Preliminary data was recently provided for a reaction sphere prototype on NASA’s zero-gravity parabolic flight vehicle. Gyroscope telemetry indicates that reaction spheres were successfully commanded at 10- to 20-ms pulses during a handful of parabolas in each flight. This is the first publicly disclosed validation of a freely rotating reaction sphere in a standalone compact package. At dimensions of

Figure 1. Testing a free-floating reaction sphere package during a zero-gravity parabola. Live streaming sensor data confirmed reaction sphere operation in real time. Inset depicts the primary hardware tested inside the free-floating package.
High-fidelity spacecraft pointing is usually accomplished with reaction wheels (RWs), which are momentum transfer devices consisting of cylindrical flywheels that are inertially coupled to the main spacecraft body. A torque in the RW system imparts a reaction torque on the spacecraft to produce a perturbation in spacecraft attitude. A properly designed attitude control system (ACS) is important for mission-critical functions such as communications using antennas, energy harvesting using solar panels, and instrument pointing. For three-axis control, at least one RW is needed in three independent axes. Control moment gyros (CMGs) are an alternative to RWs, but provide improved spacecraft agility because rotating flywheels are positioned at the ends of articulating arms. However, multiple CMGs are also usually used to avoid control singularities. Therefore, a primary disadvantage to RWs and CMGs is that these systems can be prohibitively large.

Figure 2. Data set from gyroscope containing 10-to-20-ms electromagnetic pulse train corresponding to reaction sphere rotation.
Rather than using multiple actuators, a compact reaction sphere can meet or exceed the performance metrics of conventional systems. It was originally conceived decades ago and has not been technologically feasible until recently. Reaction spheres operate similarly to single-axis RWs, but typically employ a spherical rotor that can be controlled in multiple rotational degrees of freedom. Moreover, a rotating sphere has a higher angular momentum over a RW ensemble confined to a bounding volume of similar size. This helps with extremely fine pointing requirements, e.g., long imaging exposures of celestial bodies or for tracking small objects on Earth. It also benefits spacecraft exposed to high-disturbance torques, such as atmospheric drag in Low Earth Orbit (LEO), since some missions cannot adequately dump momentum either because magnetic torque rods are too weak or because some spacecraft may not be equipped with attitude thrusters.

This work was done by Alvin Yew and Matthew Colvin of Goddard Space Flight Center, and Emory Stagmer and Michael Stoddard of Northrop Grumman. GSC-17252-1

NASA Tech Briefs Magazine

This article first appeared in the September, 2015 issue of NASA Tech Briefs Magazine.

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