Bombs Away

During World War II, US bombs were designed generally for internal carriage in the bombers of the day, from the A-20 Havoc to the massive B-29 Superfortress.  Since they were carried internally, their aerodynamic properties weren’t of any great concern. Of course, fighters such as the P-47 carried them externally on wing and belly racks, but the relatively low speed of prop planes meant the high drag design of the bombs wasn’t terribly important.

WWII 500 pound bomb

World War II era 500 pound bomb

But the transonic speeds of jet aircraft meant drag quickly became a real factor in the jet age. Jets had the promise of higher speeds, longer range, and a massive increase in load carrying capacity. The old boxy style bombs from World War II imposed too great a drag penalty on fighter bombers. Internal carriage for fighter bombers wasn’t really a practical solution. While some jets such as the F-105 could accommodate an internal bomb-bay, the space available was only enough for one or at most two weapons. That was fine for the nuclear strike role, conventional missions would still require external carriage. And that meant a more streamlined bomb would be needed.

That most accomplished of American aviation designers faced the challenge in the late 1950s, and gave us what came to be known as the “Mark 80 series” family of bombs.   the Mk 80 series all using a similar aerodynamic profile, known as “Aero 1” with a streamlined shape and an approximate 8 to 1 length to width ratio. Four main types were built.

  • Mark 81- 250 pound bomb
  • Mark 82 500 pound bomb
  • Mark 83 1000 pound bomb
  • Mark 84 2000 pound bomb

Internally, the bombs were almost identical, with the size of the bomb casing being the only real difference.

Now, you’re thinking, well, that’s pretty simple. Not a whole lot to write a blog post about though..

But the rest of the story is the flexibility and adaptability of the series. With little change, the Mk 80 family has been continues in service, and there are no plans to replace them.

gpsketch

Cutaway view of typical Mk 80 series bomb

The bomb casing accounted for roughly half the weight of the bomb, with the other half being the explosive filler. This gave it a balance between blast effects and shrapnel, and enough rigidity to penetrate most structures. It was a “general purpose” bomb. Sheet metal fins and an aerodynamic afterbody would be bolted on during the assembly process before loading on an aircraft.

mk82_500lb_bomb

From the beginning, the Mk80 series proved adaptable. Aviators could choose a fuse best optimized for a target. For instance, for buildings, a slight delay to allow the bomb to penetrate would be chosen. For troops in the open, an instantaneous, or even proximity fuse would be selected.

The Mk 80 series slim Aero 1 profile lent itself to multiple bombs being mounted on a single hardpoint of an aircraft. Multiple Ejector Racks and Triple Ejector Racks were introduced. MERs could carry six Mk 81 or Mk 82 bombs. TERs could carry three Mk 81, 82, or 83 bombs. The 2000lb Mk 84 was too large for these racks, and one bomb could be carried directly mated to the parent pylon.

In service, jets flying close air support have to get… well, CLOSE to the targets. The low drag shape of the Mk 80 series meant, however, that the bombs fell to ground almost immediately under the dropping aircraft. The large fragment pattern could easily hit the airplane and bring it down. Accordingly, a special tail kit that could be screwed onto a regular bomb body was developed, known as SnakeEye.

BombsSnakeEyes

When dropped from a fighter,  a thin wire would release the fins from their folded position as soon as it cleared the aircraft. When the fins popped out to the deployed position, the high drag meant the bomb quickly slowed. Fighters could safely drop the SnakeEye from much lower altitudes and still be clear of the blast/frag pattern when the bombs hit. If desired, the fins could be left in the closed position and dropped in the low drag configuration. Eventually, the configuration could be chosen in flight.

Experience in Vietnam soon showed the 250 pound Mk81 was simply too small for most missions. Mk 82s could be bought, stored, mounted and used in virtually any place a Mk81 could be used, so production soon ceased for the Mk 81, and existing stocks were expended.  Later, the Air Force rationalized its bomb procurement, and stopped buying the Mk83 1000 lb bomb. Henceforth, they would almost exclusively use the Mk 82 500 pounders and the Mk 84 2000 pounders.

When the Air Force became interested in pursuing laser guided weapons for precision strike, it soon decided to mount the guidance kits on existing bomb cases as a cost saving measure. The first LGBs were mounted on high drag M117 750lb bombs, but soon the kits were mounted on Mk 84 casings. The Paveway series of laser guided bombs was born. The already aerodynamic shape of the bomb casing lent itself to serving as the body of a guided bomb product. Soon, kits were developed for variants using the Mk 82, and for the Navy, the Mk 83.

A further example of the adaptability of the series was the Skipper II system. The Navy needed a weapon  to counter missile armed fast attack craft. It had the Harpoon missile, but a cheaper system with a larger warhead was desired. Since fast attack craft generally had only short range anti-aircraft systems, long stand-off range wasn’t really needed. But the short range of laser guided bombs wasn’t quite enough. Accordingly, the Navy strapped the surplus motors of obsolete Shrike missiles to the back of GBU-16 1000 lb laser guided bombs. Voila! From a simple Mk 83 dumb bomb, a precision stand-off weapon was built for comparatively little money.

The Mk80 series also forms the warhead for the JDAM series of GPS/inertial guided family of bombs that has proven so devastating in Iraq and Afghanistan.

The Mk 80 series is still in service, but there have been changes. As a result of lessons learned during the Forrestal fire (among others), bombs bought for the Navy and Marine Corps stocks are coated with a thick heat resistant ablative coating. Newer stocks of the bombs are loaded with a heat insensitive explosive filler to reduce the chances of cook off or accidental detonation.

[youtube=http://www.youtube.com/watch?v=pytjJETpvFc]

Mk 82 SnakeEyes dropped from an F-111

2 thoughts on “Bombs Away”

  1. “Jets had the promise of higher speeds, longer range, and a massive increase in load carrying capacity.”

    The B-25s used by Jimmy Doolittle’s raid on Japan each had a combat load of 2000 lbs. 16 of them were launched 650 miles out from Japan. They flew at an airspeed of 230 mph or so, dropped their loads, and flew on to crash landings in China and Russia because their range wasn’t sufficient to fly back to the ships that launched them.

    It would have taken a grand total of _two_ A-6 Intruders to carry the same combat load, and they could have done it cruising at over twice the speed. With a tanker escort, they could have topped off after take off and been able to bomb their targets and come back to the carriers from which they launched. Note that only only a pilot and bombardier/navigator from each plane would be at risk, rather than the 80 airmen who participated in the Doolittle Raid.

    We could also, however, do the raid with a single B-52 taking off from Barksdale AFB in Louisiana, refueling en route, and it would be able to put twice as much ordnance on target as did Doolittle or the two A-6s. Likewise a B-1B out of Dyess AFB in Texas. A B-2 from Whiteman AFB in Missouri would only be able to carry 50,000 lbs to target, though.

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