Volatility sets high-entropy carbide service life above 3000 K, and both wall temperature and propellant chemistry set the required hafnium fraction

Bulk phase stability does not determine the service life of high entropy carbides in rocket applications; oxidation and recession of the surface oxide scale do. A screening methodology combining a regular solution stability model, equilibrium thermochemistry, and recession kinetics is applied to (Hf,Zr,Ti,Ta,Nb)C. No compositional spinodal is predicted, so selection falls to the oxide scale; every candidate oxide is molten at a throat, so volatility, not melting temperature, controls recession. Loss is bracketed between dissociative vaporization (floor) and a steam hydroxide channel (ceiling), second order in water vapor, which raises throat recession over two orders of magnitude at a hydrogen engine. The wall temperature controls recession exponentially, and a 55 flight life needs it below about 1870 K. At fixed wall temperature the dominant lever is the water vapor pressure; exchanging the steam loadings of hydrogen and kerosene engines changes recession 22 fold, against 1.4 fold for their 105 K gas temperature difference. Hafnium enrichment is worth 8.8 fold on the dissociation channel but only 1.06 fold with hydroxides. Against an adopted 100 µm/h criterion, five of seven candidates clear a leading edge and none clears a hydrogen engine throat; the hafnium and zirconium rich carbide ranks first in both.