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The Satellite for Orbital Aerodynamics Research (SOAR) is a 3U CubeSat that has been designed to investigate the aerodynamic performance of different materials at low orbital altitudes. The spacecraft has been developed within the scope of DISCOVERER, a Horizon 2020 project that aims to develop foundational technologies to enable sustainable operations of Earth observation spacecraft in very low Earth orbits (VLEO) i.e., those below 450 km. SOAR features two payloads: i) a set of steerable fins that can expose different materials to the oncoming atmospheric flow developed by The University of Manchester, and ii) a forward-facing ion and neutral mass spectrometer (INMS) that provides in-situ measurements of the atmospheric density, flow composition, and velocity from the Mullard Space Science Laboratory (MSSL) of University College London. These payloads enable characterisation of the aerodynamic performance of different materials at very low altitudes with the aim to advance understanding of the underlying gas-surface interactions in rarefied flow environments. The satellite will also be used to test novel aerodynamic attitude control methods and perform atmospheric characterisation in the VLEO altitude range. SOAR will perform the first in-orbit test of two novel materials that are expected to have atomic oxygen erosion resistance and drag-reducing properties, providing valuable in-orbit validation data for ongoing ground-based experimentation. Such materials hold the promise for extending operations at lower altitudes with benefits particularly for Earth observation and communications satellites that can correspondingly be reduced in size and cost. The platform for SOAR is largely based on GOMX-3 heritage and the spacecraft was assembled, integrated, and tested by GomSpace A/S. The satellite was launched on the SpX-22 commercial resupply service mission to the International Space Station in on 3rd June 2021 was subsequently deployed into orbit on the 14th June 202, Article signat per 30 autors/res: Nicholas H. Crisp, Alejandro Macario-Rojas, Peter C.E. Roberts, Steve Edmondson, Sarah J. Haigh, Brandon E.A. Holmes, Sabrina Livadiotti, Vitor T.A. Oiko, Katharine L. Smith, Luciana A. Sinpetru, Jonathan Becedas, Valeria Sulliotti-Linner, Simon Christensen, Virginia Hanessian, Thomas K. Jensen, Jens Nielsen, Morten Bisgaard, Yung-An Chan, Georg H. Herdrich, Francesco Romano, Stefanos Fasoulas, Constantin Traub, Daniel Garcia-Almiñana, Silvia Rodriguez-Donaire, Miquel Sureda, Dhiren Kataria, Badia Belkouchi, Alexis Conte, Simon Seminari, Rachel Villain, Objectius de Desenvolupament Sostenible::9 - Indústria, Innovació i Infraestructura, Postprint (published version)

DISCOVERER is a European Commission funded project aiming to revolutionise satellite applications in Very Low Earth Orbits (VLEO). The project encompasses many different aspects of the requirements for sustainable operation, including developments on geometric designs, aerodynamic attitude and orbital control, improvement of intake designs for atmosphere breathing electric propulsion, commercial viability, and development of novel materials. This paper is focused solely on the description of the experimental facility designed and constructed to perform ground testing of materials, characterising their behaviour in conditions similar to those found in VLEO. ROAR, Rarefied Orbital Aerodynamics Research facility, is an experiment designed to provide a controlled environment with free molecular flow and atomic oxygen flux comparable to the real orbital environment. ROAR is a novel experiment, with the objective of providing better and deeper understanding of the gas-surface interactions between the material and the atmosphere, rather than other atomic oxygen exposure facilities which are mainly focused on erosion studies. The system is comprised of three major parts, (i) ultrahigh vacuum setup, (ii) hyperthermal oxygen atom generator (HOAG) and (iii) ion-neutral mass spectrometers (INMS). Each individual part will be considered, their performance analysed based on experimental data acquired during the characterisation and commissioning, thus leading to a complete description of ROAR’s capabilities. Among the key parameters to be discussed are operational pressure, atomic oxygen flux, beam shape and energy spread, mass resolution, signal-to-noise ratio and experimental methodology., This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 737183., Peer Reviewed, Postprint (published version)

Micro scale rarefied gas transport attracts great research interests recently due to the special phenomena arising from small spatial scales, as well as its extensive applications in microelectromechanical systems. Due to the large surface-to-volume ratio at small scale, gas-surface interaction plays an important role in micro scale rarefied gas transport, which is currently described through empirical models and incorporated in various numerical methods for rarefied gas flow simulations. The validity of the empirical gas-surface interaction models needs systematic evaluation and the accommodation coefficients which are the adjustable parameters in the empirical models require to be determined through a feasible approach. These challenges of the existing empirical models form the two objectives of this thesis: 1) to validate the empirical gas-surface interaction models in typical micro scale rarefied gas transport problems based on the resolved macroscopic flow quantities; 2) to develop simple model for estimating accommodation coefficients. First, an efficient hybrid DSMC-MD scheme is developed for the simulation of micro scale gas flows with accurate gas-surface interaction boundary condition. It applies molecular dynamics (MD) method in the thin gas-surface interaction layer and the direct simulation Monte Carlo (DSMC) method in the remaining portion of flow field. The coupling between the two methods is realized by matching the molecular velocity distribution function at the DSMC/MD interface. Further improvement in efficiency is achieved by taking advantage of gas rarefaction inside the gas-surface interaction layer and by employing the “smart-wall model”. The developed hybrid algorithm resolves the accurate macroscopic flow field and measures the accommodation coefficients at the same time. It provides the evaluation criterion for the performance of empirical gas-surface interaction models. Secondly, a systematic study on the performance of two widely-used emp

Micro scale rarefied gas transport attracts great research interests recently due to the special phenomena arising from small spatial scales, as well as its extensive applications in microelectromechanical systems. Due to the large surface-to-volume ratio at small scale, gas-surface interaction plays an important role in micro scale rarefied gas transport, which is currently described through empirical models and incorporated in various numerical methods for rarefied gas flow simulations. The validity of the empirical gas-surface interaction models needs systematic evaluation and the accommodation coefficients which are the adjustable parameters in the empirical models require to be determined through a feasible approach. These challenges of the existing empirical models form the two objectives of this thesis: 1) to validate the empirical gas-surface interaction models in typical micro scale rarefied gas transport problems based on the resolved macroscopic flow quantities; 2) to develop simple model for estimating accommodation coefficients. First, an efficient hybrid DSMC-MD scheme is developed for the simulation of micro scale gas flows with accurate gas-surface interaction boundary condition. It applies molecular dynamics (MD) method in the thin gas-surface interaction layer and the direct simulation Monte Carlo (DSMC) method in the remaining portion of flow field. The coupling between the two methods is realized by matching the molecular velocity distribution function at the DSMC/MD interface. Further improvement in efficiency is achieved by taking advantage of gas rarefaction inside the gas-surface interaction layer and by employing the “smart-wall model”. The developed hybrid algorithm resolves the accurate macroscopic flow field and measures the accommodation coefficients at the same time. It provides the evaluation criterion for the performance of empirical gas-surface interaction models. Secondly, a systematic study on the performance of two widely-used emp

Micro scale rarefied gas transport attracts great research interests recently due to the special phenomena arising from small spatial scales, as well as its extensive applications in microelectromechanical systems. Due to the large surface-to-volume ratio at small scale, gas-surface interaction plays an important role in micro scale rarefied gas transport, which is currently described through empirical models and incorporated in various numerical methods for rarefied gas flow simulations. The validity of the empirical gas-surface interaction models needs systematic evaluation and the accommodation coefficients which are the adjustable parameters in the empirical models require to be determined through a feasible approach. These challenges of the existing empirical models form the two objectives of this thesis: 1) to validate the empirical gas-surface interaction models in typical micro scale rarefied gas transport problems based on the resolved macroscopic flow quantities; 2) to develop simple model for estimating accommodation coefficients. First, an efficient hybrid DSMC-MD scheme is developed for the simulation of micro scale gas flows with accurate gas-surface interaction boundary condition. It applies molecular dynamics (MD) method in the thin gas-surface interaction layer and the direct simulation Monte Carlo (DSMC) method in the remaining portion of flow field. The coupling between the two methods is realized by matching the molecular velocity distribution function at the DSMC/MD interface. Further improvement in efficiency is achieved by taking advantage of gas rarefaction inside the gas-surface interaction layer and by employing the “smart-wall model”. The developed hybrid algorithm resolves the accurate macroscopic flow field and measures the accommodation coefficients at the same time. It provides the evaluation criterion for the performance of empirical gas-surface interaction models. Secondly, a systematic study on the performance of two widely-used emp