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This question is about Star Wars: The Last Jedi so I'm making sure the preview on the main page is spoiler-free.

In Star Wars: The Last Jedi, the bomber fleet (squadron?) arms their bombs long before reaching the primary target.

We see that arming the bombs only takes a few seconds. We also see that

arming the bombs makes the fleet extremely vulnerable, as all but one of the bombers is destroyed in a single chain reaction.

Given this, why did they not delay arming the bombs until just before they were ready to drop them? Wouldn't that have been safer and more likely to be effective?

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    More importantly, how do the bombs fall as if there is gravity pulling them down? – scott Dec 19 '17 at 0:16
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    @scott, that's already been asked and answered. scifi.stackexchange.com/q/176403/59810 This is a different question; one of strategy/tactics. – Wildcard Dec 19 '17 at 0:31
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    Because they're a bunch of disorganized rebels barely managing to fly their rag-tag fleet. None of what they do is safe, effective, or even plainly makes sense, for that matter. They're only able to hit the target because of sheer ineptitude of their opponents. – void_ptr Dec 19 '17 at 0:43
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    I don't think the bombs were actually armed and exploded in the fleet crash. Because when the tie-fighter crashed into the bomber there was only a small explosion, expected from a ship with drive and weapons, but not even close to the massive detonation witnessed later when the final bomber delivers its payload to the dreadnought. - I think they just made the bombs ready and were vunerable because they opened the hatch-doors and had no strong shields... – Falco Dec 19 '17 at 16:13
  • Frankly? That whole sequence was just... dumb on all possible levels. The only reason why it even exists is because Johnson REALLY wanted a WW2 bombing run (except at 400km/h lower speeds, B-17 top speed - 462km/h, and by my calculations SF-17 were moving under 65km/h) and he appereantly loves B-17 bombers (SF-17 Star Fortress... B-17 Flying Fortress... no points for subtlety). – Petersaber Dec 31 '17 at 19:34
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It's a way to put a plot event in terms that an early 21st century moviegoer can quickly understand. Unfortunately, they sort of missed with that one... doesn't really stand up to even cursory examination.

You might even ask - why didn't they have automatic bomb arming devices? During WW2, bombs had automatic armers, wires that got pulled out when the bombs were dropped, releasing a small propeller that spun in the airflow and completed the arming after the bomb was well clear of the aircraft. Otherwise, a fragment from an exploding AA shell or bullet from an attacking fighter could hit the fuse and touch off the bomb while it was still in the aircraft.

Note that bombs contain high explosive, the 'high' indicating that a high amount of impact force is needed to set the explosive off. It's a safety measure, so you don't get a boom until you really want one. Even a bullet or shrapnel fragment won't set off high explosives, only a blasting cap or other device capable of creating a shock wave, can do that.

One exception in WW2: The detonators for the atomic bombs were not even installed until the bomber was well clear of Tinian island, the thought being that if something went wrong and the bomb blew when it went through the first stage of activation, they didn't want to take out their main bombing base.

One other exception: the kamikaze vehicles, both aircraft and the Kaiten manned torpedo, had manual arming methods, because the bomb never left the vehicle.

Presumably, the fleet, working with technology far in advance of ours, would have solved that issue for the same reason it was solved during WW2 - early arming means higher chance of premature detonation due to the enemy shooting at you and maybe hitting a bomb... as the fleet discovered, much to it's discomfort.

The alternative to that storyline would have been to come up with a plot that had the fleet obliterated for reasons that a moviegoer might struggle with... like maybe some sort of super weapon that a contemporary viewer might not understand.

An example of dated thinking evident in sci-fi: If you go back and view 2001: a space odyssey, you will find a few things that were clearly the result of a 1968 frame of reference: the video phone being a station and not a pocket phone, for example. That can be understood - even the concept of a cell phone didn't exist in 1968. (otherwise, that film was remarkably accurate, even with what we know today)

This one, though... a rare slip-up when they should have known better.

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    I suspected as much. Actually, there was another similar one. If it takes X amount of time to close the door, and you can only see incoming ships when they get as close as Y distance away, and the ships can travel just barely faster than Y/X, then you should have some lookouts so you can close the damn doors soon enough.... – Wildcard Dec 19 '17 at 10:01
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    "High" explosive is indicative of its explosive power, not how much force is required to set it off. High explosives can have either high sensitivity or low sensitivity. – Herohtar Dec 20 '17 at 18:49
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    @Herohtar so... "That's not how explosives work!" – Wraith Leader Dec 21 '17 at 13:31
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    I mean, they could have explained it away by the fact that this is a rebel group - sure they've had funding/support from governments, but they're still ultimately a rag-tag bunch of rebels and have to source whatever equipment they can to get the job done (maybe they could have been using less sophisticated mining bombs, etc). However, the movie then goes on to point out the rebels buy their gear from the same people as the First Order, which kind of negates the possibility of this as a reason... – delinear Dec 22 '17 at 14:52
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As I've pointed out in another related question about the bomber's tactics, the bombs weren't armed until the bombers were directly over the nose of the dreadnought, their primary target.

enter image description here

Presumably some of the bombers were going to drop their bombs immediately, with some waiting a few seconds longer in order to get the maximum impact and maximum spread.

It wasn't until there was only one bomber left that they decided to hold fire until they were directly over the centre of the ship, hence the delay.

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    FWIW, while I like the details of the other answers, I think this is probably the right answer. I don't think the bombs were armed particularly early, I think it just seemed early because of that special time-warping ability of action scenes in movies, where the events are meant to be happening over the course of a few seconds but take several minutes to play out on screen (usually because there are lots of things that are meant to be happening almost simultaneously, but the director can't show us them simultaneously, so there's a lot of cutting back and forth). – delinear Dec 22 '17 at 14:56
  • @delinear, related bit of trivia: in Marvel's "Avengers," after the atom bomb countdown clock is shown (once), there are actually that many seconds intervening before it explodes. – Wildcard Jan 30 '18 at 1:25
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To ensure the bombs work. Otherwise you are dropping the equivalent of a large rock. Which is obviously a waste of money and resources, and potentially a waste any lives spent making the bombing run.

Bombs themselves are not very explosive without the detonator. The reason being exactly what you mentioned in the question, arming the bomb makes things extremely dangerous (there are a few exceptions, but not with military grade explosive).

The military tries to make its explosives as durable as it can, since you don't want soldiers blowing up from the impact of a single bullet. A block of C4 plastic explosive can withstand a rifle shot without exploding. You can even set one on fire without too much worry.

[...]

Such a bomb would have a detonator, which is far more vulnerable. The detonator serves as a mini-bomb that produces enough energy to blow up the main explosive. Here's how it works: A power source—usually a few batteries—provides an electrical charge that sets off a tiny explosion in one part of the detonator. This sets off another, somewhat bigger charge, which in turn ignites the payload
If you shoot a bomb, will it explode?

So the trade off between carrying highly explosive objects and dropping rocks is keeping them unarmed until just a moment before the drop. They then have a moment to arm everything make sure they are, in fact, going to explode. It's the middle ground between being safe and effective.

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    But arming the bombs took a couple of seconds, and was done a couple of minutes before the actual target was reached. Those minutes were traversed under heavy enemy fire. I'm not saying they should have armed the bombs after dropping; I'm saying that the timing of the order to "arm the bombs" made no sense to me. – Wildcard Dec 19 '17 at 1:40
  • @Wildcard - Well, the filmmakers certainly dragged it out a little for dramatic effect, but like I said, its a trade-off. Remember as well that the final ship deployed really late. They were supposed to drop shortly after arming. But unfortunately, everyone got hit first. – amflare Dec 19 '17 at 1:47
  • Hmmm, maybe. But the final ship deployed right into that big circular thingy. I thought that was the intended "sweet spot" (and I remember some line in the film to that effect). Based on the speed of the bombers when shown from above and the distance to that "sweet spot" I still think they armed the bombs too early. But hey, I'm not leading the court martial on the one who gave the order to arm, so whatever. ;) – Wildcard Dec 19 '17 at 1:56
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    @moo - bombs in aircraft are armed when they are dropped, not when loaded. You see the little propeller in the tail of internally-stored bombs; well that winds the detonator into position arming the bomb while it is falling. You do NOT want your bomb armed while you are flying. The fuse pins and stuff removed is yet another safety. – squire55 Dec 19 '17 at 6:35
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    Speaking of such, this would have been a better tactic for the rebels to use for their bombs. Something like the arming propeller - but works in space, of course. Maybe spin the spherical bombs as they leave the rebel bomber, and have a spinning-detector in each bomb. Actually, why not just have each bomb on a string, which pulls out a (final) arming pin? (They obviously needed some extra safety; anything to avoid the mass "friendly fire" loss of their bombers.) – squire55 Dec 19 '17 at 6:42

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