Till Frieler

With the INGA IV thruster differential pressure values between the combustion and the vacuum chamber can be recorded and examined. From the measured values and the mass flow rate of the propellant one can calculate thrust forces, using numerical methods. However, the INGA IV thruster itself does not provide direct thrust measurement. For this reason, a thrust balance for experimental studies of the thrust forces is currently designed and developed at ZARM.

In literature one can find a variety of implementations for thrust balances with different mechanical concepts, as well as various methods of data acquisition. In many concepts, the thrust force is determined by the deformation or movement of specific components in the thrust balance. To detect these deformations or movements the literature gives several examples, ranging from optical methods using interferometers or distance sensors to electrical methods, based on the usage of strain gauges. A common challenge in developing a thrust balance is the meteorological separation of the thrust force and the gravitational forces of the engine. For the INGA IV thruster these two variables differ in several orders of magnitude, which make simultaneous detection difficult.

Based on the literature an initial concept has been developed, shown below. At one end of a swinging arm thrusters (mass up to 4 Kg) like the INGA IV arc-jet can be mounted on a platform. The measurement of thrust is done via rotational movement of the arm causing flexure members in the center axis of the arm to
twist. A special leaf spring sensor based on strain gauges detects the occurring strain on the surfaces of the components. For calibration a calibration unit is being developed and mounted on the backside of the swinging arm. With a proper calibration it is possible to detect and evaluate deformation equivalent thrust forces in the rage of 1-250 mN as electrical signals on a PC - system.