Chemical and Radiative Size-Related Eects on the Performance of Liquid Rocket Engines fed with Dierent Propellant Combinations

The effects of rocket engine size on the effective exhaust velocity, for geometrically similar nozzles, are investigated as far as finite-rate chemistry and (gas-phase) radiation are concerned. Propulsion systems burning LOX/LH and LOX/LCH4 combinations are considered. The engine performance is estimated via a CFD code based on a particularly convenient formulation. It is found that finite-rate chemistry alone leads to performance steadily increasing with engine size, whereas introducing the effects of radiation reveals the occurrence of a maximum of effective exhaust velocity as a function of nozzle throat diameter. The exact quantification of radiation effects depends anyway to some extent on the modelling assumptions, as well as on the engine operating cycle.