Home / Nuclear / Radiators: throwing heat away

Radiators: throwing heat away

Every reactor in the Saturn fleet makes more heat than power, and the only way to get rid of the rest is to shine it into the dark.

See it in the tour: chapter long passage, shot 0.3.

Light is the only way out

Concordia teaches the basics: why a vacuum is a thermos and where a habitat's power and heat go. This page does the arithmetic for three machines at Saturn.

Space has no air to carry heat away. A surface sheds heat only as thermal light, and the power a flat panel radiates is its emissivity times σT⁴ per side physics. Here σ is the Stefan-Boltzmann constant, 5.67 × 10⁻⁸ W/m²K⁴. One square metre of black surface radiates 3.5 kW at 500 K and 117.6 kW at 1,200 K physics. The same panel is 33 times better at the higher temperature.

Why there is so much heat

A heat engine turns only part of a reactor's heat into power. The tug's reactor makes 5.8 MW of heat and 1.74 MW of electricity, so most of the heat has to go somewhere physics. See Reactors and heat on Titan. On Titan that heat is useful. In space it is waste.

Three radiators in the fleet's data show the scale design.

• The Gateway rejects 6 MW at 500 K from 941 m² of panel.
• The cargo tug rejects about 4.8 MW at 500 K from 790 m², its 5.8 MW of reactor heat less 1.0 MW that leaves as jet.
• The Long Passage rejects 2.1 GW at 1,200 K from 10,630 m², a fifth of her fusion power challenge.

At 500 K that 2.1 GW would need 352,544 m². Running hot is what keeps the ship a ship physics.

Why droplets

A solid panel at 1,200 K needs structure, plumbing and coolant lines, and a micrometeoroid can puncture it. A liquid-droplet radiator drops the panel. It sprays a sheet of droplets, each under about 100 microns, across a gap in vacuum. The droplets radiate as they fly, and a collector catches them and pumps them back to the heat exchanger physics. Mattick and Hertzberg described it in 1981. They argued it could weigh much less than heat-pipe panels and be less vulnerable to micrometeoroids, with the droplet generator, collector, heat exchanger and pump making up most of the mass physics.

The ship carries four wings of droplet sheets, each 150 m long and 17.7 m across challenge. Hot droplets glow, so the radiators are the part of the ship that shows its power. The engine gallery's view port looks out on them design. They also set the width of the shield, because they are the nearest large structure behind it. See Shadow shields and neutrons.

Key numbers

Sources

• Mattick and Hertzberg 1981, The liquid droplet radiator, an ultralightweight heat rejection system (IAF-81-185, NASA NTRS 19810062992)
• NIST, CODATA fundamental physical constants (Stefan-Boltzmann constant)

Related pages

• Fusion propulsion and helium-3
• Reactors and heat on Titan
• Electric propulsion
• Shadow shields and neutrons
• Inside the fusion drive