WWII gravity bomb fuses

Factual Questions
1

Somewhere around here I have a U.S. 100 lb practice bomb. These blue-painted (to indicate inert) bombs were filled with sand or water and used for bombing practice. The nose has a threaded bit, and there might be one on the other end as well (I’d have to look). I know from watching WWII documentaries that fuses were screwed into the ends prior to a mission.

First (and least important): What were some designations of WWII gravity bombs (live ones, not practice)?

Second: I’ve seen footage of crewmembers in the bomb bay removing the safeties from the fuses. How and when were the safeties removed from the bombs on fighter aircraft?

Third (and my actual question): How did the fuses work? They had little ‘windmills’ on them, which must have turned in the slipstream. Were these used to detonate the bomb at a given altitude? ISTM that all of the GP bombs I’ve seen in newsreels detonate on impact, except for some that (the narrators say) penetrate below the decks of ships to explode inside. Do the fuses generate an electrical charge to set off a detonator to ignite the main charge? If so, is the circuit closed when the spinner stops? It that’s how it worked: 1) What happened if there was a mechanical failure and the wheel stopped in-flight? 2) How did the mechanism survive the impact long enough to ignite the detonator? Was an aft fuse a common feature of WWII bombs, and was it there to ensure that the bomb did not detonate prematurely and/or to provide a back-up if the fore fuse was rendered in-op on impact?

Basically, please tell me all about WWII gravity bomb fuses.

2

FWIW, I read that the “windmill” bit is a safety device. The fuse (fuze?) is safe until the propeller has turned x number of times - in practical terms, until the bomb has travelled a pre-determined distance through the air. Rather like artillery shells aren’t armed until they’ve left the barrel.

From the diagrams I’ve seen, it was purely mechanical - the windmill turned a threaded axle, and the movement closed a gap between a striker and a detonator sufficiently that the impact would make the striker hit home.

Of course, crews in Germany are still blowing up the ones that didn’t work as intended, so failure wasn’t unknown.

3

Thanks.

You mentioned ‘diagram’, so I did some googling. I found out the thing is called an ‘arming vane’. Haven’t found any cut-away diagrams though.

4

I can’t speak to WWII fuzes, but in the 1980s a typical USAF gravity bomb still used the M104 & M105 fuzes which were a more or less Korean war era design.

Internally, the fuze consists of a small spherical (1/8") charge of impact-sensitive explosive, called the “initiator”. That stuff’d go off with a good whack from a hammer. OTOH, the charge wasn’t much more than a firecracker’s worth.

Mounted next to that was a larger (1/4" dia x 1") rod-shaped charge of a less sensitive explosive (the “intermediate” charge) & next that was a much larger (1" dia x 4") charge of a still less sensitive explosive (the “booster”).

The fuze screws into the nose of the bomb such that the final fuze charge is right next to the bomb’s multi-hundred pound charge. On impact, the initiator goes off from impact forces, which sets off the fuze’s intermediate & booster charges which in turn sets off the bomb.

And once the fuze is inserted as I’ve described, one good whack with a hammer on the nose & you would get a BOOM.

So now comes the safety magic to prevent such bumps causing explosions. Until the fuze is armed, a good whack does nothing.

Inside the fuze, that intermediate rod-shaped charge in mounted in a cylinder set transverse to the axis of the whole fuze. As assembled, the ends of the intermediate charge are about 90 degrees off from the ends of the initiator & the booster charge. So a good whack on the nose might set off the initiator, but the intermediate is protected inside a steel shell & won’t be set off, which won’t set off the booster, which won’t set off the bomb.

So this way the bomb is almost as safe to handle with a fuze as without.

The windmill (“arming vane” in the argot) turns a set of clockwork gears which rotate the internal cylinder to align the intermediate charge with the iniator & booster to complete the firing path. There is also a block safety across the front of the initiator which is stout enough to protect it from rough handling and which is retracted by the clockwork.

Once the bomb leaves the aircraft & the arming vane is exposed to the airstream, it takes 3-5 seconds for the clockwork to align the cylinder, thereby arming the fuze. After that, any good impact will trigger the full detonation.

On aircraft with internal bomb bays, the arming vanes are protected from the breeze by being inside. For fighters the solution is also very simple. A length of stout piano wire is run from a solenoid-controlled hook on the bomb rack through a fixed loop on the fuze. The end of the wire simply sticks through the vane’s rotation arc & prevents the vane from rotating. When the bomb is dropped, the wire remains attached to the aircraft and as the bomb falls away the vane is freed to spin.

Shortly after takeoff a fighter bomber flight will join up & each aircraft is carefully checked by the other for “'spinners”, fuzes whose arming wires got disloged. A spinner means you’ve got a very large, sensitive explosive strapped to your butt and hitting a bird or really nasty turbulence could instantly ruin your day. Spinners are jettisoned as soon as possible, and with some nervousness since bombs sometime bump each other as they leave the aircraft.
A typical gravity bomb has two fuzes, one in the nose & one in the tail. They both function as above although the geometry of the components is a bit different. Each has a separate arming/safing wire as described. Cockpit switches control which wires are retained by the aircraft & which are let go when the bomb is released. If both wires are let go, they stay attached to the bomb & the bomb’s fuzes will never arm. We say the bomb has been dropped “safe”, which will almost certainly result in no explosion when it hits the ground.

If both wires are pulled you get redundant fuzing, which reduces the dud rate a bit.

The third common option is to rig the nose & tail fuze differently, such that you can get different explosive results depending on which single fuze is armed. For example, add a “daisy cutter” (4’ length of pipe) on the nose fuze. If the nose fuze is armed, the bomb will explode about 4’ in the air, maximizing shrapnel & air blast. If the tail fuze is armed, the bomb will explode when it gets to ground level, maximizing cratering & ground shaking. Different targets react better (or is it worse?) to different explosions & this way the pilot can choose at the moment of attack.

There are additonal modern options for selectable tail fin configuration, nose fuzes with radar altimeters, etc. But back in WWII none of that existed and AFAIK, aerial bomb fuzes then worked as I’ve described.

5

Missed edit window. The fuze designators are M904 & M905, not 104/105. Hadn’t thought about those identifiers in 20 years. Anyhow, that’ll be enough to get your Google going.

6

Interesting. I’d always been interested in the aircraft, instead of the ordnance. You wouldn’t believe how long I’d wondered how the fuses worked. Searching on M904, I found out that they cost the Air Force less than $19 each in 2001. That’s cheap! I should get one for the collection. :smiley: (Actually it would be nice if there were any that were fully functional but contained no explosives of any kind.)

Speaking of double-sawbucks, my practice bomb has $19.98 written on it in black marker. (The surplus store I bought it from in the '80s didn’t care much about not writing on things.) I found one online that had been polished and painted gloss black as an objet 'dart for $850. Just found a plain blue one for $269.50. I should have stocked up when I had the chance!

7
  • ignorance well and truly fought. Thanks!
8

Indeed.

9

You paid a lot of money for me to gain that now-unneeded knowledge; the least I can do is share. Thanks for the kind words.

10

One of the nukes we dropped on Japan used a radar fuze to detonate the bomb (as it had to go off a specific amount of distance above the ground). I remember reading somewhere about folks in WWII setting the fuzes so that the bombs would detonate above the ground, but have no idea of how that was accomplished (or even if it was a reliable source that I read it in). The US also used timing devices for incendary bombs in Japan. The idea being that the bomb would hit a building, start a fire, and then explode when fire fighters arrived, thus making it impossible to fight the fire (as the fireman were either dead or staying clear).

11

There were primitive radar proxiity fuzes coming into use in the waning days of WWII, both for aerial bombs & for artillery. The fuzing for the nuke, being largerly free of constraints on cost or size could easily have used such.

There are (& were) a whole host of other fuzing options beyond the vanilla ones I talked about.

12

LSLGuy:
That should be a staff report.
Excellent work!

13

I noted both spellings in the various posts. In ammo-speak:

Fuse - chemical burning-type initiator. Has a compressed black powder core or column to provide a known time delay. Like the old time fuses seen burning into the mine tunnel during westerns or Indiana Jones movies. Current time fuses have an outer polyvinyl water resistant covering and just swell and bubble. The grain size, chemical composition, and packing density of the black powder is carefully maintained to get reasonably reliable delay times. Old time dependent mortar rounds may have a time fuse with a black powder time train.

Fuze - mechanial or electrical functioning. Everything from a simle point detonating fuze where impact drives a firing pin into a detonator to the latest electronically settable wonders that can be programmed as they blasts past transmitters on the way out of the gun tube or off launch rail. Mechanical time fuzes rely on a clockwork mechanism of gears to initiate the round at the proper interval rather than a chemical time train.

14

Here’s something about Naval shell Fuzes.

From the same site is an even better article onproximity fuzes.

Both of these are great articles with good detail.

15

Good question, great answer.

I second the Staff Report suggestion, would love to have read this on the SD front page.

16

Interesting. I didn’t know ordnance people made a distinction, and I’ve seen it spelled ‘fuse’ much more often than ‘fuze’.

I did find this though: