## 2 Materials and Methods
### Research Context  
Nine manufacturers are involved in the development of three different EP systems based on Hall effect thruster (HET) technology [36]. Among the electric propulsion technologies [37; 38], the HET is a technology positioned in a \"sweet spot\", since it combines a high thrust to power ratio (around 50-60 mN per kW) and high propellant efficiency (_Isp_ around 1500-2500s). A HET EP is composed of three main parts (Figure 2):  
* The thruster unit (TU), which transforms electric energy and propellant into thrust,
* The power processing unit (PPU), which feeds the thruster with electric power from the satellite bus and manages it,
* The flow management system (FMS), which feeds the thruster with propellant from a propellant tank and manages it. Figure 2 shows the main parts of an EP in a simplified schematic, highlighting the main interactions and interfaces: mechanical (red), electrical (green), software (grey) and fluid  
Figure 2: Simplified schematic of a typical electric propulsion (EP) configuration and connection between main elements (figure adapted from [PERSON] et al. [39]). how these interactions and dependencies can be one- or bi-directional, in terms of input and output. The figure highlights how an EP is a multi-technological system, presenting a mix of mechanical, electronic, fluidic and software elements. The partners involved in the project are working at system (satellite), sub-system (EP) and component levels (developing PPUs and FMSs). The project is organized around three different groups of engineering teams, each tasked to develop a HET EP in order to serve different applications and orbits. The need for the research presented in this paper is motivated by the operative conditions that determine the value of satellite propulsion technologies for \"new space\" business scenarios, which are introduced in the next session. New Space' Market Conditions that Determine the Value of Next-Generation Satellite Propulsion Systems  
Historically, the main application areas for satellite constellations have been telecommunications and Earth observation. The size of these constellations depends on the trade-off between the need for coverage and the launch cost incurred to introduce the satellites in orbit [40]. The lower the altitude the more satellites have to be built to provide coverage. On the other hand, the higher the altitude, the higher each launch cost. For a couple of decades, the biggest satellite constellation has been the Iridium constellation--residing in low Earth orbit (LEO) with 76 satellites and a mass of around 700 kg each [41]. However, the satellite constellation business has rapidly evolved due to the emergence of more entrepreneurial actors, which is now resulting in new business models. For example, OneWeb announced a partnership to produce a constellation of 648 LEO satellites to deliver broadband Internet globally [40]. SpaceX announced a similar ambition for a constellation of more than 4000 LEO satellites (called Starlink), and 895 satellites have already been launched at the end of 2020 [42]. Telesat has announced the plan to launch a constellation of 298 satellites [43]. The magnitude of the space infrastructure being proposed in these'megaconstellation' scenarios is unprecedented. For example, SpaceX's constellation is almost twice the total number of operational satellites in orbit today [44]. Despite the growing interest in developing megaconstellations, the market is at present very uncertain. For example, in 2018 Telesat reported that their ideal LEO constellation size is 292 satellites, but it could be 512 [45], emphasizing the need for the flexibility and scalability of the constellation from an economical standpoint. Also, in March 2020 OneWeb filed for bankruptcy protection because of the financial impact and market turbulence related to the spread of coronavirus disease 2019 (COVID-19) [46], with already 74 satellites launched and half of the 44 planned ground stations completed or in development.