While space warfare is not really practical on a large scale, this didn't prevent tacticians from theorizing ship-to-ship combat in the age of faster than light travel. Most of the aspects being pictured here have not been field-tested outside of military exercises, but they are the closest we have to coherent combat advice in the age of the geometry drive.

1 - The Face of Battle

Space warfare is usually conducted by FTL-capable spaceships equipped with geometry drives. From small Luciole Interceptors to large Firebases, there is a wide variety of combat ships, but all of them follow the same rough design principle. They're fusion-powered and their primary weaponry is made of high-power gimballed laser emitters (or laser grids) capable of intercepting threats and attacking enemy hulls. Secondary weaponry is made of missiles, either internally stored or installed on external hardpoints. Missiles are equipped with miniature geometry drives and can perform a single translation towards a target. More sophisticated hunter-killer missiles can perform several translations in a row, hunting down ships over several light-seconds...or dogfighting enemy missiles. Some ships can also be equipped with unguided missiles and pebble launchers for very close range engagements. Railguns and coilguns are rare and only carried by improvised warships.

Ablative armour is used to mitigate the effect of laser fire, however missiles can and will one-hit kill the vast majority of ships, even with simple kinetic warheads. Defenses rely on translations, laser interception, decoys and jamming.

2 - The Economy of Combat

Modern space combat is carried out by ships equipped with geometry drives, and thus capable of performing nigh-instantaneous faster than light translations over several light-seconds. Movement in modern space engagements is almost completely unrestrained and is not part of the economy of combat, which is a marked departure from interplanetary-era space warfare. The only restrictions of the geometry drive are that it cannot be used to jump in dense matter (such as a planet's atmosphere) or within another drive's translation envelope (typically within about a hundred kilometers of the vessel). Small geometry drives carried on missiles and drones only have an exclusion bubble of a few kilometers.

The economy of tactical warfare revolves around five resources.

-- Ammunition: while laser grids can fire as long as they are supplied with power, faster-than-light missiles are rare and expensive, and are a hard limiter on sustained firepower.
-- Situational awareness: while it is a well-known trope that "there is no stealth in space", there can be a whole world between detecting a vessel and having a clear firing solution on it. Perfect situational awareness is not possible, and sometimes not even desirable; information overload can be as damaging to decision-making than information scarcity.
-- Delta-v: faster-than-light translations conserve relative velocity and immediate momentum, which means matching up with enemy or friendly vessels requires to provide thrust. While military ships equipped with fusion drives have extensive delta-v reserves, the same cannot be said for missiles.
-- Computing power: calculating translations, coordinating missiles, intercepting projectiles with laser grids, conducting electronic warfare...all of these actions are a strain on the ship's computer.
-- Thermals: almost every battle action, from computing translations to using lasers, generates heat that has to be dissipated. The hotter a vessel is, the easier it is to detect and the slower its systems work.

All combat tactics try to deplete or counter one or more of these resources.

It is to be noted that, contrary to a popular belief, crew endurance is not part of the economy of combat. Engagements rarely last for more than a few minutes, and g-forces are not enough to inflict lasting damage on sufficiently trained crews.

3 - Alpha Strikes

The best engagement is the one that never happens. Failing that, it's a battle that only lasts for the time to launch missiles and run away. Alpha strikes are a crucial aspect of modern space combat, leveraging the operational capabilities of geometry drives to avoid a prolonged exchange of fire. They're generally carried out within a system; interstellar alpha strikes are possible but require several additional translations and burns to correctly match with the enemy, thus negating stealth.

The ideal alpha strike begins with good intelligence and, failing that, stealthy recon. When the enemy fleet has been detected, attack ships will start accelerating to match the velocity and heading of their target. An entire flotilla worth of fusion drives firing at once creates an infrared signature visible across an entire solar system. While FTL ships will outrun the signal travelling at the speed of light, enemy recon might pick up the flare and report back to the main fleet; thus, initial burns should be carried out under cover and in sensor-free areas. Once the burn is complete, ships will translate within missile range and open fire on their targets. Ideally, the first warning for the enemy should be when missiles translate within terminal burn range, at which point active defences have a handful of seconds to acquire the incoming projectiles and destroy them. Attacking ships then either disengage or open fire again to take out surviving vessels.

It is exceedingly hard to avoid alpha strikes entirely in contested systems, and most militaries consider such attacks as a fact of combat. They instead focus on mitigating the risk, by keeping heat signatures at a minimum or, on the contrary, drowning ships in IR signals and jamming bubbles. Tight surveillance may catch enemy vessels as they translate before firing their missiles, giving the defences precious time to engage. Most vessels also keep an emergency translation at hand, refreshing the coordinates for a short-range translation towards a random point at all times. When attacked, the ship can then jump away to evade without having to compute a translation from scratch.

4 - The Great Circus

The alpha strike has failed, or didn't manage to take out the entirety of the enemy fleet -- combat will now escalate as both fleets break formation and engage in a vast tactical melee spread over several light-seconds. If the two fleets are evenly matched, each formation will try to break the cohesion of opposing combat groups to get at the ships, otherwise the battle becomes a race to intercept the weaker opponent before it manages to perform a full retreat towards a friendly planet or star. Melees are incredibly confused, and the mere act of keeping in formation with a wingman vessel in such a chaotic context is considered an exploit. Ships will start by deploying recon drones to increase their situational awareness (or link up with already deployed drones), ID neighbouring vessels, then burn and engage. Anti-missile defence relies on careful positioning to trap missiles within a ship's interdiction bubble, and coordination between individual units to optimize laser coverage.

There are at least three levels of combat in a melee. Ships try to engage each other at various range with laser grids and missiles. Hunter-killer missiles run after the targets designated by their motherships, but also engage in dogfights with enemy projectiles. Finally, recon drones try to blind or mission-kill each other with micromissiles, waging their own little war. Melees are not unlike industrial age fighter jets dogfights in spirit: the primary goal is to isolate an enemy vessel and outnumber it locally. Focused laser bursts and jamming attempts are primarily targeted at mobility devices -- computer modules, radiators and engine nozzles. Missiles are used indiscriminately to kill.

In contested systems, melee generally end in draws, as the losing fleet retreats under the cover of a heavily defended moon or space station. They can, however, be extremely deadly, especially when the attacker commits to deep after a failed alpha strike.

5 - Knife fight

The most committed and immediately deadly form of warfare occurs when two or more ships decide to engage in extremely close contact warfare, less than a hundred kilometers away from each other, overlapping the interdiction bubbles of their geometry drives to prevent each other from translating. This is called a knife fight. Missiles cannot translate and are used as sublight impactors, while focused laser fire quickly melts ablative armor. This is the only situation where unguided missiles and other "pebble launchers" are useful, as the primary weapon for "drive-by attacks" at pointblank range. While it may happen as the result of an alpha strike, a knife fight generally requires mutual consent. It is very hard to forcefully lock an enemy ship in such an engagement, unless the difference in delta-v is truly tremendous.

Knife fights usually last for a few dozen seconds at best, as pointblank exchanges make very quick work of even the sturdier defenses. They leave almost no room for tactical manoeuvres and are mostly dependent on sheer volume of fire. Knife fights often result in simultaneous damage or even destruction: they're universally considered as a desperate move.

Sometimes, one of the ships may try to disengage by burning and changing course to escape the mutual interdiction bubble, forcing the other vessel to match it move for move while avoiding the deadly drive plume. This results in a "torch duel", probably the most spectacular form of space combat.

Illustration courtesy of Lilly Harper, who writes most excellent sci-fi prose on the Beacons in the Dark blog.