• low thrust measurement
• thrust balance
• hall thruster
• electric thruster
• electric propulsion

Over the past decades, electric propulsion has found increasing use in the space field and different electric thrusters have been designed to perform various tasks in space missions. In the electric thrusters development, an important role is played by the test phase, during which the various engine performances are measured. Thrust balances are among one of the main testing equipments as they allow to measure the thrust.

Thrust balances for electric thrusters are sophisticated instruments that must be able to detect very low levels of thrust. The thrust stand design process starts by defining the requirements and constraints, which are given by the thruster itself, the vacuum chamber where the test takes place, and the performances to be achieved, in terms of accuracy, reliability and long-term stability. This entire work is focused on developing a thrust stand for the HT400 Hall Thruster, under current development at Sitael in Pisa, which can generate a thrust ranging from 15 to 40 mN.

The design of a thrust balance has to take various aspects into account, related to the fact that the measurement of millinewton thrust requires high resolution sensors, but also essential precautions to eliminate or limit disturbances that are capable of generating perturbations in the order of the thrust magnitude. These are mainly due to the very small thrust to weight ratio characterizing electric thrusters, the temperature gradients generated in the thrust balance during the test, and the various harness connected to the thruster. In addition, a calibration system is indispensable to periodically monitor the thrust balance's reliability and performance.

With the aim of obtaining a high performance, low cost and low complexity thrust balance, the aforementioned cardinal points were addressed in this thesis, where the design of a double inverted pendulum thrust balance is presented. Thanks to the appropriate load cell and frictionless pivot bearings, the thrust balance will be able to measure thrust with high resolution and accuracy. The measurement errors are limited by an inclination control mechanism capable of keeping the stand horizontal with an error of 0.0001°, a thermal control system that maintains the temperature almost constant in the most sensitive regions of the stand, and an appropriate arrangement of the thruster's placing on the balance. Regarding the calibration system, the effect of the thrust will be simulated through the coupling of a magnet and a voice coil, making it a simple and reliable mechanis.
All these features have led to a high performance thrust balance for low power thrusters, capable of measuring thrust levels from 1 to 50 mN, with a theoretical uncertainty of ±0.1 mN.