Every journey burns water

Three rules

Energy is scarce

Every watt spent on one thing is a watt not spent on air, heat or water.

Water is propellant

Almost every drive throws out water, or hydrogen split from it. The rocket equation sets how much of it every journey burns.

Fusion makes travel cheaper, not faster

There is no high-thrust torch drive, so trips take months to years.

Fission

Fission is the belt's workhorse. It powers most towns, colony ships and nuclear-thermal drives, and the Brannock Company builds most of the reactors.

The fuel comes from the dead Earth. Asteroids hold very little uranium or thorium: on Earth, those were concentrated in the continental crust. Fragments of that crust now drift in the homeworld debris field, so Gravers salvage fission fuel from Earth's remains. The belt runs on its ancestors' grave.

Exodus-era stockpiles and breeder reactors stretch the supply, but the debris field is the main source. That makes fragment salvage an economic necessity as well as a moral question.

Solar

Sunlight at Ceres is about a seventh as strong as at Earth, and weaker still further out. Solar power is common in the old belt, where it is cheap and needs no fuel. On the frontier, panels produce too little, and colonies lean on fission.

Fusion

Large fusion reactors power the great cities and a few capital ships. Smaller units exist but are temperamental, expensive and short of fuel.

Fuel is the limit. Helium-3 is baked out of old regolith and counted in grams. Deuterium is separated from ice, so every reactor competes with drinking water and reaction mass.

Fusion also sorts the belt by class. Fusion-powered electric drives spend far less water per journey, so the great powers move cheaply while small colonies burn their drinking water. Being reaction mass becomes literal.

Hydrocarbons

In the belt, hydrocarbons store energy; they don't make it. Oxygen comes from splitting water, and methane is made from carbon dioxide and hydrogen. Both cost power, so making the fuel always takes more energy than burning it gives back. They are still useful:

• Emergency power. When a reactor fails, tanks of methane and oxygen keep the scrubbers running. The size of a colony's reserve is a measure of how safe it is.
• Portable power for rovers, mining rigs, suits and small outposts, where a reactor is too heavy or too precious.
• Lander fuel. Methane and oxygen are the standard chemical propellant for landers and short burns.

Fuel cells are more common than engines, because an open flame in a sealed habitat eats the air. And the belt is short of carbon: carbon burned is carbon not eaten.

Drives

The rocket equation governs everything. How much a ship can change its speed depends on two things: how fast its exhaust leaves, and how much of the ship is propellant. The relationship is exponential. Once a trip needs more speed than the exhaust gives, propellant quickly becomes most of the ship.

Steam or hydrogen

Water is dense, stores for years and needs no processing, but it is inefficient. Hydrogen is more than twice as efficient, but splitting it from water costs power, and it leaks from tanks on long trips. Poor ships burn raw steam; rich ones burn hydrogen.

No xenon

Earth's ion engines used xenon, which the belt can't get. Electric drives use water or hydrogen plasma, so even the efficient drives spend water.

Radiators

Every watt a ship uses ends as waste heat. Capital ships carry huge radiator wings, and you can tell one by its silhouette.

What a trip costs

The cheapest routes between rocks take one to two years. Doing them in months costs several times the delta-v.

Tilt is the frontier's moat. Belt rocks move at around 18 km/s, so changing the plane of an orbit is enormously expensive: a 25° change alone costs about 8 km/s. Anyone can go to the frontier, but it costs a fortune in water. Gravers working the debris field face a similar bill.

For a typical 5 km/s trip, a steam ship is about three-quarters water.

Frontier colonists either go slowly on electric drives, or fast on hydrogen they paid dearly for.

Gentle burns

Apart from chemical rockets, belt travel is gentle.

• Nuclear-thermal burns last hours to days at a tenth of a g or less, which feels like standing on a slight slope.
• Electric and fusion drives run for months at accelerations nobody can feel. People live weightless, or rely on spin.
• Only chemical rockets pull real g: landers, missiles and emergencies.

So combat unfolds over hours or days. Whoever has more water in the tanks decides whether a fight happens at all, and missiles are the only fast thing in a battle. More in War and law.