In crafts such as the LM and even the Starship, I’ve heard the walls to be ‘paper thick aluminium’ which I feel is an extreme oversimplification. However I am aware that the walls are in-fact, pretty thin. My question is, why aren’t the walls just thicker as these crafts have to sustain external damage and pressure? and how are these thin walls held together?
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4$\begingroup$ For the LM, going off thicknesses in this question and answer, “paper thick” isn’t far off. More like ~3 pieces of paper, but a pretty good analogy. $\endgroup$– fyrepenguinCommented yesterday
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$\begingroup$ There are concrete ships but I have never seen one in orbit. $\endgroup$– Peter - Reinstate MonicaCommented 18 hours ago
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1$\begingroup$ They don't have to withstand external pressure- they need to withstand internal pressure. That is a lot easier. Think of a 2 liter pop bottle which can withstand more than 100 psi internal pressure, but you can buckle the walls with your pinkie finger. And, of course, lighter is better, all other things being equal. $\endgroup$– Spehro 'speff' PefhanyCommented 8 hours ago
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3 Answers
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why aren’t the walls just thicker as these crafts have to sustain external damage and pressure?
Weight/mass. The rocket equation tells us that the fuel fraction of a rocket determines its performance, and when extreme performance is needed, the fuel fraction must get very close to 100%. A kilogram of unneeded structure costs a kilogram of payload.
If the spacecraft isn't falling apart, then the walls are evidently thick enough. Why add weight by making them thicker?
how are these thin walls held together?
Construction differs from one rocket or spacecraft to another, but the individual structural components are usually welded together.
answered yesterday
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For the kind of damage most likely in space (from micrometeoroid impacts), making the hull thicker does not help: those micrometeoroids arrive at speeds much faster than a bullet (up to 20 km/s), and they will punch through metal plates much thicker than is used on spacecraft.
Making the hull thick enough to prevent those from punching through, would make it too heavy to launch.
To defend against these, it's far more mass-efficient to use a Whipple shield: a thin plate in front of the hull, and separated from it by a few cm.
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The pressure inside a beer can at room temperature is 4.5 atmospheres. The pressure inside the ISS is 1 atmosphere. Of course the ISS is larger diameter and wall tension is proportional to diameter, but crush an empty beer can and you get the idea.
Anyone can design a spacecraft hull that won't burst. But it takes an aerospace engineer to design a spacecraft hull that just barely doesn't burst.
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12$\begingroup$ “An engineer has achieved perfection not when there is nothing left to add, but nothing left to take away” - Antoine de Saint-Exupery $\endgroup$ Commented yesterday
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3$\begingroup$ The beer can analogy became apparent when, in 2008, the last available Falcon 1 rocket whose start would make or break SpaceX started to crumble under increasing air pressure when the cargo plane descended faster than pressure equalization could take place. Vividly described in Eric Berger's Liftoff. (They un-dented it, decided it was still spaceworthy, and the rest is history.) $\endgroup$ Commented 4 hours ago