Star Wars: Imperial Propulsion Technology
Written: 1998.08.01
Last Revised: 1999.10.06
The powerful sublight drive units of a Kuat
Drive Yards Star Destroyer
There are several examples which can be used to explore the performance characteristics of Imperial sublight drives:
ANH: Battle of Yavin
X-wing starfighters easily traversed a 400,000km distance in less than five minutes before the Battle of Yavin, suggesting accelerative capabilities on the order of at least 17,000 m/s² (1700 g's). The DS1, easily the slowest vessel in the fleet, circumnavigated partway around the 200,000km diameter gas giant Yavin in 5 minutes, as seen in the following screenshots of the Rebel display console at Yavin:
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At this point, Rebel tacticians state the Death Star will clear Yavin in 5 minutes (click to enlarge) |
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The Death Star in firing position, 5 minutes later (click to enlarge) |
This means that the Death Star could cover ~200,000km in 5 minutes, for an average velocity of 670 km/s (2.4 million km/h). Perhaps more impressive is the fact that it obviously accelerated by a large amount during this interval, as indicated by a screenshot taken when the Death Star was still 15 minutes out of position:
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At this point, Rebel tacticians state the Death Star will clear Yavin in 15 minutes (click to enlarge) |
Since it moved only 100,000km in the first ten minutes and 200,000km in the next five minutes, it is obvious that it was accelerating throughout this period. If we assume that it was accelerating linearly for the entire time that it was moving into firing position, then we will find that it was accelerating at more than 100 g's during this time! For a starship this would not be particularly impressive, but for a moon-sized battle station this is a significant feat of propulsion technology. In fact, it is roughly 10% of the 10 km/s² acceleration of a Federation Galaxy Class starship (ref. TM). It should be noted that some light capital ships are capable of similar acceleration, for example the Carrack Cruiser, which is described in the SWEGVV as having identical acceleration to that of an X-wing fighter.
It has been suggested by some critics that the Rebel console display should be ignored, for the simple reason that the Death Star is clearly shown out of scale. However, there is no reason to have a display at all if it is not positionally accurate. The exaggeration of the size of individual objects is hardly unusual- tactical and strategic display systems routinely replace long-distance objects with scaled-up iconographic representations, so that viewers can see accurate positional data without having to squint at tiny dots on a display. This is done with twentieth century radar displays, as well as Federation long-range scanning display systems (as seen "The Wounded").
ROTJ: Battle of Endor
The Battle of Endor demonstrated the accelerative capabilities of Star Destroyers. The Star Destroyer group was clearly seen on the Endor shield-generator bunker's tactical display, heading toward the Rebel fleet at a velocity of at least 6E4 m/s. It decelerated to near-zero velocity relative to the Rebel fleet, in roughly 2 seconds. This means that their decelerative capability (which is equal to or less than their forward accelerative capability) is at least 30 km/s². This means that a Star Destroyer has at least 30 times the acceleration of the Death Star, and 3 times the acceleration of a Federation Galaxy Class starship.
We also saw something of the accelerative capabilities of the Executor in this battle, since it rapidly turned downwards and accelerated toward DS2, covering hundreds of kilometres (perhaps thousands) in seconds. Some have claimed that its downward motion was entirely due to the Death Star's gravity, but such an immense gravity field (hundreds or thousands of g's) should have killed all of the inhabitants of DS2. It would have torn the atmosphere from the Endor sanctuary moon.
In a particular incident in the novel Rebel Dawn, Salla Zend attempted a foolishly dangerous hyperspace microjump through the Maw, a star cluster full of black holes, neutron stars, and a few scattered main sequence star. She was violently pulled back to realspace by coming too close to a neutron star, and her ship was badly damaged as a result. When Han Solo located her on his sensors, she was orbiting the neutron star at an altitude of 1000km, although the eccentricity of this orbit is not known. It was known that she would fly into the neutron star's accretion disc in minutes, and that this would have been fatal.
Han Solo flew the Millenium Falcon into the neutron star's gravity well to intercept her ship, the Rimrunner. When he approached, he slowed down to match speeds with her ship, which was still travelling at the same speed that it had before it entered hyperspace. He rescued her, changed course to avoid flying into the accretion disc, and then accelerated out of the neutron star's gravity well.
The acceleration of gravity at that proximity to a neutron star is on the order of 20 million g's, therefore the Falcon's repulsorlifts must have been capable of exerting at least 2E8 N of force for every kg of the ship's mass (3E14 N for a 1500 ton ship), simply to keep from falling in. This establishes the minimum accelerative capabilities of a repulsorlift in a gravity well. Gravitational potential energy would be roughly -200 TJ for every kg of the ship's mass (-3E20 J for a 1500 ton ship), so this would establish the kinetic energy that the repulsorlift can generate (with the attendant requirements on the ship's power generation system). It could be argued that Han Solo's orbital velocity would have kept him from falling in, but we must remember that he had deliberately bled off most of his velocity to match speeds with the Rimrunner.
Caveats
It should be noted that we are assuming Salla Zend's ship was moving very slowly before it made the hyperspace jump, hence the conclusion that she was travelling very slowly around the neutron star. It is possible that her ship was moving quickly enough so that it would be able to briefly stay in orbit around the neutron star. This would necessitate the conclusion that she had been travelling at close to 1.4E7 m/s before she made her hyperspace jump, but this is well within the performance parameters of a typical starship.
This interpretation would raise some questions about the nature of her orbit since she was several minutes from looping back into the neutron star's accretion jet, so it would have to be an extremely flattened elliptical orbit to accomodate 1.4E7 m/s speeds. Nevertheless, if we accept this interpretation, then Han Solo only had to bleed off 6E6 m/s of the 2E7 m/s he would have gained from the neutron star's gravity field. Therefore, his ship would have only had to accelerate by 6E6 m/s to escape.
Our Star Destroyers can easily traverse our galaxy in a matter of a few weeks, with travel speeds of over 1 million times the speed of light. However, hyperdrive does have some limitations; a ship travelling in hyperspace cannot fly through a star, and it can be affected if it passes by high-energy events such as supernovae ("without precise calculations we'd fly right through a star or bounce too close to a supernova, and that would end your trip real quick, wouldn't it?"- Han Solo, ANH). Entry into hyperspace and travel through hyperspace are accompanied by interesting visual patterns, as seen below:
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The view from inside the Millenium Falcon, as it travels through hyperspace on its way to the historic Battle of Endor (click to enlarge) |
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The view from inside the Millenium Falcon, as it jumps into hyperspace (click to enlarge) |
Some Federation cultists have taken to claiming that Star Wars Imperial ships need months to traverse the galaxy, but it must be noted that Darth Vader summoned bounty hunters from the core worlds (including IG-88 who always worked in the core sectors) to the isolated Outer Rim Hoth system and those bounty hunters arrived, all while the Imperial fleet was still in the asteroid field. To travel from the core sectors to one of the outer rim sectors would be a journey of at least 30,000 light years, and the bounty hunters must have made the trip in two or three days maximum. This works out to a speed of 3.65 million times c. Further evidence for this scale of hyperdrive propulsion speed comes from page 145 of the ROTJ hardcover novelization:
"The vast rebel fleet hung poised in space, ready to strike. It was hundreds of light-years from the Death Star --- but in hyperspace, all time was a moment, and the deadliness of an attack was measured not in distance but in precision."
This scene takes place after Skywalker is captured by Vader (in the wee hours of the morning, just before dawn), and the Rebel fleet arrives shortly after Skywalker enters the Emperor's overbridge. This means that the Rebel fleet covers at least 200 light years (hence the term "hundreds") in the approximate time interval required to transport Skywalker from the surface of Endor to the Death Star in orbit. We know from ANH that a typical starship can easily achieve orbit in much less than half an hour, because the Death Star was 30 minutes out of position during General Dodonna's briefing and the X-wing crews had manned and prepped their ships, achieved orbit, and then circumnavigated several hundred thousand kilometres around Yavin to attack the Death Star before it was in position. Therefore, even if we include the time required to load Skywalker aboard the shuttle, land aboard the Death Star, and travel to the Emperor's overbridge, Vader could not have taken longer than 30 minutes to make the trip. This means that the Rebel fleet must have been travelling at speeds of at least 200 light years per half-hour. This works out to 3.5 million c. Here is a quote from Dark Force Rising, pg. 212 hardcover:
"From the labored sound of the engines, [Mara Jade] could guess they were pushing uncomfortably far past a Victory Star Destroyer's normal flank speed of Point Four Five. Possibly even as high as Point Five, which would mean they were covering a hundred twenty-seven light-years per hour."
127 light years per hour works out to roughly 1.2 million c, which is somewhat lower than the estimate based on the events of ROTJ, but it is always possible that Vader dallied around on the surface for a while before taking Luke to meet the Emperor. Some sources seem to indicate much lower hyperdrive speeds, including one quote from Heir to the Empire, pg. 35 hardcover:
"It took the Chimaera nearly five days at its Point Four cruising speed to cover the three hundred fifty light-years between Myrkr and Wayland."
This indicates that .4 hyperdrive is equivalent to a mere 26,000c in this region of space (only 8 times faster than a Federation ship), as opposed to the 1.2 million c speed of .5 hyperdrive factor, which is more than 400 times faster than a Federation ship. It is possible that this indicates an extremely steep exponential increase in speed with hyperdrive factor, but it is more likely that it is merely a region of space in which navigational conditions such as obstacles, space-time distortions, etc. slow down local travel (keep in mind that Federation warp drive is also affected by certain regions of space, as we found in "The Omega Directive" when it was revealed that there was an entire sector of Federation space in which warp travel was impossible). The speed chart from BTM (see below) clearly indicates that the speed of hyperdrive travel varies dramatically depending on where you are going. One final quote comes from the most original source of all:
From Star Wars IV: A New Hope, pg. 118 softcover: "Navigation computer calculates our arrival in Alderaan orbit at oh two hundred."
This indicates that the trip from Tattooine in the Outer Rim to Alderaan in the Galactic Core takes less than one day. Otherwise, Solo would have included a date or a number of days along with the time (if you are going to arrive at two o'clock three days from now, you won't just say that "we will arrive at two o'clock"- you will say "we should arrive around two o'clock on Monday", or "we should arrive Monday afternoon" or "we should arrive in a few days"). A trip from the outer rim to a core system will be at least 30,000 light years (and probably much more- Alderaan might be on the far side of the core), and if it takes less than one day, travel speeds must be in excess of 10 million c. This is from a canon source.
In general, the canon films tend to indicate much higher hyperdrive speeds than the official novels. Travel from Outer Rim systems such as Tattooine, Hoth, and Yavin to core systems like Coruscant and Alderaan takes hours rather than days, as described above. Also, it should also be noted that the hyperdrive-capable X-wing fighters of the Rebel Allliance have no bathroom facilities, thus further indicating that there are strict limits to how long one is expected to spend in transit. The Behind The Magic CD contains a speed chart which appears to agree with most of the official sources as well as the canon ANH film:
We can see that travel times are strongly influenced by location. In the above chart, it can be seen that a typical starship can travel from Coruscant in the Galactic Core to Bespin, Dagobah, Dantooine, Endor, and Tatooine (all located in the Outer Rim, as seen in the galactic map, also from the BTM CD) in times ranging from 19 hours to over 22 days. The reason for this range is that hyperspace routes can force slow travel if they pass through regions of the galaxy with numerous navigational hazards. A starship traveling from Coruscant to Dantooine is traveling at an average speed of more than 18 million times c, while a starship traveling from Coruscant to Tatooine is traveling at an average speed of only 600,000c. The best route for a traveler would therefore be to jump to Dantooine first, and then take the 20-hour trip to Tatooine from there. This would cut the total trip down from more than 22 days to less than 2 days.
The trip from Dantooine to Endor is even more impressive. Endor is on the other side of the galaxy from Dantooine as seen in the galactic map, a distance of more than 100,000 light years. And yet, a direct hyperspace jump between Dantooine and Endor takes only 21 hours. This requires an average speed of more than forty million times c! But the most impressive trip is still Han Solo's trip in ANH, from Tattooine to Alderaan. The BTM speed chart agrees with the estimate of less than 1 day travel time derived from ANH, with a figure of 7 hours. Furthermore, we can see from the BTM map that the distance from Alderaan to Tattooine is closer to 40,000 light years than the conservative 30,000 light year estimate used earlier, and a 40,000 light year trip in 7 hours would require speeds of roughly fifty million times c.
A common misconception is the belief that hyperdrive cannot be initiated in gravity wells. This is actually untrue- although it is not recommended, it is possible to initiate a hyperjump in the gravity well of a planet. The Millenium Falcon actually went to hyperspace inside the atmosphere of a planet to escape Byss in the split-second before the planetary energy shield closed on them (ref. Dark Empire). Again, we stress that this is not recommended for safety reasons, but it is possible.
Specialty hyperdrives can travel even more quickly. The long-range planet-destroying missiles launched by the Galaxy Gun could travel from the galactic Deep Core to anywhere in the outermost frontiers in "a few hours" (ref. SWEGWT), in spite of the severe navigational disturbances in the Deep Core. This requires speeds in excess of 100 million times c!
Star Wars Imperial sublight propulsion technology is at least the equal of Star Trek Federation sublight propulsion technology, and we do not foresee any advantage to either side in this area. However, our hyperdrive is so much faster than their primitive warp drive that we can essentially move about their territory at will, with no fear whatsoever of interception. When Han Solo flew from Tattooine to Alderaan in ANH, he was travelling at roughly fifty million times the speed of light.
Acknowledgements
William Icquatu Jr., for discussing some aspects of propulsion systems.
Brendan Stone, for pointing out the SWEGVV reference about Carrack cruiser acceleration.