While playing around with different ship building rules and methods from various roleplaying games I found, that all of them start with the question: „How big?“. The first thing you had to decide was how large you wanted your spacecraft to be. I think this comes from the fact that the very same rules had been used by the game designers to create a whole lot of spaceship designs that are part of the game universe already, giving the player a more or less reasonable framework to put in their own designs. But basically the question „how large“ is one that no real ship designer would ask at all, especially not as the first question. The first question would probably be more like „what purpose?“. Why are you designing an new craft? This question should be asked as if you were a spaceship engineer within your rpg gaming universe, not a gamemaster or a game designer outside. Is there an urgent need that a new design can cover well? Spaceships are extremely expensive and it takes a lot of effort to build one. They are fragile and precious, you should only attempt to build or design one, if there is very good reason to do so.
It depends on what society and wealth exists in your gaming universe. A very near future setting (no further than 100 years in the future) will most likely not see huge warships or zippy luxury liners, but science and exploration ships that look pretty much like the ISS with drives attached to it. Maybe there are some mining ships and transport barges, looking like flying rain barrels. If mankind already settled the solar system to a significant degree (like putting orbital stations to Jupiter’s moons) you would see a lot more traffic an variation in ship designs. Politics and quarrels over „territories“ and resources would most likely spread throughout the system, you probably will have major factions locked in a cold war, defending their claims. Or you are right in the middle of a full war situation where there is need to protect fragile stations and find a way to overcome the enemy sitting on nearly invincible planets. In a far future setting (more than 250 years from now) you will probably encounter anything. Mankind has found a way to travel the vastness of space between the stars in reasonable time, technology evolved to a fantastic level and building a spaceship is nothing more than building a car today. Ships are built for all sorts of reasons, war, trade, leisure time…
So the first question you should answer yourself is: what (main) purpose or role does my ship have?
It helps if you can divide roles into major categories like: trade, military, science/exploration, police, civilian and so on. Then you can define detailed roles within the categories. Military ships could have roles like protect (Frigates), orbital defense (Corvettes), strike/escort (Destroyer), strike (Cruiser), assault (Assault), support (Carrier), and so on. It helps to use naval ship classifications for orientation but if you only transfer navy terms into space, your space ships will not be very realistic. Space requires different roles than the oceans. Try to think about what makes sense in space, how would travel work, how the orbital stations, what is allowed or reasonable by the available technology and by the laws of physics (major pitfalls here!).
[su_box title=“Why warships at all?“ style=“soft“ box_color=“grey“]Actually, building manned warships in space is rather unrealistic. It’s just too expensive and not worth the effort as you can achieve your goals by cheaper and more effective means. If you want to erase that enemy orbital station at Io, you would simply send a missile. Or two, or two hundred. Not a slow warship with a few hundred men and women aboard, that can carry 3-6 guns and a few missiles, that will get there in about 1.8 months and most likely be shot down by the defense massdrivers. Building one such ship would cost you much more than 200 missiles, don’t even bother building a fleet. If you intend to use more subtle methods because you wish to reuse your captured target later (like Earth) you would probably not throw a mountain at it. Planetary invasion would be a rather costly and terminal act for your fleet. You can’t invade a planet by sending 2.000 missiles alone. You will have to bring in men, lots of men (it’s unclear where those numbers should come from). Planetary defenses are probably ages old, well dug in and start half the way between your starting point and Earth. Your fleet would probably be shot to pieces long before it reaches the Moon. Maybe infiltration, sabotage and such methods would work better than wasting all your wealth on a chronically underpowered fleet. Think about how politics, the balance of power, the flow of resources and wealth work in your setting. A space fleet is there to protect those values, if you don’t know them well, your fleet will look like expensive toys without a purpose. You will have to find a good reason why there are military ships at all.[/su_box]