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Most people have probably heard by now that Elon is changing Starship to a complete stainless steel build. Anyways, it got me thinking, why isn't stainless steel ever used for rocket engines (nozzle, combustion chamber)?

I am in a University student rocketry club currently building a liquid engine, so this is of particular interest to me. I am on the team designing the regenerative cooling and engine structure/manufacturing.

Copper alloys and Inconel 718 (INC718) are popular choices, but what about a 300 series stainless steel?

Compared to INC718, stainless has a slightly lower density, higher specific heat, and higher thermal conductivity. Mechanical properties such as melting temp and modulus of elasticity are similar. The greatest disparity against stainless is that the tensile and yield strength are much smaller than INC718. Is this the primary factor keeping stainless from being used in rocket engines?

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  • $\begingroup$ Sutton states it's a usable material. See page 305 pdfs.semanticscholar.org/6cce/… $\endgroup$
    – Organic Marble
    Commented Jan 22, 2019 at 21:50
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    $\begingroup$ The RS-25 nozzle is made of brazed stainless steel tubes, and at least parts of the turbopumps are made of steel: large.stanford.edu/courses/2011/ph240/nguyen1/docs/… $\endgroup$
    – Christopher James Huff
    Commented Jan 22, 2019 at 23:16
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    $\begingroup$ @JCRM I think you are correct that strength is the biggest problem: Asume it got halve the strength that would mean twice the wall thickness required and therefore twice the weight. Moreover twice as thick walls halves the heat conduction so less performance at the same thermal conductivity. $\endgroup$
    – Christoph
    Commented Jan 23, 2019 at 10:57
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    $\begingroup$ You need to do a lot more research before you can assert that stainless steel has "never been used in the 21st century". It's realm of application will be limited by thermal conductivity (lower than copper) and temperature range (lower than nickel alloys/etc), but there's certainly areas where it could still be used. And in fact, the Merlin 1A engine is a 21st century counterexample: a Falcon 1 failed due to corrosion of an aluminum nut, and stainless steel components were used to avoid this in later versions. $\endgroup$
    – Christopher James Huff
    Commented Jan 23, 2019 at 13:24
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    $\begingroup$ The Wikipedia article has some reasons to use inconel instead of stainless steel. Even for automotive use instead of rocket engines. $\endgroup$
    – Uwe
    Commented Jan 24, 2019 at 14:53

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First, I am skeptical whether it's actually true true that stainless steel is actually never used in modern rocket engines (as some commments have asked).

Stainless steels in general are not necessarily high performance materials when heated to high temperatures, in either practical strength (including vulnerability to various undesirable heat treatments, embrittlement, fatigue, etc) or in corrosion resistance, surely of some importance when chemically aggressive oxidizers are involved.

This datasheet for Inconel 718 lists the strength, at various temperatures, after various heat treatments.

Some of them are pretty favorable. Additionally, these nickel superalloys are often more chemically resistant, which is probably important when hypergolics are involved.

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