The selection of an electric propulsion subsystem architecture for high-power space missions

The arise of high-power electric propulsion is paving the way towards new horizons of space exploration. Hall thrusters represent a promising propulsion concept, able to fulfil challenging mission requirements for both commercial and exploration applications. This technology offers several benefits in terms of flexibility of operation, extensive lifetime and high reliability. However, the design of a high-power electric propulsion subsystem (E-PROP) still presents challenges to address. Filling the corresponding technological gaps will open new market opportunities, owing mainly to the extension of mission capabilities and the reduction of the overall mission costs. Therefore, investigations of innovative technology alternatives will allow to identify the most promising E-PROP architectures for various high-power mission scenarios. One of the most critical trade-off to perform is between a high-power monolithic thruster and a cluster of thrusters of lower power. Another criticality is the amount of propellant necessary to perform high delta-v missions. The high price of xenon prompted the investigation on alternative propellants, such as krypton. The propellant selection should consider the impact on different aspects of the platform design, including performance, system complexity and mission costs. Last, due to the high-power levels that the E-PROP shall manage, a different architecture can be implemented by adopting the direct-drive approach, i.e. a direct and non-isolated connection between the solar array and the thruster. However, even if the disruptive direct-drive technology allows a significant reduction in the EP system mass and cost, its implementation rises additional challenges to the design of the spacecraft power subsystem. This paper analyses the impact of innovative architecture solutions on the design of a high-power E-PROP. In the framework of this research, we first carried out an extensive investigation of possible mission scenarios and we derived corresponding mission requirements and constrains. Then, we performed three technological trade-offs: monolithic 20 kW vs 5 kW cluster configuration, Xe vs Kr propellant and direct-drive vs standard PPU. All the analysis are based on the experimental data obtained during the 5 kW and 20 kW thrusters development and characterisation at SITAEL. We characterized each design option through several figures of merit, evaluating them for each identified mission scenario. We exploited an Analytical Hierarchy Process for the trade-off analyses and a Monte Carlo method to perform the preliminary evaluation of the trade-off weights. The analyses are based on the research activities that are currently ongoing at SITAEL and PoliTo in the framework of 20 kW E-PROP development programmes. The results of the work highlight the effects of each architecture alternative on both platform design and mission performance

Magnetoplasmadynamic Thrusters

The paper starts with a brief account of the origins of MPD concept followed by an explanation of Lorentz force. The nature of an ideal Self-field MPD thruster is then discussed in terms of basic thrust law, cubic dependance and ideal scaling trends. The situation as to real Self-field MPD thrusters follows including their electrical characteristics, deviations from the ideal, the Hall and back-EMF effect and the role of Alfvén's critical ionization velocity. Onset phenomenology and the critical onset parameter are then discussed. The main theories proposed to explain the onset phenomena are summarized, such as mass starvation, instabilities, back EMF and macroscopic instabilities. A summary of onset follows with a discussion of attempts made to prevent onset. Applied field MPD thrusters are then presented, followed by Lithium propellant devices. A survey of major R&D efforts on MPD thrusters worldwide is made. Finally, some concluding remarks are made as to the status and prospects of MPD propulsion

Supporting the Vision: Alta’s Activities and Capabilities for Mars Exploration

The feasibility of affordable Mars exploration scenarios within a reasonably short time frame is crucially dependent on the capability to effectively cope with a number of key technology issues as well as with the availability of adequate skills and facilities. A number of past and present R&D programs at Alta SpA, a small company based in Pisa, are aimed at some challenging aspects of Mars missions, such as: aeroassisted EDL, ascent engine design and trajectory optimization, mini-magnetosphere generation for radiation protection in long duration crewed transfers, high Isp propulsion systems and mission design in multi-body regimes. These activities are complemented with the availability of unique facilities, including the largest space simulator available in Europe for thruster testing and plasma experiments requiring very high pumping speeds, and a variety of equipments and diagnostics related to aerothermodynamics of the Martian atmosphere. The presentation addresses the Alta experience and perspective contribution to Mars missions as an example of how a small enterprise can help support the vision for Mars exploration

GG Drag Compensation with FEEP Thrusters

Field Emission Electric Propulsion (FEEP) is presently regarded as the ideal solution for drag compensation on scientific satellites. In fact, FEEP is presently the only exisisting propulsion system capable of producing thrust in the micronewton range, with the controllability, thrust resolution and fast response which are needed in such demanding applications. Other envisaged areas of application of FEEP include small satellite attitude control and disturbance compensation on microgravity platforms. The FEEP system is under development at Centrospazio, under ESA and ASI sponsorship.This paper presents some of the most recent experimental results , devoted to the investigation of the thruster behaviour when operated in pulsed mode. This special operating mode is of particular interest in view of the use of FEEP for drag-free control of the proposed scientific small satellite Galileo Galilei

FEEP and MicroFEEP Development

Two interesting fields of applications are envisaged for a miniaturized, modular FEEP thruster: first, micronewton-level missions. For drag-free control and fine pointing of scientific spacecraft, a miniaturized device would offer additional, albeit limited, dry mass savings, and increased ease of distributing the thrusters over the spacecraft surface, as required. The second field is that of missions in the 1 - 10 mN thrust range, including attitude control and orbit maintenance of small to medium satellites. In this case, a modular FEEP thruster can lead to significant mass savings, due to both the reduced dry mass of the propulsion hardware and to the low propellant consumption resulting from the high specific impulse (about 8000 s). As for the associated high power-to-thrust ratio, this is becoming more and more manageable with the increased power availability of modern spacecraft buses. In this early phase of the miniaturized FEEP development, several engineering problems have been addressed, including silicon compatibility with the propellant (cesium, indium or other liquid metals) and propellant reservoir sealing. Electrode geometry optimization has been also investigated. Prototype microslits have been manufactured with the desired geometrical parameters, and emission tests are underway. Various accelerator electrode configurations, featuring arrays of wires and perforated thin plates, have been studied