In the context of hypersonic propulsion for civil purpose and more sustainable aeronautic industry, the air-turbo-rocket expander engine (ATR-EXP) has been studied through a lumped model. To fulfil the trajectory mission of EC-funded MORE&LESS project, based on previous works, three throttle variables has been identified. A sensitivity analysis has been conducted proving which and how the actuators and combinations of them can satisfy the control requests. The turbomachinery off design behaviour have been considered in simulating the ascendent trajectory. A control strategy has been identified compliant within a subspace of throttle variables. The system stability has been proven in certain steady state conditions.