Fundamentally, what are the pros and cons of fleet vs mega ship (warfare)?

Question

You have an advanced space faring civilization tons of resources, and you could build one massive, heavily armored mega ship, or with the same resources, build thousands or hundreds of thousands of smaller ships. And I'd like to look at a gradient here rather than binary: You can obviously divide your resources anywhere between 1 huge ship with all of your resources put into it and an an infinite number of very small ship, so 2 0.5 times ships, 4 0.25 times ships, and so on. How do the pros and cons change as you go down this gradient?

Let's assume this is a general purpose military fleet who's opponent is someone with similar capabilities, and the two factions are facing an inevitable all-out war. They can build ships as big as mountains or as small as a manned fighter drone can get.

• They aren't interested in smaller unmanned crafts because using AI to control a swarm opens a significant risk of that system being compromised and losing control of their crafts, however they do have a massive personnel force in the hundreds of thousands.

• Neither side has developed some massive weapon that the huge ship would have the benefit of carrying.

• They're fighting over one small but extremely valuable area. They don't need to span galaxies with their forces. Just one planet of great value to both sides.

Answer 1

Redundancy vs. Economy of Scale

• A single ship might be more efficient. You need only one person at the helm, etc. You need fewer engineers, physicians, etc. since 'one and a deputy' is often enough.
• On the other hand, a single megaship can only be in one place, travel one route. If you never manage to fill her cargo holds and passenger quarters to capacity, they are just wasted. If you have hundreds of smaller ships, you can send just enough for the job.
• How efficiently can you load and unload a megaship? Can it land? What is the size of her shuttles? How many can dock at the same time? Space traffic control might become a bottleneck.
• Having two or more ships in convoy means that they can help each other if one has an accident, provided that accident is something to hit only one ship at a time.
• This applies even more if the attack is intentional. I'm assuming that there is a reasonably portable weapon which can kill the largest megaship. A nuclear missile? If armor helps, things might look different.

Answer 2

A mega-ship can be in one place. Smaller ships can be deployed to many places at once. In 2017, you don't need a battleship or an aircraft carrier to fight pirates. You need a frigate. Frigates are much cheaper.

You can mothball small ships when you no longer need them. You can't mothball half of a mega-ship.

When technology improves, you can start making or upgrading smaller ships more quickly than the mega-ship. And while the mega-ship is in the drydock, you have no ship.

EDIT 1 - So, what do we do if the pirates in the example above are protected by a heavier ship? What about the frigates then?

Well, in our sci-fi universe, it depends upon the technology available

Consider the change that occurred in naval warfare in the 30s. Aircraft had improved to the point that one with a crew of three or less could cause severe damage to a capital ship. A small swarm were a danger to any ship afloat. Many things changed about aircraft, but in general they became able to carry heavy ordinance long distances. This was so fundamental that the demonstration the Japanese gave the USA on 12/7/1941 changed how the USA built and organized the entire Navy.

In the sci-fi universe, can a frigate or other small ship carry a weapon that can threaten a larger ship? In the Star Trek sort of world, I would not expect a frigate to ever shoot through the Enterprise's shields. In the modern world, the Russian "Bear" bomber is no joke. It will not fly over the USA. Instead, it will stand off the coast and puke out a lot of nuclear-tipped cruise missiles. So again we see weapons that are very portable for the amount damage they do.

We need clarification from the OP. But in a universe where nuclear weapons exist (and people will use them) and there are no shields, a larger ship is a liability.

EDIT 2 - Modern USN frigates can launch Harpoon missiles. This is really no surprise. This is another modern example of a ship having the ability to destroy a ship that's much larger. Further the frigate should also have smaller weaponry (5", .50 cal, or 20mm cannons) for more tactical use. If we up my arbitrary "frigate" to "destroyer" then all things are possible. But for 2017 pirates, a frigate is enough. For fighting a pirate frigate, it would be my expectation that a government would send six. In an equal-number fight, they'd depend upon superior training, tactics, maintenance, equipment, and discipline to carry the day.

EDIT 3 - Regarding They're fighting over one small but extremely valuable area....

What's important isn't the size of the area, but how long it takes the ships to traverse an area that size. How do they get to it? How far from the home words is it? Is there a difference in the technology to get to the star system versus the technology required to travel within the system?

For our current technology, it could take years to get a probe to the outer reaches of our solar system. Even Mars is a few years, I think. This does not make for the best story-telling. In the Honor Harrington books, the author (Weber) addresses this issue by inventing technologies that supports insane amounts of acceleration. And as long as an author is consistent, there's nothing wrong with that. (In one book, HH's poor ancient cruiser is shot up; it takes weeks to get to the in-system base... normally it would have taken a day or two tops. Yay math!) Conversely, in the Star Wars universe, ships can travel any distance apparently instantly. Defense is very difficult now. I don't like that model. Star Trek feels very inconsistent to me and I don't pay much attention to any alleged science in the show.

The short of it is that you should define how long you want travel to last, and tune your technology capabilities to fit the story. And don't explain why the technology works. A patrol for a USN submarine in WW2 might have been 6 weeks. So if that sounds good, define 3 weeks to go from Earth's average orbit to Neptune's average orbit and call it a fine day! Be sure the time to Mars (a likely stopping point) also fits your story. If it doesn't, fiddle with your technology. (Mars is 1/60th of the distance from Earth to Neptune. So assuming linear time, it would take 8 hours to get to Mars. A WW2 battleship could steam at 20 knots, so that would be 160 miles. Getting from one major station to another major station in 8 hours seems a bit quick. I'd invent a "The engines produce less magic go fu when there's more gravity..." Now it might take a week to get to Mars, and 3 weeks to get to the outer reaches.

Answer 3

The fundamental difference is the surface-to-volume ratio.

A big ship has a large volume, which creates a lot of heat (which isn't easy to get rid of in space, an often overlooked pesky detail), but might hold huge reactors or missile stores.

A small ship has a relatively larger surface, which enables easier heat shedding, and gives you more space for weapon emplacements. But its armor will probably be a lot flimsier than the megaship's.

The crewsize is a matter of technology: it's possible that a highly automated battleship might not need more crew than a fighter.

Of course, a fleet of small ships enables different tactics, ie. being in many places at the same time. Whether this is relevant depends on technology and circumstances.

Answer 4

There is one thing that comes to mind - especially after having read a LOT of science fiction based space warfare of one kind or another.

Loss of life. Space is a fairly unforgiving stone cold bitch when it comes to space battles/ warfare. Loss of life in a damaged space going vessel is very likely to be total.

SO - Pros vs Cons on single large vs many (thousands) of ships.

Single large - if the ship is severely damaged - you will very very quickly lose 90% plus of your combatant force.

Many small - for each ship that is severely damaged you will still lose 90% plus - BUT there is a high chance you will not have ALL of your fleet damaged/ destroyed.

Therefore many small gives you a likely higher survival ratio.

Of Course - it will all go very badly pear shaped for the other side whatever ships you have if you have Honor Harrington in your military. Anywhere in your military.

:D

Answer 5

There is a fundamental balance of combat which exists in all renditions of combat. This is just one of them. Many smaller craft offer flexibility, while one larger craft offer strength.

If you know exactly what needs to be done, and can pinpoint it to a single location, one large craft will give you the most bang for your buck. It can have heavier armor and better weapons. However, in practice war doesn't give you such simple easy situations. Typically an opponent, when faced with one large craft, will make sure there isn't one location where you can just park your ship and win the game. They'll adjust their tactics. They'll make it hard for you to know where to go. They'll have multiple redundant systems.

If you have many smaller ships, there are three major advantages. One is that you can spread out across multiple targets. The second is a vast increase in information processing capability. With smaller ships, you literally have a set of eyes on every ship, feeding information back and forth. It is highly likely that someone has seen something that you wouldn't have seen from onboard a monolithic ship. Third, each ship is autonomous. If they see some small goal which they could achieve but you might not be bothered by, they can go get it. While the general is interested in taking a city, the individual grunt can decide "I need to take this room out of this building, because it could be a sniper nest." That sort of distribution is massively valuable. The price of this is that you can't have the arms nor armor that a megaship could. This means that a hardened target may be invincible against your attacks.

This is why all real militaries operate on many scales. Navies have ships ranging from aircraft carriers down to frigates, and they use them synergistically. Armies divide up into squads and platoons. They rely on the small flexible units to do the work until the targets choose to harden themselves. Doing so forces them to hold still, which makes them valid targets for the slower lumbering craft or organizations.

Answer 6

Oversimplifying to the extreme, but you didn't ask for [hard-science].

Larger vessels can support larger weapons, armor, shielding, what–have–you. They are probably also less maneuverable than smaller vessels, even if you had some manner of inertial control which allowed the larger vessel to move about with the same accelerations, relative to an outside observer, as a smaller vessel.

A host of smaller vessels are able to do a number of things which larger vessels cannot:

• surround the enemy
• penetrate blockades
• modular — loss of one doesn't impair the intrinsic operability of the others, unless they used some distributed comupting (which, in your case, is unlikely)
  This one is more of a trade–off with a larger vessel, because your smaller ships are also more likely to be destroyed due to their lack of shielding.
• variable spread — the smaller vessels can gather reconnaissance from a wider number of points in their operating theater

However, there is that thing with the vulnerability of the smaller vessels. It is quite possible that, because they are unable to support the shielding necessary to adequately mitigate damage, that an attack which would cause no permanent damage to a larger vessel would destroy or incapacitate smaller ones. Smaller vessels may be more difficult to target, but they are easier to destroy.
(Of course, that might not be a concern with your world's tech.)

That's why most fleets do a little of both. I'd recommend a inverse logarithmic ratio of vessel size to their portion in your fleet: the larger, the fewer.

The answer from MolbOrg does a good job too, and probably better.

Answer 7

It's very much a matter of the technology.

In a world where offense beats defense you favor small ships. In a world where defense beats offense you favor big ships.

At various times both of these states have existed on Earth.

Look back 100 years, battleships. Armor, specifically. Double the diameter, you increase the volume 8x but the surface area 4x, thus you can double the armor thickness. Big enough ships were very hard for the shells of the day to penetrate. Thus the battleship was king.

Look at the modern era. The threat is missiles, not guns. Missiles don't have the weight limits of artillery, armor is no longer of value against them. Being bigger no longer is of value in defending yourself.

Ships are sized to carry the systems they need to carry, no bigger. That's why the carrier dwarfs it's escorts and why the carrier carries little more than point defense weapons.

Also, observe aircraft. Air-to-air aircraft carry no armor at all because it's useless against a missile. Air-to-ground aircraft may carry some armor to help against gunfire from the ground, it's useless against missiles. Thus aircraft are built as small as practical if they're going into harm's way. Bombers are big--but they're also not supposed to be exposed to anything more than light fire from the ground near the target.

So long as the missile is king ships will be small. When we move into space the situation might change, though--as the range goes up the missiles need to be bigger and bigger to do the job, missiles to shoot at missiles are far smaller than their targets, eventually the balance would shift back to the big ships.

(And it can get even more complex. Observe the Honor Harrington universe by David Weber. At the start you couldn't get a missile through the defenses of a heavy ship. Major battles were ships slugging it out with lasers, armor was of value against this and thus the big ship was king. As the series progresses missiles switch to bomb pumped lasers rather than direct attack--the standoff range makes the defender's situation much harder and the missile comes to dominate. Light ships make a comeback because of this. A while bunch of dispersed firing platforms are more useful than one big ship. They don't replace the big ships, though, because the big ships can actually guide the longer range missiles.)

Answer 8

It depends on the technologies they have. Different technologies lead to different solutions and different possibilities. The knowing of nuances of those technologies is one of the problems in attempts to figure out the optimal fleet.

It starts from ISP of engines they use(or properties of FTL if there is one) to how efficiency weapon can be scaled (bang for energy buck and energy/resource buck to build the thing)
As an example Death Star was a crappy ship technology wise, build wise, but it had a good main weapon with good bang for energy buck.

But then we also have to understand the goals of the combat, because the destruction of the planets which DS was capable of, it is good and impressive, but what if no one gives a crap about planets as places to live and considering them just as a resource of matter for their constructions. With space habitats, it is a real alternative, and destructions of a planet will make them just happy, as the way which spends them time and efforts to lift the matter from the space body.

• yes, in your question they seem to care about the planet, but in which fashion they do that. They will prevent its destruction, prevent a landing of the troops to the planet, prevent nuclear/whatever bombardment, etc.

All those and other details are important to answer the question.

Another example - if a big ship can wipe a star system in dust in 5 seconds, jump in 5 seconds in any place with FTL and you choose the big one or the fleet of the millions of small ships which are capable(any of them) of destroying the big ship in 5 seconds - go with big one, it will destroy millions of star system before it will be destroyed itself.
But if there are countermeasures for FTL - then answer is not so easy, it depends on the efficiency of those countermeasures. If you can see the enemy in FTL, predict his destination, attack while in FTL then the picture is different(and btw similar to the noFTL setting).

If we take noFTL setting or one which is just similar, by the properties, to noFTL one, then distributed systems seems to be preferable than a single big ship.

Missiles are preferable(but it depends on engines, energy sources, how efficient they really are against the big ship etc)

The distribution by size in the fleet depends on goals/task they have to solve and how mass efficient are those solutions.

As an example, a carrier in the real world have the displacement of about 100'000 tons but can be destroyed with a single rocket which weight of about 3 tons. If the probability of destruction is 1%, then you better have 1000 of such rockets, it will be just 3% by mass, and the probability of survival for the bigger ship will be about 0.004%. (in real live there will be a problem to launch that many rockets, but in space, you can have them floating, and starting on command; but IRL carrier solves another task of offering different capabilities - that is the part of the task for a fleet problem)
But if the efficiency of the bigger ship to be destroyed is not 3% by mass, and you need the same amount(by mass) of rockets, then it might depend on efforts needed to produce those rockets compared to efforts to produce the big ship.

Again the AI thing - current rockets do not have AI(as super intelligent thing), they are capable of winning the intelligence race with a rock but that's all they can - still, they are efficient in the destruction of their targets.
There are other ways to make them smarter not including AI in the equation, including an operator far far away which chooses the initial strategy of the attack.

At some point of nonmagical technology, as far as I can see myself, there is no difference in a big ship or a small ship. You just have more than your opponent and expect that what will be left is that advantage you had, so basically 1:1 ratio by mass.

As an attempt to figure things out you might read the answer, the OP in the question was more extensive in describing the available technologies, but it still not enough to answer the question.(TL;DR; missiles rocks)

Recently I saw a conclusion for Children of a Dead Earth, game, simulation that missiles again rocks, but I know a lot of people who will disagree.

But the conclusion that missiles rocks, again is not directly applicable to your situation, as it means that big ships might be not the best thing to have, but it does not make small ships automatically better than middle sized.

Advances in detection systems play a significant role in the situation but depend on the speed of travel of detected target and the goals it might have. If there is possible some sort of fog of the war situation, then having a fleet of smaller ships can be advantageous to having the big one.

etc etc etc etc

As a recommendation, go with million or few hundred millions of average size ships(100m - 1km diameter ships). It's adequate force per star system with no defense, which will allow establishing the defense and controlling the system over the time. 10 years and the system will be not easy to take, without magical technologies or overwhelming force.

As for a planet, take smaller ships and use them as missiles.

Answer 9

For a practical answer, there is Eve Online. Eve has served as a test-bed which has shown multiple ideals depending on the opponent and the effective response times of each fleet. Bigger craft require more logistics to get them to field and the turn-around on losses over time can be prohibitive. Really small craft are just unable to bring enough damage to bear on extended engagements. A mix of medium to large (note: not super) with varied damage output brings a much improved tactical response and flexibility. If you do not need to hold the field, such as when the attacker wishes to disperse the standing fleet (such as skirmish actions before the battle) small bombers in sufficient numbers, or a few large ships in hit and run actions. To hold the field, you need to define victory conditions. In Eve, it would be the ones able to consistently replace the losses to keep the damage flowing. Once one side has lost the ability to deal enough damage it has lost, and the field can then be claimed (and looted) by the victor. In a technological space battle, it is thus the missile/ammunition reserves, or the number of pilots, or the raw nano-tech/anti-matter/electro-graft material that determines your ability to keep fighting. One very large ship that serves as a factory base may prove superior to transporting replacement (small) craft

Answer 10

With the right technology, smaller ships can hit as hard as larger ones by combining their firepower. Here's an example from Star Trek: Voyager:

In this scene, the formation of 9 identical ships, all relatively small (smaller than the Voyager which is crewed by 150), are generating enough combined power to destroy an entire planet.

Larger ships are simply unnecessary. They'll also tend to be slower, easier to hit, and once a critical portion of a large ship is destroyed it's just gone. If one small ship in a megaweapon formation is destroyed, another small ship can take its place.

It doesn't even need to be this advanced, either. Firing dozens of smaller missiles or other primitive projectiles can cause as much damage as firing one big one. On the other hand, a larger ship has more room e.g. for a railgun to accelerate a projectile to much higher speeds than smaller ships could.

Here's another example from Voyager you might find interesting:

This is called "multi-vector assault mode", in which one large ship splits into multiple smaller craft. This spreads the firepower of one ship across many, allowing them to attack from multiple directions at once. Each ship can move in a different way, allowing all kinds of neat strategies you can't afford when your firepower is concentrated at one location.

So, if you can't decide, you could go the "multi-vector" route and make one ship that becomes many when necessary. Best of both worlds, really.

Answer 11

Really, it depends whether the small ships can carry weapons that are effective against the larger ships. To use a naval parallel, around the turn of the 20th century it was possible to build large battleships such as the Dreadnought class that were considered super weapons. The armour on these ships was reasonably effective against anything smaller than the guns carried on large capital ships and the main armament was considered to be lethal to smaller craft.

By the 1960s, guided anti-shipping missile technology meant that a relatively small fast attack craft or smaller warship could carry a weapon capable of posing a threat to a capital ship, rendering the large gun systems of early-mid century ships obsolete (the missiles also had greater range). Now there is no need for large gun-armed battleships or heavy cruisers and they are considered obsolete (not to mention the increased role of aircraft from WWII onwards).

Thus, there are few weapons that merit building a warship larger than a few thousand tons and larger (cruiser-sized and up) ships are in a minority. The Ticonderoga class is unusually large (approx 10,000t) because it carries two very large (~120 tubes in total) vertical launch systems. In most other cases, large naval hulls are built for other payloads (such as aircraft carriers) rather than out of a need to carry a large weapon system. Aside from specialised roles like aircraft carriers or landing ships, surface combatants larger than 5,000 tons or so are unusual.

With changing techology the cycle may continue, however, and swing back to large warships being a dominant technology.

• Point-defence systems such as Phalanx are reasonably effective at defending against anti-shipping missiles, requiring large salvoes to be launched in saturation attacks.

• Lasers can be used to blind pilots or sensors and are potentially an effective anti-aircraft defence, mitigating the current dominance of air power.

• Rail gun technology (if it proves effective) fires high-speed kinetic energy rounds with range in the 100's of km that will be difficult to counter with point defence weapons.

This could drive a swing back to large, armoured capital ships if these technologies prove to be effective anti-aircraft and anti-missile defence. Then your entry barrier will be a hull large enough to carry a rail gun system and a power plant (possibly nuclear) big enough to power it.

So, the answer is - it depends on the capabilities of the technology. In Star Wars a few squadrons of small fighters slipped through the defences of a poorly designed super weapon and destroyed it by shooting a torpedo down a reactor exhaust port. In Star Trek canon a single Constitution class cruiser was supposed to have enough firepower to trash an entire planet.

Answer 12

I can't hope to compete with most of the excellent answers here already, but nobody has yet pointed out the issue of artificial gravity. If the contested space is small enough that your craft can launch and return to a planet / moon / space station and the crew only have to endure limited flight times weightless, maybe this doesn't matter. (Although, if the space station is only a short flight away, it will quickly become an obvious target for enemy action, at which point you have to beef up it's armaments so it is basically a mega ship itself).

But if any action will require extended periods in freefall, a larger ship is going to be required to spin up to provide centrifugal force, otherwise your crew are quickly going to suffer the medical effects of extended microgravity. (Unless your universe has artificial gravity of course).

Radiation is another problem best served by a larger ship. The most effective radiation shield is a thick steel hull, maybe lined with water tanks on the inside. In a small ship, this will take up a much higher portion of your available space and mass due to the lower surface area to volume ratio. You may be able to obtain some charged particle shielding with a strong enough magnetic field (think run donut coils of wire all around the ships hull so that you get a toroidal magnetic field that doesn't permeate the interior volume), but that still benefits from lower relative surface area and probably wants a large power plant to drive.

Answer 13

A point to consider, military constructions and developments have - historically - resulted in the development of means to counter them. In a purer sense, it's dangerous to put all of your eggs in one basket, even a very safe basket, because somewhere there's a problem solver able to devise a method of defeating your basket's defenses, and before you have time to develop a new defense, your basket's been penetrated and all your eggs absconded with.

Historical/allegorical/literary examples?

• Troy - Trojan Horse.
• The Maginot Line - Belgium.
• The Whole Egyptian Army - Paint Cats on your shields.
• 13 Ships Hopelessly Outnumbered by the Japanese Navy - Abuse superior usage of terrain to force a drawn out engagement accentuating the advantages of your superior vessels, resulting in a Japanese rout and a flawless victory.
• The Death Star - Sabotage
• Scientists Have Revived Modified Dinosaurs Which Pose An Existential Threat to The State of Costa Rica On a Small Island Off the Coast - Burn it down, burn the whole thing down.

The point is, you probably can't build a thing so impressive, dangerous, or resilient that someone else can't invent a creative way of destroying it with a little lateral thinking; this sometimes applies to entire armies, but it's much easier to recover from this kind of thing when your entire military isn't a single massive vessel (or a few of them) - every problem has a solution, redundancy is the same as flexibility here, and is therefore your friend.

Answer 14

Most of the big points have already been made, but there's a rather large problem with the "Megaship" idea, namely that past a certain size structures start behaving more and more like flexible rods or giant springs.

So I'd say the fleet would be a better idea just to avoid having a ship that tears itself apart just trying to accelerate.

Answer 15

In my opinion, the correct answer would be a little of column A, a little of column B.
Larger ships definitely have their advantages. A large ship is usually more efficient in terms of materials and crew. You need skilled crew for every task required and backups for them in case things go pear-shaped for every ship in your fleet. A huge ship doesn't need much more crew for the basic functions (navigation, command, janitorial duties, etc) than a small ship. You need fuel, life-support, water and food for everyone on-board a ship, so less crews and ships means your entire navy is a little cheaper to maintain.
Larger ships are also capable of carrying a much heavier compliment of weapons and many more of them to boot. Their size means that energy-based weapons can be used much more effectively since they'll have heavier reactors to draw from. Their added bulk and size means larger weapons platforms can be mounted on them without having to design the entire ship around them.
Smaller ships, on the other hand, have other advantages. Individually, they're cheaper, easier to produce (economies of scale begin to apply here) and they're less vulnerable to enemy fire because they are both smaller targets and easier to maneuver (less mass = less inertia = easier to move). Their smaller size necessitates a more efficient design and producing more means that mistakes in the design can be caught in the early models and quickly improved upon.

Larger ships do have disadvantages. A single large ship represents a significantly chunk of your planet's GDP and if it runs into an asteriod, rail-gun shot or reactor leak, that is an investment you're never getting back. It paints a huge target on your back and represents a crippling loss to your military power if lost. As an added issue, once your ships get past a certain mass, they actually represent a danger to planets they're orbiting. When you get to Warhammer 40K sized ships, the bulk of the vessel actually becomes a kinetic kill weapon capable of anihilating a planet's biosphere when it crashes into it. That could be a pretty big problem.
Small ships aren't without flaw either. A small ship isn't going to have the supplies for long-duration flights. It'll carry a smaller compliment of weapons and said weapons will likely be less powerful.

My solution to your problem would be a combined arms approach. Several larger ships would form the bulk of your armada, each representing 5% to 10% of your total fleet's mass. These ships would be a mix between heavy siege ships and carriers. Each one could carry one or several of your heaviest weapons (think along the lines of Spinal Mass Drivers, Exotic Matter Projectors, Wave Motion guns etc). They would also function as supply vessels for your smaller ships. They'd carry extra fuel, food and so on. Naturally, each smaller ship would carry a sufficient amount to get back home but these large ships extend your fleet's operation time by several orders of magnitude.
Smaller ships, each around 0.1% and 0.2% of your fleet's total mass would make up the bulk of your fleet in terms of numbers but would ideally represent somewhere near 50% of your fleet in terms of mass. These ships are built for short-term, carrying the minimal ammount of supplies needed for their mission. They carry heavy weapons for their size and much of their mass would be dedicated to ammo, generators for energy-based weapons and so on. These ships would fall into different categories, from bomber-type ships that are a little bulkier and built around their torpedo tubes or rail-weapons. Some would be screening ships, bristling with point-defenses, jammers and sensors. Others would be search-and rescue, carrying emergency repair supplies to help damaged ships limp back to the fleet.

Answer 16

In real life, your megaship is an aircraft carrier. The carrier can defend itself using guns, torpedoes, and the fighters or bombers on board. Its size makes it less vulnerable to fire from small ships and even if it gets hit, its compartmentalized structure will ensure it doesn't get fatally damaged. Just like the Death Star. Also, its size allows for nuclear propulsion and for carrying a lot of ammunition on board.

Without additional protection from a few destroyers, submarines and other smaller ships, the carrier is vulnerable. But, if you judge by the relatively small number of support ships the aircraft carriers had during the Midway battle, it seems, at the time, the carrier was thought as the ultimate weapon. In fact, the battle itself was decided by which side lost more carriers.

In conclusion, the megaship should decide a space battle, not so much because it's big and has lots of firepower, but because it is a moving hornet nest, and any attacking ship needs to deal with its fighters.