ESA GNC Conference Papers Repository

CubeSats are increasingly used for more complex missions. In recent years, these small satellites are making their way into Earth observation and even astronomy applications. With higher mission complexity, the need for more accurate pointing increases as well. Magnetorquers alone cannot deliver full 3-axis control and thruster systems are often too complex or too large to put on board of a CubeSat. Reaction wheels have therefore proven to be paramount in bringing high accuracy pointing to CubeSats. While reaction wheels deliver high accuracy and agile pointing, their implementation also brings some difficulties. As with any mechanism on board of a spacecraft, reliability and lifetime tends to be an issue. Secondly, the reaction wheels introduce disturbances in the attitude control due to unbalance and microvibrations. Small satellite and CubeSat reaction wheels have a significantly lower heritage than their classical counterparts. Lower budgets, both financially, as in volume also have a negative impact on the delicate design and manufacturing process. As a result, the state-of-the-art of reaction wheels in CubeSats is significantly less advanced. In this paper we will present our design and development work, which is focussed on high reliability and reduction of vibrations. To reduce vibrations and increase life time, special care has been spent on bearings, alignment between motor and flywheel, and construction of the coupling such that only torque from the motor to the flywheel is transmitted, without radial or axial forces. The tolerancing of all parts has been carefully analysed. The Oldham coupling, calculation of tolerance chains and bearing design will be discussed in more detail. Next, we will present our testing procedures and results. The reaction wheels were environmentally tested in thermal vacuum and for vibrations (using a broad range of vibration levels for the most commonly used CubeSat launchers). Accelerated lifetime and spindown tests were carried out. All these tests were overseen by the ESA CubeSat team and were designed to adhere to the ECSS rules as strictly as possible. Finally, we will discuss our efforts to further reduce the vibrations that are produced by the reaction wheels. In the coming months, we will measure the produced vibrations at different rotational speeds and will use those analyses results to take measures to further reduce vibrations. The result of this study will also be discussed in the paper. As a result, the reaction wheels have high reliability and create low vibration levels. Thanks to this, they bring high accuracy pointing closer for the interesting CubeSat platform. An ADCS using these reaction wheels will be used in a CubeSat that will perform star spectroscopy. The high performance and accuracy of these wheels is one of the enablers for this demanding mission.