In aerospace industry, saving mass on spacecrafts always remain in large demand to save launch costs or increase the available payload mass. A case study is carried out designing a first concept of an additive manufactured flywheel of a reaction wheel, as it is one of the heaviest parts of wheel systems. As an objective the mass is minimized, while obtaining an angular momentum suitable according to mission requirements and maintaining recent performances. As references the SeaSAT mission and a commercial reaction wheel are used. The work includes a preliminary dimension of the flywheels design space by MATLAB calculations, where in total 15 shapes are analyzed and compared. The most promising design space is afterwards analyzed via the finite-element tool ANSYS and is defined as the reference flywheel. The reference flywheel is used for topology optimizations (ANSYS Topology Optimization), where different boundary conditions are considered. The final designed flywheel obtains 16% higher energy density than the reference flywheel and withstands the mission loads. It can be concluded that it was possible to design a flywheel obtaining less mass while keeping the expected performance.