Or, more accurately, propellant. Fuel is the stuff you have to provide energy, propellant is what you throw out of the back to provide thrust. The propellant is accelerated by the fuel. In our case, the propellant is hydrogen. There are better propellants, but hydrogen is abundant.
In the game, it’s probably easier to just call it all fuel. People get fuel.
Spacecraft should use up propellant
Players should, from time to time, have to think about filling up the tanks of their ship. If they run out of propellant, their ship becomes a simple ballistic projectile.
Spacecraft mass should vary with remaining propellant
Reactionless drives aren’t possible, according to mankind’s current understanding of the universe, and may well never be. Indeed, Pioneer’s space ships exhibit pretty little blue or magenta flames from their thrusters. Something’s leaving the ship; that something is propellant. So, as our propellant is used up, there is less propellant onboard. Less propellant means a less massive ship, which means that the ship is able to accelerate better as it uses up its propellant. This is rocketry, and it’s extremely cool.
Clearly, our little ships aren’t ‘’burning’’ hydrogen. That would use up all of the propellant (doubling as fuel) in no time, and barely get us into space. No, we’re probably accelerating it electrically. We’ll forget, for the moment, that the energy required to accelerate such small amounts of propellant to the point where the thrust becomes useful would probably vaporize the ship, and would certainly bathe the launch site in hard, hard radiation and hot exhaust, because of the Rule of Cool. Pioneer has cool spaceships, and this is a ‘’nod’’ to physics for gameplay value, not a complete conversion from fun to physics lecture.
The scale of things
My suggestion (@Brianetta) would be to provide a ship with a propellant tank equal in capacity to the mass of the ship’s empty hull, or something close to it. For maximum playability and fun, we need a lot of variation in propellant load, but we don’t want it to be used up too quickly.
These things could be tweaked, but I’m looking to balance this out between the two users of propellant; the AI, and the player. So, a 15 tonne Eagle would have a 15 tonne tank, and a 300 tonne Courier a 300 tonne tank. Perhaps individual models could override the default with a specified value.
One tonne of propellant (in an Eagle) should last and last; we’re talking about at least a complete trip from jump to station, and ‘’then’’ the rest of the tank. Bigger ships should also use a small proportion of their total available propellant for a single trip.
For the AI
Poor John Jordan has to write the autopilot. We don’t want to make his life so difficult that he walks away in disgust, so the rate of propellant use will be low enough that the AI can complete a trip without getting itself into trouble. Perhaps the tank should always be filled up half way every time the AI ship is launched. With propellant expenditure being this forgiving, an AI ship running out of it should be a very rare event, but even if it does happen, some Lua script could be activated, calling for help and perhaps offering a way to actually provide help.
For the player
The player gets all the benefits of low propellant use. We don’t want empty tanks to be the defining obstacle of the game at all. The internal tanks, which are separate from the hold, and the equipment slots, should be fillable from any station (at a fee) or from hydrogen in the cargo hold (possibly with an equipment upgrade allowing this to happen). With such large tanks, it should be different from Frontier’s method of clicking per tonne to fuel up. An auto-refueller, like Frontier’s, might seem useful, but will mean that the ship’s cargo hold is doing the job of the internal tanks. Convenience versus utility, perhaps.
The player should also be able to ditch propellant into space if they need, or want. Going to crash? Become lighter; ditch some mass from the tanks. Try not to ditch so much that you can’t make it to a safe place. In a fight? become more manoeuvrable by ditching propellant. You might not live long enough to use it all anyway.
Going exploring? Fill her right up. Got a military or courier mission with a tight deadline? Perhaps just a couple of tonnes.
Of course, it’s the player’s job to ensure that the autopilot has enough propellant. A good pilot might be able to make the same journey as the autopilot using less propellant, perhaps by taking a longer time. That can remain a matter of pride for the human.