Thrust stand for electric propulsion performance evaluation

Abstract

An electric propulsion thrust stand capable of supporting testing of thrusters having a total mass of up to 125 kg and producing thrust levels between 100 pN to 1 N has been developed and tested. The design €eatures a conventional hanging pendulum arm attached to a balance mechanism that converts horizontal deflections produced by the operating thruster into amplified vertical motion of a secondary arm. The level of amplification is changed through adjustment of the location of one of the pivot points linking the system. Response of the system depends on the relative magnitudes of the restoring moments applied by the displaced thruster mass and the twisting torsional pivots connecting the members of the balance mechanism. Displacement is measured using a non-contdct, optical linear gap displacement transducer and balance oscillatory motion is attenuated using a passive, eddy-current damper. The thrust stand employs an automated leveling and thermal control system. Pools of liquid gallium are used to deliver power to the thruster without using solid wire conenctions, which can exert undesirablc time-varying forces on the balance. These systems serve to eliminate sources of "zero-drift" that can occur as the stand thermally or mechanically shifts during the course of an experiment. An in-situ calibration rig allows for steady-state calibration before, during and after thruster operation. Thrust mcasuremcnls were carried out on a cylindrical I-Iall thruster that produces mN-level thrust. The measurements were very repeatablc, producing results that coniparc favorably with prcviously published performance data, but with considerably smaller uncertainty.l?4CS numbers:

Thrust stand for vertically oriented electric propulsion performance evaluation

“…It is a modified version of the Variable Amplitude Hanging Pendulum with Extended Range (VAHPER) thrust stand presently in use at NASA-Marshall Space Flight Center (MSFC). 4 The VTS consists of a system of linkages whereby motion of the primary member about point B causes motion of the secondary beam DEF about point E. The two members are coupled through the linkage CD, and the positions of both C and D can be adjusted. Unlike the VAHPER design, a counterbalance is attached to the primary beam at H and is needed in the VTS design to balance the thruster mass and provide a neutral hanging position.…”

“…However, there are vacuum facilities in operation that can only accommodate thrusters operating with the thrust axis oriented in the vertical direction. In the present paper we describe a vertical thrust stand (VTS) for EP thrust measurements that has been adapted from the modified hanging pendulum design discussed in work by Polzin et al 4 The commonalities and differences in the two designs are discussed and calibration data are presented to demonstrate the utility of the present design.…”

“…Recently, a variant of the hanging pendulum thrust stand, possessing a pendulum arm connecting through a series of torsional pivots to a secondary arm, was developed and tested. 4 This design maintains the stability of a hanging pendulum thrust stand, but has demonstrated increased sensitivity in performing thrust measurements on low-power Hall thrusters down to the mN-level.…”

“…Null thrust stands utilize technologies for measuring small forces, such as crystal microbalances, to obtain a direct measure of thrust. 2,3 Stands that rely on the displacement of a member are far more prevalent 4,5 and can typically be further classified as either a hanging pendulum, inverted pendulum, or torsional pendulum design.…”

“…4,11 They have been widely used to measure the performance of EP devices over a wide range of thrust-to-weight ratios; for example resistojets, electrothermal arcjets, MPD thrusters, Hall thrusters, [12][13][14] and ion engines. The higher sensitivity of the inverted pendulum configuration is obtained through a trade-off with thrust stand stability.…”