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Recently, after IFT 3 of Starship Super Heavy, I went to ponder: Can SpaceX land their Super Heavy boosters on Autonomous Spaceport Droneships to get extra performance. Currently, it separates with about 10% of its fuel left (Shown on the fuel bar in SpaceX IFT 3 flight video at around T+ 2:44). It then does a boostback burn and lands. But what if it didn't do a boostback but instead continued to coast down and do a landing burn and land on an autonomous spaceport droneship that has been modified to accommodate for the lack of landing gear .

Landing on a autonomous spaceport droneship will allow it to use all of its fuel except for about 1% for a landing burn (Courtesy SpaceX IFT 3 Video at T+ 6:58 ). It will come in a lot faster though, but given it's made of stainless steel and facing engine first, it should be able to sustain the heat of reentry at around 2500m/s (SpaceX Echostar 105/SES-11 and Kerbal Space Program Realism Overhaul/RSS)

Will it work? When will SpaceX do it? Will they do it?

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The problem is that Superheavy does not have landing legs so any landing would need a catching structure similar to that at the launch site. Superheavy is very tall so would need a very tall catching structure which would itself need to be afloat and subject to the vagaries of wave motion, so it would probably need a very large barge indeed to keep it stable. Unlike relatively shallow water barges and platforms that can be anchored in, or supported from the seabed.*

Weather in the Atlantic is often rough so that could further complicate matters with delays and cancellations and the cost in moving such a vessel and maintaining it would not be insignificant. The Superheavy would also be out of use while being transported, which although it might seem reasonable to us, would not work well with Mr Musk's plans.

Or they would need to fit landing legs onto Superheavy.

But being well, superheavy it would need some substantial legs so many tons and probably tens of tons, especially when you include the extra propellant required to lift those legs in the first place and the extra propellant required to decelerate them at the end, not to mention the propellant required to launch the extra deceleration propellant.

So I don't think it's going to happen unless there are some serious issues with catching Superheavy.

One further point although Superheavy might separate with 10% propellant remaining at the moment, this is still an experimental vehicle that they are trying to develop. The goal at the moment is just to do a controlled hover over the ocean. Once they have that in hand they will cut the margin of propellants left to as small a faction as safety will allow.

*It might be possible to install a shallow water platform somewhere in the vicinity of Puerto Rico. But the trades are complex, it probably should be much further east, but at much greater expense and then there’s the question of whether it could be used for Florida launches.

A deep ocean platform somewhere in the Atlantic looks very challenging and problematic. For example Chevron’s $500m Petronius platform is situated about 130 miles (208km) southeast of New Orleans. It is located in water depths of 1754ft (535m). The rig is a compliant tower and is the largest free standing structure in the world at 2,010ft.

The compliant tower design was chosen for its ability to withstand hurricane conditions and operate in depths of 2,000ft (610m). The compliant tower design enables it to move within an envelope of 25ft sway (7.6m), and a 10ft (3m) rotation sway at the surface.

https://www.offshore-technology.com/features/featureinto-the-abyss-the-worlds-deepest-offshore-oil-rigs/

Anything in the mid Atlantic is probably going to have to be afloat rather than being attached to the ocean floor. I doubt that the sway and rotation would be any better than on a compliant tower especially at a point 71m above the surface of the platform.

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  • $\begingroup$ For legs, won't scaled up Falcon 9 legs work? $\endgroup$ Commented Apr 14 at 7:31
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    $\begingroup$ I'm sure something could be made to work, but the mass penalty is the problem. The Falcon 9 booster only weighs around 25mt empty, where as Superheavy is around 200mt. Even though Falcon 9 landing legs are made out of advanced materials they are large and fairly massive. Scaling them up 8 fold will impose a large mass penalty (6-8 legs?) which you don't want. Also the legs would require additional launch and landing propellant of their own. $\endgroup$
    – Slarty
    Commented Apr 14 at 7:57
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    $\begingroup$ @LawnHollanderLawn the square/cube law starts biting here so this is an mouse leg to elephant leg situation (more more to scale, cow to elephant). Certainly possible but start constraining the rest of the design by needing to fit somewhere and have serious structural members to attach to. $\endgroup$ Commented Apr 14 at 9:35
  • $\begingroup$ Let's say if instead of Droneships, they used a modified Oil Rig that has a cat hing tower. Will it be any better. Type as answer for me to see and citate better $\endgroup$ Commented Apr 14 at 12:51
  • $\begingroup$ The legs also need to spread further - which means longer legs. The new heavy booster is longer - and the ratio of leg distance at the bottom to height is relevant for stability. A drone ship would have to be built like a catamaran. Makes no sense. $\endgroup$
    – TomTom
    Commented yesterday
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My view is not only can SpaceX use ASDSs for the SuperHeavy, but they will.

I've partially answered this here, so I wont repeat my full answer.

The launch site for Starship is Boca Chica, which is on the Gulf of Mexico. It is usually much less rough than the Atlantic. Port of Brownsville, a deep-water port, is 30 km from Starbase production facilities. SuperHeavy is currently moved by road 4 km to its launch site. By duplication of the current 2-way Boca Chica Boulevard this could be extended to the port (without inconveniencing locals) where an ASDS would dock.

I'd expect an ASDS for the SuperHeavy would include a Mechazilla tower and "chop sticks" to catch the empty SuperHeavy, which weighs 200 tonnes. SuperHeavy can be equipped with landing gear but has not been, partly with a view to having a high relaunch rate at some time in the future. Using a tower on an ASDS would provide greater stability. The booster would likely be secured to the tower soon after landing. Falcon 9 first stages have been known to topple on the ASDS in rough seas.

Aside from the greater launch capacity and altitudes available, SpaceX may have to invest in a SuperHeavy ASDS just to refuel its lunar lander fast enough for the Artemis mission (Full document).

SpaceX, via a subsidiary, had purchased two semi-submersible drilling rigs: Phobos and Deimos. At the time Musk indicated at least one would have a Mechazilla tower installed. The advantage of semi-submersibles is they are less affected by wave loadings than a normal ship. Semi-submersibles can be moved into shallow water by removing water ballast from their pontoons. SpaceX later determined they were not suitable, but it does make clear their intention to have Mechazilla towers at sea.

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