I was working on a few campaign concepts, and I had this idea. All players are martials and maybe halfcasters. Magic in all cases is just technology. Spaceships are represented by full casters. So you have a crew of mostly mundane people, doing scifi stuff (hey, weird nebula, let's go in.) and when they are in space combat, they use spaceships that are just heavily textured full casters. (Maybe half casters are just cyborgs?) So in game you have various models of ships- maybe from different alien races, the Wizard model has the greatest variety of weapons and can install more, the Sorcerer model can do more damage, the Druid model can terraform planets. The Cleric model can heal biological players.

This is clearly not normal usage of DND rules. I want to know potential problems that might arise, and solutions to the problems I already spotted.

-Stat conversion. What if a player attacks a ship? Or a ship attacks a player? Would 10x be fair? Should I just ignore it? Should range, damage, and health all be increased the same way?

-Ship control. Players might be left out if one person is in control. What if control is segmented? One player manages the navigation and movement, one player controls offense, and one player controls defense? Alternatively, each could take turns- one action per turn, one player moves the ship a few miles, one player shoots a fireball, and the next puts up a shield? (Maybe land vehicles should be martials...)

-Rests. Long rests might be when a ship goes to a refueling station. The players could purchase individual spell slots (fuel) and maybe get a discount for purchasing a fill to max. I will assume the ship always has enough fuel to move normally. (Spells that increase movement represents going faster.) What about short rests? Should the warlock model just be able to refuel for cheaper? (More efficient?) Spending hit die? (Auto repair?)

-Is it even worth it?

Look up Dark Matter by Mage Hand Press, or Star Wars 5e. They're both sci-fi conversions for D&D 5e, the first in a world where magic has advanced to the point that it's indistinguishable from technology, and the second modeling the world of star wars.

I've not really looked into SW5E, but you can take my review of it as a very well-balanced, pretty highly playtested, high-production publication.

Oop, edit: you can take my review of Dark Matter, idk anything about SW5E

If you want to get old school look up Star Frontiers. It was the original AD&D but in space!

Besides spending 60 dollars on something I might not use, is my idea viable, or is your answer just 'it's not worth it, spend money'.

There's also a Mass Effect 5e conversion that's hella good and I can say that from experience. It's so fun and so flavorful, for those of you that have played Mass Effect. 

But honestly, as with all big changes to the base of the game, I'd recommend a different system. Starfinder jumps out as an obvious choice, but there are plenty of tabletops designed to be sci Fi rather than converting all of DND 5e. 

Quote from HeresJohnny1 >>

Besides spending 60 dollars on something I might not use, is my idea viable, or is your answer just 'it's not worth it, spend money'.

 "Worth it" is almost impossible for us to say. If its something you'll have fun doing, then, sure, maybe the conversion is worth it. If it's for a home game, then make sure your players are on board (obviously). If its for trying to sell, I don't think anyone can ever tell what's going to hit or not.

The thing I think I don't understand conceptually is the whole ships-as-casters thing. Do you mean a person will play being some kind of AI spaceship that mechanically works like a wizard or druid or cleric? While the other players are fighters and rogues that are the ship's crew? Because I can't think how that would work. Two of the big parts of the scifi genre are space combat and exploring planets. So in space combat, what would the martial characters do while the wizard is battling another wizard ship? And then when the party lands on a new planet of space station or whatever, what would the wizard player do while the rest of the party is on the away mission? (Though as I'm thinking of it, the Imperial Radch series by Anne Leickie might be a good start, where the ships do kind of have their own consciousness, and they also have a series of androids that are kind of part of a hivemind with the consciousness and can allow the ship to go walk around on land. The first book is called Ancillary Justice.) But generally in scifi settings, I think that's why people just lean into casters being tech-focused where they don't cast invisibility on someone, they press a button on their doodad and it creates a holographic image that makes someone seem invisible. They don't cast fireball, they use a specialized grenade. They don't cast tongues, they use their doodad again as a universal translator. That kind of thing.

Sorry, I wasn't clear, I meant all the players are the crew. They can control the ship, which is represented by a spellcaster under no particular player's control.

Quote from HeresJohnny1 >>

Sorry, I wasn't clear, I meant all the players are the crew. They can control the ship, which is represented by a spellcaster under no particular player's control.

 That makes more sense, but are you keeping the standard action economy? So the shop gets to do one thing and the party has to all decide as a group what that one thing is? Cause that could get rough. 

Have you looked at either saltmarsh or spelljammer? Could be way easier to use one of those systems for ships and just say the ballista are photon torpedoes.

I'll look into it. I found a pretty good mech homebrew that might work- my players want a more ground based adventure. As for the ships, imagine it like this:

Player 1: I move it 30 space units on the map.

Player 2: I cast fireball on the moon.

Player 3: I use the dash action to move 30 space units away.

End turn.

1: I cast Magic missile on the space jellyfish that are swarming us.

So a ship has a crew capacity, the maximum amount of actions per turn, and each of it's actions would require a crew to use it. A 2 crew capacity with 3 players would go like this:

1: I move 30 space units.

2: I cast fireball.

End turn.

3: I move 30 space units.

1: I cast dispel magic on the enemy ship.

Of course, I would likely have enough capacity for the players to do stuff on every round.

Where it gets wonky is when player 1 moves to a place so that player2 can’t do what they want. And if they all coordinate, then it’s not each player making a choice, it’s a group choice to perform certain actions in a certain order. 
Imagine in a standard D&D game trying to run a single character by committee. With the right group it might work. Or you could run into fights over who should do what, or one player just taking charge while the others nod along and do what their told. 

Im not saying to not try it, just trying to brainstorm possible problems. 

Quote from Xalthu >>

Where it gets wonky is when player 1 moves to a place so that player2 can’t do what they want. And if they all coordinate, then it’s not each player making a choice, it’s a group choice to perform certain actions in a certain order. 
Imagine in a standard D&D game trying to run a single character by committee. With the right group it might work. Or you could run into fights over who should do what, or one player just taking charge while the others nod along and do what their told. 

Im not saying to not try it, just trying to brainstorm possible problems. 

 I will play test with just 2 players. Probably how many players I will have total. If one player wants to sit back and do as told for awhile, that is fine, though I will encourage interaction. My players do want more customization, and that is where spells come in. Maybe the ballista can shoot fireballs instead.

one word. 

SPELLJAMmER