A Constellation of Warships

So, you've designed a spaceship to fight your wars in space. It can travel between planets, perform multiple missions and is based on NASA reports and real, hard science. You've estimated your nation's military budget, and come out with a fleet capacity and a number of spaceships.

But then... your glorious collection of spaceships fly out in Napoleonic square formation. Or was it pre-WWI lines of battle? Maybe it's Star-Wars-esque clumps of capital ships with space fighters swarming around them. No, you need the ToughSF approach.

Wall of spaceships from Legend of Galactic Heroes 

Space is a strange place. You don't travel across a landscape, you throw yourself along trajectories. There are no pit-stops, no sinking, no up, down, left or right. In other words, this means that space warfare is fundamentally different from anything we've tried so far, be it underwater, surface, ground or aerial combat.

There are similarities to build upon, but the differences should not be ignored in favor of familiarity. 

From Leviathan

In this post, we'll look at how the peculiarities of space travel and the technological assumptions you've made will affect the designs of your warships and the formations they form in space.

Nomenclature

A fleet can refer to any large collection of vehicles. Aircraft, ships, drones, even trucks...

However, each of those vehicles and their services have specific names that are much more interesting to use. Aircraft can form Wings, artillery becomes Batteries, tanks have clatters and gunships have hailstorms...

The RapidEye satellite constellation

Spacecraft have constellations. 

Mission and Enemies

Before a constellation is launched, before it is even put together, there must be a clear idea of the opponents it will face and the mission it will undertake.

Mission configuration can be a simple as hooking more boosters together

A constellation sent to deal with a suspicious spacecraft in Lunar orbit will be very different from one sent to intercept an enemy invasion en-route from another planet.

Missions affect a constellation's design and composition mainly through deltaV and acceleration requirements. Travelling to a destination in space requires a minimum deltaV equal to the Hohmann trajectory. For a time-sensitive task, a minimum acceleration might be required. 

This is important, as all spaceships in a constellation must have the same initial acceleration, so as to depart together.

The deltaV 'budget' each spaceship has is spent in three stages: departure, maneuvers and return.

The departure stage is the most restrictive, as it requires all spaceships in a constellation to have similar and acceleration in addition to expending similar amounts of deltaV. This leads to a certain uniformity in engine type and reactor output per ton.

The maneuvers stage is the complete opposite. Depending on the weapons system and the tactics employed, this can comprise the majority of the deltaV budget or only an afterthought for transitioning from a 'Go' to an 'Abort'. 

An abort here would be skipping the Mars Injection burn and heading for a fly-by.

The return stage is conditional. In some situations, it only takes a small maneuver to make a fly-by at interplanetary speeds across your target, swinging around for a return. Others require that you perform a capture burn into orbit around your target, requiring a deltaV similar to that of the departure stage to return after the mission is completed. If your fleet has been defeated but not destroyed, there is no reason to stick around...

A space station built into an asteroid. In space, it can be considered immobile.

The targets you expect to face can change the priorities of the constellation and the equipment or loadout required to handle them. A relatively immobile, long-ranged defense station can be taken out by a massed missile attack from outside its reach. A swarm of autonomous drones will instead require direct-energy weaponry such as lasers or particle beams. This matter is a complex one, with historical, economic and technological factors in play.

Specialization and classification

Regardless of the mission or the enemies, spaceships will be more effective when specialized for their role. 

An extremely large X-ray laser weapon that is the pinnacle of laser warfare

This reality is due to the tyranny of the rocket equation. A tank on the ground, if overloaded with equipment to face a variety of opponents, will move slower. A battleship, equipped with guns, torpedoes, AA defenses and a flight deck, will have a deeper draft. A spaceship, equipped with too many weapon types, will simply not reach its target.

In other words, the spaceship's equipment have to have maximal effectiveness per kilogram. Multiple weapons systems on the same spaceship trying to handle different situations and targets will always be less mass effective than a single system designed for one, specific role. This is almost always tied to range - being able to attack earlier and from further away gives you 'first strike capability' and/or the ability to attack without return fire. In space, this is complicated slightly by the fact that kinetic rounds and missiles can drift forever.

An example of this is tank designs after WWII. 

Swiss Pz87 with a 140mm gun

In tank versus tank combat, the necessity to completely penetrate an opponent's armor at maximal range reduced the number of main guns to one. Even today, tanks do not mount more than one single weapon system, unless defensive or after special accommodations. Naval warships, despite the massive amounts of weapons systems that can be supported, focus nearly exclusively on launching large numbers of missiles and assisting them on their way. 

The result of all this optimization is that future space warships will mount a single weapon occupying the entirety of their combat payload. 

Normally, over-specification is fatal in a dynamic combat environment. The difficulty lies in the fact that facing different situations requires different weapons. To solve this, we use the combined arms concept.

In space, this would translate to multiple spaceships, each optimized for a different weapons system, configured to cover each others' weaknesses so that they may exploit their specialization fully. 

A classification system is simple to derive from the roles each spaceship and the constraints of effectiveness per kilogram optimization.

A useful classification, but only if defense, weapons and propulsion can be traded off equally 

Offensive/defensive is the simplest division of roles. Offensive craft will likely be classified depending on the type of weapon they use and their preferred tactic. Missile launchers will therefore be sub-divided into arsenal ships (very long range launch of massed missiles, with zero to negative relative velocity), lancers (short range launch of missiles at extreme relative velocities), bombers (focus on stationary or ground targets) and so on. 

Defensive craft can be classified using the type of threat they neutralize, such as shield carriers (launch drones to intercept high velocity projectiles), laser interceptors (use lasers to shoot down missiles), sensor hunters (use large cryogenically-cooled sensors to find and destroy sensor platforms) and so on. 

A second line of classification is the mission, and by extension, deltaV budget. An Earth-Jupiter warship will be much more massive than an Earth-Moon warship, even if they are identical aside from the propellant tanks. A 7-day Mars Response Craft will have quite a different propulsion module and propellant loadout than a yearly Mars Patrol Cycler. 

Further lines of classification will use characteristics such as acceleration, crew endurance or level of AI autonomy.

In all cases, classification by mass and velocity are useless at best, misleading at worst. This is because a 100 ton long-range laser can be deadlier than a 1000 ton arsenal ship loaded with ammunition, and a warship travelling at 50km/s towards the Outer Planets is tactically slower than a warship changing orbits at 1km/s.   

Fleet arrangement

Battle of Eylau, 1807

Pike squares and trenches sound outdated today, but they were tactical responses to the state of warfare and weaponry at their time. They became ubiquitous due to their effectiveness, and it took major revolutions on the way wars were fought to force their abandon.

Similarly, fleets, or constellations, in space will have tactically advantageous formations that are created in response to the threats, known and unknown, that are to be face.

The first factor to affect a fleet formation is the environment. In space, the environment of a spaceship is not material, but is a landscape of vectors and trajectories. 

A depiction of space-time curvature

During a transit between planets, the influence of gravity is negligible. It can be abstracted away into assuming that maneuvers will result in straight line trajectories for days ahead. This can be called 'flat' or 'deep space'. On short timescales, the ability to maneuver along all three axis is required. However, this is not the case on longer timescales. 

In a setting where spaceships are deltaV limited, travel is likely to be done along Hohmann trajectories. Depending on how restrictive the deltaV budget is, the approach will have to be performed along narrower and narrower corridors. Movement outside of the corridor has to compensated by acceleration in the opposite direction that becomes more and more expensive as you approach your destination. 

Accelerating along the path of travel has the least effect on your trajectory. Normal acceleration has a greater effect, and radial acceleration has the most influence on where and when you'll encounter your destination and potential targets. 

The result is that fleets will form walls, with main engines pointed in the direction of travel, and weaker thrusters pointed perpendicular directions. Weapons will face forward, competing with the thickest armor. It is pointless to have side armor, as long-range attacks will not have significant lateral velocity without incredible deltaV and propellant mass penalties. High velocity lateral attacks will therefore occur at short range, when an enemy and shoot at a significant angle without much deviation from the Hohmann corridor required to do so. 

On the contrary, settings will very lenient deltaV budgets can take perpendicular or even retrograde paths to their targets. Enemies can conversely attack from any angle. This means that fleet formations will ideally be spherical, but in practice teardrop-shaped, as attacks from the 'rear' take longer to approach combat ranges than attacks from the 'front'. 

In orbit around a planet, space is 'curved' or 'shallow'. There is a minimal altitude, under which spaceships will be burned by the atmosphere or collide will surface elevations. There is a maximal orbit velocity, beyond which a spacecraft must quickly burn in a retrograde position lest it fly out into interplanetary space. Moons, orbital installations, captured asteroids and even the planet itself form a physical landscape to hide behind. These restrictions are of little importance to spacecraft that accelerate at milli- or centi-gee rates, but a Heinlein-esque torchship with mulit-gee acceleration will feel trapped and claustrophobic. 

In low orbit, small differences in orbital altitude equal large relative velocities. Fleets will have to stay very close to each other and perform constant station-keeping if they wish to form a cohesive group. Alternatively, if the planetary body is small enough for the number of ships in the fleet, a constellation can be formed with the planetary body in the center, with spaceships supported by the preceding and following units along the orbit. 

In high orbits, space is much flatter, and warships start resembling their deep space brethren more closely. 

Despite other inaccuracies, at least they are pointing away from the planet being defended.

Combat between spaceships in low and high orbits is quite asymmetrical. Lower orbits have much shorter orbital periods, meaning they spend the majority of their time outside the reach of their opponents and behind the planet. Higher orbits are on the other hand exposed constantly. This is important for laser-dominant warfare.

However, lower orbit spaceships must fight gravity to shoot projectiles into higher orbits. Due to the distances involved, higher orbit spaceships will have a long time to evade or intercept incoming threats too. This means that they are better protected in missile-dominant warfare.

There are also some slight advantages to each position that influence tactics but do not have influence over the fleet's formation. For example, high orbit spaceships' positions change slowly, and they cannot exploit aerobraking for sudden changes in trajectory. This means that their position is predictable enough for low orbit spaceships to fire from behind the planet and 'bend' their shots onto their target's position, without ever being seen firing. Another example is laser traverse rate. High orbit spaceships only have to fire at targets within a narrow cone, meaning that their optics don't have to move as much and become more accurate...

The second, much more important factor to influence fleet arrangement is the technologies being used as a result of the author's assumptions. We will discuss this vast subject in the next part of this post.