The Eclipse 550 has been studied and even flown on a typical SUPT profile. Very underwhelming. Climb performance was a big issue, no jumpseat, lack of sortie duration(had to stop mid profile to get gas), base model lacks some instruments, side stick is not transferable to most heavies...a lot more. Fuel savings is one thing, but if you can't execute the mission, it really doesn't matter. As for Nextant option, some of what Moose said is accurate. However, I am sceptical about the $1M cost per airframe. Also, the AF didn't place the hour limit on the airframe, that came during the initial contract.