Contrail Formation Criterion for Assessment of Alternative Propulsion Technologies

Contrail emission is the greatest non-CO 2 contribution to global climate change from aviation. This study provides a consistent methodology for comparing the contrail propensity of alternative propulsion technologies, applicable to more-electric gas turbine systems, fuel cell systems with and without external cooling, and piston engines. The method accounts for distributed propulsion and boundary-layer ingestion and for alternative fuels such as liquid hydrogen. The Schmidt–Appleman theory for contrail formation is applied rigorously without invoking the perfect gas approximation. It is found that conventional use of the perfect gas approximation, neglect of fuel mass, and neglect of the latent heat of liquid fuels result in significant errors that are easily avoided with the new method. The analysis confirms that several propulsion developments intended to reduce CO 2 emissions promote contrail formation: use of hydrogen fuel, introduction of efficient fuel cell power systems (especially low-temperature fuel cell technologies), and generation and distribution of electrical power all tend to increase condensation. Boundary-layer ingestion, however, has the opposite effect, increasing the ceiling for contrail formation by several hundred meters in the present analysis, potentially providing a practical means to reduce climate impact by decreasing both fuel consumption and contrail formation.