The Power of the Empire

      Imperial Reactors and Power Technology

I.  Introduction

Nothing is accomplished without energy, and while the amount of energy you have isn't necessarily as important as what you can do with it, knowing how much energy is available is rather important to have a rough idea of what might be possible.  To be sure, with science fiction there are tools that allow for extraordinary "cheats".  Just as a mathematically-inclined caveman might find it incomprehensible that twitching one's finger could possibly result in a kinetic energy of over a kilojoule capable of killing a man, Dirty Harry (with his trigger-finger twitchy and ready) would view it as a reality of his day-to-day life.  Similarly, faster-than-light travel demands that somehow, the space-hopping characters we watch are able to violate relativity's demand that mass reaching lightspeed requires infinite energy, reaching lightspeed and beyond without infinite energy input.  Cheats are just part of the game, and while it's best to avoid assuming cheats without proof, sometimes you've just got to admit that the rules have been broken.

The point, of course, is that when it comes to one's power generation technology, how much energy one can produce in a given time is not the end-all be-all criteria of success.   But having the bigger number sure doesn't hurt.  In the below, we'll take a look at the mighty cruisers of the Empire and their reactors, so we can see for ourselves what the true power of the Empire really is.

II.  Imperial Reactor Technology

There's been a bit of confusion fomented over the years about just what makes a Star Destroyer tick.  Without a whole lot of engineering details showing up in the films at all, and with a lot of what's given about other vessels being a bit vague or confusing (e.g. what the hell could an "alluvial dampener" be doing on a starship?  What's the purpose of a "hydro-spanner"? Was Noah the captain?), there's been a lot of room for people to interject their own ideas.   Early ideas on Star Wars gave the vessels fusion-based power systems, but as the tech inflation of Star Wars in the Expanded Universe has progressed over the past few years we've seen the reactor technology 'retroactively improve' into something called hypermatter, separate and quite distinct from real-world hypermatter, that makes even antimatter weep with inadequacy.

A.  Canon Information

Despite the EU's internal confusion, the old Original Trilogy canon didn't change at all.

1.  Reactors

 We are told the following in the ANH novelization, at the destruction of the Death Star:

"Space filled temporarily with trillions of microscopic metal fragments, propelled past the retreating ships by the liberated energy of a small artificial sun." (ANH novelization, chapter 12, italics mine)

There has been much debate about this quote, previously discussed elsewhere.  Nonetheless, the quote clearly states that a small artificial sun has liberated its energy.  Ergo, many would argue that if the Death Star were powered by a small artificial sun, then undoubtedly it is powered by fusion akin to what occurs in a large, non-artificial sun, whatever opponents of the Star Wars canon might say.  That fusion is employed among the major powers of the Star Wars universe was confirmed to an extent by the novelization of TESB, wherein we're told about Luke's small power generator:

"'You ready for some power?' Luke asked Artoo, who was patiently waiting for his own form of nourishment. Luke took a small fusion furnace from an equipment box and ignited it, welcoming even the tiny glow thrown off by the small heating device, then took a power cable and attached it to Artoo [...] As power radiated through Artoo's electronic innards, the stout robot whistled his appreciation."

Of course, just a few pages later, a bit of confusion comes into the picture . . .

"Luke turned around to see the little droid standing forlornly next to the miniature fission furnace." (Ch. 8)

As seen in the film, the scenes looked like this:

Whether the unit was fission-based, fusion-based, or both, the fact of nuclear technology being the standard is seemingly confirmed.   Of course, as with our own technology different techniques are used for different purposes.  No one would use a tiny internal combustion engine to power their laptop, for instance, just as we wouldn't try to stick a huge lithium battery into an aircraft carrier to give it power.  Certainly R2-D2 doesn't seem to have a nuclear reactor on board, given that he must be recharged by one.   On the other hand, if you have fusion power available and have perfected it to the point where you can make huge reactors and tiny ones, then it's likely to be the most widely-used method of power generation you're likely to employ, just as internal combustion is for us today.

Indeed, the prequel novelizations confirm this concept quite well, and settle the matter altogether.  Despite the intervening decades of non-canon hypermatter hogwash, in the RotS novelization, we're told:

"Children on Tatooine tell each other of the dragons that live inside the suns; smaller cousins of the sun-dragons are supposed to live inside the fusion furnaces that power everything from starships to Podracers."

 That suns and power generators are both fusion-based is thus made very explicit by the narrator, even accounting for the imaginative dragon infestation that the children tell each other about.  Given that at least one child on Tatooine had the engineering skills and know-how to race and build podracers along with his friends, the claim opponents of the canon make that these are the deluded ravings of sugar-mad uneducated children is absurd. Thus, barring some unmentioned change in the intervening 20 years, a Star Destroyer runs on fusion just as much as Clone Wars vessels did.

Happily, we even get to see the "engine room" of a Clone Wars-era Republic Venator, the Tranquility.  Given that these vessels were the earliest Imperial Star Destroyers (as seen at the end of Revenge of the Sith), it is likely that Imperial Class Star Destroyers of two decades later shared similar design philosophy. The room is extraordinarily large and empty, several decks tall and dozens of meters wide.  

Note at right the tiny Ventress looking down at the spotlight-eyed droid on the walkway.

Although we could presume that this is to allow some sort of high-pressure leak a lot of expansion room, the fact that plain old air vents are present in the room with no apparent closing mechanism (merely a grate) suggests that, instead, this empty space is there for some other purpose.   Perhaps some sort of higher density leak could settle to the bottom, or perhaps the extra distance from the machinery allows some sort of radiation shielding that is sufficiently lightweight to offset the extra volume, whereas a smaller volume would require heavyweight cladding.

Whatever the case, the cavernous room is home to three columns with spherical objects atop separate pedestals.   At the floor of the room is a large hemispherical object, with another on the ceiling.  It is not clear what items are the reactors for the ship.  When charges were placed around the Tranquility engine room and detonated, crippling the ship, one of the three spherical objects appeared to fall over completely or was otherwise heavily damaged.  However, while the room did have continuing fires, at no point was there any mention of a possible reactor overload as has been made to occur when other reactors have been attacked with explosives (e.g. "Weapons Factory"[TCW2] or the Death Stars).   Presumably, then, the three spheres are not, themselves, reactor cores.


Star Wars reactors commonly feature a top hanging part and a bottom part with a skinny connection between them, plus a whole lot of extra space around them.  Note the Death Star II reactor chamber, the Geonosian droid foundry reactor from "Weapons Factory"[TCW2], and similar.



Further, the ship was apparently repaired fairly quickly after the situation ended.  Other vessels have not been so lucky.  After a power surge caused explosions throughout a battle-damaged Separatist Munificent in "Cargo of Doom"[TCW2], the vessel's "main reactor is exposed", per Admiral Yularen, and it "will implode at any moment".   This may suggest certain things about the sort of reactions involved, since a fusion reactor as we understand it that is suddenly 'open to the public' would disable itself quickly, rather than destroy itself and its surroundings.

Instead, Yularen's comment and the other similar reactor-blowing incidents suggest that Imperial fusion technology is based around some remarkably unsafe design (even for fusion).  

2.  Generators

On Naboo, a very different device is present, referred to as a generator for the power station.  Long tendrils of a white glowing plasma within some sort of tubes criss-crossed a massive chamber with the usual unsafe Star Wars walkways, and a ray-shielded room with a seemingly bottomless pit (described in the script as the melting pit) sat nearby.   The location was described in the novels:

" They were in the service corridor for the melting pit, the disposal unit of the power station's residue. The service corridor was armed with lasers against unauthorized intrusion." (TPM Ch. 22)

" He sometimes dreams of when he was a Padawan in fact as well as feeling; he dreams that his own Master, Qui-Gon Jinn, did not die at the plasma-fueled generator core in Theed."  (RotS Ch. 1)

This power generator would likely be a generator of the type we know of, one which converts energy from one form to another.  Thus the plasma, from whatever source, is being converted into electrical power here.  But, then, what a melting pit would be doing there is unclear, since it is uncertain what sort of residue would be created by a plasma-fueled generator.  

Perhaps the plasma is from a fission reactor, and the melt pit is a way of dumping radioactive waste?   But that hardly seems safe on a planet with such vast underground water habitats.  So the design and technology of this generator is somewhat confusing.

B.  Discussion

The reason for fusion's safety is that it requires extreme temperature and pressure to be initiated and continued.  A runaway fusion reaction within a reactor is thus considered largely impossible with even the most simplistic safety in the design, since a constant stream of fuel and constant powering of the confinement system (magnetic, laser, or what-have-you) is required.  As soon as a fusion reaction got too 'hot', then, it would either activate automatic cutoffs or simply destroy what was keeping the reaction going.

However, it is conceivable that there are specific types of fusion or fusion reaction chambers that might allow for extremely efficient reactors which, as a drawback, can somehow suffer from runaway reactions.  For instance, liquid lithium is currently used as a coolant and tritium breeding ground in many of our designed or experimental reactors, and lithium is also potentially useful as a fusion fuel directly.  One can thus imagine a coolant leak into the reactor core producing significant unplanned fusion events, potentially, though in most current designs this would be virtually impossible . . . it would be more like dousing a fire with water than throwing gasoline on it.   However, it is at least conceivable that a high-volume and less-safe design might have risks of this or a similar nature.

1. Fusion Reactions

As far as what we know of fusion now, "simple" deuterium-deuterium fusion reactions require millions of degrees and incomprehensible pressures to get started and to be maintained.  Once going, however, the reaction produces a Helium-3  atom, a stray neutron, and an energy of 3.27 megaelectron-volts (MeV).  A single joule is equivalent to 6.24 quintillion eV, or 6.242E18 eV, but given how many atoms are involved per gram the energy involved adds up rather quickly!  

Just as likely for the reactants above, however, is a result of a tritium atom plus a normal hydrogen atom, along with 4.0 MeV.  So assuming both occur with about a 50% chance then you end up with four deuterium atoms (deuterons), a helium-3, a tritium, a hydrogen, a neutron, and a boatload of energy.  But with fusion afoot, those reaction products can also be made to fuse if the temperature and pressure is there, in which case one ends up with even more energetic deuterium-tritium reactions along with a reaction between deuterium and helium-3.  All around, then, one ends up with 2 helium-4s, a normal hydrogen atom, a couple of neutrons, and an energy release of 43.2 MeV from six deuterons.   This is one possible fusion cycle . . . a planned series of fusion events occurring in the correct order to more efficiently make use of reactants and reaction products for maximum energy production.

Of course, there would be the issue of those pesky leftover neutrons.  One can hardly imagine Palpatine and the senators standing around at the end of Attack of the Clones like there wasn't a problem in the world if they were being bombarded by neutron and secondary gamma radiation from the Acclamators as they were taking off.  However, there would be ways to mitigate the radioactivity.   For example, neutrons can be readily absorbed by light nuclei like hydrogen.   Thus cheap, plentiful substances with a significant amount of hydrogen per unit mass like water, concrete (high in water content), polyethylene, or something like paraffin wax are useful neutron absorbers.

2. Tackling the Neutron Problem

An alternative would be a type of fusion that does not result in such large amounts of neutron radiation.  Various concepts for such aneutronic fusion have been proposed.  One involves deuterium and helium-3.   Using deuterium and helium-3 only results in run-of-the-mill helium-4 and a proton, unleashing 14.7MeV, and while other reactions could occur (such as deuterium-deuterium) that would still result in some neutron radiation, the levels would be significantly lessened (aneutronic reactions are defined as reactions where less than 1% of the energy produced is carried off by neutrons). There is the problem of the extreme temperature requirements of deuterium and helium-3 reactions (some ten times greater than D-T reactions), but this can be lessened with significant pressures.  Suffice it to say that there's a bit of a startup problem involved.

Another alternative, considered implausible for modern Earth due to fuel availability, is helium-3 fusing with helium-3.  However, helium-3 is abundant in the solar system, with vast quantities of it on the surface of the moon needing only heating to release, or minable from gas giants.  It also releases a bit less energy than deuterium and helium-3.   Helium-3 and Lithium-6 is a bit better than both, but less impressive than the cycle mentioned first. 

Carbon also has a unique fusion cycle known as the CNO cycle, commonly though to be the power source of stars larger than our sun.  It is workable at 15 million K and above, soaking up free protons in the reactor and spitting out hydrogen, helium, positrons, and neutrinos.   The cycle is fairly energetic, outputting 26.72 MeV for a full cycle, but it involves a significant startup penalty.

3.  The Steam Question

One possible reason for all the water-related terms (e.g. "alluvial dampener") is in relation to the fact that there's an awful lot of steam in Star Wars.   The generator on Naboo had bursting steam pipes near the melting pit, per the novelization.  The droid foundry on Geonosis in Attack of the Clones had enormous steam exhaust vents, and if we are to assume that like the droid foundry of "Weapons Factory'{TCW2] and the AotC version are similar then both had their own reactors.  The hangar of the Invisible Hand is said, in the RotS script, to have extensive steam piping.   The rebel base on Hoth had broken steam pipes, per the script and TESB novelization, and Cloud City seemed to be full of steam in the engineering areas.  The Return of the Jedi script and novelization features a boiler room in Jabba's palace, with the latter describing "deafening machine sounds - wheels creaking, piston-heads slamming, water-hammers, engine hums -and a continuously shifting haze of steam", and when on the Death Star II the situation turns against the Empire, the chaos is described as "Electrical fires, steam explosions, cabin depressurizations, disruption of chain-of-command."

It is not at all clear what all that steam is doing around.  It's possible that, just as naval aircraft carrier plane-launching catapults use steam generated by the reactor to push a plane into the air, steam is used in the Empire for other similar purposes.   Heating would be a possible choice, but given that Palpatine was hanging on to the hangar steam pipes by hand then heat radiation was clearly not the goal.  Whatever the case, steam technology was presumably enough of a requirement that Jabba's palace featured a boiler room.  While this might've been intended to simply give the palace a more skin-friendly humidity level than might be found naturally on Tatooine, that's not at all obvious given the heavy machinery and large room involved.

It seems plausible that in addition to electrical wiring, simple steam is used for some purposes, though it isn't clear what.

C.  Conclusion

Given the millenia of technological advancement of Star Wars, I feel quite certain they're capable of virtually any of the more efficient means of fusion, and have probably developed ways of keeping the radioactivity a virtual non-issue.  However, in spite of this concept, it is clear from the canon that their reactor designs, however efficient they may be, introduce some deleterious safety concerns.

III.  Fuelling the Reactors

As established, even children on Tatooine know that fusion powers everything from podracers to starships.   We would thus expect that any mention of the fuel for these vehicles would probably involve ultra-cold or highly-pressurized deuterium, or perhaps metal hydrides capable of storing hydrogen more easily and compactly, or similar.  Other possibilities also exist, such as the new idea of storing hydrogen in carbon nanotubes and other solid carbon forms, achieving a result similar to the metal hydrides but with more efficiency in terms of volume and weight. 

One might expect some indication of one of these sorts of techniques.  But instead, while we do get mentions of the fuel these vehicles work with, it in no way resembles what we might expect from the list provided above.   

A.  Canon Information

Let's start with this description of podracing from Chapter 1 of the TPM novelization:

"All that power, all that speed, just at his fingertips, and no margin for error. Two huge turbines dragged a fragile Pod over sandy flats, around jagged-edged mountains, down shadowed draws, and over heart-wrenching drops in a series of twisting, winding curves and jumps at the greatest speed a driver could manage. Control cables ran from the Pod to the engines, and energy binders locked the engines to each other. If any part of the three struck something solid, the whole of the assembly would collapse in a splintering of metal and a fiery wash of rocket fuel."

It's those last two words that are of interest . . . "rocket fuel".  Of course almost anything can be rocket fuel, but here we're looking for something that is liquid and flammable, given the "fiery wash".  Liquid deuterium qualifies, but only to the extent that, as it boils upon exceeding 20 K, flammable hydrogen gas is produced, a gas that very much likes to combine with oxygen.  The description above is more suggestive of flammable liquid.   Of course, it's possible that the podracer's electrical systems are powered by fusion whereas the engines use a separate fuel, but why keep a fusion reactor on the pod when it could simply sip a tiny bit of power from the engines, as a modern fighter might?

A few other consistent mentions of such fuel appear in TPM regarding podracers.  Elsewhere, observe the mention of fuel as carried aboard a snowspeeder in the text from Chapter 6 of TESB below:

"At that instant, Hobbie's burning ship crashed through the walker cockpit like a manned bomb, its fuel igniting into a cascade of flame and debris."

Here again we have behavior which could be basically on par with modern-day petroleum-based jet- or rocket-fuel.  Fighters are referred to similarly in Chapter 12 of ANH:

"One energy beam seared his port engine, igniting gas within. The engine blew apart, taking controls and stabilizing elements with it. Unable to compensate, the out-of-control Y-wing began a long, graceful plunge toward the station surface."

Hydrogen gas without oxygen does not burn, but some rocket fuels will.  Similarly, RotJ's ninth chapter gives us the following:

""Father, I won't leave you," Luke protested. Explosions jarred the docking bay in earnest, crumbling one entire wall, splitting the ceiling. A jet of blue flame shot from a gas nozzle nearby. Just beneath it the floor began to melt."

While there's no specific statement on what the gas nozzle was normally used for in the small vehicle landing bay, the implication of a refueling port is quite tempting, if not altogether likely given the usual hose-based refueling systems used for fighters.

So what's really going on here?  Is there some sort of contradiction afoot?   Does the mention of fusion in RotS suggest different technology than used elsewhere?

Well, no.  The same fellow who wrote about fusion furnaces, after all, was writing from a script containing the following:


The TWO JEDI cut their way down several floors into a large generator room.  Huge EXPLOSIONS outside the ship have caused several large pipes overhead to break, and fluid is spewing everywhere.  The Jedi get up and turn off their lightsabers.  ANAKIN dips his hand into the fluid and sniffs it.

Obi-Wan: . . . fuel.  The slightest charge from our sabers will send this ship into oblivion.  That's why they've stopped shooting.
Anakin:  Well, then, we're safe for the time being.
Obi-Wan:  Your idea of safe is not the same as mine.

They run, EXPLOSIONS rattle the ship, and pipes continue to burst around them, spilling more fuel into the hallway.  At the far end, SIX SUPER BATTLE DROIDS drop into the fuel.  The SOUNDS OF SHIELD DOORS CLOSING AND LOCKING ECHO throughout the hallway.  They pass several large power generators, which are topped with SPARKING excess power dischargers.

Anakin:  They're sealing this section off.
Obi-Wan:  Six droids coming our way!

The last of the DOORS CAN BE HEARD CLOSING in the distance.

Anakin:  Keep moving.  There must be vents . . . This way.

THEY move along a wall.  ANAKIN climbs up the side to a small vent.  The fuel gets closer to the SPARKING dischargers.

Obi-Wan:  We'll never get through that.  It's too small!

They move toward a second vent.  OBI-WAN is swimming in the fuel as it reaches to within a couple yards of the ceiling.  ANAKIN feels along the ceiling and finds another smaller vent.  He closes his eyes and tries to sense an opening, then he moves on.  OBI-WAN is forced into hand-to-hand combat with one of the SUPER BATTLE DROIDS.  It pulls the Jedi under the fuel.  Just before he is about to drown, OBI-WAN disables the SUPER BATTLE DROID by pushing him into an exhaust pipe.
The fuel is up to the Jedi's chins.  ANAKIN finds a very, very small metal grate, then pounds on it until the tiny grate breaks loose.

Anakin:  I found our escape vent.
Obi-Wan:  Anakin, this is no time for jokes.  We're in serious trouble here.
Anakin:  Only in your mind, My Master.  Look, no structure. . . .

ANAKIN grabs the side of the tiny hole and gives it a big yank, ripping a large panel loose revealing a "man-sized" work shaft.  They scramble through it as the DROIDS swim closer.


The TWO JEDI pull themselves through the narrow vent shaft until they reach a small hatch in the side of the tube.

Anakin:  Here's a way out.

As the SUPER BATTLE DROIDS reach the opening in the ceiling and the fuel gets to within a few feet of the power generator sparks, the JEDI work the keyboard on the pressure lock, opening the hatch.


The TWO JEDI climb into a small passageway and slam the hatch shut.  They make their way through the ever-shrinking shaft until they reach the end.


A hatch opens in one of the main hallways of the Trade Federation Cruiser, and the JEDI squeeze out, SLAMMING the hatch.  Behind them, ANAKIN seals the hatch with his laser sword.

Obi-Wan:  That won't hold when the fuel hits those power dischargers.
Anakin:  The blast will break the hull.  This side's pressurized.
Obi-Wan:  You still have much to learn, Anakin.


The SUPER BATTLE DROIDS climb up the vent shaft.  SUPER BATTLE DROID R77 and SEVERAL OTHER DROIDS wait in the generator room as the fuel continues to rise toward the power discharger.

Super Battle Droid R77:  I have a bad feeling about this.


The fuel hits the SPARKING power discharger, and there is a HUGE EXPLOSION.


A GREAT EXPLOSION and a flaming gas cloud spray out of the side of the Federation Cruiser.


A large bulge appears in the wall around the sealed hatch as the EXPLOSION hits.  OBI-WAN jumps back, then stands amazed.

Obi-Wan:  All right, you win.  I have much to learn.  Let's go!

ANAKIN grins at OBI-WAN, and they run down the hallway.


The above makes it clear that we're dealing with liquid fuel for the cruiser Invisible Hand, obviously, just as liquid fuel seems to be the norm for other fusion-powered Star Wars vehicles.  So instead of a contradiction, we need to figure out how one would store hydrogen in a liquid form.

Happily, we may have a name for this fuel, whatever its precise composition.  In "Rookies"[TCW1], a "highly explosive" liquid fuel called tibanna is used as a heating fuel for a base.  And, similar explosive fuel cells have been seen elsewhere, such as one of the landing bays of the listening post from "Duel of the Droids"[TCW1].   Barring a separate and distinct type of fuel, then it seems that this might be the fuel we're looking for.  

As it happens, we've heard of tibanna before.  Cloud City on Bespin from TESB was a tibanna gas mining facility, meaning that tibanna is a naturally occurring material.  The idea of tibanna gas being on Bespin might give us pause.  If Cloud City was floating amongst tibanna gas then why is it a liquid elsewhere?   However, this is not likely to be an actual issue.  First, just as water can be a vapor or liquid, tibanna might be in vapor form for the most part on Bespin.  Second, given the fact that weapons fire and ship's engines did not seem to be an issue near Cloud City, it seems likely that the tibanna gas was elsewhere in the atmosphere, presumably at lower depths or in particular cloud formations.  As a result, tibanna might likely be a liquid in its natural form and Earth-normal atmospheric conditions.

In any event, though, we can tentatively say that this liquid tibanna may be the power source for most Star Wars technology, whether via combustion for rocketry or fusion for energy.

B.  Discussion

Could tibanna be a Star Wars galaxy term for something we know?   After all, if we roll with the concept of a liquid that has a lot of hydrogen in it, then we already know of more than a few possibilities today.

1.  Fuel Properties

Whether in the form of alcohols like ethanol or methanol, or in the form of a compound like ammonia, or in the form of hydrocarbons such as gasoline, or in the form of water, liquid containment of hydrogen is not a new idea.   And while new methods of storing deuterium in a liquid form may appear in the years to come, we can try to take known liquid storage mediums and apply what we know from the Star Wars canon to try to get a rough idea of what the most likely type of liquid storage medium might be.

a.  Requirements and Possibilities

First, let's ponder the requirements we have so far:

1. Per the explosion of the Invisible Hand described in the script, the flame of the liquid when combusting must be visible, per the RotS script.
2. The vapor of the liquid must have a high flash point, given that the liquid itself had to reach the sparky-thing in the RotS script before the explosion occurred.   If the vapor is significantly heavier than air, the same result is achieved.  Mileage may vary here.
3. The liquid must be sufficiently volatile (i.e. evaporative) that it evaporates fairly quickly, given that the fellows looked dry later on.
4. The liquid must have a distinct odor, but not an especially pungent one.  Anakin had to dip his hand in it and sniff, as opposed to many fuels which can be smelled from a great distance.
5. The liquid must not be poisonous or especially dangerous on contact with skin or eyes.  Minor irritants are allowed (see note following).
6. The liquid must be such at near-room temperature . . . no cryogenic liquids like liquid hydrogen.
7. The liquid must be transparent or semi-transparent, given the hand-to-hand combat that occurs (and which, clearly, we would've seen had these scenes been shown).
8. Preferably, the other elements involved should not interfere too much with the fusion process.  Of course, since the deuterium-tritium part of the deuterium cycle discussed above requires a temperature of no less than 40 million K at  high pressure, this probably isn't too much of a problem. 
9. Preferably, for use as rocket fuel in space, a separate oxidizer should not be required.  Or, in the event that pure rocket thrust is not the use of the fuel, it should be usable as propellant in an ion engine.  Or both.

As an aside, it's really quite wonderful that we have so much information.  Rarely, and especially rarely in Star Wars, do we get such useful information about anything.  However, these requirements knock out several possibilities.

Heavy water (D2O) containing deuterium in the place of hydrogen, for instance, is right out, thanks to it having all the flammability of . . . well, water.

Nitrogen and hydrogen combinations don't work well for us.  Pure ammonia (NH3) is a gas at room temperature and is quite odiferous and irritating even in low concentrations, such as when mixed with water in household cleaners which commonly feature only 5-10% ammonia.  While some additive might negate these effects, But it would've worked nicely otherwise, given that its properties make it a very good coolant as well.  Hydrazine (NH4) or UDMH (C2H8N2) are highly toxic.

Hydrogen peroxide (H2O2) is a monopropellant (i.e. requiring no separate oxidizer, since it is one), but it is not flammable by itself and is highly toxic in pure form.

Most alcohols are also out.  Our good friend ethanol (C2H6O) is a severe eye irritant and readily evaporates in pure form, leading to flaming beverages and, more to the point for our purposes, a risk of vapor combustion in regards to the sparky thing in the Invisible Hand.  

Which is a damn shame, really, because there's absolutely nothing better than the notion of a booze-powered starship.  Except that you can then drink yourself into an engine shutdown.

Methanol (CH3OH), like hydrogen, burns almost invisibly, and suffers other failings that ethanol has.

Most other alcohols are also troubled.  Cyclohexanol (C6H11OH) is great for most of our purposes, except for the severe eye irritation.   Mitigating substances might help here, or else mixing in water.

On the other hand, there are alkanes which are similar, but simpler.  Cyclohexane (C6H12) works just as well as cyclohexanol.  Some heavier alkanes also work.

At this point, though, we're simply dealing with hydrocarbons . . . the same sort of materials that we've been burning for centuries, whether in the form of paraffin for candles or gasoline and jet fuel for our spiffiest toys.

2.  StarfleetJedi.Net and the Diesel Starship Argument

The problem we face in attempting to select a suitable known substance is that we could use many possible substances (and combinations or variations thereof) that meet most of our needs.  Another researcher addressing the topic decided that the best response was to simply make a choice . . . one not necessarily without irony.

Hence the concept of diesel starships at StarfleetJedi.Net, which is as close to the coolness of a booze-powered starship as one can get.  Quoting:

"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.

{...}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.{...}

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."

Unfortunately, diesel fuel, like many other potential choices, only satisfies some of the requirements and preferences we have for the fuel.   However, it is a captivating idea, and is close to what we need.  

C.  Conclusion

Is diesel close enough to be considered 'true'?  I think not, but it is close enough and familiar enough to make it useful as a guide to understanding the technology.

In other words, it is clear that whatever the fuel source used for Star Wars vessels, be it liquid tibanna or some other substance, it is not some strange and incomprehensible material, unknown and unknowable to modern science, but simply an excellent hydrogen storage medium in liquid form that also makes a good rocket fuel.  Understanding it in terms of diesel or gasoline or similar petrochemical fuels, with suitable caveats, does give the proper flavor of its qualities.

But frankly, I'd still prefer a ship powered by vodka.

IV.  Reactor Power Requirements

Not much is known about the power requirements of a Star Destroyer.  Effective values are abundant and can be useful for comparison, but these do not tell us anything about the actual reactor power . . . only the effect of the particular technology being examined.

For comparison, however, let's stop and ponder a modern aircraft carrier.  The USS Enterprise (CVN-65) has eight separate model A2W nuclear reactors on board.  All such naval reactors generally operate the electrical systems of the boat via steam-fed generators, as well as turning the screws.  Direct ratings are hard to come by, given that telling everyone the weight and horsepower of your boat is about the same as giving the speed, which of course is classified information.  However, some data is out there.  

Later carriers such as the Nimitz Class feature two reactors of the A4W type, rumored to have an output of around 100MWe each (that's not the direct thermal output of the reactor, but the usable energy after conversion (e.g. to motion or electricity). The next generation carrier and its two reactors are planned to rate 300MWe each, for a ship output of 600MWe . . . starting to encroach on gigawatt territory.   Given the inefficiencies in recovering the energy from the reactor with modern technology, the reactors themselves probably run at at least a gigawatt, if not more.

A suitably complex calculation based on the ship's mass, water displacement and hull shape, and known (or guessed) maximum velocity and other matters could provide a workable estimate of the usable output of the Enterprise or Nimitz reactors.  

However, we must be cautious before leaping to the conclusion that we can do the same thing for Star Wars ships.  After all, the modern space shuttle does not have a reactor powering her engines. A shuttle's primary electrical power comes from a series of fuel cells while her main engines and orbital maneuvering jets use completely separate energy sources (i.e. rocket fuel).   Or, for a completely opposite example, the sailing ships preceding aircraft carriers were propelled not by onboard sources, but by the winds of the world.

Indeed, it is a widely held assumption in science fiction and especially certain sectors of Star Wars technological analysis that for any event we see a ship perform, the effective energy required must have been provided by the reactor directly.  Thus, even for technology known to 'cheat', the reactor is claimed to have the requisite energy to have performed the task anyway, and this level of energy is then assigned to other tasks.  This would be akin to assigning Dirty Harry's trigger finger the requisite strength to flick a bullet and give it a kinetic energy measured in kilojoules, and from there determining his running speed.  It just doesn't work.

Thus, we must be cautious in attempting to assess or assert reactor power requirements.

A.  Canon Information

Canon information on the actual power requirements for Star Wars ships is perhaps even more complicated than getting information for Earth naval ships.  There are many things we could try to calculate from, but if we look closely none of them are guaranteed to offer any sort of direct relationship as we might've thought at first.   In order to figure out a reactor power output, it takes that direct relationship.  

1.  Firepower

For instance, we could calculate based on the idea that the heaviest guns of a Star Destroyer are probably capable of shots yielding as much as 1.5 megatons.  It's a laser, right, so they had to power it, right?   Unfortunately, no.   Star Wars weapons are not lasers, but are instead based on something called a "galvened particle beam", where "galven" is a nonsense word.  And, as seen in RotS, the particles or power or both of the bolts are apparently supplied to the broadside gun . . . one type of weapon used shell casings like that of a bullet, and we can assume that other weapons not using shells simply had some sort of tank or piping arrangement involved.   As noted in the ANH script, the guns "wind up their turbine generators to create sufficient power", belching smoke in the process.  While reactor energy is no doubt important in accelerating the particle beams (and maybe even doing the galvening, whatever that is), there's no way to get any useful reactor power estimate from that.   (To put the problem in a more modern-day context, the issue would be akin to trying to figure out the amount of fuel oil burned by a WW2 Iowa Class battleship by calculating the energy involved in firing a full broadside of shells, shells which were propelled by a type of gunpowder charge in a bag.  The direct relationship of shot and fuel consumption just isn't there.)

2.  Shields

Shields have also been suggested as a way of deriving reactor energy, but here too we meet problems.  How do we consider shields to operate?  Is it a direct counterforce, like stopping a projectile with an equal projectile?  Then the shield requires exactly as much energy as it is deflecting.  Is it like a wall, stopping whatever hits it without any input of energy?   Then the shield requires only enough energy to 'build the wall', however much that might be.   Is it like a magnet, putting out a magnetic field that might deflect a projectile without the magnet itself being powered, but merely using stored energy?   Then the shield could be charged over a significant period of time, so there's little way to know reactor power from that.  (Had Tsar Bomba been a total release of energy stored in a battery, then a powerful car engine could've charged it up in 31,000 years.  Absurd, but it gets the point across.)

3.  Reactor Overloads

Often the result of the success of the first item or the failure of the second, reactor overloads could also provide a gauge, perhaps.  These are generally sufficient to destroy a ship, but don't accomplish much more than that.   For instance, in "Destroy Malevolence"[TCW1] we see Amidala intentionally overload the "power system" of her Nubian Yacht (of the same type, perhaps the same vessel, seen in Attack of the Clones, and of the same type as destroyed by Jar-Jar in "Bombad Jedi"[TCW1]).  The captured vessel, commonly reported to be 40-50 meters in length, exploded in a pyrotechnic display with burning debris tossed about the docking bay, but the vessel was mostly intact and there was no evidence of any significant nuclear-scale blast effects.  


Return of the Jedi also appears to feature an ISD reactor overload.  The incident occurs during a short-range near-broadside match against a Mon Calamari Rebel cruiser.  Two heavy Mon Cal shots miss the ISD completely, but a smaller third shot does hit the area of the bridge tower at about the same time the main hull superstructure suffers a massive explosion.  The center of the ship thus in flames, the four portside heavy turbolaser batteries explode, followed by detonations apparently from the ventral bulb structure and the  hangar areas forward of that.   Within two seconds almost the entire vessel is covered in explosion and flame, at which point the scene changes.   While the event appears quite destructive, the portside aft corner of the ship is still visible, seemingly intact and unmoved.  More remarkably, the bridge tower and most of the superstructure beneath it is also visible through the darker orange flames.  To be sure, we cut away so quickly that the vessel may yet have torn itself apart, but the nature of the event strongly suggests that the ship will end up a burning hulk rather than an exploding mass of vapor or debris.   


Compare the bridge tower's survival here to the asteroid incident that sheared off the bridge tower of another ISD in TESB.

But again, the problem we face is that these events are not readily explainable.  As noted, after all, fusion shouldn't involve reactor overloads . . . the fact that it does means that these events, however interesting, are too vague, and whatever lower limits we could get from them are probably too small to matter.

4.  Antigravs

Another option is to work from the assumption that an ISD could take off from a planet and achieve orbit like its Clone Wars-era predecessors, calculate the energy of that, and figure out the power required to do so in a certain length of time.   However, Star Wars vessels employ repulsorlift technology when within six planetary diameters (ANH novel), and we don't know how that works.   People commonly imagine some box hooked to ship's power that, when activated, acts like a rocket putting out some ethereal unknown propellant, but there's no evidence for that.  For all we know it's some technobabble complex gravitic or magnetic mirror or sail system needing almost no power at all, the space equivalent to an airship flying due to helium's presence.  (Ironically, the claim that repulsors can levitate a vessel without power input is a position claimed by some of the same non-canon authors (e.g. Curtis Saxton) who so strongly proclaim extreme hypermatter power and impregnable neutronium hulls.)

We know it is more than a simple gravity-nullification field since it apparently provides propulsive force based somehow on the gravity well of the planet.  Quoting ANH Chapter 7, "Antigrav could operate only when there was a sufficient gravity well to push against-like that of a planet {...}".   

Pushing against gravity seems quite an odd concept. Magnetic repulsion of other magnets with suitably oriented fields is one thing, but gravity is not known to have any repulsive qualities.  Still, the mysteries of magnetism may be a guide.  For instance, a magnet might just sit around being a magnet, but if it is sitting atop certain ceramics that can become superconducting when cold, then the magnet could levitate itself . . . it does so because the superconductor is basically shielded against the magnetic field, reflecting it in a sense.  So let's turn that upside down.  If we chilled the ceramic to the appropriate temperature, it could levitate, only doing so "when there was a sufficient magnetic field to push against - like that of a permanent magnet."  The only energy input required would be in cooling the ceramic, but this could be done with something like liquid nitrogen cooled off-site and stored wtihin good insulation for later use.  Far from being a made-up example, there's a demonstration of that very thing right here, in the form of a model maglev train.  Give the train a push, and off it goes just as fast as you pushed it until the ceramic material warms to a non-superconducting state.  Then it will drop back down onto the permanent-magnet track.

Locally . . . that is, in the context of the train which carries the ceramic and liquid nitrogen . . . no input of electrical or heat energy is required for maintaining levitation.  Of course, what's really happened has involved a lot of energy.  For one, cooling the nitrogen requires forced heat transfer, which requires energy (otherwise your freezer would never need to be plugged in).  And there's the push to get it moving.  And, of course, the magnetic field of the track is what is really doing all the levitation work . . . the ceramic superconductor is basically just being pushed away from that field.  But from the train's perspective, it's almost a free ride.Some might even argue that the supercold liquid nitrogen is drawing energy in from the ceramic material or train as it is heated, rather than providing any energy whatsoever.

And it may not work just based on simple gravity fields . . . it may in part be a surface-repulsion effect, assuming ground vehicles operate similarly.  After all, as seen in The Clone Wars series (e.g. "Weapons Factory"[TCW2], et al.), repulsor-tanks with ground removed from beneath them quickly fall down . . . the same is true even when the ground is a plasma bridge acting as a solid surface ("Liberty on Ryloth"[TCW1]).  And yet, when Qui-Gon and Jar-Jar are under a hovertank in TPM, they aren't squished when they are part of the surface presumably being pushed against, nor are those who are under the various starships seen taking off in the films or The Clone Wars episodes suddenly smashed.  Of course, given that repulsorlifts seem to work against gas giants (e.g. the Death Star's orbit of the Yavin gas giant), that doesn't seem to work out.  

Given the superconducting ceramics example, one can imagine a unit containing a material that, when cooled or heated, exhibits unusual properties in regards to its interaction with gravity waves or the theorized graviton particle, until it wears out.  It may take a number of these units throughout a ship, or perhaps the material is simply tossed into the reactor.  Or, it may be that the material simply requires an initial charging, like a magnet, receiving it at the antigrav factory, and that discs of the material can then be installed in a ship and activated merely by flipping it over as one might flip over a mirror, until replacement is required.  (See also the addendum section on antigravs.)

Put simply, the point is that there's no way to know how antigravs work.  Our friend Occam is silent on the matter.  What little we do know inasmuch as pushing against gravity fields suggests something as simple as a gravitic sail or mirror or the gravity equivalent of a ceramic superconductor.   Unless and until we know more, assuming that the ship is magically putting out all the energy of a rocket's thrust without rocketry seems a little odd.

Speaking of which . . .

5.  Engine Thrust

Even engine thrust isn't necessarily based directly on reactor energy.  If they're all carrying around rocket fuel, for instance, this fuel could be burned directly for thrust, and the reactor could only be drawing for electrical systems, just as in the space shuttle.    Alternately, the fusion fuel supply could be fused separately to form fusion rockets, which would have no real impact on main reactor energy . . . the reactors at that point would be running assorted electrical systems but only have a tangential relationship to the engines.   

Given the apparent central location of a specific "engine room" on Venators which also feature scattered thrusters along the stern, as well as the presence of a Munificent's main reactor compared to its many external thruster engines, then it seems that we cannot presume that the reactors are actually placed near the engines to serve as fusion rockets directly spitting out the fusion exhaust, but that instead they are separate.  The thrusters could of course have their own fusion reactions afoot in separate reaction chambers.  This would explain a peculiarity of Star Wars ships, in that whenever a ship is 'alive', whether or not it is accelerating, its engines are almost invariably lit brightly.  Fusion furnaces require intense temperature and/or pressure to function . . . keeping the engines hot and ready to go, even if this is wasteful of fuel during constant 'idle', could be a necessity.

Alternately, however, the running engines could simply be some sort of thermal shunt for the main reactors.  This concept is not without merit, given that from what we've seen thus far there's no evidence for separate engine function.   That is, we have not seen individual engines shut down, except when blown apart by battle damage as occurred to an Acclamator in "Innocents of Ryloth"[TCW1].   When the ion cannon shut down a Star Destroyer in orbit of Hoth, its engines all powered down simultaneously.  If all the engines were tied to the same reactor or had some other similar single point of failure (e.g. the oft-mentioned "power converter" of unknown use), this would make sense.  

What we do know is that Star Wars engines are generally ion thrusters.  The ANH script refers to X-Wings as having "ion rockets", and the TESB script refers to the Falcon's "ion engines".  The TESB novelization refers to the ignition of the Falcon's ion engines, in reference to starting up the thrusters from a darkened state after the ship detached from the ISD.  And, of course, the RotJ novelization explains that TIE fighters are Twin Ion Engine vehicles.  Perhaps most importantly, given that all the other examples refer to very small vessels, the Revenge of the Sith novelization suggests that ion engines are the technological basis of even the bigger ship engines:  "The shimmering canopy of ion trails and turbolaser bursts was fading into streaks of ships achieving jump as the Separatist strike force fled in full retreat."  

Whereas most rockets operate by burning or annihilating something in order to produce high-pressure gasses which are then directed out of a nozzle, producing thrust, ion engines operate by taking a charged propellant and, via electrostatic or electromagnetic fields, causing the propellant's rapid departure.  Soviet-type Hall Effect thrusters, for instance, trap electrons in a magnetic field, and as propellant is fed in the electron collisions ionize the propellant, which by the design of the engine ends up departing at high velocity, taking some electrons with it, producing a net charge for the escaping gas of zero, though the plume is somewhat disorderly.

While Star Wars engines may not be Hall Effect based (though there are resemblances), the general point of an electrically-accelerated charged particle thruster seems to be the case.   This can serve to help constrain our ideas on possible engine-reactor relations.

For instance, just as steam is produced by modern naval reactors, charged particles (plasma) may be the design goal of Imperial reactors, fusion exhaust product particles which can then be transferred to the engines as propellant and converted to electrical power by special generators.   Or, the entire reactor system may simply create electricity, with engines electrically powered and separate propellant unrelated to the fusion activity on hand.   Or, the reactor may be electricity-oriented, with the fusion fuel available for use as propellant via fusion exhaust or turbines, with electromagnetic acceleration of the products.   Or, the propellant is simply a coolant for the reactor, and is accelerated away from the ship thus providing thrust and thermal control.  (Given the seeming afterburn activity of the Falcon during a moment of maximum thrust, not to mention the seeming flame from the Invisible Hand when using thrust deflectors, it may be that the ionized propellant can be genuinely ignited, as well, though this is uncertain.)

Given the aforementioned simultaneous engine failure when the ISD was hit by ion cannon shots in TESB, it seems likely that the electrical system of the ship is a single point of failure for the engines.   The "power converter" is thus probably the electricity generator fed from the reactor, and which in turn feeds the power to the ship.   The power source for the generator may, like the Naboo generator in TPM, be reactor plasma.  The propellant identity is unclear, but in any case there is a relationship between engine thrust and reactor power.  

So now we've got something.

B.  Discussion

Curtis Saxton argues that a Star Destroyer's engine glow must represent thermal power emission.  While this is true to an extent, the claim can be taken too far.  Quoting the claim:

"The idle power of a starship's engines (i.e. when the thrust streams are effectively halted) can be estimated from the power of the thermal glow visible through the engine aperture (of area A). If σ is the Stefan-Boltzmann constant and T is the effective temperature of the radiating surface, then the radiated power is P = A σ T4. For example, if a star destroyer has three online engines in which the glowing surfaces have 100m diameters, and where the glow is yellow (temperature ~ 5000K), the ship must have an idle engine power of at least 8 x 1011 W."

This represents a minimum claim of 800 gigawatts.  But, there are multiple problems with this argument.

First, the calculation requires us to believe that the entire engine bell interior is glowing hot, rather than the central aperture from which the propulsive gases are coming.  Second, as we've seen in Attack of the Clones, the entire inside of the engine bell is not glowing due to heat, but due to glowing gas being directed along it.  It is logical to presume the same of Star Destroyers.  


Even present-day ion thrusters might generate glowing gas, but the gas is so diffuse that although the ions are incredibly hot individually, the gas temperature is fairly cool.   Given that we seldom if ever see a great deal of heat distortion (some occurred in AotC, but not much considering the size of the engine bells), it hardly follows that the engine bells are white-hot.  Rather warm, yes.

It's also been claimed at StarDestroyer.Net that a Star Destroyer reactor has a peak power generation in excess of 1E25 watts, or about ten trillion terajoules per second. However, this is nonsense, since to achieve that power level with a fusion reactor would require fusing almost 200,000,000 metric tons of deuterium in one second. To get a sense of that, suppose they stored it in the form of lithium deuteride at 12 times its normal density (much more and you'll risk fusion via compression).  Stored thus, that fuel would fill a container of 1000m x 142m x 142m, or almost half of the internal volume of a 1600 meter long Star Destroyer. That's for one second, and it would have to fuse all at once!

At StarfleetJedi.Net, it's been suggested that the hyperdrive limitations of not being able to be activated near a planet are indicative of power requirements in the exawatt range.  Quoting the claim:

"If the minimum distance to jump to lightspeed away from a planet is at some local value of g, beyond which the hyperdrive may not develop sufficient power, then we can calculate the minimum power of the hyperdrive system, by rate of change in gravitational potential, as mgc. For one planetary diameter from the surface of an Earthlike planet, this is ~357 megawatts per kilogram {of ship}. For 6.5 planetary diameters, this is 17 megawatts per kilogram."

However, this argument is also a bit peculiar, since there's no indication in the canon that the hyperdrive activation limit is related to power consumption.  If that were so, then it wouldn't make sense for it to be a universal rule for all ships, as is suggested in the canon.  Specifically:

"This served as clearance radii for the effects of the simple antigrav drive which boosted all spacecraft clear of the gravitational field of the planet.

The mathematics of spacedrive were simple enough even to Luke. Antigrav could operate only when there was a sufficient gravity well to push against-like that of a planet-whereas supralight travel could only take place when a ship was clear of that same gravity. Hence the necessity for the dual-drive system on any extrasystem craft." (ANH Ch. 7)

Elsewhere in the ANH novelization we learn that antigrav range is six planetary diameters, but again, there's no indication that it is different for different ships.   Gravity tugs on all vessels equally.

Further, obtaining an exajoule per second by fusion is a costly affair.  Per certain values for D-T fusion, it would necessitate about 3000 kilograms of fuel per second.   Going by the maximum energy density for the diesel fuel suggested on that site, it would still require 3500kg/s, or by volume over 4300 liters (4.3m) per second.   Even if we assume a Star Destroyer's fuel capacity is 10% of her volume, then at such consumption rates she would burn up her fuel in just two weeks.  And while a Star Destroyer is naturally rather unlikely to be making hyperjumps from planetary orbit a million times in a two week period, the concept that the ship could suck 4300 liters of fuel and then just sip it the rest of the time (i.e. using thousands, tens of thousands, or hundreds of thousands of times less fuel for standard operations) seems a peculiar one.

C.  Engine Power Examples

Since no other good information on power requirements exists, we can take thrust examples as a guide.  However, whereas Saxton and friends seemingly use completely made-up acceleration values, we'll study the canon itself.  There are a few examples which might give us something to work with.

1.  TCW and Anakin's Ram  

One example of Star Destroyer acceleration can be borrowed from the Venator Class as seen during the Clone Wars ("Storm Over Ryloth"[TCW1]).   With engines boosted for a collision course, we get what ought to be a good example of future Imperial ship performance, barring some absurd advancement that renders all prior vessels sitting ducks.  

Note, however, that the planet was not terribly distant in this example, meaning that antigrav use could've been involved, in principle.  However, since antigravs are presumed to allow greater performance, and since the context of the scene is not indicative of antigravs, we'll see what we get.

a.  Determining Acceleration

The vessel in question is the cruiser Defender, and the damaged Defender arrives in the system via hyperdrive at an eyeballed range of about 60 kilometers from the opposing Trade Federation battleship, closing at around a couple of hundred meters per second.   Two minutes of screen time later, the Defender has closed to within an eyeballed range of ten kilometers, still moving forward but at a slow rate.  Moments before the trap is sprung, we see the following image:

The 550m wide Venator takes up about 17% of the image width, meaning it would take 5.78 of them to fill the screen.  Assuming a total field of view of between 30 and 60 degrees, then, the Venator's angular width is between 5.2 and 10.4 degrees.   Using the methods from the range pages:

 .5 A / D = tan (.5 (theta))
.5 (550m) / D = tan (.5 (5.2))
D = 275m / tan (2.6)
D = 6056m

.5 A / D = tan (.5 (theta))
.5 (550m) / D = tan (.5 (10.4))
D = 275m / tan (5.2)
D = 3022m

This gives us a range between three and six kilometers, basically.   Using the other method, which assumes the CGI is done as if filmed directly on 35mm film:

 OD / 550m = 35mm / 3.808mm
OD / 550m = 9.19
OD = 550m * 9.19
OD = 5055m

OD / 550m = 50mm / 3.808mm
OD / 550m = 13.13
OD = 550m * 13.13
OD = 7221m

This gives us a range between five and seven kilometers . . . close, but not as close as we're used to on the range page.  Presumably this is due to the CGI.   However, we do get overlap in the higher ranges, so five to seven kilometers seems likely, both by calculation and eyeball estimation.

The engines are boosted and the ship begins lumbering toward the Trade Federation battleship.   About 55 seconds later, impact occurs with the outer ring of the battleship (which of course is several hundred meters from the bridge).  At that time, an eyeball rate of speed is perhaps as high as 550 meters (about half the ship's length) per second.   Assuming a full run of 7200 meters (about the maximum distance noted) and a start from zero relative velocity, then the 55 seconds to impact requires an average velocity of 131 meters per second.  Assuming linear acceleration, that would result in a final velocity of 260 meters per second, with a very poor acceleration of 4.75m/s, or about half a gee.   Given the approximate eyeball rate of speed, however, we could presume a higher acceleration at some point in the run.   For instance, 55 seconds to get from zero to 550m/s would give us an acceleration of 10m/s, or approximately 1g.

However, that seems perilously low, even for a damaged Star Wars cruiser.  The acceleration rate of the Millennium Falcon with an afterburner boost was 210 m/s, or about 21.5g, which is 20 times that figure.  And after all, the Trade Federation ship couldn't stop the Defender, and if the Trade Federation ship had the ability to reverse thrust for even a tenth of the Falcon's maximum forward acceleration, it could've reversed and escaped fairly easily. Instead, the Trade Federation ship simply sat there and took a kilometer-long knife right in the face.

The best-case thought is that the Defender's acceleration was slowed by tractor beams, and that "we can't stop it!" as stated by the droids referred to the tractor beams being overwhelmed rather than the obvious and ordered weapons fire having little effect.   However, it would've taken a moment at least for the Trade Federation ship to react to the acceleration, and the initial acceleration of the Defender was no higher than what we would expect given the speeds seen.

Also in fairness, the Defender was damaged, though no direct engine damage is apparent, and indeed a ship with engines too heavily damaged would've been useless for Skywalker's plan.  In other words, if Skywalker planned to ram but the Defender's acceleration ability was a miniscule fraction of what a Trade Federation warship could normally muster, it would be the equivalent of one ocean-going battleship trying to ram another by having the crew hop into the water and push . . . an absurdity.

Nevertheless, let's assume that most of the talking scenes on assorted bridges were, in fact, overlapping data.  This gives us a time of about 29 seconds to get to 550m/s, for an acceleration of 19m/s.   Interestingly, this also corresponds with another value . . . if we take the 7200 meter run and average the speed to 275 meters per second, then our straightline acceleration would be 21m/s for a time of 26.2 seconds.   Given that these two separate values correspond so well, we can use 21m/s as the correct figure.

Note, however, that reverse thrust is probably not as good at all . . . after all, in The Empire Strikes Back two Star Destroyers passing in the night struck each other, apparently unable to directly slow themselves from a relative velocity of something like 200 meters per second!  Even just counting from the moment when we were first shown them both in the same frame at less than a kilometer apart, that's a full ten seconds during which they failed to avoid collision.

Similarly, when the Invisible Hand started falling toward the Coruscant surface in Revenge of the Sith, it was all the ship could do to level herself again.  The ship fell for 10 seconds, then after 20 additional seconds featuring orders to reverse stabilizers and magnetize, the "emergency booster engines", which seemed to just be thrust reversers on the main engines, were finally fired.  It still took almost another whole ten seconds for the vessel to level out.  In low orbit, the gravitational force on the vessel would have still been below 1g, but not terribly much less.  Even if we presume the vessel was dropping for the full 30 seconds and took 10 to stop the descent, then, the ship's main engines on full reverse would've offered no more than 3g, and probably less.

b.  Guesstimating Power

So, let's assume that it started from zero.  Given a middle-of-the-road Star Destroyer mass estimate of 40 million tonnes and a very rough estimation of the mass of the much less voluminous Venator at somewhere in the 10-15 million tonnes range, that would give the vessel a kinetic energy of .5 (15000000000) (550), or about 2,270 terajoules.  Considering the acceleration rate, then this would represent a peak engine power of around 87 terawatts.

If we assumed 75% of reactor power could go to engines and assumed engines that were 90% efficient (higher than current ion engine technology, mind you, but not impossible), then the value for peak reactor power would end up as about 120 terawatts.

c.  Translating to ISD

Now, the above is the only demonstrable value we have from this, however back-of-the-envelope and such it might be.  But, given that we also ought to presume, based on all the evidence, that a Star Destroyer can basically keep up with a fleeing, weaving, non-afterburning Falcon, then that would still seem a slow figure.   Even if the Falcon without afterburn was only capable of half of her maximum acceleration (i.e. 100m/s), the weaving and bobbing probably wasn't removing that advantage by a factor of ten.   So a Star Destroyer is probably capable of 25-50m/s or so.  

So, if we went with the full-size Star Destroyer and our selection of a 25-50m/s acceleration rate, then for a 40 million tonne Star Destroyer to reach the same velocity would involve a kinetic energy of 6,050 terajoules.  At the acceleration rates given, this would require either 22 seconds or 11 seconds, corresponding to an engine power of 275 to 550 terawatts.   Using the reactor power percentage and engine efficiency assumptions above, the peak reactor power of a Star Destroyer would thus come out to 380 to 750 terawatts.   

Considering that the Imperial is roughly three times larger than the Venator by volume, the former value of around 400 terawatts might have a bit firmer footing, but one's terawattage may vary.

2.  TESB Star Destroyers Comin' Right At Us

Another possible acceleration example occurs in The Empire Strikes Back.  Based on this section of the weapons range page, we see a pair of Star Destroyers moving at about 30 kilometers per second relative to the Falcon and its closely pursuing Star Destroyer, the Avenger, shortly before its 90 degree dive toward the asteroid field.  Ten seconds of screen time later, the two Star Destroyers are visible as the Falcon dives away, with Avenger right behind.  The vessels at that point are moving at a relative velocity of perhaps a hundred meters per second, or effectively zero compared to the earlier relative velocity of both pairs.

Unhappily, it seems that this example is probably unusable.  At minimum, this would require that both pairs of ships (two ISDs and the Falcon with a trailing ISD) decelerated at 1500m/s, which would require 75 kilometers of stopping distance, which happens to be the total distance scaling from the weapons range page.  However, that would have both pairs meeting at 37.5 kilometers while still moving quickly, and thus we must double that deceleration value.   

Thus we find some confusion, since 3000m/s is also more than 14 times the afterburning forward acceleration seen by the superfast Millennium Falcon.  If we took this as a valid example, it would be a profound outlier from all other examples in the visible canon, including those from Star Wars: The Clone Wars.  It would also be a profound outlier from the collision a few seconds later, apparently unavoidable despite a relative velocity of around 200 meters per second!  If the ships really could decelerate at 3km/s, then they could've stopped in less than a tenth of a second!

However, it would be within the normal range of decelerations observed when vessels exit hyperspace.  Thus, I would submit that this example is probably an example of arriving warships dropping from hyperspace and almost arriving right on top of a battle, whether by accident or design.   A similar tactic has been intentionally used in the past, as seen when Anakin exited hyperspace in a shuttle in such a way as to almost kiss the hull of an enemy ship he planned on docking with ("Grievous Intrigue"[TCW2]), not to mention the somewhat frequent Separatist technique of hypering in on top of foes ("Ambush"[TCW1], "Storm Over Ryloth"[TCW1], et cetera).

3.  Ships on the Ground and in the Air

Non-acceleration examples might also work.  For instance, Acclamators land on Geonosis in Attack of the Clones.  Palpatine and other government officials observe the departure of Acclamators at the end of the same movie.  In The Clone Wars, vessels land, take off, are boarded mid-flight in the atmosphere in battle ("Jedi Crash"[TCW1]), and other similar things all the time.   

In most such cases, unprotected individuals are within kilometers of the ships, if not directly touching them.  In the case of Palpatine, he was seemingly in the line of fire of the engine exhaust.  This tells us that the heat dissipation of these vessels, be it from their engines or their hulls or any heat sinks thereon, cannot be terribly excessive.  How does this help us?   Well, if this is the maximum energy pouring out of the tailpipe or otherwise being released from the vessel, and if we presume a certain efficiency of the ship's systems (even, say, 99%), then we can have an upper bound for the reactor energy at the time.

Using information from the legendary Nuclear Weapons FAQ, we find an easy method of calculating the range at which one can expect first degree burns from a nuclear weapon.  Unfortunately, of course, this is a ballpark figure.  While exhaust from a rocket engine will be different inasmuch as the specific relative percentages of energy emission (e.g. blast versus radiation versus thermal effects), we should have a decent guesstimate of the maximum energy coming out of the back of those ships at any given time.  This guesstimate will be imperfect, of course, given spectral issues, but at least it's something.  

As it happens, even a nuclear weapon of a single kiloton is enough to produce first degree burns at 1.2 kilometers.  Per the FAQ author's own chart, 4.3 kilometers is the range for first degree burns for a 20 kiloton device.  In both cases, of course, this is for a single energy release event -- a bomb going off -- and not sustained exposure.   The true output from the vessels would thus be far lower.

Eyeballing the Palpatine incident, we can estimate that his range from the ships was on the order of a couple of kilometers.  Overestimating, then, based on the single-event bomb detonations (rather than fully correcting for sustained energy release), we'll say that the engines would be limited to a kiloton, and probably a lot less given the even closer clones.  100 tons . . . one-tenth the bomb value . . . would probably be closer to fair, though still high.

At 99% thermal efficiency, then, the reactor must have an output no greater than 10 kilotons per second, or about 42 terawatts.  If the ship's efficiency is more like 90%, then the reactors fall to a mere kiloton per second, or 4.2 terawatts.   To be sure, even that last value is nothing to sneeze at . . . it is one quarter the total energy per second used on the entire planet Earth as of 2006.

D.  Conclusion

It is interesting, to say the least, that the usable examples above (1 and 3) roughly correspond to one another, even given the overestimations in both cases.  Considering that they are based on independent criteria (acceleration in one case, thermal radiation due to perfectly normal efficiency ideas in the other), the fact that both agree on something close to low-to-mid terawatt range power levels for Star Destroyer-type vessels is interesting.    More interesting is that the first example occurs near a planet, and thus might be even more of an overestimate if antigravs were involved at all.

If we were to roll with these examples even in spite of that, we could go for Acclamators and Venators in the 80-120TW range, and a 400-500TW Imperial Star Destroyer, without problems too severe.  As noted previously, though, this is only an intriguing possibility . . . it is entirely likely that none of these potential examples are completely valid.  For instance, technology to deal with thermal inefficiencies in ways other than simple radiation . . . such as the intriguing science fiction idea of dispensing heat via neutrino-based or similar technobabble radiation systems . . . would nullify the third example.   And, it's possible that a ship at full engine 'burn' can actually output almost all of its reactor power to the engines, depending on the situation. 

Generally speaking, though, it seems best to assume that the majority of the vessel's waste heat is emitted through the engines.

V.  Conclusion

Star Wars vessels operate via nuclear fusion, fed through some sort of liquid fuel (possibly called tibanna) with characteristics not wholly unlike a hydrocarbon fuel (such as the "diesel starship" idea fielded elsewhere).  The fusion reaction or reaction cycle in use is not clearly indicated, but there does seem to be a great danger of the reactors overloading and exploding if sufficiently damaged.  This is unlike most current fusion reactions of interest on Earth, implying either a reaction cycle that we know and have discarded, one we know but believe safe due to other constraints the Empire does not follow, or a reaction cycle that is completely unknown to us and which cannot be made completely safe, but with the benefits outweighing the risks.

Reactor energy is fed to one or more "power converter" generators, possibly via plasma, which the generators convert to simple electricity. This electricity is distributed via conductive wire in most cases, though occasional use of photonic power transmission is not unknown in large-scale applications (e.g. the second Death Star, which would presumably have other true "power converters" on hand to go from photonics to electronics).   

This electrical system powers her ion engines, though damage to the latter can overload the power converter(s).   The nature of the propellant for the ion engines is not certain (it could be the same liquid fuel, or some special as-yet-unknown solid mass, or reactor coolant, or reactor plasma.)

Based on assorted power requirement calculations, it would appear that the Clone Wars era warships of the Republic were probably limted in reactor power to 120TW or below, with the Venator being the largest and most powerful ship in that criteria.   The far larger Imperial Star Destroyers, assuming similar technology, would figure somewhere in the 400-500TW range.

VI.  Addenda

A. Running Some Numbers

Just for kicks, let's assume, for the moment, that what the non-canon commonly labels as the "reactor bulb" on the ventral side of an ISD (circled at right) is, in fact, the fuel tank, and that the small portion we see is actually part of a sphere.  This would give us a  ~200m fuel tank on an ISD, assuming it is 1600 meters in length.   4/3r tells us that this would give it a volume of 4,200,000m, the equivalent of over 2000 space shuttle external tanks (at 2070m) filled to capacity.   While that sounds like a big tank, it only represents 7.8% of the internal volume of the vessel.

First, let's assume that this is just your basic fuel tank, with deuterium for fusion stored there in a liquid form.  The density of liquid deuterium is reported variously, but normally falls within a few points of 165 kg/m.  So, given a tank as specified, the Star Destroyer could hold some 691,150 metric tonnes of liquid deuterium.  (Liquid deuterium would make sense under most circumstances provided you could readily chill deuterium to around 20 degrees Kelvin.   You could even go for solid deuterium if you knocked off just a few more degrees.  And unlike on modern Earth, where cooling hydrogen into liquid would require energy in order to keep it cool, in space all you'd need to do is keep it exposed a bit (as one might do with that hemispherical bulb), so at least in part it's cooled for free.)

For the below, we'll assume the form involving six deuterons and 43.2 MeV, being among the more powerful fusion cycles.

1.  Joules per Gram

Now, we need to get from mega-electron volts to a more useful value for our purposes.  The way to do this is to employ Avogadro's number, the number of atoms in a mole of material.  A mole of an element features a known number of atoms resulting in an amount of material which, in grams, equals the numeric value of the atomic weight.   In other words, a mole's worth of carbon-12 is 12 grams, and a mole's worth of hydrogen (with an atomic weight of 1) is 1 gram.  The atomic weight of deuterium is 2.01355, so a mole's worth has a mass of 2.01355 grams.  Avogadro's number is 6.0221415E^23, or 602,214,150,000,000,000,000,000.

So, in order to find out the number of deuterons per gram, all we need to do is to divide Avogadro's number by 2.01355 grams.  The result is a "mere" 299,080,802,562,638,126,691,663.9765 deuterons.  If we then divide this by six for each deuteron burned up in the deuterium cycle, then the result is  49,846,800,427,106,354,448,610.66.

What has that got us?  Well, for each sextet of deuterons, we were making 43.2 MeV.  Thus, a gram of deuterons makes that energy times the almost-50-sextillion sextets.   The result is 2,153,381,778,450,994,512,179,980.63 MeV, which converts to 344,993,294,752.66 Joules, or 0.34499 terajoules.

Not bad for a gram of heavy hydrogen.  And yes, the figure is artificially-specific given my total skipping of significant digits, but they annoy me and this is a hobby.  So sue me.

2.  Total Energy

0.34499TJ/g means that for a kilogram of deuterium, we're looking at 345 terajoules, which the Star Destroyer could use assuming 100% efficiency.  Of course, that would be a silly assumption.  

If the Star Destroyer has a fuel tank capable of storing 691,150 tonnes of deuterium, then the total energy possible from that tank of fuel is 238 billion terajoules, 2.38E23 Joules, or almost 57,000 gigatons of energy . . . more than enough to allow one to hold a massive barbecue.  However, a Star Destroyer can't just go fusing all its deuterium at once.  Were it to do so it would quite rapidly end up a ghost ship as anything requiring energy (such as life support, propulsion systems, et cetera) came to a sudden and grinding halt after fuel exhaustion.

3.  Endurance of a Star Destroyer

The question, therefore, is how long one would expect a Star Destroyer to be able to endure without hitting a gas station.

The question can readily cause one to tie oneself in knots, if pondered too carefully.   But if we assume that a Star Destroyer must be away from refueling for a year, then the average power consumption would have to be the total terajoules divided by 31,536,000 seconds, which results in 7561 terawatts, or about 1.8 megatons per second.  That's about 7.6E15 watts.  That would represent the consumption of 21.9 kilograms of deuterium per second in one or multiple reactors.   In any case, we're talking about the fusion of several kilograms of deuterium per second, which is at least plausible given large or multiple reactors.   Fusion thus fulfills the power requirements of Star Wars quite nicely.

Now, if you recall our earlier estimate of 400-500 terawatts for maximum reactor output, then the likely endurance of a Star Destroyer before hitting a gas station would be a much more impressive 15 years, on par with the fuel reserves of modern aircraft carriers. 

B.  Antigravity and the Perpetual Motion Counterclaim

As we understand physics, of course, almost none of these cheats we see in science fiction are even remotely possible.  Antigravity, for instance, is often said to theoretically allow for a free-energy perpetual motion setup.  The rationale is that an antigrav system requiring less energy than what would normally be expended to raise an object in a gravity field could, if turned on and off at will, allow a generator that can recover the energy of motion of a falling object to get virtually free energy, an energy profit over and above what would have to be put back into the antigrav unit.  Such generators are not unknown . . . falling water generates electricity in hydroelectric dams, for instance, and the atmosphere is kind enough to move the water back above the dam to start the process over again.

The issue is conservation of energy, which along with conservation of momentum is encapsulated in relativity as the conservation of the four-momentum.  Locally, from the perspective of the dam, the water is free energy.  Globally, however, the sun and atmosphere are doing the work of getting the water back above the dam. 

"But," you say, "a natural magnet can perform levitation tricks without energy input!"  This is true, but this is merely an effect of the stored energy within the magnet, energy which is expended when the magnet is used to move or deflect an object.  Similarly, a quick search of the internet will show you hundreds of ideas of how to generate perpetual motion and free energy from magnets, Earth's magnetic field, vacuum energy, zero-point energy, the Casimir Effect, and other natural or environmental factors.  None of it is actual free energy, and most of it isn't worth the time to consider anyway even if you can get energy out of it, especially given the nature of the salesmen.

Free energy salesmen usually explain how assorted conspiracy-theory-nut groups like the Illuminati, MJ-12, Lizard Men, or Big Oil are suppressing them, which should give you a sense of how much stock to put in the claims, and why you should probably not search the internet for such things if your mental health is important to you.  Just as people peddled elixirs of life back in the old days, "New Age" medicine now claims to have evolved past Western science and will prescribe you all sorts of 'natural' drugs/supplements that it would take medical professionals awhile to decode the full effects of, though we already know they don't do much good.  Similarly, these New Age "scientists" claim to have evolved past modern physics, and just need your money to be able to build their machines to solve the problems of the world, and to escape Big Illuminati-12 Lizard Oil or whoever.

While most theories of perpetual motion are too simple, ignoring/masking the effect of some basic physical phenomena, some of the ideas almost sound good, and have befuddled more than a few intelligent people.  Some have even required extensive calculations by professional physicists to debunk.  

Likewise, I can't help but think that such objections to science fiction antigravity (or, for that matter, FTL propulsion) on the grounds that a perpetual motion machine would develop are, like all other theories of perpetual motion, similarly too smart by half.  In the case of Star Wars antigravs that push against a planet's gravity well, we can come up with a lot of theories.  Earlier, we pondered a material that allowed antigravity to occur, and my personal favorite idea was a disc of this material that, suitably charged by whatever means, could simply be flipped over to push against the gravity field, and used for propulsion by tipping it just so, like a gravitic mirror.  The material itself might wear out over time, and of course its charge of whatever stored energy would be involved would slowly be lost over time.   This would not be a perpetual motion device . . . you could use it to get energy for awhile, but then it would be dead.  

Of course, something far more exotic might be the case, or it could be something more mundane.  Either way, the simple fact is that in the canon, perpetual motion apparently doesn't exist.  Ergo, the science fiction technologies we see in use cannot be used to create it.  But that still does not prove that antigrav and other technologies do not cheat the energy requirement for such things as getting to orbit (whether locally or, pardon the pun, globally), just as hyperdrive cheats the universal requirement per modern physics that infinite energy is required to go faster than light.