I was reading about the sea dragon rocket and was wondering why rockets today aren't made of cheaper materials like steel? And could you make a steel rocket today?
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1$\begingroup$ The concept behind Sea Dragon is different than the way rockets are/have been built; essentially, Sea Dragon is about being large but cheap instead of small but fancy/high-tech/efficient. No one's built one yet, but Ripple Aerospace is supposedly looking into it. $\endgroup$– Erin AnneCommented Jun 9, 2018 at 21:53
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$\begingroup$ Building a rocket of steel instead of aluminium would not be cheaper. A much larger rocket and much more fuel will be needed to compensate the extra weight. The resulting rocket will be more expensive, especially if one more stage is needed to deliver a payload into orbit. $\endgroup$– UweCommented Jun 11, 2018 at 12:23
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The "propellant fraction", or ratio between the mass of propellant a rocket carries and its total mass, is a very important factor in the rocket's performance. Lightweight materials such as aluminum alloys allow the propellant fraction to be maximized, leading to a smaller and overall cheaper rocket even though the cost of a given amount of material might be higher.
Steel is still in use in some rockets; the solid rocket boosters used on the space shuttle and planned for the SLS are steel-cased, for example. In this case, because the mass of the casings is discarded relatively early in flight, the cost benefits outweigh the mass penalty. Even some modern upper stages (where weight is critical) use steel; the Centaur's steel walls are so thin it has to be suspended from above or kept pressurized at all times, otherwise it would collapse under its own weight.
answered Jun 9, 2018 at 17:06
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$\begingroup$ TL;DR: steel is heavy, aluminium is not? $\endgroup$– VikkiCommented Jun 10, 2018 at 15:03
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$\begingroup$ Steel (at least, certain stainless steel alloys) is actually stronger for its weight at cryogenic temperatures than aluminum or composites, but it's denser, which makes it harder to build structures of typical rocket sizes that aren't hugely overweight...you'd be working with very thin sheet metal. SpaceX's Starship is big enough that these "gauge" issues are no longer a problem, and it might be worth updating this answer to mention it. $\endgroup$ Commented Jan 1, 2021 at 19:22