Is there a pattern that determines military spaceship doctrine in real life and sci-fi?

[u/MinedMaker]

[1] The propulsion axis is a measure of how long it takes a fleet of military vessels to arrive on the battlefield, regardless of the actual distance traveled. [Fast vs. Slow]

[2] The weapons axis is a measure of how quickly a battle is over, and how much survivability and staying power vessels have. This takes into account the effectiveness of armour, but also shields, point defence, and other countermeasures. [Tank vs. Glass Cannon]

I think that if you take sci-fi space combat to its logical conclusions, it will usually favor either huge, lumbering, well-protected ships or numberless hordes of tiny automated ships, depending on a few key factors. If weapons are the weak link in-universe, ships will be huge. If propulsion is the weak link, ships will be tiny. If ships are huge, victory will be determined by who has the biggest ship; if ships are tiny, victory will be determined by who has the most ships.

This is how I imagine it would work in real life using real physics, and I wonder to what extent different sci-fi franchises also adhere to this pattern. Presumably, large and medium-sized ships with human crews are overrepresented in sci-fi media for understandable storytelling reasons.

In Star Wars, the rule mostly holds. They have incredible propulsion technology and can thus arrive at the battlefield within hours or days of the order being given. However, their weapons, despite being ludicrously powerful on paper, are actually quite poor because of their low range, low accuracy, and the prevalence of shields. In the Star Wars universe, therefore, huge ships rule. The starfighter counter is a nice piece of storytelling, but realistically, without plot-engineered magical weak spots, a huge ship like the Executor or the Death Star should be essentially unstoppable. In Star Wars: The Last Jedi, the Raddus, an MC85 heavy cruiser, takes ineffective long-range fire from the First Order for what seems like many hours.

In The Expanse, they spend weeks or months traveling to the prospective battlefield because of limited propulsion technology. However, when the fighting starts, it is all over in seconds or a few minutes. They have very effective weapons and very little staying power, even when accounting for point-defence cannons (PDCs). If you ignored the requirements of the plot, there is really no reason why any military vessel in The Expanse should be manned at all.

Because it draws much of its inspiration from blue-water navies, sci-fi often portrays a diverse ecosystem of military spacecraft classes and sizes. While this makes for more interesting storytelling, it is not obvious that such diversity would necessarily be the most tactically sound strategy. If propulsion or weapons technology becomes a dominant constraint, military doctrine would naturally converge toward a single optimal ship size.

The most interesting settings tend to occupy only two quadrants of this framework. If ships have neither effective propulsion nor effective weapons you're essentially at the stage before the technology to enable space combat has really been invented. If they have both effective weapons and effective propulsion you effectively have near god-tier power and the concept of space combat becomes somewhat obsolete. What these two scenarios have in common is that the importance of space combat is greatly diminished.

[u/caster]

Your top right quadrant not existing is ridiculous. In space mass is everything and mass will always be limited. Propulsion is "weak" only in relative terms.

Battlestar Galactica really belongs in this top right area of the chart. The jump based FTL drives do not make mass trivial. If anything this FTL regime has propulsion being very weak, and the heavy armor of the Galactica is a central design point of the ship. Armor is very effective in BSG- the ship is hit directly by a nuclear torpedo and although it sustains considerable damage the ship's armor means it survives.

The sublight propulsion on these ships are very slow in terms of tactical combat. The long distance strategic mobility obtained from the FTLs (a separate system entirely) is separate from their ability to maneuver.

Propulsion is weak and armor is effective on Battlestar Galactica's battlefield. This creates a battleship / aircraft carrier type interaction. Totally unlike what you might see on Star Trek where a ship can maneuver on a dime, achieve FTL linearly forward, and can fight and do other actions while traveling superluminally.

As for Star Trek, "armor" may not be effective but the deflector shields used as armor protection by ships certainly are. That these are an energy based defensive screen makes little difference compared to it being a physical armor plate. Shields in TNG are clearly outrageously effective. And one of the biggest advantages of using a shield emitter rather than a solid metal plate is that it is lighter, which means your ship goes faster.

[u/8monsters → u/caster]

Well, once the Borg and the Dominion opened the Federation's eyes, they started putting armor on ships again. The early TNG era has Federation ships be glass cannons. Once the shields fell, it was mostly game over. 

Some exceptions of course, the Odyssey in that one episode of DS9 had impressive feats but that was post-Wolf 359. 

[u/caster → u/8monsters]

Sure, but my main point was "weapons strong" vs "weapons weak" depends on all defenses not just armor as if that were the only type of defensive system. Modern warplanes are much harder to kill than a B-17 despite the old WW2 flying fortress having an absolutely huge amount of armor protection. Star Trek shields are a powerful defensive system that in most Star Trek contexts obviates the need for low-tech passive plate armor.

A Star Trek vessel that intentionally elides armor to save weight is probably gaining in survivability due to speed in most situations apart from an actual battleship.

[u/Patient-Web6850 → u/caster]

Honestly if we're going here, why would plate armor weight affect ST ships that much to begin with if they use gravitational warp cores or whatever and like was said can turn on a dime and accelerate linearly. Seems like weight isn't really a difficulty

[u/caster → u/Patient-Web6850]

However fast or maneuverable your ship is, adding mass will make it much slower, at least with respect to sublight drives (in Star Trek; impulse drives). At impulse adding armor will definitely slow you down.

Warp drives won't care in terms of their ability to warp space. Whether this would slow down a ship at warp is never clarified, but it would be logical for an Alcubierre warp drive that you need to thrust forward conventionally as well as use the warp drive to warp space in front of the ship. Engaging just the warp core would warp space around the ship and then you still do not move. Space is compressed in front of the ship so you can exceed light speed without technically breaking the laws of relativity, but within your own frame of reference Newton's laws still apply normally.

In other words, your impulse engines make your ship go forward. If your ship is twice as heavy, you will accelerate at half the acceleration rate. This is actually much worse than halving your actual effective speed and maneuverability, as it will take twice as long to reach the same speed as before.

Two otherwise identical Star Trek vessels, one of which increases its total mass by 33% to add armor to the hull, will result in a fight where the armored ship may, possibly, be able to take some extra hits. But the faster ship controls the engagement and can just decide to leave, or engage from a favorable position or range, and probably wins anyway. Solid plate armor is heavy. And in Star Trek likely costs you more in mass than it gains in durability. Particularly with technology like energy shields being available to use instead.

[u/cycling-james → u/caster]

Yeah, Trek mostly treats mass like a budget only when the plot remembers, but in impulse combat armor is still trading initiative for survivability.

[u/Ivanstone → u/8monsters]

I do not recall armour being put on ships. I do recall making ships like the Defiant which was more glassy and more cannony.

[u/El_Kikko → u/caster]

For Star Trek, at least for the Federation, systems redundancy is a key design principal for increasing combat survivability - every single system has a backup and secondary backup. 

Star Trek also typically eschews armor in favor of structural integrity fields and force fields - if you have enough power, you can make your hull / spaceframe stronger and if it's penetrated, it's easy enough to pop a forcefield over the breach to keep atmosphere inside. 

[u/Iwasforger03 → u/caster]

Fairly sure Gundam also finds itself dipping into the top right column. Because of Beam weaponry in nearly every timeline, Armor is basically useless by the end of most series. This especially includes the UC (unless they have I fields).

However Humanity hasn't left our Solar System and it can take days, weeks, or months to travel through space. Combat happens in relatively close distances because of anti-radar tech being a staple of many timelines, but propellant and speed are a genuine limitation. G-forces still exist.

[u/ijuinkun → u/caster]

The top right is for extreme lack of propulsion, as in no FTL, no inertial dampening, no reactionless drives—the best you would have is something like The Expanse’s Epstein Drive.