Introduction

China's sixth-generation stealth fighter jet, the J-36, has garnered significant attention due to its unconventional design, particularly its use of three engines. This report synthesizes information from online and open source resources to analyze why the J-36 employs this configuration. The analysis explores the aircraft's air intake systems, engine configuration, strategic design choices, and their implications for performance, operational flexibility, and technological advancement. The J-36's design reflects China's pragmatic approach to overcoming technological hurdles, balancing innovation with operational requirements, and positioning itself ahead of competitors like the United States' Next Generation Air Dominance (NGAD) program.

Air Intake Systems: DSI and Caret Inlets

DSI Inlet on the Fuselage Back

The J-36 incorporates a Diverterless Supersonic Inlet (DSI) on its fuselage back, a design choice that has sparked debate due to its rarity in manned fighter jets. The DSI inlet is primarily used to manage the boundary layer—a layer of turbulent airflow close to the aircraft's surface that can disrupt engine performance. Unlike traditional inlets, the DSI uses a bump to "blow away" the boundary layer, ensuring cleaner airflow into the engine. This design is structurally simple and lightweight, making it ideal for managing the complex boundary layer on the aircraft's back.

However, the DSI inlet has limitations. Research from Northwestern Polytechnical University (NPU) China indicates that the DSI inlet with a large S-shaped duct—necessary due to the engine's placement in the lower fuselage—performs optimally at around 1.53 Mach. Beyond this speed, total pressure recovery decreases, reducing engine efficiency. The S-shaped duct introduces additional challenges, as it must maintain stable airflow during high-maneuver conditions to prevent engine stalls. NPU's proposed improvements include reducing the bump height or adjusting the lip sweep angle to enhance performance and ensure stable operation during maneuvers.

The DSI's speed limitation (effective up to 2–2.2 Mach) stems from its inability to adjust to the shockwaves generated at higher supersonic speeds. This makes it less suitable for sustained high-speed flight compared to other inlet designs. Despite this, the DSI's simplicity and effectiveness at sub-2 Mach speeds make it a practical choice for the J-36's cruising regime, particularly for fuel-efficient long-range missions.