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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?

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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.

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    $\begingroup$ Is the OP wanting to include left-over propellants in the 2nd stage itself? Then again, is that reaching much zero the definition of max payload to LEO to begin with? Would it be more if there was no need to recover the first stage? $\endgroup$ – uhoh Jan 31 '17 at 13:11
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    $\begingroup$ You'll need the leftovers to deorbit the stage after use. $\endgroup$ – Hobbes Jan 31 '17 at 13:21
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    $\begingroup$ Some of it, but not necessarily all leftovers, no matter how much there are. $\endgroup$ – uhoh Jan 31 '17 at 13:25
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    $\begingroup$ I believe the optimal strategy would be a second stage with extended nose-cone tank. That way you avoid all the overhead of staging, fairings etc. All the fuel would be "left-overs". And yes, non-recoverable stage 1 would increase that amount. $\endgroup$ – SF. Jan 31 '17 at 21:31
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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.

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