While the N+4 study is complete and the technology road maps have been delivered to NASA, Boeing is continuing work under an N+3 Phase 2 contract. This is focusing on further modeling and wind-tunnel investigation of the truss-braced wing and analysis of hybrid-electric propulsion for the Sugar Volt battery/turbine N+3 concept.

The Sugar Volt concept combines the strut-braced wing with ultra-high-bypass turbofans that can run on jet fuel and/or batteries. The removable batteries were mounted in a fairing under the fuselage, but have been moved to mid-span wing pods on the latest design. Short missions are flown mostly on batteries, and longer missions on jet fuel, giving flexibility to balance fuel versus energy use—20,900 lb. of batteries are needed for the baseline 900-nm mission, but fuel burn is reduced 63%.

Under Phase 2, the team is evaluating alternative hybrid-electric architectures. GE is updating its engine performance calculations, Georgia Tech is developing a hybrid-electric model for NASA's propulsion simulation software suite, and Boeing is updating its preferred aircraft concept. “As in Phase 1, hybrid electric continues to offer significant benefits if large battery improvements occur,” says Bradley.