Hard Sci-fi and ship based weapons.

[Minischoles]

By that same token, planetary attack missions would be one-way missions in a realistic sci-fi setting. There's no way the ships would have enough fuel to fight their way to a target planet and then fail to take it. If they take it, they can use its resources to refuel their ships and build new ones. If they don't, they're fucked. They can't even go back home.

'Twas pretty much what I was thinking. Each 'assault ship' would pretty much have what it took for a round trip and that's it. Assaults are massive undertakings with millions of personnel and thousands of ships.

Even if they are only an assault force you've got to assume they'd bring some kind of logisitical support along as well, carrying spare parts for the ships (even if the ships are near perfectly engineered odds are they're going to need spares at some point), a resupply of ammo and fuel, perhaps even a giant extraction/refinery ship if they do need more fuel, and a factory ship as well (i'm assuming if its a race advanced enough to build spaceships they're at least up to mounting asteroid capture missions for clean resources) that can manufacture basic parts and weapons.

[MichaelFerrariF1]

What are the limits on your FTL? Can you revert to sublight in orbit over the target planet, or do you have to revert outside the system and slowly make your way to the target?

It seems to me that that determines your combat range. If you can emerge in orbit over the planet, combat will probably take place at ranges of hundreds of kilometers or less. If not, you can expect extreme long range combat as defending fleets try to stop your invasion.

[Darth Wong]

The ability to bring along logistical support only increases your non-fuel mass, thus creating more problems. Now in theory, it means you can fabricate more stuff if you need it, but it's a matter of time. Loitering in an enemy star system means that you're in a race to replenish your supplies and rebuild equipment before your enemy can, and he has the planet. He has the established infrastructure. Unless you absolutely smash his infrastructure, he should be able to build more missiles, more weapons, more ships, and supply more fuel much faster than you can.

In a realistic sci-fi universe, you don't really get two kicks at the can. Your mission either succeeds, or it fails.

[Junghalli]

I'm going to give a brief critique of some of your essay's points here...

No problem, I appreciate constructive criticism, and it may be helpful to others.

First, you immediately say that stealth is impossible, therefore suprise attack is impossible. But, you didn't define the setting entirely. If you have a setting like my own where most the action takes place in Earth orbit, the rules change a little. In fact, they change a lot.

Stealth isn't really possible still, but surprise attacks certainly are - everyone is within everyone else's firing range, so all you have to do is hit the trigger. You can also possibly build ships in secret, entirely inside habitats. Someone following the money would probably put it together, but it would take accountant spies (or maybe traditional human spies on the inside) rather than telescopes to figure it out.

No disagreement here.

Kinetics at a range like this would be less useful, since most everyone are barely moving relative to everyone else.

Possibly, though you don't really need to be moving very fast for kinetics to be effective. Anything going at 3 km/s packs the equivalent energy of its own weight in TNT. I doubt many realistic ships are going to like being hit by something sizable going at, say, 8-10 km/s. Moreover, if the ships are only designed to putter around in Earth orbit they probably won't have much more delta V than the missiles, which eliminates one of the main disadvantages of missiles (unless you can put a high-performance nuclear rocket in one they will have a limited range beyond which the enemy ship can just get out of the way). On the other hand orbit would probably be a very hostile environment for missiles, because a missile moving at only a few km/s is a very easy target for a nearby laser. Orbit would likely be an arena in which lasers dominate.

You mentioned the difficulty of matching speeds to make dogfights not happen, but this also expands to something else: your ships don't have much of an option to go off course. Suddenly aborting an interplanetary mission may be impossible; once you go, you're going all the way.

Quite true. If possible I think you would give warships, at least, enough fuel for a two-way mission, probably with some extra for manuevering. But how feasible this is depends on what sort of drive and what sort of transfer orbit you're using.

In earth orbit, your high delta-v engine will never achieve its max speed; the distance is simply too short to let it run. Here, high acceleration is probably optimal; the d-v you need is pretty small anyway, so trading it off for acceleration is probably a reasonable deal. On the other hand, like I said above, if you weapons tech is good enough, everyone is in everyone else's firing range anyway, which makes acceleration go back to being less important.

Agree with this.

It still could be for civilian transit though.

Indeed. I think passenger spacecraft, for instance, will probably use chemical rockets rather than electrical ones, because nobody wants to wait a month to go from LEO to GEO. Of course, I think in a well-developed orbital infrastructure most shipping will be done through a rocketless "highway" system of rotorvators and space elevators.

You said the max range of lasers is about one light second due to targetting concerns. This depends on the size (and acceleration) of the target's side that is facing you - you might be able to hit a 200 meter wide battleship from farther out than a 10 meter wide scout. The solar panels I love would be prime targets if you see them on the wide side, even from a long distance.

True, it's more of a rule of thumb than anything else.

On defense, again, that depends on your setting. If you are in Earth orbit, there is no need for the magic fusion torch, so not all ships need to be WMDs.

Nuclear salt water rockets, D-He3 fusion, and ICF fusion are actually pretty hardish drive systems, I'm not sure if I'd call them magic. But yes, if you're only moving around the vicinity of a planet yes, you don't need such advanced drive systems, chemical, electrical, solar thermal etc. will be used. That said a massive object hitting a city at orbital velocity could still be fairly nasty, though most spacecraft would probably end up breaking apart on the way down and not doing much damage.

Even interplanetary doesn't if you use things like laser stations to move ships around. The ships themselves are just boxes on mirrors - they are propelled by lasers at controlled facilities. Of course, these lasers can also make potential interplanetary death beams...

Oh yes, the simple fact is moving a massive object between planets in a short timescale involves harnessing quantities of energy that would do very nasty things if used as a weapon. Jon's Law.

All in all, I thought it was a good essay, but some of your setting assumptions weren't as clear as they probably should be.

Thank you, and yes I admit it could have discussed a wider variety of scenarios. I was actually planning on doing sections on planetary invasions and interstellar warfare with no FTL, but I thought the thing was getting too damn long already.

[Ender]

The ability to bring along logistical support only increases your non-fuel mass, thus creating more problems.

The thought was that the logistics ship would be a separate vessel. That makes it no more challenging then creating a war ship as your design replaces missiles, armor, and guns with machine shops. You also will do less maneuvering, have less a need to accelerate at a higher rate, etc. Really, without knowing more specifics you can't say which would entail more design problems. In fact if you take the position of being totally from scratch the logistics ship would be easier because you will have already designed most of its machinery and stuff when you were building the infrastructure that let you build the warships in the first place and can recycle the designs, whereas you need to design the weapons systems anew.

Heck, the logistics ship would be easier to design then the ships that carry enough to let you invade in one go for the very reason you cite. I can move a 100,000 ton factory ship and make unlimited tanks, or bring 10,000 tanks for the invasion and have the same design issues. If you want to argue that you can transport the tanks in smaller groups then there is no reason you can't bring the logistics depot in parts as well - we did it with the ISS after all.

Now in theory, it means you can fabricate more stuff if you need it, but it's a matter of time. Loitering in an enemy star system means that you're in a race to replenish your supplies and rebuild equipment before your enemy can, and he has the planet. He has the established infrastructure. Unless you absolutely smash his infrastructure, he should be able to build more missiles, more weapons, more ships, and supply more fuel much faster than you can.

He is welcome to the planet, I'll take the rest of the system. I can always get the planet at my leisure so long as I control the rest. It represents more resources, more power, more space, and more options.

Of course if you are willing to commit what would likely be defined as war crimes there would be no need for the logistics ship. Shoot the orbital infrastructure with a charged particle beam or a dispersed x-ray or gamma laser and kill everyone on board, send in your own robots to operate the space infrastructure there.

In a realistic sci-fi universe, you don't really get two kicks at the can. Your mission either succeeds, or it fails.

How hard do you want to go here? If we assume an absolute hard scifi there isn't going to be much in the way of interstellar war, the distances make it impractical. A book idea of mine is the only way you would see what we recognize as war today - two planets tried to colonize the same system, and one got there before the other. In that case both would be coming with infrastructure to build things on their slowboats, and that is how the war would come about. Other then that it is basically genocide the natives or assimilate the invaders.

[Sea Skimmer]

The ability to bring along logistical support only increases your non-fuel mass, thus creating more problems.

Your invading a planet. Logistical support will be massive or else you’d be idiotic to even think about trying. Establishing some kind of advanced base, even if it doesn’t try to make fuel, will be essential to success just as it was in the Pacific War. You would easily have 3-5 support ships, many of which would simply be conversions of civilian vessels, for every significant warship. Seriously now, in WW2 a destroyer steaming at economical speed could still barely cross the Pacific one way, and merchant ships took several months to do so, but we still managed to support a fleet steaming at high speed off Japan. By mid 1944 the USNs service force ships (these are the ships which directly replenished naval units underway, and at the most advanced of bases) numbered more then 450 major vessels. A couple thousand more ships sailed from the US west coast to replenish the service force.

A single fast carrier task group with 3-6 carriers would have as many as two dozen oil and AVGAS tankers in direct support, plus another dozen ammunition ships, provision ships, hospital ships and general cargo vessels. Dozens of escort carriers did nothing but ferry planes out to replace of losses on the fleet carriers.

No one before or since has matched this massive seaborne supply chain, but its what you’d need for interplanetary warfare. If you can’t mount that kind of effort, and your enemy can’t either to come attack you then you’d have little reason to even try to fight a war.

Now in theory, it means you can fabricate more stuff if you need it, but it's a matter of time. Loitering in an enemy star system means that you're in a race to replenish your supplies and rebuild equipment before your enemy can, and he has the planet. He has the established infrastructure. Unless you absolutely smash his infrastructure, he should be able to build more missiles, more weapons, more ships, and supply more fuel much faster than you can.

That assumes the enemy planet is actually able to build advanced weapons, most targets would be lightly developed owing to the massive effort required to actually invade and occupy a highly developed world. Centers of strong resistance would be bypassed, or simply bombarded from orbit, ground based weapons will never be able to compete with orbital weapons in the absence of some kind of energy shield. If you can even think about attacking a planet with say the development of modern earth then your resources should be so overwhelming that sending out fuel tankers is no big deal.

Anyway, if you cant mount the proper logistical effort to attack another planet, then why even try? The enemy probably cant attack you either, and its pointless fighting over the mere void of space. A truly realistic space war would probably last for decades with a battle between tiny unmanned patrol vessels every five years or so.

[Alyrium Denryle]

By that same token, planetary attack missions would be one-way missions in a realistic sci-fi setting. There's no way the ships would have enough fuel to fight their way to a target planet and then fail to take it. If they take it, they can use its resources to refuel their ships and build new ones. If they don't, they're fucked. They can't even go back home.

'Twas pretty much what I was thinking. Each 'assault ship' would pretty much have what it took for a round trip and that's it. Assaults are massive undertakings with millions of personnel and thousands of ships.

If that is the case then static planetary defense systems become a workable solution. Armed satellites could, in theory, fire off enough missiles, that would force ships to either maneuver (thus using fuel) or be destroyed. At the very least it would increase the costs of undertaking an invasion, and increase the vulnerability of the logistics train to pre-assault counterattack.

As for ship to ship combat and weaponss, I am rather fond of Coyotes idea, to use solar collectors to power weapons. Also bear in mind, that without handwavium FTL sensors, ships will be restricted to various types of light collection. Light Telescopes, IR, Radio, and Gamma wave sensors would be how ships are detected, making stealth, in addition to maneuver and power use very important.

One thing fleet ships (destroyers and the like) would want to do in combat is keep the solar collectors to one side (toward the local star) to avoid them being shot off, while also keeping themselves from being detected, and keep themselves in a good firing position.

Were you to write out combat, fleet captains would be playing a very dangerous, and if written correctly, interesting game. And also bear in mind, a planet's defensive fleet would have to be taken out, or you risk your logistics train during a planetary invasion and would be forced to withdraw.

[Darth Wong]

The ability to bring along logistical support only increases your non-fuel mass, thus creating more problems.

Your invading a planet. Logistical support will be massive or else you’d be idiotic to even think about trying. Establishing some kind of advanced base, even if it doesn’t try to make fuel, will be essential to success just as it was in the Pacific War. You would easily have 3-5 support ships, many of which would simply be conversions of civilian vessels, for every significant warship. Seriously now, in WW2 a destroyer steaming at economical speed could still barely cross the Pacific one way, and merchant ships took several months to do so, but we still managed to support a fleet steaming at high speed off Japan. By mid 1944 the USNs service force ships (these are the ships which directly replenished naval units underway, and at the most advanced of bases) numbered more then 450 major vessels. A couple thousand more ships sailed from the US west coast to replenish the service force.

That's a good point. Instead of an interstellar attack fleet going right after the enemy planet, we should be thinking about establishing a forward presence near or inside the target star system somewhere and then creating enough infrastructure to support military operations, thus making it more of an interplanetary conflict than an interstellar one. Although the degree of feasibility here is still limited by just how realistic we're trying to be. These fleets would have to be truly staggering in size.

[Junghalli]

As for ship to ship combat and weaponss, I am rather fond of Coyotes idea, to use solar collectors to power weapons.

An internal reactor strikes me as being less vulnerable. Big soft solar panels are wide open to simply being sliced off by continuous-beam lasers. An internal reactor would be in the core of the ship, buried under layers of actively cooled thermal jacket "armor"; much safer.

Also bear in mind, that without handwavium FTL sensors, ships will be restricted to various types of light collection. Light Telescopes, IR, Radio, and Gamma wave sensors would be how ships are detected, making stealth, in addition to maneuver and power use very important.

There is no stealth in space.

Wargames like GDW's STAR CRUISER describe interplanetary combat as being like hide and go seek with bazookas. Stealthy ships are tiny needles hidden in the huge haystack of deep space. The first ship that detects its opponent wins by vaporizing said opponent with a nuclear warhead. Turning on active sensors is tantamount to suicide. It is like one of the bazooka-packing seekers clicking on a flashlight: all your enemies instantly see and shoot you before you get a good look. You'd best have all your sensors and weapons far from your ship on expendable remote drones.

Well, that turns out not to be the case.

The "bazooka" part is accurate, but not the "hiding" part. If the spacecraft are torchships, their thrust power is several terawatts. This means the exhaust is so intense that it could be detected from Alpha Centauri. By a passive sensor.

The Space Shuttle's much weaker main engines could be detected past the orbit of Pluto. The Space Shuttle's manoeuvering thrusters could be seen as far as the asteroid belt. And even a puny ship using ion drive to thrust at a measly 1/1000 of a g could be spotted at one astronomical unit.

This is with current off-the-shelf technology. Presumably future technology would be better.

Forget about not being detected. You're going to be detected the first time you fire your engines, end of story.

[Alyrium Denryle]

An internal reactor strikes me as being less vulnerable. Big soft solar panels are wide open to simply being sliced off by continuous-beam lasers. An internal reactor would be in the core of the ship, buried under layers of actively cooled thermal jacket "armor"; much safer.

Um... you realize that if they can sheer them off with lasers, they can just hit your hull and kill your ship right? I will give you the stealth. But the increased power generation ability of using a solar collector in addition to an internal reactor, increases the effective operating range of the ship, the number of shots it can fire before draining its batteries, capacitors, or whatever the hell it uses to quickly pump power from the reactor into its weapons and engines. In addition to allowing it some survivability in the horrible event that it runs out of reaction mass for its main reactor.

[Knife]

The ability to bring along logistical support only increases your non-fuel mass, thus creating more problems.

Your invading a planet. Logistical support will be massive or else you’d be idiotic to even think about trying. Establishing some kind of advanced base, even if it doesn’t try to make fuel, will be essential to success just as it was in the Pacific War. You would easily have 3-5 support ships, many of which would simply be conversions of civilian vessels, for every significant warship. Seriously now, in WW2 a destroyer steaming at economical speed could still barely cross the Pacific one way, and merchant ships took several months to do so, but we still managed to support a fleet steaming at high speed off Japan. By mid 1944 the USNs service force ships (these are the ships which directly replenished naval units underway, and at the most advanced of bases) numbered more then 450 major vessels. A couple thousand more ships sailed from the US west coast to replenish the service force.

That's a good point. Instead of an interstellar attack fleet going right after the enemy planet, we should be thinking about establishing a forward presence near or inside the target star system somewhere and then creating enough infrastructure to support military operations, thus making it more of an interplanetary conflict than an interstellar one. Although the degree of feasibility here is still limited by just how realistic we're trying to be. These fleets would have to be truly staggering in size.

Hmm, this is a good point and a good idea. Huge attack fleet with cruisers/destroyer analogues plus the thousands of 'assualt landers', ontop of a realitively small amount of what? Baseships/logistical ships warping/folding into the outer system to set up a forward base, then the 'attack fleet' folding/warping from there to targets. A forward base/ objective rally point/ fall back position.

[Junghalli]

Um... you realize that if they can sheer them off with lasers, they can just hit your hull and kill your ship right?

Not necessarily. It's possible to "armor" your main hull against lasers by covering it with a thermal jacket and putting an active cooling system under it. This will at least make laser burn-through take a lot longer and at long ranges where the laser is subjected to heavy diffusion it may prevent it altogether. Putting a thermal jacket over a solar panel will probably be simply impossible (it must be exposed to the sunlight, obviously), and giving it an active cooling system while likely doable will add a lot of mass to it (remember, it's generating megawatts to feed the lasers, it needs to be big). A solar panel will represent a large soft spot on your ship, an internal reactor will not.

But the increased power generation ability of using a solar collector in addition to an internal reactor, increases the effective operating range of the ship, the number of shots it can fire before draining its batteries, capacitors, or whatever the hell it uses to quickly pump power from the reactor into its weapons and engines.

But would it be better than simply making the reactor a bit better, if you need more power? The experimental Phoebus reactor generated 4 GWt and was specifically designed as a prototype for something that could be mounted in a spacecraft. A couple of gigawatts sounds like plenty of power for a laser to me, and you'd need a probably impractically huge solar panel to compete with that (at Earth's orbit you'd need a panel of around 2 km by 2 km, and that's assuming a much higher collection efficiency than we have now).

In addition to allowing it some survivability in the horrible event that it runs out of reaction mass for its main reactor.

I don't think that will be a serious issue except possibly in a fusion drive or open-cycle fission gas core rocket where the fuel is expelled in the exhaust. The energy density of uranium-235 fission is 89 terajoules/kg, and the energy density of fusion is hundreds of terajoules/kg. How powerful are you planning to make these lasers that there's going to be a serious issue of burning through tons of reactor fuel in one mission?

I'd also point out again that ships using solar panels will have logistical limitations: they only have power as long as the sun shines. They're probably not going to be able to operate much past the orbit of Mars, and they will have trouble with shadowed places like the backside of a planet. Depending on the strategic realities involved this may or may not be a serious liability.

[Sea Skimmer]

But would it be better than simply making the reactor a bit better, if you need more power? The experimental Phoebus reactor generated 4 GWt and was specifically designed as a prototype for something that could be mounted in a spacecraft. A couple of gigawatts sounds like plenty of power for a laser to me, and you'd need a probably impractically huge solar panel to compete with that (at Earth's orbit you'd need a panel of around 2 km by 2 km, and that's assuming a much higher collection efficiency than we have now).

You don’t need your peak power generation to be equal to the peak wattage of a laser weapon; you can simply store up power in a pulse alternator or capacitors. Solar panels are wasted vulnerable mass, except maybe as one of several onboard emergency power systems. In deep space or even the outer parts of our own solar system they simply cannot provide worthwhile amounts of power. The probe NASA recently launched towards Pluto for example, and the Voyager probes, had to be powered by RTGs because of the ineffectiveness of solar panels at such great distance. A modern naval nuclear reactor meanwhile, with a burnable neutron inhibitor, can provide a steady output for more then 30 years without refueling.

[Sea Skimmer]

One big thing is, like I said above, the solar panels might more gracefully degrade than an internal reactor when damaged.

They’ll also be degraded by such about anything, imagine what 30 years of micrometeorite bombardment will do to a solar panel. The vulnerability of a nuclear reactor can be reduce by using multiple reactors with mulitpul coolan loops for the radioators. This guards against damage from enemy weapons and provides for the basic needs for maintenance shutdowns and mechanical failure.

If the internal reactor actually takes a hit (hard if well armored), you'll probably have to shut it down lest it melt itself. If the solar panels take a hit, you just lose a section of them while the rest keep working.

Think about it, if you get hit so hard you have to shutdown a reactor that’s in the best protected area of the ship, do you really think the rest of the ship is going to keep functioning? The crew sure isn’t likely to last better then an eight inch thick steel pressure tube. If you got hit so hard not one but two separate reactors had to shutdown its pretty much certain the ship is dead. Meanwhile the thermal pulse of a nuclear bomb could screw up solar panels from tens of kilometers away.

But, the best part though (and the one that I focus on in the linked post) is that it is for cheap on mass: potentially hitting 1 kW / kg within the next fifteen years - very impressive, if it works out. Nuclear power in the future should be able to beat that, but by how much is open to debate, since the cooling system and radiation shielding really add up.

Yeah but it works with the same power output no matter what the conditions are, meanwhile solar panels will almost completely stop working by about the time you reach Jupiter orbit. For interstellar travel the panels are pure dead weight. Meanwhile the further from a star you go the better the nuclear plant will work since with no sunlight hitting the ships radiators will work better.

I’d use a nuclear reactor as primary power, RTGs and batteries for backup power, and then a few small solar panels as utter emergency power to try to keep life support running.

This is only a concern if you are really close to the planet. At GEO, the sun shines 99% of the time IIRC, but if you are down at ISS altitudes, it cycles on and off every 45 minutes.... However, you'd just keep your warship in the higher orbit lasering the stuff below.

So you’re just totally ignoring the possibly of fighting anywhere but earth orbit…. Yeah that makes so much sense.

[Minischoles]

Your invading a planet. Logistical support will be massive or else you’d be idiotic to even think about trying. Establishing some kind of advanced base, even if it doesn’t try to make fuel, will be essential to success just as it was in the Pacific War. You would easily have 3-5 support ships, many of which would simply be conversions of civilian vessels, for every significant warship. Seriously now, in WW2 a destroyer steaming at economical speed could still barely cross the Pacific one way, and merchant ships took several months to do so, but we still managed to support a fleet steaming at high speed off Japan. By mid 1944 the USNs service force ships (these are the ships which directly replenished naval units underway, and at the most advanced of bases) numbered more then 450 major vessels. A couple thousand more ships sailed from the US west coast to replenish the service force.

That's a good point. Instead of an interstellar attack fleet going right after the enemy planet, we should be thinking about establishing a forward presence near or inside the target star system somewhere and then creating enough infrastructure to support military operations, thus making it more of an interplanetary conflict than an interstellar one. Although the degree of feasibility here is still limited by just how realistic we're trying to be. These fleets would have to be truly staggering in size.

Hmm, this is a good point and a good idea. Huge attack fleet with cruisers/destroyer analogues plus the thousands of 'assualt landers', ontop of a realitively small amount of what? Baseships/logistical ships warping/folding into the outer system to set up a forward base, then the 'attack fleet' folding/warping from there to targets. A forward base/ objective rally point/ fall back position.

I'd think it would be divided almost like a modern carrier group is supported. Ships that amount to little more than thin armor around huge fuel tanks, others carrying huge amounts of food to support the invasion and ships in transit, even more filled with ammunition. Again assuming its a less developed planet you're trying to colonise and make a serious attempt at building up first you're going to want ships that are little more than floating factories for pumping out everything you're going to need (weapons, tanks, planes. Hell even spare parts can be manufactured on site to reduce the mass you need to take with you). The support fleet alone would have to be bigger than the actual battle fleet just to conceivably support it through the journey, let alone the actual invasion and colonisation.

[Sea Skimmer]

Your ship needs to go in for regular maintenance anyway; this isn't a big concern for short term missions.

So your ship is stuck in earth orbit then, not very useful for the absurd cost any spacegoing warship will have.

With laser fire, I'd believe it. If a laser focuses on the reactor section and damages the cooling system, your reactor needs to go down, while the rest of the ship may not be damaged.

If they can do that, then they can and WILL destroy the ship, and they’d probably target the crew area anyway. Why would they stop shooting once the reactor is destroyed if they have this absurdly high power laser weapon? Meanwhile solar panels could be damaged by anything. You don’t even have to destroy them you just need to cause slight surface damage and they stop working.

True. It could also destroy any other exposed equipment, like sensors or lasers - if a nuke gets that close, you're probably finished anyway.

That’s likely, but the crew and an internal power source could easily survive and at least give you a chance to escape, espically with a ship design like a I proposed, a long tube with a huge mass of armor at one end to be pointed at the enemy. Sensors and weapons could fold or retract behind armor when needed, an option you don’t really have with solar panels.

No, I'm saying that anywhere in the inner system except low orbits around planets your solar panels still get sunlight, and thus can still work. You can fight in high orbit or any solar orbit, and half the time in low orbit.

So you concede all strategic mobility to the enemy; why even bother with a spacefleet at this point? You could build orbital weapons platforms that need only small thrusters to maintain orbit for a lot less money and with vastly heavier protection.

They don't work very well in the outer system, but depending on your setting, this can be irrelevant - if you only care about Earth like planets connected with nearby magic FTL jump points (which I believe is what the OP has), you have no need to go to the outer system.

Nope it just says FTL, and even if you did have jump points I see no reason to assume that they’d always be close to habitable earth like worlds. Some might be beyond the rim of systems, others might be so close to stars that solar panels will melt (a nuclear powered ship could still use the radiators on its dark side for cooling). Who knows, others might be halfway inside of gas giants. Also this means if an enemy invades your system, seizes the jump point and then begins building an advanced base on Pluto you cannot counter attack against him.

If you have a need to go out there in the first place, then nuclear almost certainly is the better option, but there are a lot of imaginable settings and missions where there is no need to go out that far.

If you have no need for mobility or flexibility of any kind and done mind being a HUGE target then sure, solar could work, but at that point you aren’t building a proper warship and you’ve so severely constrained yourself tactically and strategically that you might as well not bother.