Reduced-Order Model for Multijet Supersonic Retropropulsion

Supersonic retropropulsion enables human-scale Mars entry, but predicting multijet aerodynamic interference forces and moments remains a challenge. Existing high-fidelity models are computationally prohibitive for trajectory analysis, while prior reduced-order models assume a single unified plume and therefore miss the topological transition between isolated and merged plumes. This work presents a topology-adaptive reduced-order model that removes this limitation, predicting heatshield forces during multijet operation at zero degrees angle of attack. The framework employs a shared-cap control volume coupled to an annular throat, using an axisymmetric method of characteristics solver to resolve individual plume boundaries. A blockage model adjusts according to whether plumes act as separate jets or merge into a single structure, resolving the ambiguity in identifying the contact surface for noninteracting nozzles, particularly in annular configurations. Comparisons against NASA wind-tunnel data for single-, three-, four-, and eight-jet configurations across a range of freestream Mach numbers and mass flow rates demonstrate that the model reproduces shielding onset and high-thrust force plateaus, reducing the mean squared error by more than 50% relative to a single-jet baseline. At approximately 1 s per prediction on one CPU, the model facilitates rapid design trade studies and characterizes the coupling between nozzle canting and control-volume topology.