The ships in The Expanse RPG are even more generic than the ones in Serenity/Cortex. So I am going to work up a modified set of rules to handle ship design and combat.
Power Plants
Apparently, all the ships have fusion reactors, even small shuttles. The reactors burn some kind of fuel pellets; deuterium doped with Helium-3 according to The Expanse wiki. The reactors get a lot of power from a pretty small fuel supply it seems, but unlike some fusion reactors, do generate some radiation. While all ships have fusion reactors, not all of them have the high efficiency Epstein drive.
Water is carried as reaction mass at least for the reaction control thrusters and main drive.
Drive Modes
Teakettle- The ship maneuvers on its reaction thrusters only, which use super-heated steam. Low thrust, very high reaction mass consumption, but stealthy. Small, short-range ships use a teakettle mode only.
Torch drive- A fusion reactor fuses hydrogen and expels it for thrust. The torch drive delivers high thrust comparable to an Epstein drive but consumes at least a magnitude more fuel. High thrust, but high fuel consumption also. In hindsight, torch drive ships probably need liquid hydrogen fuel for the drive, and some water for the reaction thrusters.
Epstein drive- The Epstein drive is a vastly improved torch drive, magnetically accelerating ionized plasma from the reactor to generate thrust. According to one source, 1 ton of engine delivers about 140 tons of thrust.
Someone complained about ships docking or landing with their fusion drives and tearing up pads and such. Ships could dock in teakettle mode and minimize damage.
Ship Construction
Ship design is going to be my modified Serenity rules, with a few tweaks for the Expanse setting.
Epstein drive equipped: 20% +1.5% per G of thrust
Torch drive only: 20% +1% per G of thrust
No drive, just reaction thrusters (max 2G): 20%
Power plant fuel is pretty small, and storage is part of the reactor tonnage. A ship carries a few months' worth of fuel pellets. Reaction mass is more significant, but it's water, so it's easy to deal with. An Epstein drive consumes 1 ton of reaction mass/7 hours per 1000 tons of of thrust (abbreviated TT). A 2000 ton ship boosting at 1G consumes 2 tons in 7 hours. A torch drive consumes ten times as much, 10 tons in 7 hours or .7 tons per hour. A ship in teakettle thruster mode consumes 25 times as much as an Epstein drive, 25 tons in 7 hours or .28 tons per hour.
EDIT- I am working on updating Traveller and just realized I am contradicting myself here. I said the Epstein drive uses ionized plasma from the reactor, then said water is the reaction mass. Obviously, I messed that up. BUT, if you feed water through the reactor and heat it, the water breaks down into hydrogen and oxygen, and you can heat that to an ionized plasma and feed that plasma through the Epstein drive.
The Expanse game has Assets and Complications (Qualities and Flaws), just like Serenity, so I will translate those over. Most of the Serenity Assets and Complications will be valid, Born to the Black is a significant one as most ships larger than a light freighter or small warship cannot land.
Price is 5 credits per ton times G thrust times complexity modifier for an Epstein drive ship. 3.75 credits for a torch drive and 2 credits for reaction thrusters only ship. A 1000 ton, 20G thrust, Very High (x2.4) Complexity ship is 240,000 credits base price before gear.
Weapon systems need to be detailed but will mostly be as per Serenity. Ranges will be vastly longer. Almost all ship to ship guns are railguns, there are no plasma guns (but there are plasma warheads). Lasers supposedly exist, but we haven't seen any other than comlasers on ships and some battlefield weapons.
Let's look at a ship near and dear, the Rocinante:
1000 ton Martian Corvette class Torpedo Frigate
Agility d8 Strength d6 Vitality d8 Alertness d8 Intelligence d6 Willpower d6
Skills: Pilot D4, Sensors D4, Gunnery d4
Hull, Controls, Reactor and 20G max Epstein drive- 500
12 crew and room for 6 marines (8 tons per)- 144
2 launch tubes and 20 torpedoes- 22
6 PDCs and magazines- 12
W2 Armor- 100
Medical bay- 8
Reaction Mass- 172 (60 days at 1G)
Cargo- 42
Assets
Good Juice (m), Improved Stores (m), Expert Medical System (m)
Complexity: 48, Very High x 2.4
Price: 276,626 credits
Assets and Complications
New Assets
Good Juice (m)- Superior acceleration couches and medication, +1 step bonus to resisting injury from high acceleration. Resupply costs are increased to keep this advantage.
Improved Stores (m)- The ship's food supplies are high quality and have extra capacity. Resupply costs are increased to keep this advantage
Expert Medical System (m)- The ship has a high quality autodoc which can perform major surgery and is equipped with superior medications. Resupply costs are increased to keep this advantage. Requires purchase of a medical bay.
Fuel Efficient (m)- +20% burn time
New Complications
Bad Juice (m)- Inferior acceleration couches and medication, -1 step penalty to resisting injury from high acceleration.
Born to the Black (m)- Unlike Born to the Black (Major), the ship can land, but only on airless moons or planets with a thin atmosphere (Mars is probably the limit and might be pushing it, Earth and Titan are out of the question).
Skeletal Frame (M)- Take with Born to the Black (M). The ship has a hull of exposed girders like the Dewalt. The ship is lighter and cheaper. Repairs are faster and easier since there is no hull to remove and replace. But most repairs must be done in EVA. Ship components are more exposed to damage; flying any distance through Saturn's rings, for example, could cause minor damage to half the ship's systems. The ship cannot be armored without buying off the cost and weight savings, plus a penalty.
