The wave rotor combustor can significantly improve gas turbine engine performance by implementing constant-volume combustion. The periodically open and closed combustor complicates thermodynamic analysis. Key cycle parameters depend on complex gas dynamics. In this study, a consistent air-standard aerothermodynamic model with variable specific heat is established. An algebraic model of the dominant gas dynamics estimates fill fraction and internal wave compression for typical port designs, using a relevant flow Mach number to represent wave amplitudes. Nonlinear equations for thermodynamic state variables are solved numerically by Newton-Raphson iteration. Performance measures and key operating conditions are predicted, and a quasi-one-dimensional computational model is used to evaluate the usefulness of the algebraic model.