Auxiliary Thrust Jet Engines: Design, Integration, and Performance Analysis

Fixed-wing surveillance aircraft built around electric or small internal-combustion propulsion offer good endurance but are speed-limited, which restricts their ability to reposition rapidly over long standoff distances or evade emerging threats. This paper presents the design, integration methodology, and performance analysis of an auxiliary thrust micro-turbojet engine intended to supplement the primary propulsion system of a long-range, high-speed intelligence, surveillance, and reconnaissance (ISR) aircraft. The proposed architecture retains an efficient primary propulsion system for extended loiter and cruise, while a belly-mounted auxiliary turbojet is engaged during transit, dash, and evasive-maneuver phases to substantially increase true airspeed and reduce time-to-station. A thrust-drag-range simulation model, adapted from the Breguet range equation and a simplified thermodynamic thrust-lapse model, is used to evaluate the performance envelope across altitude, Mach number, and auxiliary-engine duty cycle. Under the first-order model's simplifying assumptions, engaging the auxiliary jet for 20–40% of a representative mission profile is predicted to extend effective mission range by approximately 28–46% relative to an electric-only baseline, at the cost of increased fuel-mass fraction and thermal signature during the dash phase; these figures have not yet been validated against bench or flight-test data. The paper further discusses structural mounting considerations, fuel-system sharing strategies, control-law implications for a dual-propulsion digital electronic control unit (DECU), and the principal design trade-offs of the hybrid configuration. The findings support auxiliary jet augmentation as a viable path to combining long dwell time with high transit speed in medium-class surveillance aircraft platforms.