Spaceship defence

Defences are measures that decrease the probability, lethality, or viability of a threat. They are commonplace in space warfare.

Armour
Spaceships require armour to reduce the lethality of radiation, debris, and weapons fire.

However, armour is heavy. It will either have to be lifted into space from the surface or constructed elsewhere and moved into orbit if the spaceship is being built in space. This is a problem; every kilogram of mass requires a significant investment of time, money and resources to be launched. There is a point at which the usefulness of armour is outweighed by the cost of adding it.

Materials
Possible materials for use in spaceship armour.

Steel
An alloy of iron, carbon, and often other elements. It has a density of around 7.8g/cm3; reasonably heavy. It has a melting point between 1,100 and 1,300 degrees C depending on composition, and loses reliable strength above around 600 degrees. It has reasonable thermal conductivity.

It is used so widely for many good reasons. One is its cheapness, as iron and carbon are plentiful and processing is simple. It is also very versatile, useful in many different roles. Its tensile and shear strength are among the best of all materials, it has high surface hardness, is not vulnerable to shock, and does not burn. It also has a lower fatigue cycle limit (a steel structure can be designed which will never develop fatigue cracks.). With a very high vaporisation energy, it would also be effective against energy weapons.

There are very few disadvantages. It is quite heavy, but not extremely so. As armour materials go, it is one of the most promising.

Aluminium
A reactive, lightweight metal, aluminium has a density of around 2.7g/cm3 and a melting point of 660 degrees centigrade. Its reactivity means that it forms a hard, unreactive oxide layer on its surface when exposed to air, which protects the metal underneath. It is a good thermal and electrical conductor. It is a good reflector and has about a third of the stiffness of steel.

Aluminium is the most widely used metal other than iron (and iron-based alloys). It makes up significantly more of the Earth's crust than iron does, but is expensive to extract due to its reactivity. It must be extracted through electrolysis requiring enormous quantities of electricity and large, high-maintenance facilities which use up 5% of the USA's electricity.

Aluminium alloys resist most forms of corrosion better than steel (though the materials which it cannot resist react catastrophically with it) and has a higher strength-to-weight ratio than steel. It is stronger kilo for kilo than steel. However, metre for metre, it is actually weaker; any given size of steel beam will be stronger than one of aluminium. It changes size rapidly with temperature. It suffers rapid surface erosion, so is not good for high-wear purposes. It also has no lower fatigue cycle limit, and so is certain to eventually fail under stress. This is why planes are retired after a few years in service.

The Royal Navy experimented with aluminium hulls for warships, but a problem soon became apparent- aluminium fires start at a low temperature, burn hot, and are very hard to extinguish. The Apollo spacecraft were made with aluminium skin and cabin, but coated with a phenolic coating.

Titanium
Titanium is a strong, corrosion-resistant, low density metal. Native titanium has a density of 4.5g/cm3 and a melting point of 1668 degrees centigrade. It has low thermal and electrical conductivity for a metal. Like aluminium, it is a reactive metal that forms a hard surface layer when exposed to air. This is made from titanium dioxide and titanium nitride, which are hard and inert. It is about 60% denser than aluminium but usually more than twice as strong, and while 45% lighter than common steel grades it is just as strong. It has a fatigue limit like steel.

It is often seen (particularly by Hollywood) as a 'supermetal', though this is not quite fair. While reliable and useful, it is only good for certain purposes, and is rarely effective on its own. Many grades of steel have almost three times its tensile strength and titanium loses strength at a lower temperature than steel. Some grades of steel have nine times its tensile strength.

Titanium is the seventh-most abundant metal in the crust, but like aluminium its reactivity makes extraction and processing difficult and energy-intensive. It is extracted by the Kroll process.

Titanium alloys are used in aerospace, armour, spacecraft, and missiles where steel's mass is too prohibitive. Some titanium alloys are almost as strong as medium carbon steel.

Magnesium
Another reactive metal with a density of 1.7g/cm3 and a melting point of 650 degrees centigrade. Free magnesium burns very hot and bright if heated to about this temperature. The oxide layer that forms on its surface is impermable and reasonably hard, but it is quite soft and has low strength. It is very common, being the eighth most abundant element in the crust, and is usually extracted from seawater by electrolysis.

Magnesium can be alloyed with zinc, aluminium and manganese to produce a viable structural material. Many aircraft contain half a ton or more of magnesium alloys in vital spots and make substantial weight savings. In fact, magnesium is the third most-used structural metal after iron and aluminium.

Carbon
Carbon can be used in several different forms. It has a very high vaporisation energy for its mass, making it ideal armour against energy weapons.

Carbon fibre is a lightweight material with both a high strength-to-weight ratio and high rigidity, having a tensile strength slightly greater than titanium alloys and medium carbon steel. It is corrosion-resistant and chemically stable, while also non-flammable. Maintenance requirements are very low.

However, it is brittle and highly expensive. Only in niche applications (usually aerospace or racing) is it worth the cost.

Carbon nanotubes are the strongest (tensile) and stiffest (elastic modulus) materials yet discovered. They are very low mass, at about 1.3g/cm3 and so also have the best specific strength. They are also exceedingly hard and are very effective conductors of heat and electricity.

They are not nearly so strong against compressive or bending forces, however. Speculative production methods may lower its cost to $20 per gram (compare to .46 cents per gram of iron ore).