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Fusion propulsion and helium-3
The Long Passage burns about 370 kg of fusion fuel on a crossing, and throws nearly 4,000 tonnes of hydrogen out of her magnetic nozzle to do it.
See it in the tour: chapter long passage, shot 0.7.
Mass becomes energy
The Long Passage runs on one reaction physics:
D + ³He → ⁴He (3.6 MeV) + p (14.7 MeV)
A deuterium nucleus and a helium-3 nucleus fuse into ordinary helium-4 and a proton. Weigh both sides. The fuel weighs 5.030131 atomic mass units and the products weigh 5.010428. The missing 0.4% leaves as 18.3 MeV of motion, by E = mc² physics.
Per kilogram of fuel that is 3.5 × 10¹⁴ J physics. To get it, the plasma runs at hundreds of millions of kelvin, hot enough for the nuclei to overcome their electric repulsion physics.
Why helium-3
Both products of the main reaction carry electric charge. A charged particle cannot cross a magnetic field line. It spirals along it. So superconducting coils can hold the burning column off every wall and steer the products out of the stern. That is a magnetic nozzle challenge.
A neutron has no charge, and no magnet can catch it. The main reaction makes none. Side reactions between deuterium nuclei still make some neutrons, so the shadow shield stays physics. The reactor sits about 400 m from the habitat drums, behind it design.
Fuel and reaction mass
The fuel is the small part. A crossing burns about 371 kg of deuterium and helium-3 challenge. Most of what leaves the nozzle is hydrogen: 3,935 tonnes of it, fed into the exhaust beside the fusion products. This is mass addition design. Extra mass lowers the exhaust speed and raises the thrust for the same power. The design exhaust speed is 200 km/s.
The ship leaves at 10,000 tonnes and arrives at 6,065, a mass ratio of 1.649, for a 100 km/s mission challenge. The hydrogen fills 58,354 m³ in six tanks, each 18 m across and 44 m long. Thrust is 63.7 kN, an acceleration of 0.00065 g. The engine burns for 142.9 days of a crossing of about one year over 8.5 AU illustrative.
Fuel from the outer system
Titan supplies the bulk. Hydrogen from electrolysed ice becomes the fusion transports' reaction mass design, so the ship refills at Saturn instead of hauling her return reaction mass out from the inner system. The settlement sells helium-3 only in experimental quantities fiction.
Radiators
The reactor makes 10.6 GW. Six tenths of it, 6.37 GW, goes into the jet challenge. A fifth arrives as hot waste heat. Space has no air to carry heat away, so the ship must radiate it. Hotter radiators shed far more heat per square metre, so these run at 1,200 K. They are liquid-droplet sheets: glowing streams of hot droplets sprayed between collector bars, 10,630 m² of them challenge. Their orange glow is fusion's real visual signature, and it shapes the ship's silhouette.
Key numbers
| Quantity | Value | Basis |
|---|---|---|
| Reaction | D + ³He → ⁴He (3.6 MeV) + p (14.7 MeV) | physics |
| Mass lost / energy released | 0.4% / 18.3 MeV | physics |
| Fuel energy | 3.5 × 10¹⁴ J/kg | physics |
| Fuel burned per crossing | about 371 kg | challenge |
| Hydrogen reaction mass | 3,935 t | design |
| Exhaust speed | 200 km/s | challenge |
| Departure / dry mass | 10,000 t / 6,065 t | design |
| Thrust / acceleration | 63.7 kN / 0.00065 g | design |
| Fusion / jet power | 10.6 GW / 6.37 GW | challenge |
| Radiators | 10,630 m² at 1,200 K | challenge |
| Ship length / width | 650 m / 122 m | design |