This page is devoted to the common mile-long destroyer known to the forces of the Galactic Empire as the Imperator-class star destroyer, and colloquially known as Imperial star destroyer to rebel operatives. There exist other kinds of star destroyers in the Imperial Starfleet, but this commentary ignores them in order to focus exclusively on the most thoroughly observed class. [NB. This page does not deal with destroyers of Victory or Allegiance classes, nor any other class of Imperial warship.] Star destroyers are the smallest kind of Imperial warship seen in the STAR WARS cannon, but the existence of numerous other designs which are smaller and larger indicates that they are actually medium-sized warships.
The ship class was named after the Imperator in the Mandel blueprints of 1978, long before the present generation of references. With the exception of a few modest references in LucasArts games, the first-wave references have been unreasonably ignored by newer works, despite being generally just as accurate. The Mandel plans definitely precede the recent "Imperial-class" referencing.
The "Imperial-class" term created in the earliest second-generation spin-off books is an irrational description betraying an ignorant misinterpretation and misapplication of naval nomenclature, in addition to culpable neglect of the prior Mandel reference. Referring to a starship as an "Imperial-class star destroyer" is equivalent to talking about a "Japanese-class submarine" or nonsense of that kind. The "Imperial-class" designation is very commonly used in recent publications: it is a noxious weed propagating in the recent literature, reflecting conformity to lightly researched sourcebooks written during the merchandising lull of the 1980s. In these commentaries the common but unsupportable term is rationalised intra-textually as rebel slang, and the more formal and self-consistent "Imperator-class" is preferred instead.
In either case, the vessel may be abbreviated to the universally-understood acronym: ISD.
In its roleplaying references, West End Games lists two variants of the star destroyer with slightly different performance characteristics. The rebels know these models as the Imperial-I and Imperial-II class ships. The official nomenclature of Kuat Drive Yards and the Imperial Navy may be different, but the histories of the Galactic Civil War have (naturally) been written by the victors. Many elements of guerilla fighter-jockey slang seem to have been uncritically adopted by New Republic historians.
The movies show two or three variants of this star destroyer. They are chiefly distinguished by the features of the command tower, hangar facilities, and the sublight drive nozzles. (For more information, refer to Star Destroyer Bridge Towers.) Good arguments can be made in favour of accepting the first two types under the common ISD-I and ISD-II labels. The third possibility, seen only as a tower, is probably the Imperial communications ship at Endor.
The earliest known vessels seen resemble Devastator, Lord Vader's flagship during the time period surrounding the construction of the first Death Star. (NB. The precise age of this ship is unknown; it could be anything up to twenty years old, which was the age of Palpatine's regime as of A New Hope.) This ship shows little luxury, with a scarcity of viewports on the forward face of its command tower, and the bridge is well hidden and protected. The primary thruster nozzles each feature triple baffle vanes, presumably for more effectively vectoring thrust particles.
No later than three years afterwards, a slightly different model of destroyer was used in Lord Vader's fleet, escorting the great command ship Executor. The Avenger, commanded by the ill-fated Captain Needa, can be taken as archetype of these newer star destroyers. On this ship the linear array atop the bridge tower was almost always seen sitting entirely below the level of the scanner globes in a horizontal orientation. The triple-baffles on the primary thrust nozzles were replaced by a more slender ring baffle.
The number of star destroyers in service is immense. The hull number of the star destroyer Entor (CVS 1049) indicates either that in excess of a thousand star destroyers of this class have been built, or that this is the 1049th built by its particular shipyard. According to the reports of Rebel Alliance historian Arhul Hextrophon (in The Imperial Sourcebook) the Galactic Empire has a about three to five dozen or so destroyers in an average sector group fleet, of which there is at least one for each of the Empire's thousands of sectors. This indicates that the local territorial fleets alone account for tens of thousands of star destroyers galaxy-wide. It should be noted that though Hextrophon's reports have proven to be error-prone on many occasions; they consistently tend towards gross underestimates of Imperial naval and military strengths. The true number of star destroyers is probably much greater than the values he implies. In addition to the vessels assigned to particular sectors, there must be uncounted numbers attached to Imperial High Command and elite roving forces such as those of some Grand Moffs, and higher officials such as Lord Darth Vader.
A solitary star destroyer can be expected to deal with small or poorly organised threats to galactic security, especially criminal or minor rebel threats arising in remote regions. Against more formidable foes, these ships gather in squadrons or assume a true destroyer role in battle formation with heavier but less numerous capital ships such as the great star cruisers, battlecruisers, battleships and elite command ships which constitute the backbone of the Imperial Starfleet.
Example of the ISD-II star destroyer subclass.
Shape of star destroyer seen in Roleplaying Game references. Like that of the Executor, this representation is imperfect: the platform of the turbolaser turrets has been subdued and the turrets wilfully omitted; the ventral bulb also seems to be placed too far back by several hundred metres.
The common Imperator-class star destroyer is generally acknowledged to be about 1600m or one mile long. Unlike dimensions quoted for some other important warships of the Imperial Starfleet, this length is not in serious dispute. However it is useful to briefly consider the available measurements and lines of evidence.
Fragmentary pieces of evidence exist concerning the original intentions of the artists working on the star destroyer models seen on film.
The prototype model created early in the production of A New Hope has guns and other features which imply a size of only a few hundred metres. This is supported by an early starship scale comparison diagram. The final detailing of the film model was much finer, including proportionately tiny turbolaser batteries and viewport lights. The size of the interior sets of Princess Leia's blockade runner implies that this vessel is of a size which requires the destroyer to be on the order of a mile long (in order to fit it into the docking bay). Nevertheless, the original smaller dimensions were accidentally reproduced in the otherwise excellent published blueprints by Mandel.
Cut-away star destroyer blueprints, and sketch for A New Hope. In this undersized version, each gun turret has only two barrels, and there are only six of the heavy turrets on the whole ship, rather than eight. Smaller guns can be seen in silhouette across the dorsal hull surfaces.
Early vehicle scale diagram by Joe Johnson. The blockade runner and stardestroyer kept the same relationship to each other, but they became much larger compared to the Millennium Falcon and the other small vessels. Incompatibility of this preliminary diagram with the final scale of the films is proven in several different ways. The blockade runner is much too small to fit the interior corridors seen in the Tantive IV. The destroyer is too small in relation to the Millennium Falcon for the freighter to fit on the back of the command tower. The destroyer is too small for a Lambda-class shuttle to fit through the aperture of the forward launch bay, as Lord Vader's did in Return of the Jedi. The hangars within the bays probably could not fit a shuttle or freighter anyhow.
The blueprints of Geoffrey Mandel are rare but official publications predating the second-generation products which were initiated with the game references of West End Games. Most of the Mandel's labelling is logical and inspired, though outdated art and prototype work for A New Hope has been too influential with regard to the ship's scale.
Blueprints and sketch of the ISD from the recent Essential Guide to Vehicles and Vessels.
There is one piece of evidence that ordinary ISD's are in fact a few miles long rather than exactly one mile. This hint is mentioned in the context of a discussion of the ISD design in Stephen Sansweet's STAR WARS: From Concept to Screen to Collectible. It seems possible that the Lucasfilm artists working on the original film designed the ship to be five or six miles long. If so then this original intention takes precedence over the one-mile figure given in Roleplaying Game and other recent references. The size of the ISD is dealt with on pp.42-43 of the Sansweet's book. A caption to the illustration on p.43 is quite explicit:
The final version of the Star Destroyer, built in several different sizes, was meant in filmic terms to be about six miles long.
On the other hand From STAR WARS to Indiana Jones supports the one-mile theory. This book has the best credentials of any published reference, when it comes to matters of scaling, because it is directly based on props from the Lucasfilm Archives which were restored and put on exhibition in Japan under the sponsorship of one of the authors. The book is sufficiently faithful to primary reference materials to quote the original correct "eleven times ISD" length figure for Lord Vader's flagship Executor.
Professional model makers at ILM and elsewhere maintain consistency in their creations by conforming to a certain set of standardised scales employed by model kit companies for models depicting real-world There is good evidence that the effects modellers of STAR WARS esteemed the classic scales too. The model of the Avenger created for The Empire Strikes Back was 2.59m long and was detailed with thousands of parts from warship model kits. These warship kits are produced in several standard imperial or commercial scales. One such scale is 1/620; it was used by some manufacturers at the time when the classic trilogy was filmed. Scaling the star destroyer in accordance with its constituent and decorative parts yields a total length of almost exactly one mile long: 1606m. To within the precision of our knowledge of the model's size, this is perfect agreement.
A handful of methods exist for determining the scale of a star destroyer by comparison to objects whose size is independently known or at least limited by other empirical evidence.
Two distinctive apertures on the ventral surface of a star destroyer are docking bays: partially enclosed volumes where smaller ships can be conveniently moored in vacuo without gravity. The aft docking bay (primary bay) is approximately 185m long and 160m wide. The foreward (secondary) bay is a few times smaller, with dimensions of about thirty-seven by fifty-six metres. The smaller spacecraft launched and received from these sites include vessels as tiny as TIE fighters and unmanned probe droid hyperspace pods. Craft as large as dropships, ground assault landing barges and even some lesser combat starships can be accommodated.
Each bay has at least one hangar set into a vertical inner wall. These seem to match the standard form for hangars at Imperial spacedocks, space stations and large capital ships everywhere: a rectangular entrance of the order of forty to sixty metres wide, sealed with an atmospheric shield and leading to the main boxlike landing area. Hangars are designed for receiving, servicing and launching shuttles, starfighters and other small craft capable of resting in a standard-gravity air-filled environment. They are the launch point for probot hyperspace pods. Some star destroyer hangars seem to have a huge armoured shutter door which can be lowered to physically seal the entrance. These doors may be shut when a hangar is not in use or when dangerous combat conditions might jeopardise the integrity of the atmospheric shields.
The region surrounding and between the two docking bays is thought to be almost entirely dedicated to docking and hangar operations. Large vertical repulsorlift transit shafts in the hangars are believed to lead to the main internal storage hangars between and to either side of the bays and on decks immediately above. (This is well supported by both the classic blueprints of Mandel and the crossection of an older star destroyer class in Starfall.) It also seems likely that forward hangars of the primary bay and aft hangars of the secondary bay may be directly connected in some star destroyer models. (Indeed the bounty hunter starship Slave I was once released from an aft engineering waste disposal chute of the Avenger, presumably after being transported internally from the hangars.)
Different models of star destroyer have different hangar configurations within the bays. In the older Devastator-type ships the secondary bay has a hangar on the aft wall but none on the forward side. In the Avenger and newer ships the reverse is true: the secondary docking bay's hangar is situated on the forward wall. In all star destroyers seen to date the primary bay has two hangars on its foreward wall, though occasionally one of the hangars may be covered with its huge metal blast door or left inactive and unlit.
One of the most distinctive features of the primary docking bay on all models of star destroyer is the network of paired straight and curved rails on the bay's "ceiling". These appear to be paired tracks for the movement of one or two large pincer-like mechanisms across the whole space of the bay. The rails are spaced about thirty metres apart. The symmetry of the tracks hints that there might be two pincers, but at least one is always hidden. The rails lead off to the edges of the docking bay at several points, including into the hangars. It seems possible that the pincers park in the hangars. Presently there are no clear views of the pincers available, but they consist of a broad hand mechanism (as thick as a Corellian blockade runner) and movable twin claws which are approximately as wide as a TIE cockpit. The pincer acts to secure large vessels in the docking bay (such as Princess Leia Organa's Tantive IV captured by the Devastator) or sometimes as a launching point for TIEs (or so it superficially appeared during Lord Vader's arrival at Death Star II). It seems possible that the claws have an airlock mechanism and air tubes for the transfer of pilots to and from starfighters, or for delivery of troops and personnel to a larger docked ship.
There are five other mobile structures on the rails of the primary docking bay which might be related to boarding functions. All of them are rectangular, a few dozen metres wide with a side ratio of approximately 1:3. They are thin and do not protrude far beyond the rails. In one of the best available images, four of these objects are aligned in port-starboard sense and arrayed on the starboard extreme of the track. The remaining unit is instead aligned with the axis of the ship, sitting on one of the middle sets of rails. Two vaguely cylindrical features hang down from this unit; they may be boarding tubes or devices to launch starfighters, other small vessels.
Two distinctive pieces of apparatus are mounted near the corners of the forward end of the primary docking bay. Each is about forty metres across and bears what seems like a small parabolic dish about twenty metres in diameter. Situation in such close proximity to both of the major bays suggests that these structures perform a vital function related to the control of bay traffic. The dish shape is suggestive of powerful scanners, which would be an asset to flight control. On the other hand, the objects may be the most powerful and externally prominent of the vessel's tractor beam projectors. Essential Guide to Weapons & Technology hints that tractor beam projectors are at least one of the components of these features. If so then they cannot be the only tractor beam projectors on the ship, because they lack line-of-sight to much of the interior of the primary bay, and virtually all of the interior of the secondary bay. This includes all of the known hangars entrances. This coverage is inadequate because we already know that TIE starfighters must be guided into a hangar by a dedicated tractor beam. The nature of the two dishes near the docking bays remains open to debate.
The docking facilities of a star destroyer are lined with lights, probably for the safety or for the comfort of pilots of incoming vessels. The docking bays are illuminated from within by powerful floodlights situated in the corners between the ceiling and the port and starboard inner walls. These visibly indicate the position and extent of the bay ceiling in relation to the pilot. The individual hangars are also brightly lit, particularly around the aperture where the atmospheric containment shield is projected. Although tractor beams and flight controllers obviate the need for manual docking under most circumstances, unaided landings may be necessary on occassion (perhaps in some heavy combat conditions, or after the destroyer has taken severe damage).
Ventral views of the Devastator. The first two images directly reveal the primary and secondary docking bays in true proportion to the rest of the ship.
Close views of the primary docking bays of the Devastator (first two images) and Avenger (latter image). The forward end is at the right side of these images.
View of primary and secondary docking bays of the Devastator, sighting towards the aft. Note the presence of aft hangar in the secondary bay and the absence of any obvious aft hangar in the primary bay. The second image features the primary (rear) docking bay. Note the two foreward hangars, the pincers and rails. The third image features Devastator's secondary (forward) docking bay. The foreward wall is visible in this shot; the absence of a forward hangar is obvious.
Representations of the docking bay area of a mark-II destroyer from games such as Balance of Power are usually accurate concerning generic features and proportions, but twin dish apparatus are absent from their position forward of the main bay.
A star destroyer hovers in escort over the Executor, casting its shadow on the giant flagship. Captain Needa departs the star destroyer Avenger (top of frame) in a twin-pod TIE shuttle.
The secondary (forward) docking bay of a star destroyer of the same type as Avenger. Note that the hangar is in the foreward wall, unlike the configuration of Devastator-type ships.
A star destroyer has three major propulsion systems. One is the hyperdrive, which achieves a crossing of the lightspeed barrier for supralight travel; the other two are concerned with movement through realspace at sublight speeds.
The hyperdrive systems of Imperial warships are exceptionally powerful and well-tuned compared to those of civilian starships. Hyperdrive acceleration systems are coupled to excellently calibrated inertial damping systems so that crew and officers can work and stand freely during jumps without experiencing the tiniest jolt. (Compare this with the performance of the star freighter Millennium Falcon, where passengers must "strap in" for the jump.)
In a single jump a star destroyer expends more energy than the total power consumed by some nations of civilised planets over all history. If this is taken to mean the electric energy consumed by an industrialised nation in a pre-hyperdrive civilisation like Earth today, then the quantity involved is at least a few times 1021 joules, (ie. thousands of millions of TeraJoules). However if we take it to mean a nation with the higher-technology power consumption seen in STAR WARS, having existed through a history of dozens of millennia rather than mere centuries, then the proper figure is at least thousands of times greater. This is not surprising, since this energy level is comparable to the total power output known for the star destroyer turbolaser batteries (as described in a later section). [Note: hyperjump energy calculations are mainly due to other investigators - ie. Stephen Cumblidge.]
In deep space, sublight propulsion is primarily achieved via the seven thrust nozzles at the stern. Subatomic particles are accelerated to immense momentum in the drive systems of the ship and hurled out the nozzles at relativistic speeds, imparting a forward accelerative thrust to the vessel. This general kind of propulsion system is often called "ion-drive". Three primary nozzles, approximately ninety metres in diameter, are generally lit when the ship is properly functioning. Four auxillary nozzles may also be lit, providing extra acceleration. This is most often used in combat situations.
The fact that the auxillary thrusters of the Devastator were left unlit during the pursuit of Princess Leia Organa's Tantive IV may suggest something about the confidence of the warship's commanders or the relative speeds of the two vessels (the Alderaanian ship was probably already suffering serious damage).
During the Battle of Endor at least one star destroyer left its primary thrusters unlit but kept the auxillaries active. This may be because the battle involved opposing warships standing at rest relative to each other, rather than engaged in a chase, and so only slight low-acceleration course adjustments were be needed.
The details of the nozzles show subtle differences across the known sub-classes of star destroyer. On the Devastator and similar ships triple rectangular baffles protrude a few dozen metres over the nozzle's aperture. They may have some role in vectoring the exhaust flow to allow the ship to turn by accelerating in a direction off the vessel's principal axis. Their operation is not directly physical, they must be a kind of electromagnetic plate projecting fields to channel and deflect the flow of electrically charged particles. The baffles are absent in several of the newer ships involved in the events surrounding the Battle of Hoth (eg. the Avenger). Instead these latter ships have a single ring-like baffle per nozzle.
The disappearance of the triple baffles reflects a genuine variation of star destroyer sub-class, not just an accident of film production with different models. This is decisively proven by the addition of the baffles to the detailed destroyer model (originally made for the Avenger) used to depict the Devastator in the IMAX film Special Effects.
In the immediate vicinity of a planetary body, such as the Endor moon, another form of sublight propulsion and support is more effective. Repulsorlifts are an application of artificial gravity technology, employing a kind of antigravity effect. These devices are invisible on the ship's exterior, consisting of repulsorlift coils hidden inside the vessel. Within approximately six planetary diameters of the surface, a starship is deep enough into the gravity well for repulsorlifts to be useful as the primary drive system. This may account for the relative inactivity of the conventional sublight thrusters during the Battle of Endor.
There exist very few indicators of the performance characteristics of star destroyer propulsion systems. It is abundantly clear that the linear acceleration of an Imperator-class vessel exceeds that of the Millennium Falcon: the destroyer always outruns the freighter in a straight-line chase. However, the destroyers have considerably less turning ability. [See, for instance, the near-collision during the chase from Hoth in The Empire Strikes Back.]
The moments before the Battle of Endor provide one of the best demonstrations of accelerative capabilities. The tactical screen in the power generator bunker shows the fleet of star destroyers moving into position near the Death Star II in order to trap the rebel aramada which is due to arrive. This shot lasts for 94 movie frames, about 3.92s. Scaling by the horizontal diameter of the DS2 (106 pixels in laserdisk, assumed to be 900km), the horizontal interval between the screen's vertical lines is about 97km (183 pixels for 16 intervals). The destroyers cross 189km in 92 frames, implying a mean velocity of 49 km/s in the plane of the screen. The true velocity could be greater if there is a component of motion perpendicular to the plane of the screen. This speed may be further underestimated because of the dead-time between screen updates: refreshes of ship positions occur about once per second, and one was due at about the time when the movie scene was cut. At this speed and heading, the fleet could have hit the surface of the battle station within several seconds. The destroyers must halted within this time. This implies a minimum deceleration on the order of a thousand G (depending on the unknown velocity component and geometry in the perpendicular direction). The true deceleration is most likely a few thousand G.
In one fatal chase in the vicinity of a black hole [Starfire Rising, Marvel SW #54], a star destroyer indirectly proved an upper limit on its tolerance of sublight acceleration. The destroyer was fine, and its crew apparently comfortable, until the differential of gravitational forces across the ship's length exceeded some millions of G, when the ship disintegrated suddenly. This signifies the failure of the inertial compensators and tensor fields that normally counteract external forces (or effects of applying engine thrust). Field failure at this point implies that the engines were designed for a maximum thrust considerably less than a million G: possibly in the range of thousands of G. (It is worth noting that the destroyer's crew remained locally comfortable right until the inertial compensators failed across the miles' length of the ship.)
One rare demonstration of turning ability occured during the Battle of Hoth. Tyrant was struck by the rebel ion cannon and lost control. It's engines misfired in a spasmodic fashion for several seconds. Unbalanced thrust caused a turn upwards and to starboard by what appears to be almost forty-five degrees. (The turn was not directly imparted by the ion blasts, which hit well above the destroyer's center of mass. The tendency of the ion blasts would be to cause a downwards turn.) Since this was merely a random, uncontrolled turn, deliberate maneuvring should be capable of reorienting the ship at a similar rate.
The star destroyers in the near-collision in TESB did not turn more than a few degrees per second. Their relative velocities were slowed to less than a few hundred metres/second, and the thrust of the main drives must have been lowered. Perhaps the ships can't turn effectively when their engines are in a low-thrust mode? Otherwise star destroyers are incapable of transverse accelerations of more than a few G.
In Dark Empire a New Republic star destroyer crashed on Coruscant. It was ruined, but that probably had more to do with the physical impact than with the surface conditions. Since star destroyers can withstand accelerations of thousands of times the gravity of a habitable planet, soft oceanic landings should be possible. The fact that this is not often attempted must be partly due to physical awkwardness, and the environmental damage caused by a warship in atmosphere. It may also indicate that a star destroyer's lateral (as opposed to longitudinal) acceleration, provided by its repulsors, may be not so great compared to the local gravity. If the ship was facing upwards, it could launch without difficulty; being on its belly may be the problem. The command-ship Lusankya involves technology similar to a star destroyer, and yet it could only lift off a standard planetary surface (from a cold start) with aid from a single-use specialised giant repulsorlift sled. The tentative result of these considerations is that the acceleration of medium and large Imperial warships may have maximum limits that are much more stringent than their phenomenal fore/aft acceleration. Of course the immobility may actually be due to other unconsidered disadvantages of operations near a planetary surface other
Sublight thruster nozzles of the Devastator, while inactive. The triple baffles on the primary thrusters are clearly visible.
Thruster nozzles of what appears to be a ship of Avenger's type or later. Note the absence of the triple baffles on the primary nozzles.
Imperial destroyers and the Executor advance rapidly upon the Death Star II as seen on the monitors in the base on the sanctuary moon. Every second they moved a distance of at least one interval. There were at least 22 vessels, without accounting for possible superimposition of ships. battle station. The last link is to a 24fps QuickTime animation of the onscreen motion.
A star destroyer cannot cope with a force differential greater than some millions of G across its length. [Marvel SW #54]
Han Solo compares the sublight acceleration, supralight acceleration and turning ability of his freighter with that of a star destroyer.
The star destroyer Tyrant is the first of the Imperial warships blockading Hoth to be struck by the rebels' heavy ion cannon. As the ship's systems fail and the exhaust nozzles flicker dimly, a rebel transport ship and fighter escort make an escape.
During a search for the Millennium Falcon in a particularly dense and unstable asteroid belt, one of Lord Vader's star destroyer escorts suffered serious collisional damage. A fast-moving asteroid of approximately 70m diameter slipped into contact with the destroyer before the gunnery crews had a chance to blast it. The object exceeded the capacity of the deflector shields around the command tower, which were already undermined by continual pummelling and whatever effects had earlier been suffered at the hands of the Hoth rebel ion cannon operators. Both the asteroid and the tower were obliterated. The remainder of the ship flew onwards. [Michael Wong originally investigated the kinetics of this event. I independently repeated this work, but with a more refined estimate of the asteroid's velocity.] Judging by the observed size and velocity of the asteroid, and assuming a typical ferrous asteroidal composition, the kinetic energy involved in the collision was in excess of
E > 5 x 1014 J.
This can be considered an upper limit to the effectiveness of the deflector shields of the star destroyer (for instantaneous impact absorption) which was already seriously battered by asteroids and possibly an earlier collision with two other destroyers. The shields of a destroyer in peak condition would be more powerful. The book Anakin Skywalker: The Story of Darth Vader explicitly states that all the Imperial vessels withstood "multi-megaton" impacts on their shields, a megaton being equal to 4.19 x 1015J.
The maximum capacity of a warship's shields should be designed to be similar to or greater than the total firepower of a comparable opponent vessel. If shields were much weaker than weapons then their use would be pointless; if they were very much stronger then weaponry would be superfluous.
A similar collision between another destroyer and a smaller asteroid left the ship unscathed. The asteroid struck near on the portside of the primary docking bay. Upon impacting with the shields it crumbled within a fraction of a second, indicating a lower-limit estimate on the shields' capacity to absorb sudden blows. The asteroid was approximately 10m in diameter and had a kinetic energy of at least
2 x 1012 J.
Later, when the Millennium Falcon swooped across the surface of the star destroyer Avenger, some of the big ship's guns missed their target and accidentally struck other parts of the destroyer's dorsal hull. The blasts were harmlessly dissipated by the energy shields. Assuming that these weapons were at the same power as the guns used to casually vaporise asteroids [refer to weaponry section] the instantaneous capacity of the dorsal energy shields is not less than a figure on the order of 32TJ or 260TJ.
The shield rating of an Imperator in the LucasArts computer games is approximately 48 times the destructive output of a starfighter's standard proton torpedo. The torpedo is a nuclear missile device, and inclusion of "proton" in its name hints (inconclusively) at a hydrogen-fusion basis. As such, its output would not be less than about 100kilotons, even by the primitive standards of Earth's technology. (A deliberate effort of profoundly high technology would be needed to create a viable fusion explosive with less yield.) Therefore the total instantaneous surge capacity of the star destroyer shields is about 2x1016joules, if you believe the computer games. The games do sacrifice several aspects of physics for the sake of gameplay (most notably the non-Newtonian propulsion dynamics). Since the game intends to satisfy and flatter players as fighter pilots, the capabilities of warships seem to be dramatically handicapped in comparision to starfighters; A realistic star destroyer is probably more durable.
A large asteroid collides with a star destroyer, seemingly obliterating the command tower. This vessel is of the same type as the Avenger, but it cannot be the same ship (because Avenger is observed with an intact tower much later). If the five destroyers of Vader's original taskforce were the only ones searching the asteroid field then this ship receiving extra asteroid damage could be the Entor, which named in a report on the capture of a giant space slug as a zoological specimen. If, as the novel suggests, there were twenty ships in the search, then this could be a different vessel.
An asteroid is destroyed in impact with the Avenger's deflector shields near the primary docking bay.
The aggressive combat role of a star destroyer is one of the most important factors dictating the specifics of the vessel's design. The angular dagger-like form, sloping and thinning towards the bow and sides, ensures maximum visibility of guns to targets in almost every direction, with minimum obstruction from other emplacements and hull sections. Enemies above, below or to the sides are faced with at least half of the destroyer's weapons. Targets in the forward arcs are vulnerable to nearly all of the guns simultaneously: all guns in the brim trenches, on the major hull plates, and on the dorsal superstructure; only some extreme stern-mounted guns would be blocked.
The weapons of different emplacements appear to fire independently. The firing rate of one gun does not seem to affect its immediate neighbours. Although only a few shots are fired simultaneously in the films, in the Thrawn trilogy star destroyers were known to produce hails of turbolaser fire from nearly all guns.
An Imperator-class star destroyer has weapons of several different sizes. These different classes of weapons represent difference balances between performance criteria such as firepower, fire rate, reliability, and accuracy of fire-control and kinematic-anticipation targetting computers. Mandel's conservative Star Destroyer Imperator Class blueprints indicate that the star destroyer has at least three different sizes of cannons. This is probably based on the Devastator model, but the stated number for the largest guns (32) is an underestimate by half, compared with the later, better-observed ISD-II sub-class. Direct observation of star destroyers in action indicates that there are at least this many gun types, which shall be descrived in turn below:
In The Empire Strikes Back the shots from the Avenger appear in three different sizes: the smaller bolts are the size of those from light artillary or rifles; the medium bolts are slightly longer than the Millennium Falcon; and the larger ones are three to four times as long. These different lengths probably indicate the firepower (or power setting) of the originating guns. In A New Hope the medium bolts are used by Devastator against Tantive IV and small bolts (with fire rates of several per second) were used against Millennium Falcon.
In this view along the axis of the star destroyer Avenger we see that almost every gun on the ship has a line-of-sight to the target. The only exceptions will be stern-mounted guns and any which sit behind the command tower. At this range some of the heavy turret guns may obscure each other, but the problem would be alleviated at distances further from the destroyer's hull.
Turbolaser bolts of the long kind.
More turbolaser bolts of the long kind, used to hound the Millennium Falcon out of an asteroid field. The bolts are several time the length of the freighter; perhaps two hundred metres long.
Turbolaser bolts of medium length. They are approximately 50m long.
Turbolaser bolts of smaller length. These seem to be only as long as shots from a blaster rifle.
An indication that the effective range of turbolaser fire exceeds the effective range of a star destroyer's tractor beams, at least in the vicinity of a stellar-mass black hole.
On both sides of the dorsal superstructure there are four large batteries set in disk-shaped broadside turret mounts. Each pivot mount is approximately fifty metres wide and has a number of turbolaser or ion cannon barrels.
On the ISD-II subclass the turrets are identical, and each carries four turbolaser barrels on either side of a central partition. Therefore each battery has eight barrels and there are 32 of these guns on each side of the ship, for a total of 64. Each barrel is about fourteen metres long and over a metre in diameter. This is somewhat larger than the common turbolaser batteries defending the Death Stars' surfaces.
The earlier ISD-I destroyers carry six bulkier turrets with less numerous but heavier guns. The forward three pairs of turrets are double heavy turbolaser emplacements, for a total of twelve guns; and the rear turrets are double ion cannons.
Although there is no close-up footage of these weapons in use, their nature is evident in production art and on the star destroyer concept prototype. (There are distant views of one star destroyer apparently firing ship-length bursts from its heavy turrets in ROTJ.) The guns' location permits good fire coverage over much of the sky, except the ventral side of the ship and a small dead zone behind the command tower. Being on turret mounts implies that these weapons are mainly intended or grouped firing against one or more enemy warships, rather than for bombardment of targets on a stationary planetary surface. (Firing against a fixed point favours taking a simple manoeuvre to reorient the whole ship; firing against multiple moving targets requires that the guns be able to move independently.)
The turrets' disuse in the films' foreground battles can be explained as situational bad luck. In the battle between Princess Leia's Tantive IV and the star destroyer Devastator, the Corellian blockade runner carefully stayed below the midplane of the destroyer, below the big guns' fire horizons. Destroyers entered the Battle of Endor at long range but subject to holding orders; they made no initial attacks. Later they were at point-blank range to the rebel ships, making geometry unfavourable to long-range guns.
The prime weapons of a star destroyer must have a range of at least several thousand or several tens of thousands of kilometres, the typical distances of the blockade orbits around Hoth in The Empire Strikes Back. The destroyers were positioned to intercept any escaping ships, but were also prepared to bombard the rebel base once the shields were lowered.
Medium and close views of the portside heavy turbolaser batteries. Each circular rotating mount has eight barrels.
A distant star destroyer apparently firing bolts from its heavy flank turrets.
Heavy batteries of similar size are mounted in the brim trenches of ISD-I destroyers, within the narrowest and aftmost of the notches which indent the lips of the brim. There are four barrels on each of these emplacements, mounted on what seems to be a single solid rotating base. In Star Wars Incredible Cross-Sections they are called "lateral quad-laser batteries". It seems likely that the purpose of the notches is to allow the guns to aim directly further above or below the ship's plane than the lips of the brim trench would otherwise permit. These cannons partly compensate for the lack of very heavy guns on the ventral faces. The emplacement appears to have two barrels of at least ten metres' length. Curiously, the gun is sometimes seen only on one side of the ship, but the "missing" gun might merely be lost in shadow. Emancipator, the reconditioned Accuser employed by the New Republic at the Battle of Calamari, completely lacked trench notches and probably lacked these heavy brim guns. It is unknown whether the design of this subclass normally excludes notches and brim turrets, or whether Emancipator's condition is simply the result of patched battle damage and redesign.
Direct side views of the narrow brim notches of ISD-I and ISD-II star destroyers. The first image is from Devastor, and shows the four-barrel cannon clearly. The rest of the images are of an ISD-II; the big guns known on the older ship are not seen.
Ventral views of ISD-I star destroyers; the heavy brim-trench cannons are visible in the smaller notches.
Cut-away star destroyer production sketch and blueprints for A New Hope. In this undersized version, each gun turret has only two barrels, and there are only six of the heavy turrets on the whole ship, rather than eight. Smaller guns can be seen in silhouette across the dorsal hull surfaces. However the overall vessel is very much smaller compared to its weaponry, due to the upscaling of the ship late in film production.
Further medium guns were seen on the dorsal surfaces of the ISD-I design. Three triple-gun turrets are mounted on the ridge of the ship, just forward of the lowest, forwardmost terrace of the dorsal superstructure. The guns appear to be about ten metres long, but may be slightly longer than the barrels of the trench quad guns. They are given the label "axial defence turret" in STAR WARS Incredible Cross-Sections.
Two visibly large and important ventral turbolasers are mounted just beyond the forward lip of the primary docking bay of the ISD-I design. The barrels sit near the corners of the docking bay, seemingly amidst the machinery surrounding the twin dishes that are commonly regarded as tractor beam projectors. In fact it is even possible that the turbolasers are mounted inside the dishes. (The dishes are discussed in the docking bays section.) The cannons presumably were designed for defending the vulnerable docking facilities, but were also active during the pursuit and bombardment of Princess Leia's Tantive IV by the star destroyer Devastator.
Two frames from behind, showing a turbolaser bolt released from a weapon mounted amidst the machinery at the forward portside corner of the main docking bay. Compare the two images to spot the motion of the bolt.
Turbolaser bolt fired from starboard weapon at the edge of the primary docking bay. It can be difficult to discern at a distance in the earlier frames, but comparison with the later frames allows it to be spotted and traced.
Numerous other cannons are distributed over the brim, dorsal and ventral planar surfaces of the ship. Against the full length of the ship, these guns are almost invisibly small. Assuming that the normal turbolaser cannons are at least several metres long, like the main anti-fighter defenses of the Death Stars, they are probably associated with the numerous tiny lumps which dot the planar faces of both dorsal and ventral surfaces of the ship. During the pursuit of Tantive IV, fire rates of one shot per two seconds were seen.
At least one of the guns fired from the stern region, implying that a normally aft-oriented weapon is free to take a dramatically different facing. It seems that these guns are free to fire in almost any direction with a clear line of sight.
Fire patterns and sizing considerations indicate that these guns are isolated, rather than operating on group clusters or batteries. They may be very similar to the lighter guns of the Death Star, artillery-like pieces mounted indoors on a pivot with the barrel itself pointing out into space through a window sealed with an atmospheric containment shield.
Designs for smaller guns in the brim trench; produced for Return of the Jeci but not seen in detail in any film. They were later drawn in a view from the trench of a destroyer in the Byss cordon in Dark Empire II.
One bolt fired forward from a cannon mounted near the stern of the star destroyer Devastator.
During its hunt for the Millennium Falcon, the star destroyer Avenger used its minor guns in the brim trench and on the hull to clearing asteroids in its path [TESB]. These observations indicate a lower limit on the energy delivered by a single blast from one of the smallest turbolasers. These were probably not full-power shots, but were merely intense enough to remove the asteroids with minimal trouble and waste. The asteroids were on the order of several meters to several dozens of metres in diameter, and composed chiefly of iron and similar metals. Realistically, the asteroid composition may have been silicate (eg. like granite or basalt) or dominated by iron-like metals.
The energy delivered by the turbolaser shot must raise the asteroid material from the initial temperature to the melting temperature (Tf), then supply the latent heat of fusion (Lf) to change the state frm solid to liquid, and then raise the temperature to a boiling point (Tv) and provide a latent heat of vaporisation (Lv). The boiling point is pressure-sensitive; in principle it might be close to the melting point because vaporisation of the exterior of the asteroid occurs in a nearly perfect vacuum.
Solid iron has a density 7870 kg/m³ and a heat capacity of 449 J/kg/K near room temperature, latent heat of fusion Lf=2.67x105 J/kg and Tf=1811K. The heat capacity increases with temperature, but we can use these figures to determine a lower limit on the amount of energy needed to melt a ball of iron in space. With a diameter of 10m, starting from 0°C (arbitrarily chosen, though the real initial temperature was probably lower) the energy required to melt an iron asteroid is > 3.94 TJ. This whole quantity of energy is delivered to the object within the duration of the turbolaser bolt, no more than a fifth of a second.
Asteroid diameters of 5m - 20m have been suggested [Stephen Comblidge, Peter Chung: private correspondence and newsgroup contributions] implying minimum melt energies of 0.49 TJ to 32 TJ respectively. This is comparable to the energy yield of a kiloton of TNT, 1 kt = 4.2 TJ. For further comparison, a small and primitive fission bomb might have an explosive yield of 20kt.
The latent energy of vaporisation may alone exceed the amount of energy needed to melt the body, let alone the amount needed to raise its temperature to the boiling point. In terrestrial conditions, the latent heat of vaporisation of a 10m diameter iron ball is approximately 25.8 TJ. Estimates of the melting plus vaporisation energy [by Eric Vandersall] are 32TJ and 260TJ respectively for 20m and 40m diameter asteroids at an initial temperature of about 206K. These are uncertain as detailed values, since the heating process process is non-equilibrium, supersonic and takes place in vacuo. Nevertheless, these results are plausibly indicative order-of-magnitude estimates. Therefore, considering the maximum observed fire rate of at least 1/s [in the asteroid field and in battles throughout the saga] we can estimate a lower limit on the power of each of the small anti-fighter point-defence cannons as between 250TW and 2000TW. The sixty-four visibly large dorsal heavy cannons must have considerably greater power than even this lower limit.
A turbolaser blast from the star destroyer Avenger vaporises an asteroid. The time interval between the first and last frames is much less than a second. In the first two images we see the surface material on the destroyer's side of the asteroid being progressively vaporised before the material on the other side or in the asteroid's interior.
The literature of STAR WARS novels contains several passages which suggest the scale of weapon yields on star destroyers and other Imperial warships. Slave Ship p.248 describes warship weapons with structural mounts designed to cope with the equivalent of explosions "in the gigatonnage range." This hints that the individual guns, which may be the kind mounted on destroyers, have yields of a similar scale: ~1018J (ie. millions of terajoules) or more.
A single star destroyer is known to have sufficient firepower to reduce the surface of a habitable planet to molten slag. This operation is known by the grim command code "Base Delta Zero". Such was the fate of the planet Dankayo, site of an important Rebel Alliance administrative and intelligence facility in the years between the Battles of Yavin and Hoth, when the star destroyers Avenger, Devastator and Relentless arrived. The atmosphere was lost. The topsoil layers (at least) were vaporised to disassociated atoms and would eventually recondense as a silicate analogue of snowflakes and rain. The remaining surface was left as cratered slag, like the molten face of a newly forming planet.
Considering a plausible heat capacity (about 732 J/kg/K) and density (about 2.5g/cm3) for rock, melting the crust of a habitable terrestrial world to a depth of only one metre (an insignificant fraction of the whole crust) involves an enormous amount of energy. Raising the temperature of this material by merely one degree would require the uniform injection of about 1x1021joules of energy. Reaching melting point requires a temperature rise on the order of one and a half thousand degrees. Then further heat is required to accomplish the change of phase from solid to liquid at the melting point. Including this latent heat, the combined power of the turbolasers of a star destroyer firing throughout a circumplanetary orbit of what must be at least a few minutes' duration cannot be done without a total energy injection of
E(melt) > 1.7 x 1024 J.
There will be losses and inefficiencies because some of the heat will radiate away into space, be conducted into the deeper planetary interior or be expended in blowing off atmosphere and surface material. The true energy input required for this kind of bombardment may be a hundred or a thousand times greater. Neglecting these complicating considerations for the moment, we can calculate a lower limit on the total sustained power of all the turbolasers.
The time taken to complete the operation depends on how quickly a unit area of planetary surface can be shot to melting point, and how long it takes for the star destroyer to move around the world to work on the unscathed hemisphere. In Return of the Jedi a fleet of star destroyers took no more than a minute or so to move around the Endor moon, so the firing effectiveness probably controls the timescale of a Base Delta Zero operation. It cannot be too time-consuming (more than an hour or a few hours), otherwise the operation would be tactically impotent. Enemy forces would have sufficient time to evacuate assets on the side of the planet which faces away from the assaulting destroyer.
Assuming that the attack takes as long as an entire hour to complete, corresponding to a rather leisurely orbit, the total sustained power output of all the turbolasers is not less than about half a billion TeraWatts.
P > 5 x 1020 W = 5 x 108TW.
Melting of the crust to depths greater than merely one metre will require proportionately greater energy input.
Interestingly, this value is several orders greater than the previous section's estimate for the shield capacity of a star destroyer which has already been battered by several hours of exposure to a dense rain of asteroids. The difference may reflect the extreme exhaustion of the destroyers near Anoat, or it may reflect a distinction between the capacities and capabilities of combat particle deflector shields, navigational particle deflector shields, and energy shields.
A flotilla of star destroyers could render a planet's surface uninhabitable within less than an hour, but this isn't an easy option in all cases. Firstly, a planet has value to both sides of a conflict, as the habitable worlds of the galaxy are the living spaces over which territorial wars are fought. In most cases the conditions of victory involve the capture of a planet with resources intact. Secondly, some planets are protected from conventional bombardment by regional or global shields that are affordable to governments and even guerilla groups like the rebels on Hoth. Defeating the advantage of a completely shielded advanced world like Alderaan or Coruscant requires either infiltration / sabotage tactics or a Death Star.
The Imperial Star Destroyer has enough firepower to reduce a civilized world to slagImperial Sourcebook, p.61
These colossal, wedge-shaped behemoths, bristling with turboweapons and carrying entire TIE squadrons within them, each possess more firepower than the entire planetary forces of most worlds, and can reduce a planet surface to smoking debris in a matter of hours.Star Wars Technical Journal, vol.2, p.17
... to rendezvous at Dankayo and reduce the tiny base to molten slag. Even before the last of its atmosphere drifted away, before the dense clouds of atomized topsoil could begin to settle, Imperial transports Elusive and Timely, as well as a complement of TIE fighters, moved in to perform "mop-up" operations and a through search of Dankayo's now evenly-cratered surface.Scavenger Hunt, p.3
Transmitted from Dankayo to Alliance Com Buoy 965C shortly after the Imperial attack
As instructed, I have remained behind until the last of our transports departed safely into hyperspace. Imperial Star Destroyers have so thoroughly blasted Dankayo that I fear for my safety, even in this deep-planet survival shelter.Scavenger Hunt, p.20
"Sir, what about bombardment? Is there a stage for that?"
"Blasting a planet from orbit is easy — you don't need me to tell you how to do that. Limited orbital strikes would occur during the invasion stage. Just hope you are never given a Base Delta Zero order, lieutenant. Ah, yes, another question?"
"Sir, what's the Base Delta Zero order?"
"Base Delta Zero is the Imperial code order to destroy all population centres and resources, including industry, natural resources and cities. All other Imperial codes are subject to change, as you well know, but this code is always the same to prevent any confusion when the order is given. Base Delta Zero is rarely issued. ...."A World to Conquor, Star Wars Adventure Journal #2, p.256
A Victory-class star destroyer bombarding an unshielded planet's surface to slag in a Base Delta Zero operation. [STAR WARS Vehicles trading cards]
Princess Leia contemplates the prospect of Imperial retribution against the primitive world of Shiva, which has thrown off an attempted occupation. [Marvel SW #54: Starfire Rising]
The star destroyer statistics printed in roleplaying game references do not mention any kind of missile weapons. This omission is inconclusive, since the statistics also miscount the number of heavy turbolasers and mistate their fire arcs (really 64, each of which can target forward, some rear arcs, and mainly-dorsal arcs to either port or starboard). It would seem strange for a vessel a mile in size to not possess some missile weapons, since even a mere starfighter is able to carry and benefit from proton torpedoes. They may be nearly valueless against fast-moving starships, but missiles could be useful against slow targets such as space stations and ground facilities.
The literature contains examples of star destroyers using missiles. For instance General Calrissian's Emancipator used missiles during the desperate Battle of Calamari in Dark Empire.
The Star Destroyer Imperator Class blueprints give the ship a standard compliment of several missiles with yields in the range equivalent to 5-10 Megatons of TNT. This means a few times 1016joules per device. For comparison, large modern nuclear blasts are twenty Megatons or just a few times higher. Mandel's blueprints are very conservative, making the destroyer's missiles not much different from present-day technology. Perhaps he has underestimated the power or the number of the weapons (as he did with the number of heavy turbolasers), or perhaps they're really only a minor auxiliary weapons system. Certainly the total sustained output of the turbolasers would be greater than the total missile armament specified by the blueprints.
The location of missile launchers is harder to determine than the cannon weapons, because these weapons can be entirely internal. A launch tube could be positioned inconspicuously on almost any part of the hull with little effect on the potency of the missiles. Missiles do not require a line-of-sight to their target at the time of launch; they can adopted a controlled curving trajectory to swerve around the body of the destroyer, if necessary. However it would probably be most convenient to have a forward facing. Considering the lack of small depressions in the well-scrutinised main dorsal and ventral hull plates, the most likely location for launch tubes is probably the brim trenches.
A squadron of star destroyers docked at a major Imperial Navy space station, with several Interdictors on patrol nearby.
The star destroyer Avenger pursues the Millennium Falcon, prize prey for Lord Darth Vader.
A star destroyer hovers in escort over the Executor, casting its shadow on the giant flagship. Captain Needa departs the star destroyer Avenger (top of frame) in a twin-pod TIE shuttle.
Various views of the star destroyer Avenger from the renegade freighter Millennium Falcon, as it perches hidden in a sensor blind-spot and then drifts off in the pre-jump rubbish stream.
For further details on the treatment of continuity, refer to Continuity, Canon and Apocrypha.
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