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Challenging space missions include those at very low altitudes, where the atmosphere is the source of aerodynamic drag on the spacecraft, that finally defines the mission's lifetime, unless a way to compensate for it is provided. This environment is named Very Low Earth Orbit (VLEO) and it is defined for h < 450 km . In addition to the spacecraft's aerodynamic design, to extend the lifetime of such missions, an efficient propulsion system is required. One solution is Atmosphere-Breathing Electric Propulsion (ABEP), in which the propulsion system collects the atmospheric particles to be used as propellant for an electric thruster. The system could remove the requirement of carrying propellant on-board, and could also be applied to any planetary body with atmosphere, enabling new missions at low altitude ranges for longer missions' duration. One of the objectives of the H2020 DISCOVERER project, is the development of an intake and an electrode-less plasma thruster for an ABEP system. This article describes the characteristics of intake design and the respective final designs based on simulations, providing collection efficiencies up to 94 % . Furthermore, the radio frequency (RF) Helicon-based plasma thruster (IPT) is hereby presented as well, while its performances are being evaluated, the IPT has been operated with single atmospheric species as propellant, and has highlighted very low input power requirement for operation at comparable mass flow rates P ∼ 60 w . [ABSTRACT FROM AUTHOR]

Operating satellites in Very Low Earth Orbit (VLEO) benefit the already expanding New Space industry in applications including Earth Observation and beyond. However, long-term operations at such low altitudes require propulsion systems to compensate for the large aerodynamic drag forces. When using conventional propulsion systems, the amount of storable propellant limits the maximum mission lifetime. The latter can be avoided by employing Atmosphere-Breathing Electric Propulsion (ABEP) system, which collects the residual atmospheric particles and uses them as propellant for an electric thruster. Thus, the requirement of on-board propellant storage can ideally be nullified. At the Institute of Space Systems (IRS) of the University of Stuttgart, an intake, and a RF Helicon-based Plasma Thruster (IPT) for ABEP system are developed within the Horizons 2020 funded DISCOVERER project. To assess possible future use cases, this paper proposes and analyzes several novel ABEP-based mission scenarios. Beginning with technology demonstration mission in VLEO, more complex mission scenarios are derived and discussed in detail. These include, amongst others, orbit maintenance around Mars as well as refuelling and space tug missions. The results show that the ABEP system is not only able to compensate drag for orbit maintenance but also capable of performing orbital maneuvers and collect propellant for applications such as Space Tug and Refuelling. Thus, showing a multitude of different future mission applications. [ABSTRACT FROM AUTHOR]

A common practice in the field of differential lift and drag controlled satellite formation flight is to analytically design maneuver trajectories using linearized relative motion models and the constant density assumption. However, the state-of-the-art algorithms inevitably fail if the initial condition of the final control phase exceeds an orbit and spacecraft-dependent range, the so-called feasibility range. This article presents enhanced maneuver algorithms for the third (and final) control phase which ensure the overall maneuver success independent of the initial conditions. Thereby, all maneuvers which have previously been categorized as infeasible due to algorithm limitations are rendered feasible. An individual algorithm is presented for both possible control options of the final phase, namely differential lift or drag. In addition, a methodology to precisely determine the feasibility range without the need of computational expensive Monte Carlo simulations is presented. This allows fast and precise assessments of possible influences of boundary conditions, such as the orbital inclination or the maneuver altitude, on the feasibility range. [ABSTRACT FROM AUTHOR]

Several small, unconnected satellites flying in formation are a frequently discussed option to replace large satellites due to advantages such as increased flexibility, reduced costs and enhanced reliability. Using differential aerodynamic forces as a means to maintain the formation despite present perturbations, to perform formation reconfiguration or to initiate a rendezvous maneuver is a promising propellant-less alternative to chemical and/or electric propulsion systems. Subjected to uncertainties, dynamic variations and non-linearities its implementation in a real mission is, however, challenging. A common practice to either design suitable reference trajectories for robust control strategies or to gain deeper insights into the methodology is to use linearized relative motion models and a constant density assumption. Following this practice, multiple previous studies developed and enhanced analytic control algorithms to zero out the relative position and velocity of two spacecraft utilizing differential aerodynamic forces. Even though aerodynamic lift expands the controllability to all three translational degrees of freedom, it has been frequently neglected due to the low lift coefficients experienced in orbit so far. And even if considered, analysis has shown that the feasibility range of using the differential lift for the eccentricity control of the in-plane motion is extremely limited compared to using differential drag. However, two different possible options, namely applying enhanced algorithms to decrease the eccentricity before even initializing the eccentricity control phase as well as using advanced satellite surface materials targeting specular or quasi-specular reflection and thereby increasing the differential acceleration forces, have been suggested to increase the feasibility domain of the methodology. In this article, the effectiveness of the proposed adjustments is analyzed in a Monte-Carlo approach by applying them to the analytic control algorithms introduced in the literature. Since these are based on the linearized Schweighart-Sedwick equations, so is the presented analysis. Moreover, additional modifications to the algorithms are made to reduce the maneuver time of one of the respective control phases. The results show that the analyzed options noticeably improve the success rates of the maneuvers. Especially the feasibility range of the lift-based control algorithms is enlarged considerably. In addition, the implemented modifications are shown to consistently reduce the maneuver time of the respective control phase. Consequently, the presented analysis improves the current state of the art of the analytically designed rendezvous trajectories and provides valuable new insights into the methodology of using differential aerodynamic forces as a means of relative motion control. [ABSTRACT FROM AUTHOR]

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-80s, 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 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 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. [ABSTRACT FROM AUTHOR]