Characterization of novel ceramic composites for rocket nozzles in high-temperature harsh environments

This paper presents the results of experimental tests for the characterization of Ultra-High-Temperature Ceramic Matrix Composite (UHTCMC) materials for near-zero erosion rocket nozzles. Two dedicated test set-ups were developed for preliminary screening of material candidates in a representative environment, characterized by relevant heat flux and temperature. The experimental set-up was based on a lab-scale 200N-class hybrid rocket engine, employing gaseous oxygen as the oxidizer and High-Density PolyEthylene as fuel; the configurations included free-jet test, in which small button-like samples were exposed to the supersonic exhaust jet of the rocket nozzle; and chamber inserts, in the shape and size of an annular element, placed inside the rocket combustion chamber. Computational Fluid Dynamic simulations, for modeling heat transfer and combustion chemical reactions, complemented the experimental observations and supported the characterization of test conditions. Samples with ZrB2-SiC matrix and continuous or chopped carbon fibers, sintered by either Hot Pressing or Spark Plasma Sintering were tested. Free-jet test samples demonstrated a substantially improved erosion resistance with respect to conventional graphite and in one case a negligible material recession. UHTCMC samples erosion was associated to the occurrence of a rapid rise in surface temperature, which achieved values over 2900 K. Chamber inserts, besides confirming the outstanding erosion resistance of UHTCMCs with respect to traditional materials (i.e. C/SiC), proved that long-fibers samples with sufficient porosity are more likely to withstand thermal shocks typical of the rocket combustion environment