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Since SpaceX lands their rockets on drone ships (aka ASDS, barges), I'm curious what if anything would prevent rockets from being relaunched from the ship.

Obviously the ship would need infrastructure to refuel the rocket and connect a new payload / fairing.

Besides this, are there any physics problems with launching? IE:

  • Could the platform be made strong enough to survive the rocket blast?
  • Would the barge sink into the water from the lift-off thrust?
  • Would the barge be too unstable to launch off of?
  • ???

There'd be some huge advantages in terms of launch cadence and "owning" the launch range ...

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    $\begingroup$ While sea launch is possible, one wouldn't want to land on a ship full of fuel, nor would one want to take off from a flat surface without a flame trench. $\endgroup$
    – user20636
    Commented Aug 7, 2018 at 19:27
  • $\begingroup$ Great idea! Sea Launch thought so too. en.wikipedia.org/wiki/Sea_Launch Economically it didn't work out so well for them though. $\endgroup$
    – Organic Marble
    Commented Aug 7, 2018 at 19:43
  • $\begingroup$ Barge landings are needed when the first stage has a large downrange travel distance. If you relaunched from the barge you landed on, you'd need another barge another few hundred km downrange to land on, and if you wanted to relaunch from that one, you'd need another, and so on. $\endgroup$
    – Russell Borogove
    Commented Aug 7, 2018 at 21:28
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    $\begingroup$ Re "There'd be some huge advantages in terms of launch cadence and "owning" the launch range ..." Not really. This question asks us to ignore the absolutely huge infrastructure problems and focus on the more or less non-existent physics problems. The physics problems are indeed minuscule; after all, this has been done before with Sea Launch. $\endgroup$
    – David Hammen
    Commented Aug 8, 2018 at 3:57
  • $\begingroup$ @JCRM Presumably another ship would hold the fuel for refueling. Re: flame trench, is a tower infeasible for some reason? $\endgroup$
    – Dan Sandberg
    Commented Aug 8, 2018 at 20:18

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The barge would need some kind of flame trench, and you'd need some kind of rain bird system to absorb the acoustic energy reflected off the barge surface, otherwise you risk damage from the rocket's own exhaust and loudness.

You'd also definitely want to put a fairing on top of the interstage, meaning you need a crane (maybe on a second servicing vessel and not the barge itself).

Fuel would also need to be loaded from a secondary service vessel - you really wouldn't want to land on top of several hundred thousand kilos of RP-1 and LOX.

EDIT

I'm sure it would be possible to design and build a floating platform that could handle landings and launches (and that's the vision for point-to-point BFS flights), but it won't look anything like the current drone ships (would look more like a drilling platform). And we have to remember that the drone ships (along with Mr. Steven) are a hack - they only exist because the Falcon booster can't make it all the way back to shore for certain missions. In an ideal world you wouldn't be dealing with sea-based launches or landings at all; the logistics are more challenging that it's worth.

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  • $\begingroup$ Couldn't the rocket be rotated by an arm so that it was over the water itself? Yeah, I assumed other ships would be aiding for refueling and fairing replacement. $\endgroup$
    – Dan Sandberg
    Commented Aug 8, 2018 at 20:21
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    $\begingroup$ @DanSandberg - The question hand waves the real problems away, and now you're waving not just a hand but an entire arm! $\endgroup$
    – David Hammen
    Commented Aug 8, 2018 at 20:54
  • $\begingroup$ @DanSandberg, that would make things worse. Now, in addition to all the stresses of launching, you need to deal with one of the most corrosive substances in the world: hot salt spray. $\endgroup$
    – Mark
    Commented Nov 17, 2023 at 0:35
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There are no fundamental physics problems with launching from a floating platform in the sea - the comany Sea Launch has been doing it for years, though in their case it's more a converted oil drilling rig than a barge.

What makes it implausible is the huge "etc" that you missed out in "Obviously the ship would need infrastructure to refuel the rocket and connect a new payload / fairing."

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  • $\begingroup$ Is the oil platform they used rigidly connected to the sea floor or is it floating? If it's floating, then I agree there's obviously no physics problems :) $\endgroup$
    – Dan Sandberg
    Commented Aug 8, 2018 at 20:28
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    $\begingroup$ It was floating. $\endgroup$
    – Organic Marble
    Commented Aug 9, 2018 at 0:19
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The engineering issues you mention would not present any serious problems - in general anything you can build on land could also be built on top of a sufficiently large ship, and between Sea Launch and missile submarines it’s been done before.

What really makes it implausible is that on top of the high infrastructure cost, it wouldn’t provide the operational advantages you suggest. Refueling at sea is easy enough, but for a useful flight you need to either ship the payload to the rocket or the rocket to the payload, and the rocket is already on a ship.

Once reusability is effectively unlimited, it may make sense to refuel the rocket and get it back to the launch site in ten minutes rather than a week. There would need to be significant pressure on the supply of either rockets or landing ships for it to be worthwhile though - I suspect the paperwork involved isn’t quite as simple as for a ferry flight under general aviation rules.

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The first problem I see are the frangible legs. Even if Falcon 9 could retract them (not sure if it can; doubt it) they absorb part of the shock by crumpling a part of internal structure, which then needs to be replaced during the refurbishing process. So even if you could re-launch it, you couldn't re-land it again, as the internal structure of the legs is already damaged and won't absorb the shock of the second landing.

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    $\begingroup$ Previous versions of F9 had to have the legs removed and replaced, but I believe the new block 5's legs are designed to be retractable $\endgroup$
    – Jack
    Commented Aug 7, 2018 at 19:53
  • $\begingroup$ Pretty much all parts are inspected before a launch turnaround too... It would honestly be stupid not to. $\endgroup$
    – Magic Octopus Urn
    Commented Aug 7, 2018 at 20:11
  • $\begingroup$ @MagicOctopusUrn: Let's assume they reach enough confidence to re-launch without need for such inspection. Like you don't check every part of your car before you commute to work every day. $\endgroup$
    – SF.
    Commented Aug 7, 2018 at 20:31
  • $\begingroup$ My car isn't designed with a specification to hit a max Q daily AFAIK ;). Fair enough though-- that wasn't the assumption OP sounded like he made though. $\endgroup$
    – Magic Octopus Urn
    Commented Aug 7, 2018 at 20:49
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    $\begingroup$ @jack : the legs are retractable, but that doesn't mean they can be reused without swapping out the crush core. $\endgroup$
    – Hobbes
    Commented Aug 8, 2018 at 5:47
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Obviously the ship would need infrastructure to refuel the rocket and connect a new payload / fairing.

That's a start. You cannot wave those huge issues away. The physics problems you instead want us to focus on are miniscule. After all, Sea Launch has had a 89% success rate out of 36 attempted launches. That's not great, but it's not bad.

The real problems lie in the areas of engineering: Infrastructure, supply chain, and people. Those engineering issues equate to money. Reducing the cost of launching a vehicle into space has been the driving force behind SpaceX from day one. One key problem with launching from sea is that it drives up costs, massively, in terms of infrastructure, supply chain, people, ...

Another key problem: The outlined approach in the question doesn't solve the problem. The reason SpaceX uses its barges is because sometimes the launch vehicle doesn't have enough reserve fuel to return to the launch site. A separate barge would be needed for landing with a sea launch from a highly upgraded barge.

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    $\begingroup$ Yeah, I think it's a good point that the infrastructure costs that I hand-waived around would probably exceed any savings in terms of owning the range. I had assumed a separate barge for refueling and fairing replacement. Couldn't a low-V launch be done that returns the rocket to the launch site? Otherwise I agree that the leap-frogging barge problem would be a big issue. $\endgroup$
    – Dan Sandberg
    Commented Aug 8, 2018 at 20:27

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