Development of a ultrathin reaction wheel for modular nanosatellites

The M.Sc. thesis aims at the design and analysis of an innovative, ultrathin, low-cost reaction wheel prototype integrated in the primary structure of a 1U CubeSat. The wheel exceeds performances of present state-of-the-art and commercial reaction wheels for CubeSats, in terms of mass and volume, retaining competitive saturation momentum. The reaction wheel is integrated in one face of a 1U primary structure (termed “tile”) and thus the tile must satisfy CubeSat mechanical requirements (CubeSat Design Specification Rev. 13, 2014), which impose a maximum thickness of 6.5 mm above structure ribs, hence the ultrathinness requirement. The tile is composed of a stack of seven FR4 printed circuit boards, soldered together. The FR4 panel facing CubeSat interior and payload is termed inner plate and hosts electronic components and circuitry for wheel motor. The FR4 panel towards space environment is termed outer plate and hosts two solar cells and their circuitry. Intermediate panels have holes, so that the internal part of the tile is hollow to contain the reaction wheel, which rotates about a copper shaft thanks to commercial ball bearings. The shaft is soldered to outer plate directly and to inner plate through a copper collar, that absorbs the dimensional variations of the above mentioned components by deformation. Shaft, wheel and collar have been designed and built autonomously. PTFE rings are glued in wheel cavity to avoid impacts among the wheel, inner and outer plates. The wheel is driven by an Axial Flux Permanent Magnet (termed AFPM) brushless motor, which in a first tile prototype has been autonomously designed, partially analysed but not built and tested, although its feasibility is clearly shown. Firstly, a thorough study of motor working principle, characteristics and performances has been completed. Motor magnetic circuit has been designed in order to avoid ferromagnetic material saturation. Saturation momentum and other main performance parameters have been evaluated. Since motor efficiency was not acceptable, a final prototype was designed and analysed, employing a commercial, thin AFPM brushless motor. Saturation momentum has been evaluated for the second prototype as well. For the first prototype, with the autonomously designed motor, two commercial, miniaturized ball bearings are mounted on the shaft and support the wheel. They allow for gyroscopic torque transmission from the wheel to primary structure. The commercial motor features one internal ball bearing, thus the final prototype has just one commercial ball bearing, mounted on the shaft and supporting the wheel, to allow for gyroscopic torque transmission. Mechanical tolerancing of wheel and shaft for both prototypes has been conducted to assure adequate play between PTFE rings and wheel and acceptable wheel misalignment. A structural finite element analysis has been performed on the final prototype to verify stresses, deformations and resonance frequencies of the entire tile for launch conditions expected on a Vega launch vehicle. A thermal analysis has been accomplished on the final prototype as well, to verify compatibility with components maximum temperatures. Thermal mathematical model heat sources include the motor, in nominal working conditions, and solar and terrestrial radiation in low Earth orbit.