There is a high chance that a large variety of metals is in your proximity at this very moment. Metals are found and used virtually everywhere, from the iron in your red blood cells to the rare earth metals in the screen you are reading these lines from.
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Many of the greatest advances in technology can be traced back to the exceptional characteristics that can be achieved by manufacturing parts from metal or alloying different metals to obtain even more superior materials.
Apart from the materials themselves, the manufacturing techniques evolved from hammering copper in approximately the 6th millennium BC <1> to, more recently, 3D printing of titanium.
One of the greatest advancements of the 20th century is certainly the human venture into space. The requirements for the launch vehicles to deliver scientific or commercial payloads into a stable orbit around the Earth are complex and often strongly differ from common engineering applications.
For example, the structural materials need to sustain high forces during the phase of maximum aerodynamic pressure at the ascent, low temperatures in the liquid fuel systems, high temperatures in the combustion and exhaust section and hydrogen embrittlement <2>, in case hydrogen is used as fuel. If all of that wasn’t enough, all components used need to be extremely lightweight. The reason for massive parts being unacceptable for launch vehicles is rooted in the very foundation of rocket science, the rocket equation.
Why it’s all about the mass
In 1903, Konstantin Tsiolkovsky applied the conservation of momentum to rockets and came up with his rocket equation <3>, shown in Figure 1. From this equation, it can be seen that the structural mass of the rocket plays a critical role.
A rocket is merely a transportation system with the sole purpose of delivering a payload. This objective is achieved by expelling the propellant of the rocket at the highest attainable velocity. The Δv needed to reach low earth orbit is about 8 km/s <4>. The propellant fraction required to gain such Δv is in the range of 83-94%, depending on the type of propellant <5>.
The remaining part of the mass (mf) is shared by the structural materials of the rocket and the payload. Hence, the lower the structural mass of the rocket, the higher the mass for the payload can be for a given amount of fuel.
Many metals come with a high density. As components cannot be manufactured infinitely thin, a high-density material typically leads to massive parts. You might wonder, what about the available low-density metals, for example, aluminium, magnesium and lithium?
All of these metals do find their applications in space structures to some extent, but they have some drawbacks in common. Namely, a comparably low melting point and a high chemical reactivity, which make them unsuitable for parts in contact with cryogenic fuel or the hot exhaust gas. Composites and ceramics offer high strength and chemical stability, but they are often too brittle to survive the mechanical loads. It is up to some well-known metals like nickel, chromium, cobalt and iron to save the day, despite their comparatively high density.
The usefulness of these metals was highlighted in a recent interview with Elon Musk, where he announced to build his Starship and the Super Heavy rocket booster out of stainless steel instead of an advanced carbon fibre structure. In the new design, the stainless steel 301 is used in a multi-purpose integration of structural material and heat shield.
The wide operational temperature range, from cryogenic temperatures to 1100 K, enables this steel to outperform aluminium and carbon fibre structures and make it, comparing the mass needed, the lightest of the three.
Structures, pipes & tanks
Generally, the greatest mass of the rocket structure is concentrated in the propellant tanks, which are often integrated to bear structural loads as well <6>. Thus, the propellant tanks have to sustain moderate pressures at cryogenic temperatures while withstanding the strongly varying mechanical loads during the ascent. A common design is a stiffened aluminium alloy shell that can support its own weight even when not pressurized from within.
Historically, the 2000 series of aluminium alloys has been deployed for structural tanks. This material series is comprised of aluminium-copper alloys with a weight percent fraction of copper in the range of 0.9-6.3% <6,7>.
Within these alloys, the intermetallic compound CuAl2 causes the strengthening effect, while silicon, lithium, and trace amounts of manganese, magnesium, and titanium are added to improve forgeability and to inhibit corrosion under stress. A favourable characteristic of aluminium alloys is their increase in tensile strength at cryogenic temperatures, which makes them especially attractive for this application.
To feed the fuel from the tanks to the engine and to connect other auxiliary pressurized systems, feedlines and pipes are used. Metals for these components need to have a high ductility in order to allow for the necessary curvatures. Also, maintaining strength and ductility at cryogenic temperatures and the chemical compatibility with the conducted fluid are important.
The hydrogen embrittlement mentioned earlier is especially critical for pipes. The corrosion resistant 321 stainless steel is a prominently used material for rocket pipes <8>. This steel is rich in chromium and nickel and stabilized by 0.3-0.7% of titanium. Other suitable materials are the nickel base superalloy Inconel 718 and the stainless steel A-218, which are both deployed in the space shuttle main engine (SSME). An overview of the major materials that are used to manufacture the SSME is presented in Table 1.
Rocket engines – engineering masterpieces
Engines are without question the most delicate parts of the rocket. The thermal gradients in a rocket combustion chamber are incomprehensible: liquid hydrogen at 20 K is used to cool down the inner combustion chamber wall that is facing exhaust gas with temperatures exceeding 2000 K <9>, all over a distance of less than a millimetre.
The working principle of a liquid rocket engine is intriguingly simple. Thrust is approximately equal to the mass flow rate times the exhaust velocity. These two quantities are optimised to the very extremes in rocket engines. To achieve a high mass flow rate, a dedicated turbopump is deployed for each propellant.
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A titanium-based alloy has proven to be the ideal material for the turbopump blades and casing. The turbopumps are powered by the combustion of fractions of the propellant. The exhaust gas from this combustion is piped into the main combustion chamber, where it is mixed with the main flow of the high-pressure propellants. The mixture is then ignited, expands, and the only way out is the open end of the nozzle.