Originally posted by 1stGenCRXer
Rotating mass is rotating mass, if you're spinning it, it costs power, simple as that.
When a regular road car at 60 mph puts off a contrail, the humidity is pretty high, since I'm in a bay area, we get humidity levels at and above 100% frequently, clouds pop up out of nowhere, and it normally rains that night 
Actually... no it's not that simple. This is an oversimplification of the problem. The amount of acceleration gained by the removal of fractions of pounds of rotating mass is insignifigant when compared to the mass of vehicle reduction between a full and empty fuel tank.
Relative Humidity can NEVER go above 100%. It is completely impossible. It's a ratio of how much water is in the air divided by the amount of water that it could have. Therefore you see that it is IMPOSSIBLE to actually contain more water than it has the potential to hold and subsequently, a relative humidity number above 100% makes no sense at all.