Had never seen this one before. Not only is the concept of a DC-9 in this military role exotic, this one raises the bar by using the UDF - UnDucted Fan - engine, which was a sort of hybrid between a turbofan and a turbopro. It promised up to to 30% reduction in fuel burn without decreasing in-flight cruise speeds. Seen here is an MD-81 demonstrator with the General Electric GE36 UDF at the 1988 Farnborough Air Show, shortly before the project was cancelled due to the lack of orders. 📹Phil Whall