I wonder how useful SpaceX' second stage would be as a propulsion stage docking in LEO or so with separately launched payload. It seems to carry 92 tons of propellant on the launch pad. How much could it retain after reaching LEO or some other specific trajectory of common interest, on a Falcon 9 and a Falcon Heavy respectively? Is there any good reason not to use the second stage itself as an in-space booster?
Should be pretty close to the payload to LEO: 22.8 tons. Maybe a bit more if you design a new second stage with larger tanks and a nose cone.
Not good to use 3/4 empty 2nd stage as a booster. But maybe FH will be able to deliver almost full 2nd stage to LEO and recover all thee cores if used without payload I hope.