Pulsed Thrust Measurements Using Laser Interferometry
Interferometric proximeter system for electric propulsion thrust stands
E. A. Cubbin, J. K. Ziemer, E. Y. Choueiri, R. G. Jahn — Electric Propulsion and Plasma Dynamics Laboratory, Princeton University
Overview
An optical interferometric proximeter system (IPS) for measuring thrust and impulse bit of pulsed electric thrusters was developed. Unlike existing thrust stands, the IPS-based stand offers a single system with electromagnetic-interference-free, high-accuracy (<2% error) measurements across a very wide impulse range (100 µN·s to above 10 N·s), covering all known pulsed plasma thrusters. It is also theoretically capable of measuring steady-state thrust as low as 20 µN for microthrusters.
Key Highlights
<2% Error
High-accuracy thrust measurements across the full impulse range
Wide Range
100 µN·s to above 10 N·s — covers all pulsed plasma thrusters
10 nm Resolution
Two-sensor laser interferometer with 10 nm position accuracy
EMI-Free
Optical measurement immune to electromagnetic interference
20 µN Steady-State
Theoretically capable of measuring thrust as low as 20 µN
Dual Thruster Demo
Validated with an ablative PPT and a quasi-steady MPDT
Measurement Principles
Performance measurements rely on either total delivered impulse or instantaneous thrust, both determined by analyzing thrust-stand dynamics. A swinging-arm type stand is modeled as a damped spring-mass system. For impulse bit measurements, the dynamics before and after the delivered impulse are analyzed. For instantaneous thrust, measurements during the pulse are used. The effective mass (calibration constant) is determined by applying a known impulse and observing the response.
Interferometric Proximeter System
Light from the laser source is split into two beams, each reflected back by a corner cube. The beams are recombined at the beam splitter, passed through a lens to diode sensors, and recorded on a computer. When path lengths differ by a non-integer multiple of the laser wavelength, a phase difference produces constructive or destructive interference. With one corner cube moving, the position is resolved to 10 nm. Multiple fringes are facilitated by slightly offsetting the two beams at the diode sensors.
Applications Demonstrated
The wide application of the thrust stand was demonstrated with two thruster types: an ablative pulsed plasma thruster (APPT) — currently one of the only viable high-specific-impulse propulsion options on small spacecraft with under 200 W available power — and a quasi-steady magnetoplasmadynamic thruster (MPDT), originally used to simulate steady-state high-power thrusters. For MPDTs, the system resolves instantaneous thrust during the pulse to estimate equivalent steady-state thrust, crucial when the current pulse differs from a perfect rectangle.
Pulsed Thrust Measurements Using Laser Interferometry — PDF
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