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Won't it be easier and save a lot of money to just catch it or land it in a basket?

So rocket coming down can open arm longer than the diameter of the high basket where it goes down through, and gets stoped by the arms resting on the basket, then the engine can be safely turned off.

Why don't they just do that?

Landing it on the ground and trying to make it stand proven very difficult and cost them a lot of money.

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  • $\begingroup$ Upon a request from another member, I am reopening the question. How can this be unmarked as duplicate? $\endgroup$ – Mocas Mar 9 at 18:23
  • $\begingroup$ This question has the rocket with arms sticking out, catching on the top opening. It ends up hanging from the top, not laying in a net. I think this is an actual proposed/suggested technique, but with arms on the ground catching the rockets arms (i.e. grid fins).I think this question can have its own answer so shouldn't have been so quickly closed. voting to reopen! twitter.com/elonmusk/status/1344327757916868608 $\endgroup$ – uhoh Mar 9 at 23:13
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    $\begingroup$ Vote to be reopened $\endgroup$ – Mocas Mar 9 at 23:41
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    $\begingroup$ Note that this is the (current) plan for the SuperHeavy booster, except the tower will catch it by the extended grid fins. As for why they didn't do it with F9, they may not have had the confidence in their guidance software to put it down that precisely. Also, they knew they were going to have downrange landings, so they had to have a freestanding landing mode anyway. $\endgroup$ – John Bode Mar 11 at 16:46
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They plan to do something very similar with the SuperHeavy - a booster; catch it using two movable arms on the launch tower, hanging it by the grid fins.

The Spaceship is meant to be capable of landing on the Moon and on Mars. In the classic chicken-and-egg problem style, there are no means to deliver or build such heavyweight landing infrastructure there without establishing a decent base first - and what else can be used to establish it, than the Starship?

Since it must be able to land "in the rough" there, if it's unable to do the same on Earth, it just falls short of its fundamental requirements. Using fancy ground infrastructure to assure it can land on Earth won't resolve the fundamental problem of it being able to land on Mars, and once it has the capacity to land on Mars, it will land on Earth without the fancy tower just fine.

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  • $\begingroup$ "there are no means to deliver or build such heavyweight landing infrastructure there without establishing a decent base first". Why? First people to arrive on Mars can land with all of their equipments and all the necessary materials needed to build this infrastructure using parachutes. They build this infrastructure and then rockets can land easily and cheaply on Mars. It would be cheaper to deliver things to Mars by a rocket going there throwing them to land by parachutes and come back to Earth than landing on Mars and take off again. What is different about delivering by rocket landing? $\endgroup$ – Mocas Mar 13 at 17:15
  • $\begingroup$ @Mocas "First people on Mars can land..." how? In what spacecraft? Only Starship is currently a viable option for human trip to Mars. $\endgroup$ – SF. Mar 13 at 19:53
  • $\begingroup$ As mentioned in the comment, by an aircraft that goes down parachuting. Just like Perseverance, just bigger one. $\endgroup$ – Mocas Mar 13 at 21:57
  • $\begingroup$ @Mocas You're neglecting the fact the spacecraft needs to sustain the crew for about a year of space travel. Parachute barely slows it down to subsonic speeds in 0.006bar and that's just a small part of the braking - most is done through heatshield, and the ending - by rocket engines. There are currently not even plans of a spacecraft (other than Starship) capable of making this trip. So you'd need to design an entire new Mars-capable spacecraft practically from scratch just to enable Starship landing. Building a base is not a walk in the park. Before this is done Starship would be obsolete. $\endgroup$ – SF. Mar 14 at 11:14
  • $\begingroup$ Also the spacecraft would not be reusable, and delivering all the materials would take many flights. The economy of recoverable spacecraft goes out through the window. $\endgroup$ – SF. Mar 14 at 11:15
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Won’t it be easier and save a lot of money to just catch it or land it in a basket?

Scaling size, volume and mass can make huge differences to engineering calculations. Consider how easy and cheap it would be to catch a toy plastic rocket thrown towards a large net. No real problem. Now consider catching a grand piano thrown off the top of a sky scrapper. It should be possible, but it’s not going to be that easy or cheap, huge net huge supports etc and plenty of calculations.

Now imagine something more than two orders of magnitude bigger than that, that’s taller than Sydney harbor bridge and weighs in at 100+ tonnes. Easy and cheap are no longer appropriate words.

But might it be easier and or cheaper than the alternatives? It might be. But the only way to find out is to do a detailed engineering assessment. The answer that was obvious at small scale is no longer obvious at large scale.

There are many complicating factors for example:

• How safe is it compared to the alternatives? What would the FAA say and what is the risk to the SpaceX launch pad / infrastructure? (Consider the piano in the above example what would the police department and skyscraper owner have to say and how would that complicate the matter?).

• Every second the rocket engine is burning it eats hundreds of kg of propellants. How quickly can that rocket engine be turned off compared to alternative means of landing? And that is no easy calculation.

• The rocket being caught is also easily damaged being made of steel that’s only a few mm thick, how accurately can it be landed? How thick must the supports be? What mass of reinforcement is required in the area that takes the strain?

• The rocket is also landing over a huge plume of fiery rocket exhaust. How fire resistant is the catching device?

Part of the reason why it has proven so difficult to land Starship is that they need to turn the rocket from horizontal to vertical very quickly. They would still need to do that even if they were to catch it in a basket.

In summary landing in a basket will also be very difficult and cost them a lot of money. They have certainly considered it and it might even be a good idea in some circumstances (perhaps For tankers and low Earth orbit cargo Starships), but the calculations are not simple or clear cut.

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  • $\begingroup$ Sorry, but your example doesn't quite match the case here, the piano is not capable of reducing its falling speed to zero m/s. $\endgroup$ – Mocas Mar 12 at 20:48
  • $\begingroup$ Still I think you raise several important points that directly address the core of the OP's question, +1 $\endgroup$ – uhoh Mar 12 at 23:50
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Update!

The Super Heavy launch process will still involve use of its engines to control the velocity of its descent, but it will involve using the grid fins that are included on its main body to help control its orientation during flight to “catch” the booster — essentially hooking it using the launch tower arm before it touches the ground at all. The main benefit of this method, which will obviously involve a lot of precision maneuvering, is that it means SpaceX can save both cost and weight by omitting landing legs from the Super Heavy design altogether.

Another potential benefit raised by Musk is that it could allow SpaceX to essentially recycle the Super Heavy booster immediately back on the launch mount it returns to — possibly enabling it to be ready to fly again with a new payload and upper stage (consisting of Starship, the other spacecraft SpaceX is currently developing and testing) in “under an hour.”

Notably, Super Heavy BN1 isn’t fully representative of the boosters that will support Starship’s first orbital launch attempts. For unknown reasons, SpaceX appears to have forgone the installation of any kind of landing legs on the first pathfinder and prototype. CEO Elon Musk has expressed a desire to avoid the need for legs entirely by catching Super Heavy boosters (and possibly even Starships) with a tower outfitted with giant arms, but it’s virtually impossible to imagine that such a wholly unproven recovery mechanism will be ready for full-scale testing – let alone operational use – later this year.


31-Dec-2020 Elon Musk tweet:

We’re going to try to catch the Super Heavy Booster with the launch tower arm, using the grid fins to take the load.

While any Musk tweet must be taken with some degree of skepticism, even those tweets that don't include references to 420, this references both a kind of small target (the tower arm) which is distinct and very different from catching it in a net, and identifies the things sticking out of the sides of the rocket (grid fins) which must be strong enough for the rocket to hang on, both as shown in the OP's drawing.

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