A miniature force probe for cold gas thrust measurements

The rapid growth of the space industry is driving the development of electric propulsion systems spanning thrust from hundreds of mN down to the μN range. Conventional plume diagnostic tools are limited by size and versatility, with them being difficult to adapt across testing facilities and thrusters. Moreover, plasma-based sensors, such as Faraday or Langmuir probes, are blind to neutral species, limiting their use for cold gas thruster plumes. To address these limitations, a miniature MEMS-based force probe was developed to directly measure the total momentum flux of impinging particles, enabling total thrust reconstruction. In this work, the calibration and the thrust reconstruction methods for the force probe are developed and validated using a cold gas thruster. Thrust measurements over a range of 200-950 μN show good agreement with a conventional thrust balance within ±5% in the far field and up to ≈30% in the near field. After accounting for the accommodation factor, assuming diffusive particle reflection, the residual discrepancy is attributed to non-uniform pressure distribution across the proof mass and transitional flow regime effects. For the reference case 1 mg s-1 of N2, the local pressure measured at 16.5 cm from the thruster exit plane ranges from 8 to 25 mPa, corresponding to a total thrust of 600 μN. These results demonstrate that the MEMS force probe is a valid, versatile instrument for thrust measurements and plume diagnostics, providing spatially resolved momentum flux information relevant for the characterization of next-generation electric propulsion systems.