Study on the effects of combustion chamber pressure on plume flow and infrared radiation characteristics of same-class-different-type rocket engines

Inferring rocket engine type from infrared signals of exhaust plumes is challenging because the measured radiative characteristics are jointly affected by engine parameters, flight conditions and observation conditions. To support engine type identification, this study investigates the intrinsic relationship between engine parameters and plume radiative characteristics for the scenario of Same-Class-Different-Type, referring to engines using identical propellants but differing in operating or structural characteristics. A forward numerical modeling approach constrained by theoretical engine design principles is adopted, with LO_X/CH₄ engines selected as representative cases. Two Same-Class-Different-Type conditions are considered, denoted as Category 1 and Category 2. In Category 1, the nozzle exit size and exit pressure are fixed while the chamber pressure is varied. In Category 2, the nozzle geometry is fixed while the chamber pressure is varied. The results show that plume radiative intensity increases with chamber pressure in both categories, but with distinct growth behaviors. In Category 1, the growth rate gradually decreases as chamber pressure increases, whereas in Category 2, it increases progressively. A power-law relationship is identified between the integrated plume radiative intensity ratio and the chamber pressure ratio. The fitted exponent ranges from 0.142 to 0.356 for Category 1 and from 0.886 to 1.129 for Category 2, depending on spectral band and flight altitude. These results provide a quantitative basis for engine type inference under the Same-Class-Different-Type scenario. The main limitation is that the present analysis focuses on idealized engine parameter variations and intrinsic plume radiation, while the effects of complex flight trajectories and sensor specific observation conditions require further investigation.