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Episode IVThe Next Generation
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The various references to power plants in the six movies, put together, paint a very intriguing picture.

Fusion power is clearly the rule of the day, from starships to portable heaters. At the same time, liquid fuel is used for both starships and ground vehicles (the Invisible hand, AT-STs, landspeeders, podracers, etc); we know all these references that it is flammable.

Were it used alone by starships, we could presume it to be a propellant; if fusion power generators as small (and presumably inexpensive) as the heater seen in TESB were viable, there would be no reason to power an AT-ST chemically. Together, these bits of data from the movies and their novelizations combine to tell us that Star Wars starships run fusion engines that fuse hydrocarbons. See Details

To put it simply, Lucas has invented - intentionally or not - the diesel starship. A raw output of 70-230 terajoules per liter (100-280 TJ/kg) is quite enough for the purposes of any ship's actions in Star Wars.

The question of what reactor power is available, and how much fuel storage ships actually have, is open. We have no firm upper limits from the movies, but the hyper limits described in ANH suggest a lower limit of 17-357 megawatts per kilogram of starship. If the hyper limits are an obstacle that could be overcome with more power, these lower limits also serve as upper limits once adjusted for possible efficiency.

See DetailsThe solution to this mystery lies in the chemistry of hydrogen. Raw hydrogen or deuterium is a very inconvenient fuel, with an exceptionally low density. It's also a pain to store; hydrogen gas easily escapes seals, and hydrogen freezes only at exceedingly low temperatures. Metals in contact with hydrogen react chemically with it, becoming brittle hydrides. A stable metallic hydrogen would have the density required, but forming it requires dangerously close to fusion pressures, and its stability is questionable. And solid pellets of metal are a pain to store and feed into a reactor in a controlled fashion.

Water is often cited as a good method of storing hydrogen. Water is one ninth hydrogen by weight, giving it a higher volumetric density of hydrogen by weight than pure hydrogen under all normal ranges of temperature and pressure. Heavy water, water containing deuterium instead of "regular" hydrogen (protium), has 0.2 grams of deuterium per milliliter.

However, hydrocarbons are even better. A "heavy" decane would have 0.225 grams of deuterium per milliliter, while being lighter than heavy water, less corrosive, more compressible under sudden shock; it doesn't expand when it freezes, bursting pipes and tanks; it has over twice the temperature range that it stays liquid in. All these features make hydrocarbons a logical form to store hydrogen in.

As an added bonus, if your fusion engine is a robust model that can fuse more than just hydrogen, the carbon part of hydrocarbons returns more energy than the oxygen part of heavy water. The only disadvantage to using hydrocarbons as fuel for fusion engines is the mass.

Considering the engineering difficulties in burning hydrocarbon fuel in a fusion furnace, Star Wars fusion engines are clearly highly developed. It is thus reasonable to guess that Star Wars engines get nearly the maximum theoretical ~230 TJ/liter out of their fuel.