So far as an astronomer understands the 'witchcraft' of spacecraft design, very light weight and durable alloys of titanium, aluminum and magnesium are commonly used.

Aluminum - Most commonly used conventional material (used for hydrazine and nitrous oxide propellant tanks). Low density, good specific strength. Weldeable, easily workable (can be extruded, cast, machined etc). Cheap and widely available. Doesn’t have a high absolute strength and has a low melting point (933 K).

Magnesium - Higher stiffness, good specific strength. Less workable than aluminium. Is chemically active and requires a surface coating (thus making is more expensive to produce).

Titanium - Melting point 1933 F. Light weight with high specific strength. Stiff than aluminium (but not as stiff as steel) Corrosion resistant. High temperature capability. Are more brittle (less ductile) than aluminium/steel. Lower availability, less workable than aluminium (6 times more expensive than stainless steel). Used for pressure tanks, fuels tanks, high speed vehicle skins.

Ferrous Alloys like Stainless Steel - Have high strength. High rigidity and hardness Corrosion resistant. High temperature resistance (1200K). Cheap Many applications in spacecraft despite high density (screws, bolts are all mostly steel).

Austentic Steels - Non-magnetic. No brittle transition temperature. Weldable, easily machined. Cheap and widely available. Susceptible to hydrogen embrittlement (hydrogen adsorbed into the lattice make the alloy brittle). Used in propulsion and cryogenic systems.

Beryllium - Stiffest naturally occurring material (beryllium metal doesn’t occur naturally but its compounds do). Low density, high specific strength High temperature tolerance Expensive and difficult to work Toxic (corrosive to tissue and carcinogenic) Low atomic number and transparent to X-rays Pure metal has been used to make rocket nozzles.

Inconel’ (An alloy of Ni and Co) - High temperature applications such as heat shields and rocket nozzles. High density (>steel, 8200 km m-3).

Aluminium-lithium - Similar strength to aluminium but several percent lighter.

Titanium-aluminide - Brittle, but lightweight and high temperature resistant.

In the near future we may use combinations of plastics and various hybrid materials such as metal-matrix composites that will greatly reduce the weight of a spacecraft, and greatly reduce launch costs. In fact, carbon fiber technology has already been used to replace many spacecraft components, except for the outer spacecraft bulkhead itself. Bulkheads are still made from titanium, aluminum or other conventional metals and alloys because of the tremendous thermal and pressure demands. The B-2 Stealth Bomber used carbon fiber materials in its wings, but the forces it would be required to take and the thermal loading are quite small compared to a spacecraft launched from the ground, or re-entering the atmosphere from space.

Fiber-reinforced materials such as carbon, aramid and glass composites have the highest strength and stiffness-to-weight ratios among engineering materials. For demanding applications such as spacecraft, aerospace and high-speed machinery, such properties make for a very efficient and high-performance system. Carbon fiber composites, for example, are five times stiffer than steel for the same weight allowing for much lighter structures for the same level of performance. In addition, carbon and aramid composites have close to zero coefficients of thermal expansion, making them essential in the design of ultra-precise optical benches and dimensionally stable antennas. Some carbon fibers have the highest thermal conductivities among all materials allowing them to be incorporated as heat dissipating elements in electronic and spacecraft applications.

Even carbon nanotubes and fibers are being developed with spacecraft and rocket technology in mind. So, we have a lot to look forward to in this exciting arena. Every pound saved is a major reduction in launch cost to the tune of $5000 per pound. This means that if you replace a pound of aluminum with a pound of some exotic new alloy, you have almost $5000 per pound in new allow cost to play with because aluminum counts for only one percent of the $5000!

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