Power Technologies
Background
While the standard fuel for starships and other spacegoing craft across human space is Helium-3, other options do exist. The most common alternative is the use of Deuterium-Deuterium reaction chains to provide electrical and thermal power. While commonly used in the 21st century alongside fission power before cheap Helium-3 became available and out on the rim well into the 22nd for similar reasons, D-D reactors became something of the past until the ZOCU war.
During the war, Core interception of HEC Helium-3 convoys disrupted ZOCU operations, both civilian and military. However, Deuterium is commonly available all across the galaxy, being readily seperable out of water. Crash programs were instituted to replace Helium-3 burning reactors with ones that would require Deuterium fuel only. While these were technically successful, they could not be refitted to enough ships and aerospace craft to continue fighting once a total blockade was imposed. Equally as bad, D-D reactions are both strongly neutronic and much less efficient than He3. In order to get comparable energy densities they must run much 'hotter', resulting in rapid irradiation of the reactor vessels.
Power Technologies
Deuterium Fusion
In the aftermath of the ZOCU war, some states seeking energy independence acquired ZOCU D-D technology or developed their own. While building Deuterium-Deuterium plants did have the desired effect, the downsides of this technology have not been solved. As stated above, D-D reactions are both strongly neutronic and much less efficient than He3. In order to get comparable energy densities they must run much 'hotter', resulting in rapid irradiation of the reactor vessels.
+1 Deuterium Core: This upgrade is slot-free and allows a unit to pay its He3 upkeep in Supplies - in effect, replacing the irradiated fusion reactors with new ones. Ships with Deuterium cores may opt to pay with regular He3 - their reactors are robust enough to handle the fuel differences.
Note that as this actually represents damage to a (replaceable) component, ships traveling on Deuterium power do not need to pay any supplies until they have accumulated 20 Transit-weeks or 1 full upkeep payment. Thus over short trips it is entirely feasible to travel and return, paying the travel cost only afterwards. However, for simplicity payment can also be made before.
Plutonium Fission
The earliest long-duration spaceflight programs used heavy element fission, as reliable fusion had yet to be developed. While this technology is well studied and capable of achieving high power outputs, it generally falls short of fusion power in terms of raw output. Advanced development of plutonium-fueled gas-cooled fission plants has led for some Rim states to build fission piles once again.
+0 Plutonium Reactor This upgrade is slot-free and allows a unit to pay its He3 upkeep in Supplies - in effect, supplying the ship with plutonium. Ships with Plutonium Reactors may not pay with regular He3, as their fission reactors use helium as a coolant opposed to a fuel.
Monopole-Catalyzed Fusion
The other power technology in common use is monopole-catalyzed fusion reactors. A relatively recent innovation, monopole cores can make use of the proton-proton chain that occurs inside stars; with a high energy density and the fuel being simple hydrogen, they have much simplified logistics. Unfortunately the monopoles decay over time, requiring intermittent 'refueling' operations.
+1 Monopole Core: This upgrade is slot-free and converts He3 upkeep into Monopole upkeep; ships pay x2 their He3 upkeep while all other units pay x1.5. This only needs to be paid once per year and covers all fuel upkeep a unit may need during the year. It does not cover supply upkeep and a ship with a monopole core may not operate on He3 fuel.
+1 Stable Monopole Core: Based off of rare strategic resources, stable monopole cores act nearly identically to standard monopole cores, except stable monopoles do not decay. Ships require x5 their He3 upkeep while other units require x4. This payment only needs to be done once and covers all fuel upkeep.
Monopoles are always bought at 5 points per 1 SD, and are unaffected by Deep Core Mine. Stable monopoles cannot be bought at nation creation, but may be found or bought off of markets.