New structural technologies
The challenges of launcher architecture in the context of New Space

3.1 The return of steel

Steel is making a remarkable comeback in the New Space sector, thanks to its intrinsic characteristics, which once again make it a suitable material for large-scale space applications. Its high mechanical strength enables it to withstand the loads and stresses inherent in launches and the extreme conditions of space. For example, high-grade stainless steel, such as type 304L or 316L, is used in the manufacture of rocket fuel tanks, providing the corrosion resistance and sealing essential for propellant storage. The emblematic example is SpaceX's Starship. It should be noted that this is really a return to basics, as the Saturn and Ariane 1-4 first stages already made extensive use of high-performance steels, not to mention solid rocket boosters.

Let's clear up a misunderstanding about steel right away. Comparisons between materials are in fact complex, taking into account the variety of stresses, couplings and bifurcations that may be non-linear. This allows us to make comparisons more precise.

In orders of magnitude compared to light alloys, steel's density is three times greater, but its main mechanical tensile properties – the simplest generic test – are also far more favorable. In the aerospace sector, and for lightweight structures in general, it's performance per unit mass that counts.

Let's take a closer look at why – despite the near mass equivalence of steel and aluminum in simple traction – aluminum has, until now, been used more than steel. For the same membrane flow, the aluminum shell will be three times thicker but of the same mass, compared to the steel solution. But when we consider compression or shear, i.e. as soon as buckling can occur, then thickness plays a major role, since quadratic flexural rigidity depends on the cube of thickness. So it's easy to see why, as soon as a complex load appears, light, thick materials have the advantage. This is also one of the reasons for the success of composites.

However, there's an additional element to the equation: pressurization. New means of active structural control make it possible to optimize pressurization in large structures, and steel is once again a leading structural material.

In addition, like aluminum, steel offers excellent thermal and electrical conductivity, making it suitable for cooling applications in space equipment, while regulating potential differences. Steel heat sinks effectively manage the heat dissipation generated by electronic components on board satellites and space probes.

Another advantage of steel over aluminum is its availability and ease of fabrication. It can be formed and assembled flexibly, enabling the manufacture of space structures of various sizes (at least if relatively mild steels are involved). For example, steel structures are used in the construction of satellites and space probes, offering an ideal...