The chopsticks actually don't catch the booster by the grid fins, there are little struts that stick out from the rocket that don't stick out nearly as far.
I thought the same thing before the first catch, if you go look at the catch footage you can see the booster resting those on the chopsticks.
One of the things that came up in one of the livestreams was that some of the changes to the starship heat shielding were to test a couple of different spots for those struts - because the booster doesn't do orbital-speed reentry, starship itself does, so to catch that, you need to avoid burning off the struts...
> So the struts (plus supporting structure) are lighter than the legs? Why is that?
Besides the other answers you've received, the lugs hold the booster from (near) the top. This means that the body of the booster is in tension during and after landing. Legs, on the other hand, support the landing load and weight after loading in compression. The booster is basically a thin-shelled tube, which is limited in compression strength (for a given wall thickness) by buckling; in tension, the strength approaches the strength of the material, so less additional reinforcement is needed in the structure to support landing loads.
The booster is already strong enough to support itself in compression, because that's what it does during ascent and the landing burn. The entire bottom structure of a rocket (the "octaweb" for F9) is basically made to transfer the thrust compression loads of the engines into the tanks.
Pressurized with what? They've already used their fuel for the landing. They can't put anything else in the tanks without worrying about contamination for the next flight.
They autogenously pressurize the tanks - they heat up the cryogenic propellants with the engines and use some of the gas to pressurize the tanks. In Starship’s case it’s methane and oxygen.
Pressurization gases? The fuel goes from tanks to engines (engine pumps) because tanks are under pressure, right? Even if the liquids are spent - they are rarely spent in full - the gases remain.
Which doesn't show the design constraints but who wants those - edit and it's not an image of the booster? Elon mentions a design feature missing from the diagram: https://x.com/elonmusk/status/1093643894917492736
I would personally guess you'd need to be very careful with your implied load bearing connections between the tanks at x Kelvin and the skin at redhot reentry temperatures...
Good luck on buying spaceY.com and competing against those engineering fools at SpaceX ;)
I am mocking unreasonably, and I know I would find similar comments in my own internet history. I am hoping you will learn to be a little less thoughtless in your armchair. We all assume other rocket-science engineers must not know what they are doing but usually that just shows our own ignorance.
Let me explain once more :) . The original post which I was replying to was
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> So the struts (plus supporting structure) are lighter than the legs? Why is that?
Besides the other answers you've received, the lugs hold the booster from (near) the top. This means that the body of the booster is in tension during and after landing. Legs, on the other hand, support the landing load and weight after loading in compression. The booster is basically a thin-shelled tube, which is limited in compression strength (for a given wall thickness) by buckling; in tension, the strength approaches the strength of the material, so less additional reinforcement is needed in the structure to support landing loads.
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Note how the author says that a thin-shelled tube is limited in compression strength by buckling. Technically it's correct, but practically if you put some extra pressure in that tube - which, after all, has also airtight caps on both ends - then the tube becomes much stronger, and is able to withstand reasonable forces during landing.
That's what I noted, and I can repeat that. I am quite sure SpaceX engineers considered that possibility, and I think they rejected that because they felt they see an even better result. I'm trying to see that here.
I also suspect that you don't know my qualifications in the area, and referring to armchair ones just so. It's interesting how many different and widely qualified people participate in HN discussions.
You know, that's completely unimportant. The important parts are that 1) Starship stage is under pressure when landing and 2) pressurization makes a thin-walled metal cylinder much stronger resisting buckling. Details of how Starship works and how pressurization is historically used to increase strength are just to support these two points. But if you already have these two points, you should admit that the argument "Starship can't land on legs because there's too big of a risk of buckling" has some counterarguments. And the overall decision isn't as clear as we'd like to have it.
The same, its just a much higher proportion of gas rather then liquid. Basically on the pad its mostly full with liquid, as it launches, it pumps back part of the gas created in the engine back into the tank. That called 'Autogenous pressurization'. So they don't need an extra gas like helium, as for example Falcon 9 needs.
The big issue during landing is that you need to make sure that the engine doesn't suck in gas. That causes bubbles and can destroy the engine. This was actually the failure that caused some of the earlier SN flights to explode or not produce enough power from the engine.
You need to either have header tanks, like the booster. Or some kind of method to push the liquids into the right place.
If you want to deep dive into the whole problem, 'CSI Starbase' on youtube has a brilliant series on all the engineering problems with all of this. Its a very complex problem.
Ok, so I was right: Does this end up being more efficient because more of the loading is in tension instead of compression?
This bugged me because everyone was saying the deletion of legs was key, but to me the struts are basically legs mounted up high. It's taking advantage of tensile loading that promotes the weight reduction.
the other big difference is that legs need to extend below the engine which means they need up move, which makes them much bigger and more complicated than the catch pins
The legs would need to be much longer (because you can’t push the engine nozzles all the way into the ground and still hope for good things.) Longer structure means more mass, but also larger torque which need to be handled with the support of the structure.
Legs need to move to deploy. The struts are just there, static things are much simpler. Simpler things weigh less.
Legs need to contain shock absorbers. With the struct solution the shock absorber is in the chopsticks. It doesn’t matter how much the shock absorber weighs when you don’t need to carry it up with you.
It would be good to see the numeric analysis of variants here. Legged landings are surely possible - say, with longer legs (twice as long as struts?), possibly static (legs are always deployed, even when launching), with shock absorbers which aren't that heavy... Would be good to see good and bad qualities next to each other.
The pins/struts are a 2 point system that double as the booster lift points in general operations. The booster mostly hangs in tension which the existing tank structure can support. I would guess they share some of the structure beefiness with the grid fins.
Legs require at least 4 points, probably more. Shock absorption hardware, ability to unfurl to an acceptable width. Require reinforcement (cross bracing) near the base of the tanks to handle the loads pushing inwards toward the center of the tanks.
You can technically imagine two legs with really wide feet, allowing some perpendicular stability. I wonder if one-leg lander could be imagined. 3-legged landing scheme was used in Surveyors, first American automatic Moon landers, and was surely considered for Appolo LEMs, but rejected. So there could be additional, secondary reasons when choosing the number of legs.
4 legs have no additional redundancy over 3. One leg failing will still result in the booster tipping over. They do push the maximum angle of tipping before your CG is no longer supported out farther though.
I thought the same thing before the first catch, if you go look at the catch footage you can see the booster resting those on the chopsticks.