A consideration of geometry in very-low Earth orbit satellites

Abstract

There is increasing interest in operating Earth observation and telecommunication platforms at altitudes below 300km, in a region known as Very Low Earth Orbit (VLEO). VLEO provides several advantages over higher altitude orbits, such as reduced payload size and mass, as well as reductions in the payload power requirements, and improvements in the downlink data rate. Operating in VLEO, therefore, provides the opportunity to reduce the overall cost of a platform for a given mission.

Despite its advantages, operating in VLEO is not without its challenges. One such challenge is the higher levels of atmospheric drag a satellite experiences in these lower orbits. The drag can be compensated for using a dedicated propulsion system, however, this means that the operational life of the platforms will be limited by the amount of fuel it can carry. It is therefore desirable to find ways to maximize the life of the satellite as much as possible. This thesis presents work that examines aspects of a satellite’s geometry and identifies aeroshell profiles that will minimize the drag experienced by the platform in VLEO while maintaining its usability.

The approach taken for this work is to approximate the results of the Direct Simulation Monte Carlo (DSMC) simulations using a Radial Basis Function (RBF)-based surrogate model. The DSMC simulations are used to calculate the aerodynamic forces on the satellite’s body but are computationally expensive to run. By carefully selecting the sampling locations using an adaptive sampling approach and through effective interpolation of the data using RBF, the number of simulations required to explore the design domain can be limited. This means that a more complete examination of the parameter space of the aeroshell profiles could then be performed.

Using multi-objective optimization, a set of Pareto-optimal satellite geometries was identified. This set traded-off minimizing the drag against maximizing the internal volume of four aeroshell profiles: Blunted Wedge, Elliptical profile, Double-Conic nose and Rounded-Conic nose. When the aspect ratio of the body was fixed, it was seen that the Elliptical aeroshell profile generally performed the best of the four profiles tested, particularly for higher aspect ratio bodies. This is because the Elliptical profiles provided a good compromise between low drag and large volume. However, for a given internal volume it was seen that a Blunted Wedge profile was able to achieve a greater reduction in the drag experienced when compared to a similarly sized cuboid body. It was also shown that, in general, increasing the length of the satellite body to incorporate an aerodynamic profile is always beneficial.

Another factor to consider was the higher abundance of Atomic Oxygen (ATOX) in the VLEO environment. Since some payloads require access to space, they may be more susceptible to the corrosive nature of ATOX. The work presented approached this problem both analytically and with simulations as this helped to identify relationships and simplifications that could be applied in future work. It was also shown that the maximum ATOX flux inside a simple rectangular pit is experienced at the rim of the forward-facing panel and was approximately half the flux seen on the front of the satellite in the direct flow. It was shown that the angular distribution of particle through a point in space approximated a normal distribution in the orbital environment. This meant that the flux distribution on the forward-facing surface could be approximated by the cumulative frequency distribution of this normal distribution.

Date of Award	12 May 2022
Original language	English
Awarding Institution	University of Bristol
Sponsors	Engineering and Physical Sciences Research Council & Thales Alenia Space UK
Supervisor	Lucy Berthoud (Supervisor) & Christian B Allen (Supervisor)