Aerodynamic Stability for Optimal CubeSat Drag Sail Operations

In the interest of reducing the looming threat of space debris, the United States and the European Space Agency adopted a rule requiring the deorbit of satellites in Low Earth Orbit (LEO) within 5 years of the end of their life. Adherence to this regulation can be a challenge for non-propulsive and low-budget CubeSat missions. The combination of high orbital velocities and the existence of a thin atmosphere means an aerodynamic drag force will eventually deorbit any satellite in LEO. The magnitude of this force is dependent on the cross-sectional area exposed to the flow, however. Many small missions cannot generate a large enough drag force to deorbit within 5 years. A team of students at the University of Colorado Boulder have designed a drag-sail system capable of attaching to a CubeSat, deploying a cross-sectional area of at least 2 square meters, and increasing the drag force until the CubeSat re-enters and burns up in the Earth’s atmosphere. This drag-sail system stows to a volume of 1U (10cm x 10cm x 10cm), carries its own independent power system, and has safeguards to deploy even if the host CubeSat is unresponsive. Once deployed, however, the attitude of the sail- that is, its pointing and rotation behavior- is affected by various factors including solar pressure and micrometeorite impacts. As the drag-sail system utilizes a thin film, certain attitude orientations render it useless Due to the low cost, volume, and energy requirements of the drag-sail system, it does not carry an active method of attitude control. In order to allow the system to maintain its required cross sectional area, the team has developed a method of passive stabilization by use of aerodynamic forces. This method uses a set of ‘angled panels,’ in which the thin membrane sail is offset at an angle instead of being perpendicular to the flow. Nominally, the center of pressure is aligned with the center of mass. Any attitude perturbations result in an adjustment of the center of pressure which generates a restoring torque. Results of modeling analysis show that the drag-sail system can remain within a 15 degree cone of its optimal direction, even when presented with realistic perturbations. The use of aerodynamic stabilization is an effective, low-risk, and inexpensive method to optimize the operations of any drag-sail system.