Numerical Predictions for On-Orbit Ionospheric Aerodynamics Torque Experiment

Abstract

Accurate modelling of the aerodynamic interaction between the space environment and Low Earth Orbit (LEO) objects improves our ability to understand, predict and control their motion. A neglected aspect of the LEO aerodynamics problem is the charged aerodynamic interaction of these objects with the ionosphere, i.e. ionospheric aerodynamics. This work describes a prospective on-orbit ionospheric aerodynamic experiment (IEX) that aims to improve our fundamental understanding of ionospheric aerodynamics and provide in-situ data to support ground-based experiment and numerical efforts. IEX involves the charged aerodynamic control of a formation of two 6U CubeSats. Through the establishment of an optical inter-satellite link (ISL), ionospheric torques induced through the asymmetric charging of High Voltage Panels (HVP) will be measured via the integrated buildup in reaction wheel (RW) speed required to maintain ISL quality. The purpose of this work is to provide preliminary estimates of RW speed buildups to establish measurement detection threshold and ISL control loop feedback rate requirements for the experiment. Here, induced torque estimates are made using the Particle-in-Cell (PIC)/Direction Simulation Monte Carlo (DSMC) simulations using the PIC-DSMC code, pdFOAM. These torque predictions are applied to a digital twin simulation of the satellite formation featuring couple dynamics, space environment and flight software models using the astrodynamics framework, Basilisk, and in-house space environment interaction analysis tool, rayMAN. Given a 500 km altitude circular orbit with a 42 o inclination with ion number densities (ni) ranging between O(10 11 − 10 12 ) m −3 , pdFOAM simulations predicted an induced torque of O(−0.25) µN m given a HVP surface potential (φP ) of −100 V for both the high and low density cases. An unexpected result was a net thrust prediction for the low-density cases caused by indirect thrust forces that arise from plasma sheath driven ion acceleration. Uncertainties surrounding appropriate boundary conditions for wake surface are hypothesised to have contributed to this result and are a key focus for future work. Considering an induced torque of −0.25 µN m and accounting for aerodynamic, solar radiation pressure and gravity-gradient torques, Basilisk simulations predicted an integrated buildup of 25 − 50 RPM over a 10 minute period. Future work discussed includes: attitude determination errors resulting from uncertainties in the ISL, controller errors from discrete-time effects and magnetically induced torques in the digital twin simulation.

Magnetic Disturbance Rejection in Spacecraft using Electromagnetic Propulsion

“…Basilisk handles the spacecraft state propagation and supports adding Python modules to handle simulation tasks specific to this research. Basilisk has been used in previous research that requires 6-DoF propagation and the addition of new modules such as control laws [64] or external torques [65]. The simulations are run on a machine using the Ubuntu 22.04 operating system with an Intel Core i7 processor.…”