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The testing of atmosphere-breathing electric propulsion intakes is an important step in the development of functional propulsion systems which provide sustained drag compensation in very low Earth orbits. To make satellite operations more sustainable, it is necessary to develop new materials which withstand erosion, long-lasting propulsion systems to overcome drag, and tools that allow for ground-based testing. Among the tools to enable these innovations is the Rarefied Orbital Aerodynamics Research facility at the University of Manchester. Here, a description of the facility is provided together with two different methodologies for testing sub-scaled intake designs for atmosphere-breathing electric propulsion systems. The first methodology is based on measurements of the pressure difference between the two extremities of the intake, while the second uses a gas sensor to measure the collection efficiency of the intake. Direct Simulation Monte Carlo models have been used to assess the viability of the proposed testing methodologies. The results of this analysis indicate that either methodology or a combination of both can provide suitable measurements to assess the performance of future intake designs.

Plasma-based CO2 conversion is a promising pathway towards greenhouse gas recycling. In the corresponding research field, various types of plasma reactors are applied for carbon dioxide dissociation. So far, spatial inhomogeneities of the specific energy (SEI) distribution in plasma generators, e.g., induced by non-uniform heating or an inhomogeneous mass distribution, are not the focus of the investigations. In this work, the spatial inhomogeneity of mass-specific enthalpy in the plasma jet of the inductive plasma generator IPG4 at the Institute of Space Systems (IRS) is examined. For this, the mean mass-specific enthalpy as well as the radial distribution of the local enthalpy are measured using plasma probes. Moreover, the influence of the determined specific enthalpy inhomogeneity on the CO2 splitting performance is quantified. It is shown that an inhomogeneous radial distribution of the specific energy can significantly lower the carbon dioxide conversion, compared to a homogeneous case. With regards to IPG4, the performance reduction is 16 %., Comment: 30 pages, 12 figures

Differential drag is a promising option to control the relative motion of distributed satellites in the Very Low Earth Orbit regime which are not equipped with dedicated thrusting devices. A major downside of the methodology, however, is that its control authority is (mainly) limited to the in-plane relative motion control. By additionally applying differential lift, however, all three translational degrees-of-freedom become controllable. In this article, we present a tool to flexibly plan optimal three-dimensional formation flight maneuvers via differential lift and drag. In the planning process, the most significant perturbing effects in this orbital regime, namely the J2 effect and atmospheric forces, are taken into account. Moreover, varying atmospheric densities as well as the co-rotation of the atmosphere are considered. Besides its flexible and high-fidelity nature, the major assets of the proposed methodology are that the in-and out-of-plane relative motion are controlled simultaneously via deviations in the yaw angles of the respective satellites and that the planned trajectory is optimal in a sense that the overall decay during the maneuver is minimized. Thereby, the remaining lifetime of the satellites is maximized and the practicability and sustainability of the methodology significantly increased. To the best of the authors knowledge, a tool with the given capabilities has not yet been presented in literature. The resulting trajectories for three fundamentally different relevant formation flight maneuvers are presented and discussed in detail in order to indicate the vast range of applicability of the tool.

This paper discusses the design and the performance achievable with active aerodynamic attitude control in very low Earth orbit, i.e. below 450 km in altitude. A novel real-time algorithm is proposed for selecting the angles of deflection of aerodynamic actuators providing the closest match to the control signal computed by a selected control law. The algorithm is based on a panel method for the computation of the aerodynamic coefficients and relies on approximate environmental parameters estimation and worst-case scenario assumptions for the re-emission properties of space materials. Discussion of results is performed by assuming two representative pointing manoeuvres, for which momentum wheels and aerodynamic actuators are used synergistically. A quaternion feedback PID controller implemented in discrete time is assumed to determine the control signal at a sampling frequency of 1 Hz. The outcome of a Monte Carlo analysis, performed for a wide range of orbital conditions, shows that the target attitude is successfully achieved for the vast majority of the cases, thus proving the robustness of the approach in the presence of environmental uncertainties and realistic attitude hardware limitations., Comment: 33 pages, 14 figures, 3 tables, accepted for publication in AIAA Journal of Guidance, Control, and Dynamics (2022-01-04)

We present the validation of ADBSat, a novel implementation of the panel method including a fast pseudo-shading algorithm, that can quickly and accurately determine the forces and torques on satellites in free-molecular flow. Our main method of validation is comparing test cases between ADBSat, the current de facto standard of direct simulation Monte Carlo (DSMC), and published literature. ADBSat exhibits a significantly shorter runtime than DSMC and performs well, except where deep concavities are present in the satellite models. The shading algorithm also experiences problems when a large proportion of the satellite surface area is oriented parallel to the flow, but this can be mitigated by examining the body at small angles to this configuration (${\pm}$ 0.1{\deg}). We recommend that an error interval on ADBSat outputs of up to 3\% is adopted. Therefore, ADBSat is a suitable tool for quickly determining the aerodynamic characteristics of a wide range of satellite geometries in different environmental conditions in VLEO. It can also be used in a complementary manner to identify cases that warrant further investigation using other numerical-based methods., Comment: 30 pages, 15 figures. Submitted to Computer Physics Communications

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.

The inductively-heated plasma generator IPG6-B at Baylor University has been established and characterized in previous years for use as a flexible experimental research facility across multiple applications. The system uses a similar plasma generator design to its twin-facilities at the University of Stuttgart (IPG6-S) and the University of Kentucky (IPG6-UKY). The similarity between these three devices offers the advantage to reproduce results and provides comparability to achieve cross-referencing and verification. Sub- and supersonic flow conditions for Mach numbers between $Ma = 0.3 - 1.4$ have been characterized for air, argon, helium and nitrogen using a pitot probe. Overall power coupling efficiency as well as specific bulk enthalpy of the flow have been determined by calorimeter measurements to be between $\eta = 0.05 - 0.45$ and $h_s = 5- 35$ MJ/kg respectively depending on gas type and pressure. Electron temperatures of $T_e = 1 - 2$ eV and densities $n_e = 10^{18} - 10^{20} m^{-3}$ have been measured using an electrostatic probe system. At Baylor University, laboratory experiments in the areas of astrophysics, geophysics as well as fundamental research on complex (dusty) plasmas are planned. Most recent experiments include the study of dusty plasmas and astrophysical phenomena and the interaction of charged dust with electric and magnetic fields. In this case, dust can be used as a diagnostic for such fields and can reveal essential information of the magneto-hydrodynamics in low-temperature plasmas., Comment: Accepted for publication in Acta Astronautica

Renewed interest in Very Low Earth Orbits (VLEO) - i.e. altitudes below 450 km - has led to an increased demand for accurate environment characterisation and aerodynamic force prediction. While the former requires knowledge of the mechanisms that drive density variations in the thermosphere, the latter also depends on the interactions between the gas-particles in the residual atmosphere and the surfaces exposed to the flow. The determination of the aerodynamic coefficients is hindered by the numerous uncertainties that characterise the physical processes occurring at the exposed surfaces. Several models have been produced over the last 60 years with the intent of combining accuracy with relatively simple implementations. In this paper the most popular models have been selected and reviewed using as discriminating factors relevance with regards to orbital aerodynamics applications and theoretical agreement with gas-beam experimental data. More sophisticated models were neglected, since their increased accuracy is generally accompanied by a substantial increase in computation times which is likely to be unsuitable for most space engineering applications. For the sake of clarity, a distinction was introduced between physical and scattering kernel theory based gas-surface interaction models. The physical model category comprises the Hard Cube model, the Soft Cube model and the Washboard model, while the scattering kernel family consists of the Maxwell model, the Nocilla-Hurlbut-Sherman model and the Cercignani-Lampis-Lord model. Limits and assets of each model have been discussed with regards to the context of this paper. Wherever possible, comments have been provided to help the reader to identify possible future challenges for gas-surface interaction science with regards to orbital aerodynamic applications., Comment: Journal paper (accepted for publication in "Progress in Aerospace Sciences") Replacement: Corrected typos in Equations

Challenging space missions include those at very low altitudes, where the atmosphere is source of aerodynamic drag on the spacecraft. To extend such missions lifetime, an efficient propulsion system is required. One solution is Atmosphere-Breathing Electric Propulsion (ABEP). It collects atmospheric particles to be used as propellant for an electric thruster. The system would minimize the requirement of limited propellant availability and can also be applied to any planet with atmosphere, enabling new mission at low altitude ranges for longer times. Challenging is also the presence of reactive chemical species, such as atomic oxygen in Earth orbit. Such species cause erosion of (not only) propulsion system components, i.e. acceleration grids, electrodes, and discharge channels of conventional EP systems. IRS is developing within the DISCOVERER project, an intake and a thruster for an ABEP system. The paper describes the design and implementation of the RF helicon-based inductive plasma thruster (IPT). This paper deals in particular with the design and implementation of a novel antenna called the birdcage antenna, a device well known in magnetic resonance imaging (MRI), and also lately employed for helicon-wave based plasma sources in fusion research. The IPT is based on RF electrodeless operation aided by an externally applied static magnetic field. The IPT is composed by an antenna, a discharge channel, a movable injector, and a solenoid. By changing the operational parameters along with the novel antenna design, the aim is to minimize losses in the RF circuit, and accelerate a quasi-neutral plasma plume. This is also to be aided by the formation of helicon waves within the plasma that are to improve the overall efficiency and achieve higher exhaust velocities. Finally, the designed IPT with a particular focus on the birdcage antenna design procedure is presented