power generation system. Here, two the most challenging problems are heat dissipation and radiation shielding (in case of manned missions) and both of them have been addressed and deeply examined along HiPER project. With a constant effort devoted to their further development, space nuclear generators are expected to be fully mature in about then years, then making possible a whole new set of space missions (such as the ones requiring a large amount of power very far from the sun).

VI. CONCLUSIONS

HiPER - High Power Electric propulsion: a

Roadmap for the future - was a 3-year collaborative project aimed at laying the technical and programmatic foundations for the development of innovative EP technologies - and of the related power generation – in order to fulfill future space transportation and space exploration needs. Different research and development phases have been progressed in parallel, leading for the first time to a complete overview of the problem, considering three different thrusting technologies (Hall, GIEs and MPD thrusters) and two different power generation systems (solar and nuclear). In harmony with the mission analysis team, near term and mid-long term scenarios have been sketched, demonstrating the feasibility of the ideas here proposed and showing the outstanding benefits that can be obtained by using high power electric propulsion for future missions. From scientific exploration missions, to manned missions toward Mars, from missions targeting near-Earth asteroids, to interplanetary travels to Jupiter and beyond, everything seems to clearly show the importance of continuing on this way, devoting any necessary effort to make this new-frontier technology a solid reality.

Acknowledgements

Authors wish to sincerely thank Dr. Cosmo Casaregola, technical responsible and coordinator of the activities throughout the first three years of the programme, as well as all the partners that participated to the project.

This work has been partly funded by European Commission (Grant agreement N. 218859), monitored by the technical officer Ms. Paula Mota Alves of Research Executive Agency.

Bibliography:

[1] Cosmo Casaregola, “HiPER: a Roadmap for Future Space Exploration with innovative Electric Propulsion Technologies”, 31st International Electric Propulsion Conference, IEPC-2009-066, University of Michigan, USA, 2009

[2] Cosmo Casaregola , “Future Scenarios for Space

Transportation and Exploration based on High Power Electric Propulsion Technologies”, 47th Joint Propulsion Conference, AIAA-2011-5516, San Diego, CA, USA, 2011

[3] Emanuele Ferrando et al., “Study Results on a

Solar Electric Power System for High Power Electric Propulsion (HiPER) Applications”, IAC-11-D2.4.7, IAF, Cape Town, 2011

[4] Richard Blott et al., “High Power Nuclear

Electric Propulsion study (HiPER) Nuclear Electric Powered missions”, IAC-10.C.4.5.6, IAF, Paris, 2010

[5] Richard Blott et al., “High Power Nuclear

Electric propulsion”, 32nd International Electric Propulsion Conference, IEPC-2011-061, Giessen Justus-Liebig-University, Germany, 2011

[6] Stephan Zurbach et al., “Performance

Evaluation of a 20 kW Hall Effect Thruster”, 32nd International Electric Propulsion Conference, IEPC-2011-020, Giessen Justus-Liebig-University, Germany, 2011

[7] Michele Coletti et al., “Dual Stage Ion Optics, a

Parametric Study”, 42nd Plasmadynamics and Laser Conference, AIAA-2011-3741, USA, 2011

[8] Michele Coletti et al., “Discharge Hollow

Cathode design for a 4-Gridded Ion Engine”, ESA Space Propulsion Conference 2010, SP2010_1842617, AAAF, San Sebastian, 2010

[9] Riccardo Albertoni et al., “Experimental Study

of a 100-kW class Applied-Field MPD Thruster”, 32nd International Electric Propulsion Conference, IEPC-2011-110, Giessen Justus Liebig University, Germany, 2011

[10] Marco De Tata et al., “100-hr Endurance Test

on a Tungsten Multi-rod Hollow Cathode for MPD Thrusters”, 32nd International Electric Propulsion Conference, IEPC-2011-108, Giessen Justus-Liebig-University, Germany, 2011

Recent Developments in High Power Electric Propulsion: Outcomes of HiPER Project Activities

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