In order for a satellite formation to maintain its intended design despite present perturbations (formation keeping), to change the formation design (reconfiguration) or to perform a rendezvous maneuver, control forces need to be generated. To do so, chemical and/or electric thrusters are currently the methods of choice. However, their utilization has detrimental effects on small satellites' limited mass, volume and power budgets. Since the mid-eighties, the potential of using differential drag as a means of propellant-less source of control for satellite formation flight is actively researched. This method consists of varying the aerodynamic drag experienced by different spacecraft, thus generating differential accelerations between them. Its main disadvantage, that its controllability is mainly limited to the in-plain relative motion, can be overcome by using differential lift as a means to control the out-of-plane motion. Due to its promising benefits, a variety of studies from researchers around the world have enhanced the state-of-the-art over the past decades which results in a multitude of available literature. In this paper, an extensive literature review of the efforts which led to the current state-of-the-art of different lift and drag based satellite formation control is presented. Based on the insights gained during the review process, key knowledge gaps that need to be addressed in the field of differential lift in order to enhance the current state-of-the-art are revealed and discussed. In closer detail, the interdependence between the feasibility domain / the maneuver time and increased differential lift forces achieved using advanced satellite surface materials promoting quasi-specular or specular reflection, as currently being developed in the course of the DISCOVERER project, is discussed. Keywords Satellite aerodynamics • differential lift • differential drag • formation flight control • propellant less control Acknowledgements This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 737183. This reflects only the author's view and the European Commission is not responsible for any use that may be made of the information it contains. The author would like to thank the reviewers, the DISCOVERER team as well as several colleagues from IRS for their valuable feedback and suggestions.
The applied-field magnetoplasmadynamic thruster AF-MPD ZT1 was successfully put in operation at IRS. The thruster operated in steady-state mode with Argon as propellant at low electric arc power of 6 kW. Discharge voltage and power increased as expected almost linear with applied magnetic flux density. The variation of mass flow rate ratio between anode and cathode gas towards higher cathode gas fraction showed increasing thrust and thrust efficiency at applied magnetic flux density of 0.1 T and are comparable with DLR's X13 thruster. Additionally a steady-state 100 kW AF-MPD thruster SX3 was developed, set in operation and preliminary characterized at IRS. The SX3 thruster was operated at relatively low arc powers up to 30 kW and applied magnetic flux density of 0.1 T generated by the modified ZT coil. Due to low arc current and magnetic flux level, the AF-MPD ZT1 thruster achieved thrust of 70 mN and exhaust velocity up to 10 km/s at 6 kW arc power and up to 6 % thrust efficiency. The SX3 thruster reached thrust of 362 mN and thrust efficiency more than 12 % in steady state-operation at 25 kW arc power and at applied magnetic flux density of 0.1 T. However both thrusters have been operated at limited magnetic fluxes only. For SX3 a total operation time of more than 3600 s together with 30 ignitions could be accumulated. The electrodes, however, do not show significant erosion nor a respective degradation. The performances of thrusters provide an outlook for future investigations on AF-MPD thrusters at IRS and give a hint to improvement in thrust efficiency of presented devices with the new applied-field coil, which will be manufactured in the future to produce magnetic flux densities up to 0.6 T allowing further increase in thrust efficiency up to 30 %.
After decades of traditional space businesses, the space paradigm is changing. New approaches to more efficient missions in terms of costs, design, and manufacturing processes are fostered. For instance, placing big constellations of micro-and nano-satellites in Low Earth Orbit and Very Low Earth Orbit (LEO and VLEO) enables the space community to obtain a huge amount of data in near real-time with an unprecedented temporal resolution. Beyond technology innovations, other drivers promote innovation in the space sector like the increasing demand for Earth Observation (EO) data by the commercial sector. Perez et al. stated that the EO industry is the second market in terms of operative satellites (661 units), micro-and nano-satellites being the higher share of them (61%). Technological and market drivers encourage the emergence of new start-ups in the space environment like Skybox, OneWeb, Telesat, Planet, and OpenCosmos, among others, with novel business models that change the accessibility, affordability, ownership, and commercialization of space products and services. This chapter shows some results of the H2020 DISCOVERER (DISruptive teChnOlogies for VERy low Earth oRbit platforms) Project and focuses on understanding how micro-and nano-satellites have been disrupting the EO market in front of traditional platforms. Satellites Missions and Technologies for Geosciences 2 Figure 1. Micro-and nano-satellites launched between 1997 and 2017, classified by sectors (own elaboration).
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