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Electric propulsion

The inter-moon tug pushes with 85 newtons for 120 days and carries 100 tonnes from Titan's orbit to Enceladus.

See it in the tour: chapter ocean beneath ice, shot 8.1.

Push with charged atoms

A Hall thruster ionises xenon and pushes the ions out with an electric field physics. Rocket chemistry cannot match the speed: a flame is limited by how hot a chamber can get, and an electric field is not. NASA Glenn and Aerojet's high-voltage Hall accelerator reached 2,700 s of specific impulse at 58% thrust efficiency in test physics.

The tug's thrusters run at 2,500 s, an exhaust speed of 24.5 km/s, and turn 60% of their electric power into jet physics. Methalox gives 345 s and methane nuclear-thermal gives 600 s. The fusion transport's design exhaust is 200 km/s challenge.

Thrust is the price

Power divided by exhaust speed sets the thrust. The faster the exhaust, the less push per watt. The tug has 1.74 MW of electric power and gives 85 N design. The fusion ship gives 63.7 kN and the shuttle 300 kN.

So the tug never lifts off a moon. It works only in orbit, where nothing pulls it down and a gentle push never stops adding up.

A spiral, not a hop

The Titan to Enceladus leg costs 7.05 km/s physics. A low-thrust spiral spends exactly the difference between the circular speeds of its two orbits around Saturn: Titan at 1.22 million km, Enceladus at 238,000 km. The tug burns for 120 days design. The app shows the spiral at 10,000× so that you see why it takes months.

Each leg of the network is flown by the engine whose exhaust speed suits it. When the needed speed change is far above the exhaust speed, the propellant mass explodes (the rocket equation), so the fastest leg needs the fastest engine. physics design

The rocket equation, kindly

At 24.5 km/s a 7.05 km/s leg is a small fraction of the exhaust speed. The tug's mass ratio is 1.358: 35.8 t of xenon (23 m³) for 100 t of tug and payload design. That ratio covers about 7.5 km/s, so the leg has margin.

A methalox stage on the same leg would need a mass ratio near 8, about 700 t of propellant for the same 100 t. Electric propulsion trades time for mass.

Power and heat

A fission reactor makes 5.8 MW of heat. Brayton conversion turns 30% of it into 1.74 MW of electricity physics. The rest is waste heat, and space has no air to carry it away. The tug radiates it from 790 m² of panels at 500 K design. They form the 40 m span of the radiator wings on a hull 70 m long design. The fusion ship runs hotter, at 1,200 K, and needs 10,630 m².

Imported xenon

Titan makes methane, oxygen and hydrogen. It does not make xenon. The tug's propellant is imported and flows inward through the Gateway with the other cargo design.

The plume is a steady blue-violet glow from xenon ion lines, a soft cone with no flame physics.

Key numbers

Quantity	Value	Basis
Specific impulse / exhaust speed	2,500 s / 24.5 km/s	physics
Thruster / conversion efficiency	0.6 / 0.3	physics
Titan to Enceladus spiral	7.05 km/s	physics
Trip time	120 days	design
Thrust	85 N	design
Electric power / reactor heat	1.74 MW / 5.8 MW	design
Mass ratio / xenon	1.358 / 35.8 t	design
Tug and payload (dry)	100 t	design
Radiators	790 m² at 500 K	design
Length / span	70 m / 40 m	design