An elastic pendulum thrust stand for evaluating six-degree-of-freedom thrust of 10 mN-class propulsion systems

Understanding the six-component thrust vector of an onboard propulsion system is essential for ensuring high-quality thrust and torque in advanced CubeSat missions, as propulsion systems may produce unintended thrust components such as side thrust, disturbance torques, or swirl torques. This paper proposes and demonstrates an elastic pendulum six-degree-of-freedom thrust stand designed to measure all thrust components of a propulsion system for CubeSats. Conventional six-degree-of-freedom thrust stands struggle to measure thrust in the mN class or lower due to the low thrust-to-weight ratio of propulsion systems. The proposed thrust stand addresses this limitation by using springs to support the propulsion system, forming an elastic pendulum to improve the effective thrust-to-weight ratio. The linearity and functionality of the thrust stand were confirmed through the mechanical characterization of the stiffness matrix. A null-balance method, in which the thrust stand is controlled by six voice coil motors, was implemented for six-degree-of-freedom thrust measurement. The thrust stand was demonstrated by measuring a 10 mN-class cold-gas thruster, showing measurement uncertainties of within 0.82 mN for thrust vector components and within 0.13 mN m for torque components. These uncertainties were derived based on the uncertainty propagation of individual components of the thrust stand.