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Radiation
Radiation in Kerbal Health accumulates and gradually but permanently reduces maximum Health Points that a kerbal can have. Radiation is measured in banana equivalent doses (bananas, for short) per day. By default, 1e7 (10,000,000) bananas reduce max HP by 25%. Therefore, 4e7 bananas will kill any kerbal, even if his/her health is full. As of v.1.0.0, there is no way to reduce accumulated dose, so watch it carefully!
There are two types of radiation in Kerbal Health: cosmic radiation and artificial radiation. The former affects all assigned kerbals, but its strength depends on the location of the vessel. Artificial radiation is produced by certain craft parts such as nuclear engines and reactors.
The main radiation formula is this:
Radiation = Exposure x (LocationCoefficient x CosmicRadiation + ArtificialRadiation)
Exposure is measured in percent and is 100% by default. Kerbals on EVA are more vulnerable to radiation and have 1000% exposure. Exposure can be lowered with Shielding (see below).
Cosmic Radiation consists of Solar Radiation and Galactic Rays. Galactic radiation is uniform everywhere while solar radiation follows the square root law:
SolarRadiation = NominalSolarRadiation x SunDistance ^ (-2),
where SunDistance is measured in Astronomic Units (distance from the Sun to the home planet). At 1 AU from the Sun, both solar and galactic radiation are 5,000 bananas/day each.
Artificial Radiation is created by certain parts. The stock game only contains one such part: NERV engine, which produces 250 bananas/day. RTGs are not radioactive, because they use Plutonium-238, which is much safer than Uranium. You can check a part's radioactivity by holding mouse over it in the Editor.
Location Coefficient determines the fraction of cosmic radiation that reaches your vessel: the lower, the better. It is always 1 (or 100%) in interplanetary space, but elsewhere it can be reduced by magnetosphere, atmosphere and occlusion.
Most planets and some moons have magnetospheres that stop some radiation. Kerbin's magnetosphere is considered standard and it works as follows:
- In space high: 50%
- In space low: 10%
Some celestial bodies have stronger magnetospheres and some have weaker or none (see below). Moons by default lack a magnetosphere of their own. However, magnetospeheres' effects are additive (or rather, multiplicative): if you are in a SOI of a moon that has a magnetosphere, its Location Coefficient will be multiplied by that of its parent planet. See a table below for data on stock celestial bodies.
Atmosphere is usually very effective in shielding the surface from radiation (high atmosphere less so than low atmosphere). Kerbin is again considered standard with the following effects:
- Stratosphere (flying high): 30%
- Troposphere (flying low): 2%
Note that these numbers are multiplied by the planet's low magnetosphere coefficients. So, if in Kerbin's case in-space low coefficient is 10%, only 3% of cosmic radiation will reach the vessel in the upper atmosphere and only 0.2% in the lower. Again, some planets and moons have thicker atmospheres than Kerbin and some have thinner.
A celestial body itself is the best radiation shield and prevents all radiation from passing through it. So the closer you are to it, the more radiation will be stopped by the planet or moon. On the surface (sea level), it will be 50%. Note that Kerbal Health doesn't take day or night time into account.
Many planets and moons have their own health configuration that determines the strength of their magnetospheres and shielding effects of their atmospheres. Both these values are exponential. This is defined in the KerbalHealth.cfg file and can be changed manually or with MM patches. Below is the table for stock bodies assuming default radiation settings.
- Moho
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- Magnetosphere strength: 1 ** In space high: 50% ** In space low: 10%
- Eve ** Magnetosphere strength: 0.5 ** In space high: 71% ** In space low: 32% ** Atmospheric absorption: 2 ** Stratosphere: 2.8% (including magnetosphere's effect) ** Troposphere: 0.01%
- Kerbin ** Magnetosphere strength: 1 ** In space high: 50% ** In space low: 10% ** Atmospheric absorption: 1 ** Stratosphere: 3% ** Troposphere: 0.2%
- Duna ** Magnetosphere strength: 0 ** In space high: 100% ** In space low: 100% ** Atmospheric absorption: 0.25 ** Stratosphere: 84% ** Troposphere: 38%
- Dres ** Magnetosphere strength: 0 ** In space high: 100% ** In space low: 100%
- Jool ** Magnetosphere strength: 2.5 ** In space high: 18% ** In space low: 0.3% ** Atmospheric absorption: 1 ** Stratosphere: 0.1% ** Troposphere: 0.01%
- Tylo ** Magnetosphere strength: 0.5 ** In space high: 71% ** In space low: 32%
- Eeloo ** Magnetosphere strength: 0.2 ** In space high: 87% ** In space low: 63%
The only way to reduce the amount of radiation your kerbals receive is by reducing their Exposure with Shielding. The bigger the vessel, the more Shielding you will need, but you will never achieve 0% exposure due to diminishing returns:
Exposure = 2 ^ (- Shielding x (CrewCapacity ^ (-2/3))
Example: A simple 1-man pod with a shielding rating of 1 will have an exposure of 50%. It means that 50% of radiation (both cosmic and artificial) will reach the kerbal inside. A shielding rating of 7 will bring this value to about 1% (because 2^-7=0.0078125). But a 2-seat vessel will require a shielding rating of 11 to achieve the same exposure. These figures are shown in the Health Report window in the Editor.
There are three ways to increase Shielding. First, it is provided by a special resource called, ehm, Radiation Shielding. You can add this resource to most crewed parts and to some others too. 1 unit of Radiation Shielding weighs 0.1 t and provides, you guessed it, 1 level of Shielding. Just note that, by default, this resource is empty, so you'll have to add it to the parts in the Editor if you want radiation protection.
If it's not enough, Shielding is also provided by certain craft parts: heat shields (ablator is not required), panels and very large cargo bays. You can further strengthen them by adding more Radiation Shielding.
Lastly, for the most creative, some other resources can also be used as radiation protection: Lead and Water (or WasteWater) at the moment. When used with right mods, it allows you to even produce your own makeshift shielding in-situ.
Use this tool to calculate radiation effects. You need to copy it to your Google Drive or download it as an XLSX to be able to edit.