NPL is involved in the development of instruments for measuring thrust at the milli-Newton and micro-Newton level as well as consultancy in this area. The work is aimed at providing traceable thrust measurement for electric propulsion and cold gas thrusters for the space industry.
Electric propulsion is becoming increasingly important as a means of fine attitude control, drag compensation and formation flying of both scientific and commercial satellites. With the increasing demands being placed on thruster performance particularly by science missions, ground testing and verification of electric thrusters is becoming essential.
Here we describe the research work carried out at NPL to provide traceable micro-thrust measurement facilities for customers including QinetiQ and the European Space Agency (ESA).
NPL has developed a number of thrust and impulse standards for measurement of thrust, thrust noise and impulse of electric propulsion devices. A range of instruments have been developed which together cover six orders of magnitude of thrust from below a micro-Newton to around 0.5 Newton. Significant features of NPL's family of thrust stands include:
- Built-in traceable calibration capability with direct traceability to International Standards.
- Controllable measurement bandwidth enabling thrust noise and thruster step-response to be measured.
- Excellent vibration and tilt isolation to minimise the effect of the local environment on the measurement.
The design philosophy behind the thrust balances produced by NPL has always been to produce a device with a traceable calibration (ie a calibration that can be compared to national standards by a series of well defined and documented steps). In order to achieve this, all the designs have been based on pendulum with a force feedback nulling servo system. The design of the force feedback system ensures that the output signal is directly proportional to the applied thrust. Crucially, the scale fact relating output signal to applied thrust is determined only the force actuator. Our force actuators are design in such a way that they can be calibrated with direct traceability to International Standards thus providing a simple means of calibrating the instrument.
Current research activity is aimed at reducing drift and noise and linearising the thrust stand frequency response, through a combination of design optimisation and data processing.
- Hughes, E B, Oldfield, S 'Thrust balance for ground testing of electric thrusters.' 4th EUSPEN International Conference, Glasgow, UK, May - June 2004, pp 432-433.
- Nicolini, D, Chesta, E, Gonzalez del Amo, J, Saccoccia, G, Hughes, E. B, Oldfield, S, 27th International Electric Propulsion Conference, 2001.
- Hughes, E.B, Oldfield, S, Traceable, High-accuracy thrust measurement for electric propulsion, 28th International Electric Propulsion Conference, Toulouse, 2003.
- Hughes, E. B and Oldfield, S, 'Traceable calibration of the 3-axis thrust vector in the milli-Newton range.' 4th International Space Propulsion Conference, Sardinia, 2004.