I know that for a period of time the B-52 had a tailgunner, but nowadays many heavy bombers lack any turret defense.Why?
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15$\begingroup$ Related: Why don't bombers have close-in weapon systems (CIWS)? and Why do heavy bombers not carry air to air missiles? $\endgroup$– foootCommented Jan 16, 2018 at 18:08
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$\begingroup$ Related : U.S. Air Force Scientists are Working to Arm the B-52 Bomber with Laser Weapons $\endgroup$– J...Commented Jan 16, 2018 at 21:22
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8$\begingroup$ An interesting (although as it turns out, false) myth about bomber guns is about the tail gun on the B-58 Hustler. It was computer controlled, and used radar signals for data input. But, at supersonic cruise, the muzzle velocity of the rounds fired from the gun was (supposedly) less than the aircraft true air speed, so the bullets, although fired backwards at a approaching fighter, would still be travelling forwards when the target aircraft literally ran into them, from behind. Actually, the muzzle velocity of tail gun was 3380 ft/sec, and max speed of B-58 was 1600-1799 ft/sec. $\endgroup$– Charles BretanaCommented Jan 17, 2018 at 2:22
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5$\begingroup$ You can only shoot what you can see, and today's enemy is often out of sight. $\endgroup$– ReactgularCommented Jan 17, 2018 at 16:39
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1$\begingroup$ Also, current & foreseeable future enemies don't have jet fighters. $\endgroup$– jamesqfCommented Jan 18, 2018 at 19:51
8 Answers
There are really two reasons. First, fighter tactics have evolved over time, and second, the role of the B-52 has evolved over time.
Fighter tactics
Fighters used to depend (heavily) upon getting fairly close to an enemy, and shooting it down with a machine gun. Over the years, guns have become less and less dominant, and instead fighters tend to use missiles from outside a machine gun's effective range.
Role Change
The B-52 was designed from the beginning to carry nuclear weapons. Nonetheless, it's never been used to actually bomb an enemy with a nuclear weapon.
Early in its life, the B-52 was used to drop conventional bombs on enemies though. During the Vietnam conflict (in particular) B-52s were used to drop thousands of conventional 500 pound bombs on various targets.
The Linebacker II campaign was, perhaps, especially notable. During Linebacker II, 15 B-52s were lost--all of them to surface to air missiles (SAMs) not enemy fighters.
Linebacker II did see some limited success from tail guns as well: two B-52s shot down two MiGs. No B-52 (nor any other aircraft of similar size) has shot down an enemy fighter since.
Now, B-52s act as an aerial platform for launching stand-off weapons. Its normal operation is to fly in low (where it's difficult to detect on radar), and launch from a substantial distance from the target. Since it can be a substantial distance from the target, an enemy would need to defend a much larger perimeter (e.g., a large number of fighters basically blanketing a large area) to stand a decent chance of intercepting it.
Even if we assume an enemy fighter were to intercept the B-52, we get back to the situation with fighter tactics outlined above--they'd be unlikely to get within range of its tail gun anyway.
There are times the B-52's are still used for launching weapons at relatively close range, but only after achieving sufficient air dominance that doing so poses only minimal danger to the B-52s (where "minimal" basically means around the same level as training missions).
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1$\begingroup$ "an enemy would need a large number of fighters basically blanketing a large area to stand a decent chance of intercepting it" - or AWACS, or a target well inside their air defence region. $\endgroup$– RichCommented Jan 17, 2018 at 1:25
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2$\begingroup$ What do you mean by stand-off weapons? $\endgroup$ Commented Jan 17, 2018 at 10:13
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6$\begingroup$ @DavidGrinberg Stand-off weapons are missiles: weapons that you fire at a distance from your target and the weapon makes its own way there. $\endgroup$ Commented Jan 17, 2018 at 11:59
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3$\begingroup$ B-52s aren't just used for stand-off weapons: they've dropped a lot of dumb bombs in Afghanistan (and, as I recall, Iraq). However, that doesn't change anything of significance in your answer, since the Taliban has no aircraft and the coalition had total air superiority in Iraq, so guns on the B-52 wouldn't have been useful in either situation. $\endgroup$ Commented Jan 17, 2018 at 12:01
I was in the United States Air Force (USAF) working B-52's when the tail gunner position was retired. The reason was that it was deemed ineffective. As the answers here point out, fighter aircraft evolved such that medium and long range air-to-air missiles were the primary weapons. This kept fighters out far enough that the tail gun cannot hit.
But another very important reason was that the electronic countermeasures on the B-52s evolved such that it became the primary defense and was very effective.
Another major factor was that the fire control system was dependent on a radar transmitter/receiver. That in itself is a problem, because the radar signal could be detected and identified. When you are flying into enemy territory, you really don't want to do it with a radar transmission signal that says "BUFF OVER HERE WITH LARGE PAYLOAD THAT WILL DO MASSIVE STRATEGIC DAMAGE IF WE GET THROUGH".
It needed to be re-engineered to be more stealth, and the USAF didn't assess the time and cost to do that would pay off.
Flexible gunnery is useless against surface-to-air and air-to-air missiles. Which is the backbone of today's air defense systems.
Instead bombers have other means to distract said missiles.
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2$\begingroup$ That's begging the question, isn't it: why is it useless? They still mount guns (a close-in weapon system, CIWS) against surface-to-surface and air-to-surface missiles. $\endgroup$– ChrisWCommented Jan 18, 2018 at 13:32
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$\begingroup$ Because in those cases the defender is either slow moving or static, much easier. $\endgroup$– user14897Commented Jan 18, 2018 at 13:50
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$\begingroup$ But if missiles aren't effective due to those countermeasures, then wouldn't that encourage the target to intercept the bomber with fighters that use guns? $\endgroup$ Commented Jan 19, 2018 at 12:37
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1$\begingroup$ @jamesdlin - I didn't say those countermeasures are effective. Not all missiles are easily distracted. It's an ever evolving cat and mouse game. $\endgroup$– user14897Commented Jan 19, 2018 at 12:41
Because the defending nations stopped sending fighter planes at them to shoot them down at close range (where those guns were effective) and instead launched Ground to Air or Air to Air missiles at them from a larger distance.
This meant that it's more important to defend against missiles than against fighters.
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$\begingroup$ @DavidK the fighters would usually make high speed passes through the bomber formations, firing their guns as they did so. They did get within dogfighting range, even if there was not much turning involved either by the fighters or bombers. $\endgroup$– jwentingCommented Jan 17, 2018 at 11:43
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$\begingroup$ @DavidK there were dogfights between light and medium bombers and heavy fighters trying to intercept them in WW2. Think Me-210s vs. B-26s or Mosquitoes. $\endgroup$– jwentingCommented Jan 17, 2018 at 13:08
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$\begingroup$ @jwenting Never mind, I think the edited answer is perfectly acceptable. $\endgroup$– David KCommented Jan 17, 2018 at 13:36
Gun turrets are for defending against aircraft that are within gun range. However, modern fighters will usually lock on and fire missiles long before the fighter is within the range of a gun turret.
Nowadays, you can lock onto planes that you can barely see or even not see at all. On the other hand, you have a very limited effective range with machineguns.
Also, keep in mind that shooting down a plane from another plane is very hard (check videos on gunner training in WWII). Now imagine trying to do so on very fast planes like nowadays fighter jets.
To sum things up, most of the time, you won't even be in range to be able to use your machine guns and, even if you were, it would probably be really hard to defend yourself effectively.
Keep in mind that adding these machineguns weights down the aircraft and might even hinder the installation of more important equipment like flares or jammers.
There was a tailgunner on the B-52 because the use of canons by jet fighters back in the Vietnam war was more common than nowadays.
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$\begingroup$ A WW2 20mm and/or a .50 cal machine gun fired at 600 rounds/minute and was human-aimed; a modern CIWS fires at 4000 rounds/minute and aimed by radar and computer ... not very comparable IMO. $\endgroup$– ChrisWCommented Jan 18, 2018 at 15:21
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$\begingroup$ That means you need an engine for the turret, you need the program to aim the gun (which might require a more expensive computer) and you need to pack up enough ammo to back up these 4000rpm. All of these problems make the plane either more expensive and much more heavy, all of that for a mere novelty. $\endgroup$– Hawker65Commented Jan 18, 2018 at 15:24
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$\begingroup$ They put a canon on modern fighter planes (e.g. the F-35), I wouldn't have expected they'd be too weighty for a modern heavy bomber. $\endgroup$– ChrisWCommented Jan 18, 2018 at 15:41
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$\begingroup$ Yes, but that still would be a novelty. It's already more or less a novelty on modern fighter planes but at least they can do some strafing run to suppress some targets. The weight you don't use for a tail canon can be used in flares, jammers or something more useful and relevant. $\endgroup$– Hawker65Commented Jan 18, 2018 at 15:48
Modern fighters are usually armed with missiles, with which they can engage from far beyond the range of guns. At least one North Vietnamese fighter was shot down by a B52D's rear gun armament during the Vietnam War, however.
I believe that another reason is to do with the speed of modern bombers. At transsonic speeds old-style gun turrets would be ripped off the plane. On helicopters and other non-transsonic aircraft one can still find modern equivalents of the gun-turret. Have a look at e.g. the Mil Mi-24 https://en.wikipedia.org/wiki/Mil_Mi-24
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$\begingroup$ B-52s had tail gun turrets until 1994, so this can't be the reason. $\endgroup$ Commented Jan 19, 2018 at 16:14