Antonio Arcos Rueda*
The increasing interest in the exploitation of very low Earth orbits (VLEO) has led to innovative operational concepts, including the use of aerodynamic orbit and attitude control methods. Aerodynamic forces and torques are the main source of perturbation that a spacecraft will experience at these lower altitudes in VLEO. Rather than solely using traditional attitude control actuators (reaction wheels, CMGs, and magnetorquers) or thrusters for orbit control, aerodynamic control can therefore be used as an integral aspect of operational attitude and orbit control.
A range of attitude and orbit control methods using orbital aerodynamic effects have been proposed in the past. In some cases, these methods have been demonstrated in-orbit and ultimately used for some operational purpose. Notable examples include the GOCE mission which utilised an aerostable geometry to assist the drag-compensating propulsion system which was required to accurately map the Earth’s gravitational field, and the ORBCOMM constellation which used differential drag techniques to assist the deployment of the different satellites into their intended orbital slots.
However, more complex aerodynamic control has yet to be developed and demonstrated. For Earth observation (EO) applications, the ability to provide precise and stable pointing in the presence of disturbing forces and torques is necessary. Rapid slewing capability is also often desired, requiring platform agility and the ability to offset or reject unwanted aerodynamic torques. Combinations of aerodynamic control and traditional attitude control actuators may provide the necessary performance, whilst the aerodynamics can also help to maintain these actuators, for example through momentum management. Concepts for orbit maintenance and re-tasking using aerodynamic forces have also been proposed, but are at present limited by the available material technologies which do not currently allow the generation of meaningful lift-forces in the VLEO environment.
Investigation of materials which may have improved gas-surface interaction (GSI) properties in the VLEO environment is being addresses in other aspects of the DISCOVERER project. This project study the development of novel controllers utilising aerodynamic forces and torques and establishes their feasibility for operational use in VLEO. The VLEO environment is first described, from which a modelling toolbox was created enabling simulations of the spacecraft attitude and orbit motion to be performed in the presence of the expected perturbing forces and torques (eg. gravity, aerodynamics, solar-radiation pressure, magnetic field interactions).
Three reference aerodynamic platform concepts were developed to which the aerodynamic control methods could be applied. Two are nominally aerostable designs, the first a shuttlecock which features an aerodynamic skirt which extends behind the satellite body, and the second an arrow or feathered configuration which features aerodynamic fins. The third geometry, a “disc satellite”, was designed to be neutrally stable in the nominal configuration and takes the form of a cylindrical body with two panels extending from the flat end surfaces. For each geometry, steerable aerodynamic control surfaces were specified, enabling the generation of varying aerodynamic forces and torques and therefore control in one or more of the spacecraft body axes.
In orbit control, investigate the use of aerodynamic drag and lift for formation flight and rendezvous purposes are summarised. Also, it investigates the performance increase that could be achieved with the development or identification of novel materials that have improved GSI characteristics and can promote specular reflection properties, thus enabling the generation of useful lift forces.
In attitude control, combinations of synergetic aerodynamic-based control and traditional attitude actuators (reaction wheels) were selected to investigate the development of pointing and trim manoeuvres. Aerodynamic control was also chosen to perform the momentum management of the reaction wheel with the intention of avoiding saturation of the actuators in the presence of disturbing environmental torques. In order to perform the simulations of these control manoeuvres a modified PID with in intelligent integration action and gains selected using a linear-quadratic regulator (LQR) was implemented. A range of other control methods were considered, but the modified PID ultimately chosen for simplicity of implementation and robust control behaviour in order to first prove the feasibility of the selected aerodynamic control methods for the representative platform concepts.
The results of the presented case studies demonstrate the feasibility of aerodynamic pointing control and momentum management using the developed controller logic and the concept platform geometries. It was noted through the analysis of the different concept geometries that the selection of aerodynamic control surfaces is critical in providing the available control authority to perform the necessary manoeuvres. The shuttlecock geometry was not able to provide control authority in roll, while the neutrally-stable disc satellite was unable to provide control authority in pitch.
Finally, a number of improvements to the developed controllers were proposed in response to the achieved results. These include further development of the panel selection methodology to include a Jacobian formulation which may provide more efficient computation of the aerodynamic torques, and anti-saturation logic for the aerodynamic actuators to demote selection of high-drag configurations which would contribute more to orbital decay.
Within the context of DISCOVERER, the opportunity to perform in-orbit demonstration of aerodynamic control manoeuvres exists using the aerodynamics test satellite SOAR (Satellite for Orbital Aerodynamics Research). Following a consideration of the specific requirements and limitations for implementing control methods on this test satellite, a proposed set of manoeuvres for demonstration have been presented. These manoeuvres include aerodynamic-assisted pointing, aerodynamic trim, and momentum management tasks and are intended to provide proof-of-concept of operationally-relevant aerodynamic control in the VLEO environment. These manoeuvres have been taken forward for specific development and implementation on the satellite hardware within the scope of DISCOVERER.
*DevOps Engineer at Deimos Space