M. Jaramillo1 and J. H. Koo1,2
1 Dept. of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712, USA
2 KAI, LLC, Austin, TX 78739, USA
Abstract
Ablative material is required in solid rocket motors (SRMs) since the formation of a char layer provides thermal protection to key structural components during motor firings. In general, the char strength is very difficult to evaluate: to date, there is no standard to evaluate the strength of a char. There is an urgent need to identify, design, and adopt a reliable and mature approach to quantify char strength.
Elastomeric ablatives (EAs) are used as insulation material of the SRMs. Koo et al. developed a new class of EAs based on thermoplastic polyurethane elastomer nanocomposites (TPUNs). If compared to current industry standard EPDM/Kevlar®, TPUNs exhibit comparable ablation and insulative characteristics, easy fabrication, and recycleability. The evaluation of the char strength of TPUNs is of particular interest as its potential as a replacement for EPDM/Kevlar®. Due to the nature of the charred TPUNs, ordinary testing method such as Rockwell hardness is not feasible, thus necessitating the creation of a testing protocol and a sensor.
This study encompasses the improvement of a compressive testing sensor [1] and protocol for evaluating the char layer strength of EAs and also the modification of the method to accommodate for a shear testing protocol. The test procedure consists of measuring the amount of force required to crush an area of the charred sample for a specified penetration depth. For both the crushing and shear tests, the energy dissipated is quantified. The test is fully automated to ensure repeatability of each measurement and to remove the potential for human-induced error.
[1] M. Jaramillo, J.H. Koo, A.E. Edd, and M. Wells, “An Experimental Investigation of Char Strength of Polymer Nanocomposites for Propulsion Applications,” Proc. SAMPE 2011 ISTC, SAMPE, Covina, CA, Oct. 2011.