Analysis of Energy Utilization and Losses for Jet-Propelled Vehicles

The global control volume-based energy utilization balance for an aerospace vehicle is extended to allow for the analysis of jet-propelled vehicles. The methodology is first developed for analyzing the energy utilization and entropy generation characteristics of jet engines without airframe considerations. This methodology, when combined with separate energy utilization analysis for an unpowered airframe, allows for the assessment of a powered vehicle. Wake entropy generation for a powered vehicle is shown to be the summation of the wake entropy generation associated with the propulsion system (no airframe) and the unpowered airframe. The fundamental relationship between overall entropy generation and the flight conditions required for maximum range and endurance of a powered vehicle are also derived. Example energy utilization results obtained for a modeled turbojet engine in off-design operation are provided; wake and engine component entropy generation characteristics are directly related to engine operation and flight conditions. This engine model is then integrated with a legacy (twin-engine) Northrop F-5E Tiger II airframe. The overall entropy generation temporal rate for the vehicle is minimized, as predicted by our analysis, at flight conditions corresponding to maximum endurance. For flight conditions corresponding to maximum range, the overall entropy spatial rate is minimized.