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The Death Star trench was used as a plot device in Episode IV to facilitate the Rebel attack on the Death Star using small, one-man starfighters: the trench created a dead zone in which most of the Death Star's surface turbolasers (the ones not in the trench) could not fire on the starfighters. (Out of universe, the trench run was inspired by the films 633 Squadron and The Dam Busters, according to this answer.)

But, in-universe, why would the Empire design the Death Star to have a trench? What purpose does it serve? The equatorial trench was installed early in the Death Star's construction, as seen in Episode III:

Death Star being constructed in Episode III

The equatorial trench was probably installed early to help hold the Death Star's superstructure together during construction. By why build a trench rather than just a level surface?

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Null
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    To house thermal exhaust ports?! – Skooba Jun 17 '16 at 15:12
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    Per the New Hope junior novelisation "The trench contained ion engines, hyperdrives, and hangar bays". No indication is given why they needed to be in a trench. – Valorum Jun 17 '16 at 15:13
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    Well I'm stumped. I've found plenty of sources confirming what's in the trench, but nothing that seems to justify why a trench is required. – Valorum Jun 17 '16 at 15:27
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    The design was thought out as defending against capital ships, right? "Does not consider a small fighter a threat" and all that. So... would this make those systems a harder target? Doesn't feel like a great thought, but its something. – Radhil Jun 17 '16 at 16:02
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    I'm with Radhil on this one. Something on a flat surface can be easily targeted from vast distances, by much bigger weapons. Housing them in a trench conceals them and makes you attack them with smaller ships and weaponry, that have to get past the gauntlet of your defenses to even get a chance to target them. – PoloHoleSet Jun 17 '16 at 16:32
  • There are definitely turbolasers in the trench in the first Death Star. Notice the trench wall in the background of this image: http://www.stardestroyer.net/Empire/Tech/Beam/Turbolaser.jpg And we know the turbolasers could hit fighters in the trench because they were ordered/required to stop firing when the TIE fighters entered the trench to attack the rebel fighters to prevent accidental friendly fire. The trench was probably actually built to recess the thermal exhaust port and other items from the surface. Note that it was very difficult to attack the port since it was in the trench. – Todd Wilcox Jun 17 '16 at 17:38
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    Also, any equatorial trench that the first Death Star might have had was not the trench that was flown along to reach the exhaust port. If you watch the briefing carefully, you'll notice that the area of the Death Star that the display zooms into is closer to one of the poles than the equator. Note that my answers and comments are made in the context of M-canon I found a cleaned up briefing video – Todd Wilcox Jun 17 '16 at 17:40
  • @ToddWilcox I mentioned that the surface turbolasers (i.e. the ones not in the trench) wouldn't be able to target the fighters. I'll edit to clarify, though. The briefing is also a good point, but the Death Star diagram doesn't look accurate -- the superlaser dish appears to be cut by the equatorial trench, but that's not the case. In the actual assault the fighters look like they approach the equatorial trench. See this video (about 2 minutes in), for example. – Null Jun 17 '16 at 18:01
  • I was going to say that sub-conscious urges instilled in the Empire by supernatural forces of their Creator, created a geas for a design that could be easily represented by a snap-together plastic model, seams and all – infixed Jun 17 '16 at 18:08
  • While it doesn't speak to the question of why a trench is there in the first place, my personal impression of both the discussion of the trench in the briefing and the flying through it in the battle is that the trench did the rebels no favors. E.g., in the briefing Dodonna says, "You're required* to maneuver down the trench to this point*" (emphasis mine), and it seemed difficult to pull out of the trench after firing the torpedoes, and there was also discussion of being forced to bunch up in the trench, inhibiting maneuverability. – Todd Wilcox Jun 17 '16 at 18:11
  • Maybe it wasn't so much that they designed a trench, but that they didn't design anything to fill it with. If you're building a battle station the size of a moon, there's only so many bottomless chasms you can put in before you just start leaving holes on the surface. – DaaaahWhoosh Jun 17 '16 at 18:44
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    i always assumed it was manufactured in two halves and the trench was the seem where they joined. – Octopus Jan 25 '17 at 19:41
  • It is definitely a rain gutter... (?) – xDaizu Apr 18 '17 at 08:51

2 Answers2

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"The trench contained ion engines, hyperdrives, and hangar bays". – Valorum

"In the briefing Dodonna says, 'You're required to maneuver down the trench to this point.' " – Todd Wilcox

Just as the trench is protecting the hanger bays from the sun, it protects sensitive equipment that is required to protrude from the surface from errant mishaps, by simply employing a giant trench to recess them into.

Which in order to assault and have the correct angle to fire upon anything in the trench, you must make yourself an obvious target flying a given trajectory. It's like a camel's eyes in a sandstorm.

"The trench was probably actually built to recess the thermal exhaust port and other items from the surface. Note that it was very difficult to attack the port since it was in the trench." – Todd Wilcox

enter image description here

Aye, changing course, Commander... Issue the alert: SPF 5000.

Note that the equatorial trench isn't the only one.

Several polar trenches are shown in the rebel schematic animation. They radiate out from somewhere near the pole to approximately halfway towards the equator. –theforce.net

This is all for the most part, speculative. But having more than just the one trench suggests that they have some use, whatever it may be, other than being a flat-out "battleground written in to the story."

To tie this into Lucas' inspiration, in aviation it's hard to account for drift, so you plot your course along any natural landmarks you can find. Ideally, like a river leading up to a dam, or in this case a trench. Which when followed takes you exactly where you need to be and keeps you on course for your target.

Conventional weapons require the proper trajectory when launched. Which makes you, in essence, a sitting duck because that's where they're going to have put the guns:

The plant is in a seemingly impregnable location beneath an overhanging cliff at the end of a long, narrow fjord lined with numerous anti-aircraft guns. –633 Squadron

Vader flippantly decides that he has to go out there and save the day, all the while the batteries in the trench were doing their job just fine. (Way to go, you burnt-bacon smelling Nurf herder.)

Equatorial and polar trenches are installed early in a Death Star's construction because, not only do they house vital equipment, but they also make for the 'spine' of the superstructure, onto which everything else added.

The first thing you need for any project is a place to warehouse delivered supplies and docks to receive personnel. The first steps in construction (planetside) are the foundation and the framing. In space, they're one and the same: a superstructure.

It is also possible that the 'trench' is simply an artifact left over from construction, but was designed to be put to good use afterwards. The giant 'U' shaped band around the DS might have been required to be oversized, to support itself and the subsequent addition of sections, until completion.

Mazura
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This is addressed in the Rogue One: The Ultimate Visual Guide factbook. In short, the trenches, including the famous Meridian Trench contain cooling elements essential to the operation of the Death Star's main weapon and reactor core, providing additional coverage from stellar or hyperspatial radiation (which they would presumably collect if they were on the surface).

enter image description here

Valorum
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  • Good reference, +1. However, I'm not going to change the accepted answer since Mazura guessed correctly and made some other points about the benefits of a trench. – Null Dec 18 '16 at 20:38