>>14286761>In a world where we have fusion freighter stages that are 45% structure, 5% fuel, and 50% water by mass, and offer a delta V of ~30 km/s per water tank load when carrying its own mass again in payloadTo put real numbers to this so it's less confusing the read:
2000 tons total wet mass
1000 ton payload on the front
500 tons of water as reaction mass
50 tons of fusion fuel
450 tons of vehicle (including reactor, tanks, maneuvering engines and associated systems, radiators, anything else you can think of)
After the burn the vehicle has a mass of 1499.5 tons (500 tons of water reaction mass used up, 0.5 tons of fusion fuel used up). If the fusion engine operating in water-boosted mode produces thrust at an Isp of 10,000, the vehicle achieves a delta V of 28,245 m/s. Assuming it is moving between two destinations where it can refill with water, it can perform this trip a total of one hundred times before it needs to refill its fusion fuel tank. If it carried no water reaction mass at all, the engine Isp would rise dramatically but since the mass fraction would decrease the delta V of the vehicle would only go to 33,246 m/s and would consume the entire 50 ton fusion fuel tank. Basically what this means is that using water reaction mass allows you to effectively achieve a massive Isp in terms of actual fusion fuel burned, which saves cost.
For extremely high delta V flights where refilling reaction mass is not an option, you instead fill the vehicle up with fusion fuel. The 1000 ton payload can now be pushed to a maximum delta V of 315,366 m/s, which is getting into feasible interstellar mission territory.
