>>12770370Maybe its first stage, they plan on making it reusable right? Steel is really good for that because it actually saves mass on thermal protection, but it's difficult to work with the correct thickness of steel as you go to smaller stages. When you scale rockets up everything gets easier because the relative thicknesses of the walls at a given mass fraction stay the same while the actual thickness of material increases and therefore everything gets more stiff on a human scale. Stiff, durable panels of material are easier to build things out of than paper thin sheets that you can dent with your finger or rip by hand. The Space Shuttle external tank for example had a better mass fraction than a can of soda, and that was only possible because at the scale of the external tank the walls could still be over a centimeter thick of aluminum, whereas a soda can is nearly unworkably thin (we use dies and presses to smear out the metal that thin, imagine the size of a press you'd need to use the same method to build a rocket tank).
Anyway since steel is denser but stronger than aluminum you need a slightly heavier but much thinner panel for the same job, which effectively means it gets more tricky to work with faster as you go to smaller scales. The first stage of Neutron may just be big enough that it makes sense to use steel regardless however, after all it's going to be a bit bitter than the original Atlas and that used steel as well (balloon tanks in fact, even thinner than actually necessary for a first stage).
As for the second stage, unless they're going for full reusability I'd stick with carbon fiber if I were them. They already have the materials tech, the upper stage should easily fit into a commercial autoclave, and if the stage is expendable there's no reason to worry about thermal protection etc.
