Fission Engine

Do not confuse with the fanmade Expanded Fission Engine or Fissile Fuel Cell Engine. An older technology still encountered occasionally, particularly in the Periphery, fission engines use nuclear fission by splitting heavy atoms for energy. Most often, the goal is thermal energy used to drive either a gas or steam turbine to generate electrical power which is then harnessed for various ends.

History
The use of nuclear fission for propulsive power was an early goal of nuclear research, one attained by the predecessors of the Western Alliance in under two decades from the end of World War II. At first limited primarily to large maritime applications (particularly submarines) and initially used by military vessels for reasons of cost, the idea of using fission engines for smaller-scale work was an enduring one that blossomed as technology advanced. Despite the drawbacks (primarily safety and the sheer weight of the engines themselves), fission engines are not an uncommon choice for powering IndustrialMechs and support vehicles. Military use is much rarer, particularly on smaller units like BattleMechs or combat vehicles, and was pioneered by the Taurian Concordat in 2882.

Benefits and Drawbacks
Fission engines have certain distinct advantages compared to Internal Combustion and Fuel Cell engines. They are very fuel efficient, providing ground vehicles with a functionally unlimited range defined more by a time measured in months to years, and the efficiency is such that the rules do not track fission engine fuel use for such units. Fission engines are a high power energy source, meaning that units using them do not need power amplifiers for certain energy-intensive applications and are even able to use heavy Gauss rifles. Finally, fission engines require a somewhat simpler, less advanced industrial base to build and maintain than even the simplest fusion engines do, although the details of this point are obscured by the broad classification by Technology Rating; they are both classed as D.

Fission engines are not without their drawbacks. A caveat to their fuel efficiency is seen in aerospace operations, where the fission reactor's own fuel duration is unaffected, providing a considerable endurance of power, but propellant use is much less efficient. Fission engines use radioactive fuels by definition, making a damaged engine a danger to the unit's operator(s) and requiring heavy radiation shielding for normal safe operation that makes them one of the heaviest engine types, even compared to internal combustion engines. The size of fission engines frequently makes them more costly than an equivalent but safer and lighter fusion engine. Safety concerns also make them a controlled item.

Manufacturing
Many entries on this list represent best guesses based on support vehicle entries, which do not always note manufacturing location or engine brands and models. Customarily, the name and, if known, world of the vehicle's manufacturing company is used in these cases and marked with a question mark. Fission engines are manufactured on the following planets:

Fanmade extras
Fission Engines are universally and objectively awful compared with Fusion engines. They are 33% more expensive, have the same Tech requirements; they are also always heavier, and have half the amount of heatsinks, which doesn't even make up for their increased weight. They also require additional shielding on vehicles with makes them heavier still, even heavier than ICE. All in all, they are completely useless as written, and there would be no reason to take one given that Fusion prices are lower for being better in literally all regards, with the same tech rating.

These rules and extra equipment aim to offset that imbalance somewhat, thus giving something in return to its drawbacks to fission tech.

Fan variants

 * The Expanded Fission Engine is meant to provide Fission engines with more usability, since they are otherwise strictly inferior to Fusion ones, while also being more expensive and having the same tech rating. This version occupies more slots, equivalent to an XXL engine, and weights 1/3 of the base one, while not being nearly as expensive as a Fusion XXL.
 * Fissile Fuel Cell Engines (FFCE) are Fuel Cell Engines that use fissile nuclear fuels such as Plutonium to power up a vehicle or 'mech. The fission provides power without a reactor.

Fission tech downgrade
 ''Fission Engines are classed as D on the tech scale, which puts them in the same classification than Fusion. However, Fission Engines were already used in vehicles from the middle 20th century, when Fusion wouldn't be viable, let alone miniaturized, for a long time. Fission Engines would be more available to primitives, rogue actors, periphery, and deep periphery states.''

As such, with this optional rule, Fission Engines should be considered C-level tech.

Fission C-bill discount
 ''Fission Engines would be easier to construct than Fusion ones due to the lower concentration of forces involved in splitting atoms rather than joining them. They would also not require lostech facilities during the darkest times of the Sucession Wars, when Fusion engines were explicitely scarce. As a result, many Fission engines would be built, making them cheaper due to greater supply. Even when Fusion engines are reintroduced to a greater extent, their superiority would drive down the price of pre-existing Fission engines.''

When playing with this optional rule, C-bill cost of Fission engines is halved, so they cost 2/3rds of a Fusion one rather than 4/3rds. This discount doesn't apply to Expanded Fission Engines, which would still cost 5/3 of the regular C-bill cost of a basic Fusion engine.

Fission Criticality
 If playing with this optional rule, Fission Engines always use the Stackpoling rules, while Fusion engines don't.

Fission Deliberate Meltdown
 If this optional rule is active, you can use the full turn of a Fission-toting unit to try to deliberately melt down the reactor. This produces 1 heat in heat-tracking units for every full 50 of the Fission engine's rating (no rounding). Then, if your unit is above its heat limit, roll 2d6. If the result is equal or lower than your unit's pilot Piloting skill (yes, bad pilots have higher chance of blowing up; this is deliberate), the unit Stackpoles. Non-heat tracking units always roll the 2d6 but only Stackpole on a natural 2 or 3.

Destruction Waste Radiation
 If playing with this optional rule, after the unit is destroyed (including by stackpoling explosion), an area is contaminated with radiation for 100 years (so permanently, for normal gameplay purposes). This radiation deals 1d6-1 damage to infantry and Battle Armor for each turn they end inside it. To non-infantry units, it deals 1 damage per turn they end inside it to their center structure (body or center torso, for example), bypassing armor, unless that armor is Ablative Armor, in which case the armor is damaged.

The size of the area contaminated depends of the rating of the engine:
 * 10-100: Only contaminates same hex.
 * 110-200: Contaminates same hex and 1 hex radius.
 * 210-400: Same hex and 2 hex radius.
 * 410+: Same hex and 3 hex radius.

Contamination of a ground hex can be cleared permanently by firing 1 ton of water through Sprayers. However, water hexes aren't immune to contamination. In the case of water hexes, the contamination clears automatically in 10 turns.