Performance scaling of a novel Modular Hybrid Propulsion System for aircraft flight performance flexibility and enhancement

Abstract

A novel propulsion system concept, the Modular Hybrid Propulsion System (MHPS) is introduced that has the potential to enable a step-change in aircraft performance and flexibility over traditional carbon-fuel propulsion systems. The MHPS describes a hybrid propulsion system where the electrical components are interchangeable on a mission-to-mission basis. Scale-agnostic performance relationships are developed for the MHPS concept for a conventional takeoff and landing (CTOL) aircraft. Takeoff, climb, cruise, and payload performance metrics are formulated in terms of hybridity (proportion of electric power output and energy storage) and modularity levels (proportion of total aircraft mass that is interchangeable). For takeoff, increasing the power hybridization value decreases takeoff distance when the electric motor (EM) specific power is greater than the carbon-fuel (CF) engine specific power for a parallel hybrid configuration. For a ratio of EM specific power to CF specific power of 3, increasing the power hybridization from 0 to 1 decreases takeoff distance by 73%. Cruise scaling indicates that the relationship between energy hybridization and cruise time is dependent on the battery to fuel specific energy ratio and EM to CF engine efficiency ratio. Specifically, the product of the two ratios controls whether or not increasing energy hybridization will lead to an increase in cruise time. When the product of ratios is less than 1, increases in energy hybridization decrease cruise time. If the product of ratios is greater than 1, behavior is dependent on battery and fuel mass proportion, there is either an optimum energy hybridization value for cruise time or increases in energy hybridization increase cruise time monotonically. The delineation between the two trends is dependent on the relative amount of energy storage (battery + fuel) mass on the aircraft. Specifically, the usage of carbon-fuel reduces aircraft mass in-flight which is an added benefit when maximizing cruise time. This results in carbon-fuel usage being more beneficial than battery usage even when the product of the battery to CF specific energy ratio and EM to CF efficiency is equal to 1. It is found that the greater the potential fuel mass, the larger the product.