Magnetohydrodynamic drag force measurements in an expansion tunnel using a stress wave force balance

Magnetohydrodynamic (MHD) aerobraking has the potential to signifcantly reduce peak heat loads for high-speed planetary entry, due to increases in total drag force in the early phase of the trajectory. Establishing accurate techniques for measuring MHD drag force in ground testing facilities is a necessary frst step for investigating this phenomenon. The objective of this paper is to demonstrate the methodology required for using a stress wave force balance (SWFB) to measure MHD drag force in an expansion tunnel. Whilst the SWFB is an established technique for measurement of short duration forces in impulse wind tunnel facilities, conventional designs exhibit unusually low signal-to-noise ratios in MHD ground testing fow felds, due to electromagnetic interference from the plasma in the shock layer. This paper investigates the efect that sting design, amplifer setup, and strain gauge location have on the signal-to-noise ratio of a SWFB. Experimental validation of each design was undertaken in the X2 expansion tunnel with a high enthalpy (∼ 16 MJ kg−1 ) argon test fow by comparing the results to those obtained using an accelerometer-based force balance. The results demonstrate that maximum signal-to-noise ratio is obtained when: the sting is made from a low stifness material such as polycarbonate to maximise strain; the charge amplifer is located near the strain gauge inside the model and is grounded at the data acquisition system to minimise electrical noise; the strain gauge is located close to the applied load to delay the return of refected stress waves from the free end. MHD drag forces measured in this study varied up to approximately 3 N, and measurement uncertainty was found to be approximately ±0.2 N. Overall, successful adaptation of the stress wave force balance technique to MHD ground testing was achieved, and it has been shown to be a viable alternative to accelerometer-based techniques