The bomb that sank the Arizona

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paul.mercer
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The bomb that sank the Arizona

Post by paul.mercer » Mon May 11, 2020 9:34 am

Gentlemen,
I was watching a documentary on the exploration of the inside of USS Arizona by ROV the other day and it was stated that the ship was sunk by a 16" shell in an encased streamline casing dropped from around 10,000 feet that went clean through all the decks and exploded in a magazine, presumably the one that held the cordite charges. The Arizona was a WW! built ship of around 30,000 tons so presumably was not so heavily armoured as later ships (although it seems that the US navy managed to do what the RN could not and fit 12 x14" guns in a ship of less than 35000 tons!) I digress, given the expertise that has been expressed on the subject of shell v armour and plunging fire, in this Forum. i wonder if you might answer yet another of my questions.Would later ships like the KGV's Bismarck. Tirpitz the Iowa's and Yamato's have withstood the same bomb, bearing in mind it would presumably be falling vertically as opposed to normal battle fire?

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jabeque
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Re: The bomb that sank the Arizona

Post by jabeque » Mon May 11, 2020 11:56 am

Basically the context is very different from a projectile. The angle of incidence was almost 90º. Plunging-fire is about 30-45º in the most favourable case and with very low possibility of impact.

In the 20s, it was known that a piercing bomb, dropped high enough, would beat any possible armoured deck system. The problem was that the possibility of impact to a moving ship was very low. Therefore, the dive-bombing, was developed.

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Re: The bomb that sank the Arizona

Post by OpanaPointer » Mon May 11, 2020 12:01 pm

The shell was machined down to allow the fins to be fitted. Speculation is that this overheated the shells and degraded the hardening.

paul.mercer
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Re: The bomb that sank the Arizona

Post by paul.mercer » Tue May 12, 2020 9:05 am

gentlemen,
Thanks for your replies,
You state that this was known about in the 20's, but surely armour and armoured decking had improved considerably since then and the ships that I mentioned were all presumably built on the basis that they would not only have to fight others with similar or more powerful armament as well as having to contend with larger aircraft carrying heavier bombs? When Yamato and Musashi were sunk it was with a combination of bombs and torpedoes and I have always assumed (rightly or wrongly) that it was the torpedoes that did most of the damage. Did the US developed a purpose built armour piercing bomb that could penetrate their decks or did they follow the Japanese idea of modifying a 16" shell or are we to assume that no WW2 battleship was proofed against a heavy bomb falling almost vertically?
I have to say that looking at pictures of h two Japanese ships which appear to have so much anti aircraft armament that they resemble the spikes on hedgehogs, that the fact that this could not protect them meant that the day of the battleship was truly over.

paul.mercer
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Re: The bomb that sank the Arizona

Post by paul.mercer » Tue May 12, 2020 9:06 am

Gentlemen,
Sorry, double post again, must keep fingers off mouse!

Mostlyharmless
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Re: The bomb that sank the Arizona

Post by Mostlyharmless » Tue May 12, 2020 10:21 pm

There are a fair number of bomb hits on modern battleships in WW2 and we can make some guesses about what would happen.

For example, the IJN scored a single 500 kg bomb hit on Prince of Wales with a level bomber from about 2,500 metres, which was stopped by the 160 lb on 40 lb armour over the machinery although the explosion did cause PoW to sink faster. A bomb from 2,500 metres is moving at similar speed to a bomb dropped from a dive bomber at about 5,000 ft because the dive bomber is diving at about 300 knots. Thus this is comparable to the FAA hits on Tirpitz with 1,600 lb bombs of which all but one were stopped and the one that penetrated was too damaged to explode.

However, if the aircraft is higher when the bomb is dropped, penetration is increased. Thus three hits on Scharnhorst at La Pallice with 1,000 lb bombs from 14,000 ft all penetrated (rather too well as they went through the bottom). This inspired the H41 redesign and (according to that famous source Wikipedia) the deck was increased in thickness from 120 mm (4.7 in) to 200 mm (7.9 in) and the sloped armor at the edges was thickened from 150 mm (5.9 in) to 175 mm (6.9 in). Interestingly, there was a 1942 proposal to strengthen the main deck of Montana to 8 inches to help those ships resist bombs dropped from medium to high altitude.

The Littorio class suffered hits from British and American bombs in the 1000 lb or 2000 lb class, which all avoided the main deck. Two hits on the very strong turret roofs were defeated by the slopped 200 mm armour whilst the other hits caused flooding.

Finally, there were bigger bombs such as Fritz-X or even Tallboys, which were rather destructive.

paul.mercer
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Re: The bomb that sank the Arizona

Post by paul.mercer » Tue May 26, 2020 9:07 am

Gentlemen,
Thanks again for your replies.
just as an afterthought, as we know the RAF used 6 ton tallboys to sink Tirpitz. Thinking about the success the Japanese had t Pearl Harbour with their modified 16"shells dropped from around 10,000 feet and bearing in mind the huge carrying capacity of a Lancaster, would they have been better off adopting the Japanese shell modifications and carrying 6 or more heavy bombs which would have almost certainly given a larger percentage of hits or,
on the other hand, would they not be so effective as as Tirpitz was much more heavily armoured than Arizona?

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Re: The bomb that sank the Arizona

Post by Steve Crandell » Mon Jun 01, 2020 4:47 am

This is the Wiki explanation of the hit which destroyed USS Arizona. It gives citations and I think we can trust it as far as it goes:

"Two competing theories have arisen about the cause of the explosion. The first is that the bomb detonated in or near the black-powder magazine used for the ship's saluting guns and catapult charges. This would have detonated first and then ignited the smokeless powder magazines which was used for the ship's main armament. A 1944 Navy Bureau of Ships report suggests that a hatch leading to the black powder magazine was left open, possibly with flammable materials stocked nearby. The Naval History and Heritage Command explained that black powder might have been stockpiled outside the armored magazine.[52] The alternative explanation is that the bomb penetrated the armored decks and detonated directly inside one of the starboard magazines for the main armament, but smokeless powder is relatively difficult to detonate. Thus the 14-inch powder bags required a black powder pad to quickly ignite the powder. The time elapsed from the bomb hit to the magazine explosion was shorter than experience suggested burning smokeless powder required to explode.[53] It seems unlikely that a definitive answer to this question will ever be found, as the surviving physical evidence is insufficient to determine the cause of the magazine explosion."

I believe all the inboard battleships were targets of these same type of bomb. Arizona was hit by 4 of them, and "only" one destroyed the ship. Hits on the other battleships obviously didn't destroy them.

paul.mercer
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Re: The bomb that sank the Arizona

Post by paul.mercer » Mon Jun 01, 2020 8:36 am

Many thanks for the info Steve,
I didn't know that the cordite required a black powder charge to ignite it and always assumed it was by an electrical charge, it seems quite funny that after all the years of improvements on naval guns that they still had to rely on gunpowder to set the thing off, was this true on later ships?
I believe the Arizona was WW! or just after design, which probably did not have the later type of armour on her decks which may have explained the bomb going straight through which was the conclusion of the people who were allowed to explore inside her with a ROV. Perhaps those 16" 'bombs' would not have been so effective on Tirpitz as i presume she had a lot thicker armour - obviously not proof against a 6 ton bomb though!

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Re: The bomb that sank the Arizona

Post by Bill Jurens » Mon Jun 01, 2020 3:36 pm

The black powder was required because due to it's very small grain size it produced a very large energy input very quickly. It would have been very difficult to evenly ignite all of the much larger nitrocellulose propellant grains otherwise. Actually, although few appreciate this, in many circumstances nitrocellulose is actually easier to ignite than black powder is. Ironically, in many of the big guns the black powder ignition charge itself required ignition from an even smaller charge of nitrocellulose contained in what amounts to a small blank rifle cartridge, usually initiated electrically.

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Re: The bomb that sank the Arizona

Post by Bill Jurens » Mon Jun 01, 2020 3:39 pm

Without tooting my own horn, readers may like to refer to the three-part (I think) series of articles on the loss of Arizona that Christopher Wright and I published about 10 or 15 years back in Warship International.

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paul.mercer
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Re: The bomb that sank the Arizona

Post by paul.mercer » Wed Jun 03, 2020 9:08 am

Thanks Bill, where would one find these articles today?

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Re: The bomb that sank the Arizona

Post by Bill Jurens » Thu Jun 04, 2020 2:43 am

So far as I know, all of these articles from Warship International are now on the JSTOR website. You may have to pay to access them. It may still be possible to get copies from the INRO archives. The material appears in three issues, No. 39/3, No. 39/4, and No. 40/1

Here is what I think is a full list taken from the Warship International index:

WI 39/3 Sep 2002 The US Navy's Study of the Loss of Battleship
Arizona Part One: A Documentary Review

Christopher C. Wriqht; William J. Jurens
WI 39/4 Dec 2002 The US Navy's Study of the Loss of Battleship
Arizona: Errata to Part 1 (W.I. No. 3, 2002)

C. C. Wright; W. J. Jurens
WI 39/4 Dec 2002 The US Navy's Study of the Loss of Battleship
Arizona: Appendix 1 - A Technical Analysis of the
Arizona Explosion Film

C. C. Wright; W. J. Jurens
WI 40/1 Mar 2003 USS Arizona: Part 3 of 3 C. C. Wright; W. J. Jurens
WI 40/1 Mar 2003 USS Arizona: Annex A - List of War Damage
Analysis

C. C. Wright; W. J. Jurens
WI 40/1 Mar 2003 USS Arizona: Annex B - Development of
Propellent

C. C. Wright; W. J. Jurens
WI 40/1 Mar 2003 USS Arizona: Annex C - Extract from Boise
Damage Report

C. C. Wright; W. J. Jurens
WI 40/1 Mar 2003 USS Arizona: Annex D - Main Battery C. C. Wright; W. J. Jurens

Hope this helps...

Bill Jurens

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