7.2.3 Attitude Control

High precision pointing will be accomplished through the reaction wheels and momentum dumping through thrusters.

The sizing of the ACS subsystem is based on the approximation of the Asterius spacecraft as a solid, homogeneous, and rigid cylinder, with dimensions corresponding to those of the actual vehicle. The centroid of the cylinder is located at the coordinates (0, 0, 3.2) meters with respect to a coordinate reference frame having an origin at the base of the cylinder (with the z-axis being the axis of symmetry). The moments of inertia for the cylinder approximation are $I_{xx}=I_{yy}=8.174\times 10^4$ kg m and $I_{zz}=1.45\times 10^3$ kg m .

The attitude control accuracies have been modeled after those of Cassini. The stability requirement during staring mode for Cassini is 4 $\mu$rad over 0.5 s ($2\sigma$) [6]. This same requirement was adopted as a base requirement for the attitude control approximation for Asterius. The resultant moments necessary to fulfill this requirement are M_x=M_y=2.616 N-m, and M_z=0.046 N-m. These moments will be generated primarily with the reaction wheel assemblies.

Asterius carries sixteen Olin MR-111 hydrazine thrusters for the spacecraft's attitude orientation and pointing. The specific thruster design is included in the propulsion subsystem of the spacecraft. The thrusters will be used for attitude control, orbit maintenance, torque unloading, and low $\Delta V$ burns. The complement of thrusters is mounted on the SOM, where they are then arranged in four clusters of four.

The RWA's will serve as momentum exchange devices, providing high precision accuracy. When the RWA's become saturated with stored momentum, that momentum will be unloaded through propulsive thruster firing as momentum dumps.

The RWA's will be placed along three orthogonal axes (angled relative to the spacecraft axes) with a fourth added for redundancy. The resulting assemblage composes a four-for-three redundant package with only three wheels operating at any given time. The fourth wheel will be positioned such that it if any one of the original three fail it will be able to amply serve as a substitute.