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There were multiple mentions of the future ideal first stage ASDS landings - if the mission won't have enough deltaV to do a RTLS, then just land on the barge, refuel and "hop" back to the launch site.

How much fuel is needed for such a hop and how can it be done? Would it be most effective to go "back" to space and endure a "full" reentry and landing again? Or might it be possible to return through atmospheric-only flight (I am thinking about multiple engine ignitions maybe)?

Would it make any sense to fill the first stage full of fuel to work as ballast and allow hover or would it burn most of it anyway to get to the destination (rocket equation is such a tyrant)?

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  • $\begingroup$ Ballistic trajectory is a no-brainer because "hover" would be way too costly in terms of fuel lost to gravity drag. Now what kind of ballistic trajectory, that depends on the distance to cover; almost certainly leaving the atmosphere would be unnecessary. The fuel needed would be matched to that need - I'm practically sure you can't land Falcon 9 while it's mostly full - too much landing mass, the needed braking power of the engines would be destructive to the rocket. $\endgroup$
    – SF.
    Commented Jan 16, 2016 at 15:45
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    $\begingroup$ How would the braking power of the engines be destructive to the rocket? It would be exactly the same power used on ascent. $\endgroup$
    – Russell Borogove
    Commented Jan 16, 2016 at 16:49
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    $\begingroup$ I'd guess it would be close to a mirror of the trajectory that got it there in the first place: powered ascent to apogee on the order of 140km, out of the atmosphere. $\endgroup$
    – Russell Borogove
    Commented Jan 16, 2016 at 17:03
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    $\begingroup$ Is this really more cost effective than recovery? $\endgroup$
    – Brian Lynch
    Commented Jan 16, 2016 at 18:08
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    $\begingroup$ I don't see how it can be cost-effective; the only way flyback-from-barge would make sense is if flight schedules got really hectic and you didn't want to take a stage and a barge out of service for days at a time. $\endgroup$
    – Russell Borogove
    Commented Jan 16, 2016 at 18:26

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It's likely that the most efficient flight path is the mirror of the one that got the 1st stage to the barge in the first place - a powered ascent phase to high altitude (a little ways out of the atmosphere) followed by a ballistic coast phase, hypersonic reentry, and powered descent and landing.

I did some number crunching, making a lot of unwarranted assumptions, and came up with a figure of about 200 tons of fuel for the return flight -- half the tankage of the 1st stage.

The return trajectory might be shaped somewhat differently from the ascent, and drag effects would be very different (even if SpaceX puts a small conical fairing on the stage for the return flight), so I could be off by a lot, but I think it has to be something like half the tank.

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  • $\begingroup$ Thats a bit more than I expected, my unsubstantiated estimate would be ~30%. I suppose any "atmospheric" trajectory does not offer any benefits without a lifting body. Thank you. $\endgroup$
    – jkavalik
    Commented Jan 16, 2016 at 18:11
  • $\begingroup$ Yeah, it was more than I expected as well. $\endgroup$
    – Russell Borogove
    Commented Jan 16, 2016 at 18:15
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It seems to me very unlikely that this would actually occur. One would have to get the fuel out to the barge, fill the rocket, monitor the mission to return, and run the risk of losing the rocket, etc. The cost to have the barge return is minimal in comparison, and in fact, the cost is probably about the same regardless of if there is a booster on top or not.

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  • $\begingroup$ I selected the other answer as it tackles the question with some numbers (I understand it's mostly a guesstimate). But I agree with your assessment of the general situation. Thank you. $\endgroup$
    – jkavalik
    Commented Jan 25, 2016 at 7:00
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Hop back to the launch site? What???? They either land it on the barge or at the launch site. If they land it on the barge they just sail the barge back!!

The landing is hard enough without them having to do it TWICE per flight. The barge is of course movable so flying the rocket back would be ridiculous!

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