# Flexible Three-Dimensional Stress Sensor for Embedded Monitoring of Solid Rocket Propellant

## TL;DR

A flexible 3D stress sensor is developed for monitoring solid rocket propellant stress during its lifecycle.

## Contribution

A novel flexible three-dimensional stress sensor with liquid metal elements for embedded propellant monitoring is introduced.

## Key findings

- The sensor prototype achieved a sensitivity coefficient of 1.5%/kPa at 300 kPa pressure.
- The sensor detected multiaxial stress differences when embedded in simulated propellant.
- The sensor showed a stability error of 0.17% and a maximum hysteresis error of 3.98%.

## Abstract

Solid rocket motors (SRMs) play a pivotal role in space exploration owing to their reliability and high thrust-to-weight ratios. SRM propellant health monitoring is in critical demand owing to the complex operational scenarios throughout the entire life cycle of SRMs. To achieve in situ detection of three-dimensional stress, this study introduces a novel flexible three-dimensional stress sensor (FSS). First, a liquid metal pressure-sensing element with a variable cross-section was designed and numerically modeled. The performance of the FSS under different loading conditions was analyzed using finite element modeling. The sensing element prototype was fabricated using mold casting and liquid metal injection methods. The fabricated sensing-element prototype with an area ratio of 1:5 exhibited a sensitivity coefficient of 1.5%/kPa at a pressure of 300 kPa, a maximum hysteresis error of 3.98%, and a stability error of 0.17%. Finally, the FSS was developed by integrating multiple pressure-sensing elements and encapsulating the force-concentrating layers. The fabricated FSS prototype was characterized using simulated propellant experiments. Via comparison with the simulation results, the FSS was found to detect multiaxial stress differences when embedded within a propellant.