Cooling Starships
by Bryn Monnery


If we stick with our 2300 universe as being realistic, then one major factor has been left out of starship design. Getting rid of a starships waste heat. Much of the hard numbers of this article are derived from the well researched CORPS: VDS.


All energy generated by a starships reactor ends up as waste heat, and this must be purged from the ship. Essentially, if you have a 150MW fusion reactor, you need to get rid of 150Mj of thermal energy every single second. If using active sensors, screens or beam weapons, some if radiated off as radio waves etc., but in the rest must be dumped.

For terrestrial craft, dumping heat is easy. There are 3 thermal transfer processes, conduction, convection and radiation. Unfortunately, since space is a vacuum, only radiation works.

Thermal Radiators

These replace hull masking, and ALL ships require radiators.

The surface area required is dependent on the TL and the output of the ships power plant
TL Square Meters per MW
OC 40
NC/ OM 30
NM 25

The radiators are 10cm thick, and so consume internal space equal to 0.1x area, mass equal to their volume, and cost Lv100 per square meter.

This immediately creates problems for a few ships, mainly the tight military ones like the Kennedy. The Kennedy's 150MW reactor requires 3,725 square meters of NM radiators.

Starships may have more than the required amount of radiators, and doing so makes them more stealthy, as the energy is spread over a larger area, it lowers the energy density, possibly below the opposing sensors detection threshold. Divide the actual radiator area by the required, divide the power plant output by this number to get the effective power plant output for sensor purposes.

example: I have a frigate with a 5MW fuel cell. This requires 125 square meters of radiators. I actually install 250 square meters, so my effective power output is 5/2 = 2.5MW, rounded to 3MW. This equates to a radiated signature of 3 rather than 4.

Heat Sinks

In addition, you can store heat in heat sinks. This is not a long term solution, unless you have a large ship with a small power plant (in which case radiators would be fine)

Heat sinks are really a tactical system, and are mentioned on Andy Goddards site. Essentially the waste heat is stored and radiated away slowly later. This involves essentially going "all stop" to dump the heat.

Storage capacities (per ton of sink) are:
TL Storage Capacity (in MW turns)
OC 1.8
OM/ NC 2
NM 2.1

Any portion of the power plants output can be stored, and only the remainder is used to calculate signature while in "stealth mode".

e.g. Our Kennedy has a 150MW power plant. We want two stealth modes, one radiating 10MW (as per the reduced SC signature), and one "ultra-stealth" where the power plants entire load is stored to enable stealthy approaches. The first requires 140MW to be stored, so 140/2.1 = 66.67 tons per minute. The second mode is 150/2.1 = 71.43 tons per minute. Bearing in mind the original masking mass, and that the radiators mass 372.5 tons. We have 900 - 372.5 tons of heat sink = 527.5 tons. We decide on 7 minutes of ultra stealth (signature 0), so the radiator/ sink system in total masses 872.5 tons.

Heat sinks mass 1 ton per cubic meter, and cost Lv1000 per cubic meter.

Cooling Problems

If the cooling needs of the vehicle are not met, eventually the power plant will be into melt down. Divide the power plants mass (in tons) by the cooling shortfall (in MW), and multiply by 100. This is the time in seconds until meltdown. For a Kennedy with a 1MW shortfall for example it will take 5.8 days to reach critical.

Turning down the powerplant

The other option, both tactically, and in the event of an emergency, is to turn the power plant down, or off. Fuel cells are totally variable, the power may be any proportion of their maximum. The same for nuclear reactors, down to around 25% of their output (at which time the reactor gets poisoned, just like Chernobyl). MHD's are set to produce power at a set rate, and may only be turned on or off. Fusion reactors may be scramed (dump the core), but not turned down.

In the case of scraming a reactor, the restart times (in minutes) are:

Fusion: sqrt (mass) * 3

Fission: sqrt (mass) *30

MHDs: sqrt (mass) * 0.3

Fuel Cells: instant