Like a lot of my posts, this one is inspired by a discussion online about recent events. In this case, the release of Spelljammer:
I'm not familiar with Spelljammer system personally. From what I can tell, it's basically D&D in space. My story world is basically an Steampunk/Atompunk romp with Wizards in space. Needless to say, the problems with Spelljammer are going to be exactly the same sorts of problems I am going to run into when it comes to user acceptance.
Fortunately for me, I decided early on that the wizards in my world would be flying around in semi-realistic spacecraft. Thus I can pull ideas from some of the grittier hard sci-fi genres. There is a lot of prior work on the subject. I have been getting ideas from the Expanse Roleplaying game, Warhammer 40k's Rogue Trader system, and I've even dusted off my old copy of Star Fleet Battles.
There is a balance between easy to play and satisfying detail.
The Expanse game is on the lighter side of detail. The complete rules for spacecraft cover 27 pages, 6 pages are devoted to combat, with another 2 pages detailing a sample battle. Basically ships are complicated and expensive things that characters can't generally afford, and if they are given one it's strictly for story purpose.
Rogue Trader devotes 40 pages to spacecraft, 12 pages are devoted to combat. Like in the expanse, ships are rare and expensive. The selection between the two games of ship types and basic weapons is remarkably similar.
And then... there is Star Fleet Battle. If you are familiar with Battletech, its the same basic idea. Ships have a character sheet, with damage boxes and armor boxes divided across different areas. The entire rule book is 224 pages. The rules stop at around page 156, and the rest is filled with world building sections on the various factions and sample missions.
Star Fleet Battles has a "Cadet's game" which only uses about 10% of the rule book. Which, interestingly enough, would but it at about the same page count as the Expanse. So, long story short, space combat for a tabletop role playing game is probably going to require at least 25-30 pages, and then for only the most basic of systems.
In Sublight we don't have to deal with hyperspace or warp drives. Faster than light travel simply isn't possible. Any loophole exploited in the laws of physics that would allow for faster than light travel or time travel instead cast one into an alternate universe. So, not exactly commercially exploitable, nor militarily exploitable, but fine for adventurers looking to take a one-way trip.
What Sublight does have is an active shipbuilding economy. Several economies, in fact. There are three major factions, each with a different focus:
Krasnovia prides itself on self-sufficiency. All of the major shipyards in Krasnovia are state-run enterprises. Which basically means players can have any ship that they want. So long as it conforms to one of the existing platforms they have developed to support their war machine. Transports are optimized to carry traffic between Krasnovian ports. Smaller craft hop from one port to another on Luna (or one of the Luna-like moons around the Solar System). Those smaller ships dock with the mega-interplanetary transports that go from Earth to Jupiter to Saturn (and occasionally Uranus and Neptune).
Krasnovian Military craft are specialized for surface attack support, and for intercepting attacking spacecraft as the enter Lunar orbit. Most are small and heavily armed. Their capital ships are mainly kept in port unless training or responding to an active situation.
Krasnovia also maintains a small fleet of scout vessels (200-1000 ton size range). These are captured or purchased from ISTO or the Circle Trigon. Their role is mainly to collect intelligence by mingling around the margins of the other two major powers.
The Circle Trigon is the faction of Rapacious Capitalists. Their ships are built for commerce. But commercial transactions are as often settled with bullets as credits. As such every Circle Trigon transport is armed. The major reason they are immune from invasion is by virtue of the fact that any attacker would not be battling an army. They would be battling all of millions of nuts who arm their ships and carry a magic wand on their hip at all times.
Circle trigon ships are mass produced. They are available in a variety of sizes, though the bulk of ships transiting the inner system tend to be in the 200-4000 ton size range. These "lighters" don't require a substantial crew, can carry an economic payload, are highly maneuverable, and have plenty of extra reactor capacity and volume to utilize for after market weapons.
The Circle Trigon faction is powered by trade, almost to a fault. There are many factions within factions battling to control trade routes and natural monopolies.
ISTO is the organization that is evacuating the Earth, and setting up new worlds in the Asteroid belt. They are the lastest faction to join the game, and as such they tend to have the highest average technology. The Krasnovians have a few evil genius powered vessels. Magnates in the Circle Trigon have high-tech yachts. But ISTO is mass producing their fleet with a budget somewhere between a moon-shot and a cold war naval power.
Due to arms limitation treaties, ISTO has a somewhat small dedicated armed fleet. This fleet mainly exists as a deterrence to Krasnovian agression, as well as a stick to fling at miscreant factions of the Circle Trigon. Like the Krasnovian Space Fleet, ISTO Spacey mainly exists in-port. Their power is concentrated in 12 massive fleet carriers, with the rest of the ships existing to support or escort those 12 battle groups.
ISTO is scattered around the Asteroid belt. Most settlements are far too sparsely populated to rely on the free market to keep supplied. As ISTO has a deep commitment to avoid stripping the Earth of any more resources, each settlement is producing some vital material. To maintain lines of communication, ISTO runs its own dedicated fleet of trasports: the Merchant Spatial.
Like in the Circle Trigon, many of these ships are in the 200-4000 ton range. Several classes also have the capability to land and take off from the surface of the Earth. (An extravagance that only a Mogul could afford in the Circle Trigon. Krasnovian ships and crews, having developed in Lunar gravity, would be crushed on Earth gravity.)
These "frigates" and "light transports" are about as well armed as a typical privateer in the Circle Trigon. Mainly because they often have to do battle with some of the more "enterprising" segments of Circle Trigon society. By ship count, tonnage, and personell, the ISTO Merchant Spatial is actually several times the size of ISTO Spacey.
The Merchant Spatial operates 12 logistics hubs. These craft are built on the same engineering plant as the massive fleet carriers, but are filled with farms and shopping malls instead of waves of assault craft and missiles. They act as portable cities for the smaller settlements, providing state-of-the-art medical and entertainment facilities, as well as court systems, government offices, and higher-education testing facilities.
As we can see, every faction has access to small, but plausibly capable, craft in the 200-4000 ton range. Big enough to travel the Solar System. Small enough to be run by the players and a couple of NPCs. Common enough that there would be Mos Eisley like settlements all over the Solar System catering to them. In conceptual size, the smaller ships are on about the same scale as Serenity or the Millennium Falcon. The larger ships are comparable in size to a modern naval frigate.
Players who are going for the more comic-book character style adventures would probably be members of Krasnovian special forces, travelling around in a captured pirate vessel or ISTO frigate. Each member of the party an elite specialist who was trained from birth to have some critical skill. The party has some sort of epic mission that requires a starship. Straying from that mission, or utterly failing at it, could make them outcasts. Operating from a captured or stolen ship, they will often need to procure black market parts. Or, perhaps the crew needs to "privateer" between missions to subsidize their endeavors.
Players who want a life or Piracy would pick a crew from the Circle Trigon. They operate as "independent merchants", who probably got their start by borrowed some money from the mob. Or perhaps they have rich patron who regularly calls in some favors. While on one hand there are advantages to staying on the right side of the law, the question is always "and who is really the law in these parts?"
Privateers are responsible for the maintenance and repair of their own ships and crews. Picking profitable cargoes and side-jobs are essential for remaining solvent. And depending on the legality of their enterprises, they may also need to keep a lawyer on retainer, pay fines, etc.
Players who want the "Star Trek" vibe would be sail for the Merchant Spatial. They get a general issue fleet vessel. The fleet buys the ships, maintains the ships, and handles crew recruitment. In exchange, the captains of these vessels have to perform the missions that are assigned to them by the fleet. Mission range from escorting larger transports, shuttling valuable cargoes, interdicting pirates, dealing with mad-science experiments gone wrong, and possibly even solving crimes in remote settlements.
In times of peace, any style of party can travel to any of the other faction's ports. In times of war... things would get complicated.
Combat is space has 2 major components: ship-to-ship battles and boarding actions. Boarding actions are just like regular combat (in rules to be expressed later.) Ship-to-ship battles are what we are going to be developing here.
The combat mechanics are built around a rock-paper-scissors style of technologies and vulnerabilities. There is no universally impervious set of shields, nor is there a singular "can opener" style of weapon. We have both magic and basic technology at our disposal.
Weapons are evolved from basic tools needed for basic spacefaring. The most important part of spacefaring is propulsion. Every ship (of any worth) has on board some form of thermonuclear reactor. That reactor is capable of generating enormous amounts of energy...
...for propulsion. Massive warships (like the ISTO fleet carriers) have a special device known as a wave motion cannon. WMC's use complex gravitational fields to direct streams of nuclear plasma into a focused beam which can obliterate almost anything in the path of the beam. But it's impractical to install a WMC on any ship smaller than a million tons or so.
On smaller ships the only useful direction that most of the energy a reactors produces is out the exhaust nozzle of the main engines. This energy excites propellant which flies out at a fraction of the speed of light to produce thrust. If your opponent is close enough (within 100 meters of so), you could use your engine as a plasma lance. But it would be a fool who would allow an enemy to get that close.
Keeping the reactor and engines from melting requires a cooling system. Steam generated by that cooling system can be directed through a turbine to produce electricity. The amount of electrical power produced is a fraction of the propulsive power of the reactor. But even a small ship's waste heat collector can generate megawatts of power. That power can be stored, using a variety of technologies, for release in quick bursts, in the ship's Energy Storage Array (ESA)
Direct Energy Weapons can impart momentum on mass, heat up matter, and block streams from other Direct Energy Weapons. Some can be tuned to disrupt the electrical systems on an enemy vessel. DEW doesn't work well against armor or structure. Ships are protected by energy reflective tiles and coatings. Their structures are metal which conducts energy very well. However DEW can deflect PEW, and even evaporate SPEW. DEW's usefulness is limited to the amount of power stored in the ship's energy storage array. The array is charged by diverting power from ship's reactors.
PEW flings solid projectiles. Shots range in size from as small as 50g to as large as 5 metric tons. Larger rounds can be fired from a rail gun, which is powered by the ESA. Most rounds are propelled by chemical explosives. Projectiles can be solid shot or explosive. Special implosion rounds will detonate any thermonuclear fuel they come into contact with. However implosion rounds are only issued to the armed forces of Krasnovian and ISTO, and are only intended for use in times of war. Per the Geneva Convention, every PEW round fired must auto-explode after 10,000 meters or 5 minutes.
Non-rail gun PEW is stored in a magazine. Hits to the magazine will cause a catastrophic explosion.
SPEW are sub-50 gram projectiles that are fired in random patterns. Their chief use is to stop incoming missiles. However, peppering an enemy vessel with SPEW can cause damage to delicate systems like radiators. (But only if their DEW systems are completely depleted.)
SPEW ammunition will explode when hit. Most SPEW launchers are self-contained, limiting damage from a magazine hit to the launcher itself. If reloads for a SPEW are stored aboard, they represent the same hazard as the magazine for PEW weapons.
On most vessels, SPEW operates automatically as soon as the vessel detects an incoming missile. SPEW can also be ordered to fire on an incoming shuttle craft or and adjacent combatant. SPEW fires for a complete battle round. A standard SPEW unit has enough ammo to fire 6 times. Reloading SPEW can be done as a damage control action.
Missiles come in a variety of sizes. They are powered by a chemical engine. They are designed to crash into an enemy vessel at great speed, and deliver a payload of mayhem. Missiles can be equipped with a variety of seekers and warheads. Missiles list a flight time, which is how long (in seconds) it's engines will fire and provide propulsion. Once a missile has expended it's fuel, if it calculates it has no chance to hit, it will destroy itself.
Ships can be equipped with armor. Armor is solid material, and requires no power to operate. Each armor plate has a mass rating, a proof rating, and a hit point rating. The mass rating indicates how much heavier the ship will be with the plate installed. Proof rating indicates the amount of damage the plate will repel completely. The hit point rating indicates the number of points above the proof rating the armor will absorb before failing completely.
Radiators are the means by which ships leak waste heat from their reactors into space. By their very nature of operation, radiators must be located on the skin of the ship above any armor belt. Each block of radiators has a heat point rating. 1 hit point of non-energy damage to the skin of the ship does 1 hit point of damage to a radiator in that section.
The framework of a vessel can absorb only a limited amount of damage before failure. Each major section of a ship has a given number of hitpoints. When that area goes to zero hitpoints, the section and all equipment in that section are compromised. Compartments in the section are considered open to space. Loss of all sections triggers to the catastrophic structural failure of the vessel itself.
Vessels smaller than 5000 tons are considered a single section.