Reports say that SpaceX now aims to recover the Super heavy booster by catching it with the help of the launch tower. What are the engineering challenges for this and how will it benefit ?
This was simply a throwaway tweet by Elon Musk in response to a fan rendering of a Super Heavy landing.
Here is literally everything we know:
This is the sum total of all publicly available information. Note that all three of those tweets are also linked in the article, so the article already contains all known information.
The original plan was to land Super Heavy directly back on the launch mount. The plan until that tweet 14.5 hours ago was to land Super Heavy directly next to the launch mount, then lift it onto the launch mount with a crane mounted on the top of the launch tower.
SpaceX is iterating fast, and an idea thrown out yesterday by Elon Musk might already be obsolete today.
Compared to the original plan, the major advantage is that you can rotate the launch tower before catching the booster so that the exhaust does not hit the launch mount directly, then move the booster over to the launch mount. The disadvantage is then, that this adds an additional step of lowering the booster onto the launch mount.
Compared to the interim plan, the major advantage is that you don't need landing legs, nor any of the associated mechanisms for lowering, retracting, etc.
The major challenge is to catch a 24-storey building plummeting to the ground in mid-air!
Even though Super Heavy will be able to hover, unlike Falcon 9, doing that doesn't make sense: while you are hovering, you are constantly fighting 1g of gravity, and you're not going anywhere, so you are wasting propellant. All the propellant you are wasting while hovering, you need to bring with you during launch. That extra propellant mass needs even more propellant to push it up through the atmosphere and against gravity, which needs more propellant, which … well, we all know how the Tyranny of the Tsiolkovsky Rocket Equation works.
So, you really want to do a hover slam / suicide burn maneuver like Falcon 9 does, except instead of canceling your velocity right at the time when your altitude above ground level is 0, you want to cancel your velocity right at the time when your grid fins touch the catching mechanism.
This is not impossible, SpaceX is demonstrating precision suicide burns on almost every launch. Also, they need to have a crane that is capable of lifting the Super Heavy anyway, so all that is needed is for the crane to be able to absorb the additional dynamic loads of a Super Heavy booster landing. And they already have experience with catching stuff from the sky.
The control will probably need to be more precise than what they currently do with Falcon 9, but not as precise as the original plan of landing directly back on the launch mount.