It is a law of thermodynamics that anything that generates energy also creates heat. In spacecraft this is especially true. With even small craft requiring the same energy output of a city, the dispersion of waste heat becomes a critical factor in their operation. The early ages of space travel birthed ships that dedicated massive amounts of structure and mass to the dispersion of waste heat. These thermal radiators took the form of massive panels that gave the ships and space stations the look of ancient sailing vessels. Since then however advances in materials and technologies have provided for much more efficient and less cost prohibitive thermal radiation methods.
Deep space is cold, and the further a ship ventures from a star or planet the colder it gets. As a result the simplest way to remove excess heat from a generator is to use it heat the crew. This of course is not always practical, especially in combat conditions where defensive and offensive systems alike generate tremendous amounts of waste heat that would boil the crew should they be exposed to it for any sustained period of time. The life support system, in fighters primarily, also contains emergency thermal vents where waste CO2 is heated to the supercritical state by waste heat and vented into space. This is not the way most preferred way to deal with waste heat and so is normally used only in extreme cases.
Nanosheet Thermal Radiator (NTR):
The most common type of thermal radiators used on combat spacecraft is their own outer armored shell. The outer layers of armor used by the majority of combat starships are composed of nanosheet, these sheets of material are composed of interlaced carbon nanotubes which are 250 times stronger then steel at 10% the mass. As one of the most thermally conductive materials known, aside from blackbody (discussed later), nanosheet lends itself to the development of heat sinks. In the case of space craft, nanosheet armor is often used to supplement or completely replace traditional thermal radiators.
Of course a combat spacecraft could not allow its entire surface to become a giant thermal radiator as this would reveal them to passive sensors with ease. Instead only certain areas of the hull are used to disperse heat, though the entire hull can be used in emergency situations. On transatmospheric fighter craft the areas of the surface most commonly used for thermal radiation are the wings. These provide a great deal of surface area and spacing away from the hull to prevent injury to the crew. Most fighter designs compress the used surface area further to the leading and trailing edges of the wings and any additional control surfaces (Figures 1&2). This area can be increased as needed to encompass the whole of the wings and, in extreme cases, the main fuselage.
Capital ships, which have much greater surface area, tend to spread out the thermal radiator hull panels all across the hull creating small pockets of thermal energy instead of one massive thermal zone. When large areas are required however they tend to be near the areas that generate the most heat, typically the engines, near the power core, or near heavy weapons emplacements. While this creates a vulnerability to thermal seeking sensors it is considered to be a necessary precaution against failure of the thermal transport system.
Black Body Radiator (BBR):
Black Body Radiators are some of the most advanced and thermally efficient radiators in use, this is can effectively radiate nearly all possible wavelengths of energy. Despite the name, black bodies are not actually black as they radiate energy as well. The amount and type of electromagnetic radiation they emit is directly related to their temperature. Black bodies below around 700 K (430 °C) produce very little radiation at visible wavelengths and appear black (hence the name). Black bodies above this temperature, however, begin to produce radiation at visible wavelengths starting at red, going through orange, yellow, and white before ending up at blue as the temperature increases.
Black Body Radiators (BBRs) are made from highly compressed and super dense weaves of nanotubes, making them far more efficient and expensive then standard nano-sheet radiators. BBRs are typically only found on small areas of any ship and usually only around areas that are going to have high thermal activity, and must radiate that energy quickly. This includes areas around weapons, engine exhausts, maneuvering thrusters, external power plants, thermal vent ports, etc… BBRs are usually not found in large areas of fighters because of how much heat they generate when operating, far more then conventional nanosheet radiator plates. Some fighters are however fitted with emergency BBR sails that deploy when all other thermal radiators have exhausted or overloaded. Large capital scale vessels will often feature them as radiator fins around engines, near weapon systems, or in the case of Centauri designs, coating their Anti-Matter Collider Rings and in massive thermal radiator vanes.
Zero Kinetic Energy Plug (ZKEP):
ZKEPs are heat sinks made of exotic Phase Change Materials (PCM) with no internal kinetic energy. These typically solid plugs are stored at absolute zero prior to installation. Once their cooling systems are deactivated waste heat pours into them. These heat sinks are able to absorb tremendous amounts of heat as their temperature rises from absolute zero and even more so when they undergo a state change from their solid to a gaseous state. The ZKEP is divided into a number of hexagonal cells (typ), which are mechanically connected to the heat source via a thermal shunt in the middle of the cell. Heat is applied to each ZKEP cell one at a time until all cells are activated and reach a uniform temperature (Figure 5). Once all cells in a ZKEP reach critical temperature, at which the PCM can no longer absorb heat, it is cut off from the heat source and the next ZKEP activates.
ZKEPs are found almost exclusively on fighters and other small craft for use during combat conditions or when it is necessary to limit a craft’s heat signature. ZKEPs are swapped out as part of a light craft’s routine maintenance and new ZKEPs installed even if they are not used so that they can be serviced and refrozen. In the event that a ZKEP is heated too rapidly or overloads they are designed to outgas out into space. These outgassing events can be quite violent and produce a noticeable heat plume so are avoided wherever possible. ZKEP venting is commonly seen after atmospheric insertion when the skin of the craft is too hot from reentry to effectively radiate heat, forcing ZKEP use and emergency venting (Figure 6).
(Example of ZKEP venting)
(Image property of Darkhorse Comics and Star Wars)