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.

Flying a satellite at very low earth orbit is a technological challenge. It presents advantages, such as increase resolution in optical payloads, reduce costs of launch and enhance the use of air breathing propulsion and specular materials. The density of the atmosphere at these altitudes is much higher, behaving as a free molecular flow. This has severe implications in the increase of drag torques and forces that has to be analyzed in depth. We analyze the effects and the perturbations to small satellites, affecting their dynamics, performance and lifetime by implementing and analyzing realistic models of the environment at VLEO.

A combined approach for the simulation of reactive, neutral, partially or fully ionized plasma flows is presented. This is realized in a code framework named “PICLas” for the approximate solution of the Boltzmann equation by particle based methods. PICLas combines the particle-in-cell method for the collisionless Vlasov–Maxwell system and the direct simulation Monte Carlo method for neutral reactive flows. Basic physical and mathematical modeling of both methods is addressed, and some application examples are presented in order to demonstrate the capabilities and the broad applicability of the solution strategy.

The numerical simulation of ion thruster optics is one of the most common applications of particle-based methods in the field of electric propulsion. Over the years, powerful tools for simulation of ion trajectories and charge exchange erosion have been developed and validated. However, not fully conservative physical models have to be applied with caution, since they are likely to fail when a kinetic modeling of both ions and electrons is relevant, which is mostly the case when electron fluid models (e.g., the Boltzmann relation) lose their validity, such as in the simulation of ion beam neutralization. For this, a more sophisticated approach based on high fidelity Particle-In-Cell (PIC) methods is expected to be necessary. Highly parallel computation concepts are required to attain feasible simulation times also for more complex geometries. Recently, the ion thruster operation at electron back-streaming limit was simulated using the efficient hybridizable discontinuous Galerkin Poisson solver of the 3D PIC-DSMC code PICLas. The shown accordance between fully kinetic simulations and those using the common Boltzmann relation (BR) justifies to focus for validation and sensitivity analyses on BR cases. Therefore, new optics simulations for a RIT-mu X EBB thruster of Airbus are performed. Experimental validation is achieved by comparing simulated grid currents at several operating conditions. Parameter studies show the influence of plasma parameters and collision models for charge exchange. For a scattering-based collision model, good quantitative agreement is achieved for a reasonable assumption of neutral gas temperature. Also the erosion pattern from experiments is reproduced qualitatively.

A high-order hybridizable discontinuous Galerkin spectral element method (HDGSEM) for Particle-In-Cell (PIC) schemes is presented for the simulation of electrostatic applications on three-dimensional unstructured curved meshes. The electrostatic Poisson equation is solved and optionally a Boltzmann relation for the electron species can be used which leads to non-linear source terms. The hybridizable formulation reduces the total number of unknowns of the field solver, allowing the simulation of large problems. The implementation of the HDGSEM solver in a PIC code is described and validated using several test cases with successively increasing complexity. It is shown that the high-order convergence properties are retained on curvilinear meshes, likewise when material jumps are introduced. The simulation of an ion optic illustrates the applicability of the presented method for complex geometries and large problem sizes.

An atmosphere-breathing electric propulsion system (ABEP) captures the residual atmosphere of a planet and uses it as propellant for an electric thruster to counteract the drag. The system would theoretically allow orbiting for unlimited time without on-board propellant storage. A new range of altitudes, e.g. 120-250 km in Earth orbit, the Very-Low Earth orbit (VLEO), for permanent orbiting can be accessed, thereby enabling new scientific missions. ABEP can be conceptually applied to any planet with atmosphere. IRS has several decades of heritage on the development of inductively heated plasma generators (IPG). Such devices are electrodeless, removing the issue of electrode erosion that reduces performance over time (see RIT, HET). Aggressive gases such as O as propellant, highly present in VLEO, will cause even faster erosion. IRS is currently developing an inductive plasma thruster (IPT) for ABEP application within the H2020 DISCOVERER project.

Using several small, unconnected satellites flying in formation rather than a single monolithic satellite has many advantages. As an example, separate optical systems can be combined to function as a single larger (synthetic) aperture. When the aperture is synthesized, the independent optical systems are phased to form a common image field with its resolution determined by the maximum dimension of the array. Hence, a formation is capable of much finer resolution than it could be accomplished by any single element. In order for the formation to maintain its intended design despite present perturbations (formation keeping), to perform rendezvous maneuvers or to change the formation design (reconfiguration) control forces need to be generated. To this day, using chemical and/or electric thrusters are the methods of choice. However, their utilization has detrimental effects on small satellites’ limited mass, volume and power budgets. In the mid-eighties, Caroline Lee Leonard published her pioneering work [1] proving the potential of using differential drag as a means of propellant-less source of control for satellite formation flight. This method consists of varying the aerodynamic drag experienced by different spacecraft, thus generating differential accelerations between them. Since its control authority is limited to the in-plane motion, Horsley [2] proposed to use differential lift as a means to control the out-of-plane motion. Due to its promising benefits, a variety of studies from researches around the world have enhanced Leonard’s work over past decades which results in a multitude of available literature. Besides giving an introduction into the method the major contributions of this paper is twofold: first, an extensive literature review of the major contributions which led to the current state-of-the-art of different lift and drag based satellite formation control is presented. Second, based on these insights key knowledge gaps that need to be addressed in order to enhance the current state-of-the-art are revealed and discussed. In closer detail, the interdependence between the feasibility domain and advanced satellite surface materials as well as the necessity of robust control methods able to cope with the occurring uncertainties is assessed.

In the context of the validation of PICLas, a kinetic particle suite for the simulation of rarefied, non-equilibrium plasma flows, the biased hypersonic nitrogen flow around a blunted cone was simulated with the Direct Simulation Monte Carlo method. The setup is characterized by a complex flow with strong local gradients and thermal non-equilibrium resulting in a highly inhomogeneous computational load. Especially, the load distribution is of interest, because it allows to exploit the utilized computational resources efficiently. Different load distribution algorithms are investigated and compared within a strong scaling. This investigation of the parallel performance of PICLas is accompanied by simulation results in terms of the velocity magnitude, translational temperature and heat flux, which is compared to experimental measurements.

Challenging space mission scenarios include those in low altitude orbits, where the atmosphere creates significant drag to the S/C and forces their orbit to an early decay. For drag compensation, propulsion systems are needed, requiring propellant to be carried on-board. An atmosphere-breathing electric propulsion system (ABEP) ingests the residual atmosphere particles through an intake and uses them as propellant for an electric thruster. Theoretically applicable to any planet with atmosphere, the system might allow to orbit for unlimited time without carrying propellant. A new range of altitudes for continuous operation would become accessible, enabling new scientific missions while reducing costs. Preliminary studies have shown that the collectible propellant flow for an ion thruster (in LEO) might not be enough, and that electrode erosion due to aggressive gases, such as atomic oxygen, will limit the thruster lifetime. In this paper an inductive plasma thruster (IPT) is considered for the ABEP system. The starting point is a small scale inductively heated plasma generator IPG6-S. These devices are electrodeless and have already shown high electric-to-thermal coupling efficiencies using O2 and CO2. The system analysis is integrated with IPG6-S tests to assess mean mass-specific energies of the plasma plume and estimate exhaust velocities.

Flying at VLEO has several advantages such as the increase of the resolution of images recorded by optical instruments, the increase of geospatial position accuracy, the improvement of the signal to noise ratio and the reduction of energy consumption by active payloads. However, the drag produced by the interaction of the atmospheric gas particles with the surfaces of the spacecraft requires an extended knowledge of orbital aerodynamics. The aim of this work is to carry out a study from the principles of orbital aerodynamics to the definition of requirements for a set of satellite platforms covering Earth Observation applications taking advantage of operating in Very Low Earth Orbit (VLEO) and making use of aerodynamic technologies. Four platform concepts were defined: optical coverage platforms, optical Very High Resolution (VHR) for high performance platforms, low cost optical VHR platforms and Synthetic Aperture Radar (SAR) platforms. In addition, the main orbit and attitude control operations to be done with these concepts were analyzed. A relation between the different mission concepts and the performances to be obtained with enhanced aerodynamics was established to identify which of the four platform concepts could perform as a commercial platform to guarantee the use for different applications.

Challenging space mission scenarios include those in very low Earth orbits, where the atmosphere creates significant drag to the S/C and forces their orbit to an early decay. For drag compensation, propulsion systems are needed, requiring propellant to be carried on-board. An atmosphere-breathing electric propulsion system (ABEP) ingests the residual atmosphere through an intake and uses it as propellant for an electric thruster. Theoretically applicable to any planet with atmosphere, the system might allow drag compensation for an unlimited time without carrying propellant. A new range of altitudes for continuous operation would become accessible, enabling new scientific missions while reducing the required effort for the launcher by achieving these low orbits. Preliminary studies have shown that the collectible propellant flow for an ion thruster in low Earth orbit (LEO) might not be enough, and that electrode erosion due to aggressive gases, such as atomic oxygen, will limit the thruster’s lifetime. In this paper we present the advances on the design of an inductive plasma thruster (IPT) for the ABEP. The IPT is based on a small-scale inductively heated plasma generator IPG6-S. IPG have the advantage of being electrodeless, and have already shown high electric-tothermal coupling efficiencies using O2 and CO2 as propellant. IPG6-S requires a scaling of the discharge channel to meet with power requirement and expected collected mass flows, as well as optimisation of the accelerating stage, to provide the required thrust to the spacecraft. Tests have been performed to verify some of the parameters and are as well presented within this paper.

We present a load balancing strategy for hybrid particle-mesh methods that is based on domain decomposition and element-local time measurement. This new strategy is compared to our previous approach, which assumes a constant weighting factor for each particle to determine the computational load. The timer-based load balancing is applied to a plasma expansion simulation. The performance of the new algorithm is compared to results presented in the past and a significant improvement in terms of computational efficiency is shown.

Orbiting at lower orbital altitudes, where the residual atmosphere is source of aerodynamic drag requires an efficient drag-compensation system for satellite lifetime extension. One solution is proposed by using Atmosphere-Breathing Electric Propulsion (ABEP), a system that collects atmospheric particles and directly uses them as propellant for an electric thruster. Challenging is also the presence of reactive chemical species at low altitudes, such as atomic oxygen. This is an erosion source of (not only) the propulsion system components, i.e. acceleration grids, electrodes and discharge channels of conventional EP systems such as Radio frequency ion thrusters (RIT) and Hall-effect thrusters (HET). The thruster for an ABEP is proposed to be an Inductive Plasma Thruster (IPT) based on an electrodeless design. Hereby the first step is an efficient plasma source working on atmospheric propellant. Starting from IPG6-S as test-bed, a small scale inductively heated plasma generator at IRS [1]–[3], the mechanisms of RF power absorption by plasma in the low-pressure inductive discharges are analysed numerically and experimentally. The application of a relatively low external magnetic field is reported to enhance plasma density and power transfer efficiency [4]. Performances of both magnetized and unmagnetized plasma source, in terms of plasma resistance and density, are evaluated for different frequencies, input power, magnetic field intensity, pressure, temperature, plasma density profile, discharge channel and antenna dimensions. Investigations on plasma parameters such as its resistance RP and the absorbed power, are based on numerical simulations and supported by theoretical and experimental results. In particular, the application of a magnetic field is foreseen to improve the coupling by increasing both RP and absorbed power. A preliminary design of the plasma source for the IPT, currently under development, is also presented. 7 Russian-German Conference on Electric Propulsion 21 26 Oct 2018