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


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.


Possible materials for use in spaceship armour.


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.


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 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.


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 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).

Armour TypesEdit

Vehicle armour comes in several forms. In World War I and World War II, rolled homogenous armour (a type of armour steel) formed the bulk of defences (supplemented with plate alloys and cast steel). Vehicles equipped with sufficient armour could withstand even the strongest anti-tank guns. However, naval armour had already been through several generations before RHA was replaced on tanks.

The original battleship armour was wrought iron plate, first hammered and then rolled. Ships tended to have four to five inches of iron backed by 0.5 to 1 metre of wood- the wood prevented spalling, cushioned the shock, and distributed the force over a wider area to weaken penetration. A disadvantage of this system was the weight, but with the support of the sea this was not prohibitive, though it made military craft significantly slower than unarmoured vessels until steam turbines were employed. Laminated armour was tested, but offered little advantage so was not used except where there were no facilities to produce plate of the correct thickness, such as during the US Civil War. Steel was tested, but the steel of the time proved too brittle.

Steel armour followed, decreasing the thickness and so the weight of the armour. As it was harder, it could deflect and resist shot with greater ability. However, armour-piercing shells soon appeared, and presented a major threat to even steel armoured ships.

As such, compound armour was developed. It was formed of a hard, brittle high-carbon steel backed by a softer low-carbon wrought iron plate. The front plate caught the projectile, and the back plate caught splinters and held the armour together even if the steel was to break. It was also cheaper than wholly steel armour. It was temporarily usurped by nickel-steel armour due to the rise of chrome-steel shot and the production of nickel-steel. However, this was then replaced by case-hardened steel, which used similar principles to compound armour. Elements are infused onto the surface of a low-carbon steel, forming a harder fronting layer.

Harvey armour came next. Steel plates were covered in charcoal and heated to 1200 degrees, so the carbon content of the face increased, making it harder. It did not solely enrich the face of the plate, though, also increasing the carbon content up to an inch in. It then went through annealing, forging, and heat treatment. All major navies employed it, the US navy using nickel steels and the Royal Navy using normal steels as they both had approximately the same resistance to penetration.

This was made obsolete by Krupp Armour, which was manufactured initially by a similar process to Harvey armour but much chromium was added to increase hardness. Carbon was added using carbon gases rather than charcoal, increasing the depth of enrichment. 10.2 in of Krupp Armour was as effective as 12 in of Harvey armour, so it was taken on by all major navies.

Shortly after the beginning of the 20th century, Krupp then introduced Krupp cemented armour, with some changes in alloy composition. The back of the plate was made more elastic, reducing spalling and cracking. The US Navy's class A Midvale armour was very similar but with a slightly higher but with a greater carbon content.

STS (Special Treatment Steel) was a high-percentage nickel steel used by the US Navy in WWII and beyond where direct impact protection was required (except gun mounts and conning towers). Slightly ductile, it could be used as a structural steel as well as in armour. However, it was comparatively expensive.

It is clear that the development of armour is both complex and progressive. The armour outlined above was more than sufficient for ground and sea vehicles (aircraft tended to use duralumin, an aluminium alloy, and then alclad, which was laminated with a duralumin core and aluminium outer layers. While not as strong as steel, it was the best material that could be used for its weight). Things changed little until the emergence of shaped charges and improved kinetic penetrators in the later parts of WWII.

On tanks and other ground vehicles, composite armour replaced steel plate. It was designed to combat HEAT rounds, which fired a jet of molten copper that was very effective at armour penetration. Composite armour is formed of layers of metal, ceramics, plastics, and air that while light offer great resistance to penetration. The best known and most common is the British Chobham armour, which sandwiches a layer of ceramic between two steel plates, bringing significant increases in resistance to HEAT rounds even when compared to other composite armours. It was employed by the M1 Abrams due to its effectiveness. This was well demonstrated in Operation Desert Storm where no Challenger tanks were lost to enemy fire (one equipped with Dorchester armour was damaged by an IED, but while the driver was injured, he was soon back in service).

AMAP (Advanced Modular Armor Protection) is a German composite armour concept designed for modularity so that it can provide specialised or all-round vehicle protection. It utilises advanced steel alloys, titanium-aluminium alloys, ceramics and nano-ceramics in a composite system that provides double the protection of RHA of the same mass. The nanoceramics also offer multi-hit capability. It is currently employed by later generation Leopard 2 tanks.

Another type of vehicle armour is reactive armour, designed to combat shaped charges and long rod kinetic penetrators. Explosive reactive armour is formed by a layer of high explosive sandwiched between steel plates. When impacted the explosive detonates, damaging the penetrator. Metallic jets are disrupted, lessening penetration. Kinetic penetrators are deflected and broken up.

NERA is another type of reactive armour, but with a layer of rubber rather than explosive. This dissipates energy on impact and causes the plate to bulge, thickening the effective armour depth. A technology in development is electric reactive armour, designed with two conductive plates seperated by space or an insulator to produce a high-power capacitor. It is charged by a power source. On impact, the penetrator closes the circuit and causes the capacitor to discharge, transferring significant quantities of energy into the object and possibly causing it to vaporise or turn into plasma, diffusing the attack.

Weapon-Specific CountermeasuresEdit

When defending against certain weapons systems, other precautions can be taken. Laser defence is likely to involve surfaces which are reflective even when ablated, as this will lessen the energy absorbed. No mirror is 100% efficient so mirrors will not significantly weaken the effects of lasers but they may slow them slightly.

Defence against gamma rays and x-rays such as those produced by nukes is best done by using the densest materials you can get, such as depleted uranium or lead. However, their mass may prove prohibitive. Steel offers good defence against these and carbon presents reasonable defence.

Particle beams should be countered with substances containing as much hydrogen as possible (lighter elements are better). Heavy metals will make these worse as x-rays will be produced on impact. Water, lithium hydroxide, and paraffin are all good armour against particle weaponry. Beryllium, while expensive, is a very good neutron reflector, and steel and graphite are two other neutron reflectors. Both carbon and boron make good defence against particle beams.

Ship DesignEdit

Warships should be designed with survivability and repairability as main priorities. Because of this, redundancy is key.

Depending on the size of your warships, secondary control centres, power plants, and life-support systems are likely to be present in either just battleships or, if your spaceships are larger, battleships, cruisers, destroyers, frigates, and anything else you want to make more battleworthy. Duplicates should be located with some seperation so a single shot is unlikely to take both out. This means that a warship will still be capable of fighting even if a large portion of it is blown up or off. Also, the CIC should be located at the centre of the ship where all the rooms and armour are defending it (definitely not on the very top, asking to be shot off, a la Star Trek).

Naval vessels are compartmentalised using bulkheads. Breach the hull and only one compartment is ruined. Each room acts to contain the damage and also makes atmospheric breaches far less of a problem.

An isogrid superstructure formed of triangular stiffening ribs to support the hull and armour is a good idea. They are in use in many modern spacecraft where low mass, high stiffness, high strength and high damage tolerance are required, and they present good strength however stress is applied.

More controversial is the shape of the ship. A sphere offers the smallest surface area (and therefore presents the least surface area on average) to be hit for a given volume. A very narrow ship offers a miniscule surface area to be hit if face on but presents a large target if attacked from the side or above. As targetting systems will likely be able to hit human-sized targets at this range I see no real benefit to any specific shape in terms of defence, so you should probably determine that by considering other factors (such as firing arcs and whether it will ever enter atmosphere).

Point DefenceEdit

Point defence systems are active systems used to defend an object or asset, such as interceptors, CIWS, AA guns or missiles, and the hardkill countermeasures of active protection systems such as reactive armour and interceptor missiles.

Some detail on point defence can be found on the Weapons page.

Kinetic Point DefenceEdit

Railguns and coilguns are too slow-firing and energy-intensive to make good point defence, but conventional kinetic weapons prove effective at intercepting other projectiles. Examples of such technology include CIWS and the AK-630

They are capable of firing thousands of rounds per minute. A single round could conceivably neutralise a nuclear missile and is very likely to disable an antimatter missile. The fragments and debris may still impact your ship at high velocity so you may wish to shoot those as well.

They will have a range of several kilometres. This may not be enough; a big nuke or AM warhead detonating out of range of your kinetic PD could kill your spaceship through radiation. Even bigger warheads could destroy your ship from that range. Kinetic CIWS can also only aim at one target at once and take a short time to train on a new target. Also, as the bullets are unguided, missiles carrying out erratic manoeuvres are significantly more difficult to intercept. Having multiple small or medium-sized kinetic PD systems that overlap can solve many of these issues.

Laser Point DefenceEdit

Point-defence systems based around lasers will not be using small lasers- the bigger the laser, the better the point-defence it provides. They have the advantages of light-speed beams (making missile erratic movement worthless and ranges much greater), very very short time taken to train on a new target, and the ability to go through anything given long enough dwell times.

Effective burn ranges for megawatt-class visible laser are in the order of thirty thousand kilometres, increasing in effectiveness as the object closes on your ship. Depending on the laser and the targets, such a system may be able to neutralise hundreds of projectiles before they reach the ship.

Missile Point DefenceEdit

Missile CIWS have ranges far greater than kinetic CIWS. Range is limited by fuel (for acceleration and seeking) only, and they have very high accuracy. A missile fired from a CIWS system can disable another missile far larger than itself. They can also fire missiles at multiple targets at once, allocate how many missiles to fire at each target, and have no real training time required. A hot launch, where the ship launches the missile even before its rocket kicks in, is favoured here.

Evasive ManoeuvresEdit

A surefire way not to get hurt is not to get hit (either by the weapon or its effects). If you don't want to get hit, move around out of the path of enemy weaponry, and do not do so predictably or they will shoot at where you will be and hit you.

Vessels will have to accelerate erratically in many directions. Limits on acceleration are imposed by the manouevring thruster's thrust, the fuel available, the limits of the human body and the limits of the ship's systems and superstructure. Naturally, the further away from the enemy you are, the easier it is to dodge. Accelerating directly away from or towards the enemy ship won't help but you can thrust in any other direction. Your main drive may be equipped with cascade vanes which redirect the exhaust, or gimballed nozzles for thrust vectoring, letting you use your most powerful engines to change course. Attitude thrusters are much smaller rockets, usually with independent fuel tanks, that are placed seperately from the main drive (usually so that the force acts through the ship's centre of mass).

Ships take a long time to turn around due to their large moments of inertia- likely a minute or more for a reasonably sized space battleship, so they can't dodge in any direction quickly.

When fighting laser- and kinetic-armed vessels, the two main targetting problems are lightspeed and weapon lag. Weapon lag is the time taken to train your weapon on the target and fire. If your vector is known to the enemy, however, a hit is almost certain. Erratic evasive manoeuvres lessen the effective range of lasers by a factor of ten or more, and that of kinetics from indefinite down to a few hundred kilometres.

Missiles don't really have to concern themselves with your evasive manoeuvres because they actively home in on you and they can accelerate faster than you can. You are unlikely to be able to get them to waste all their fuel changing course, either.

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