Saturday, March 17, 2012

8 December 2010 – The Disney Bomb


No, this isn't about "Tangled" or "TRON: Legacy".  I haven't seen either film yet.

The Fixx were right; one thing does indeed lead to another.  Late last week I found myself searching online for information on a certain WWII-era British free-flight unrotating artillery rocket.  I didn’t have a lot of luck until I stumbled across a fairly heavy .pdf file of a 300+ page report by the US Army Chemical Material Destruction Agency.  This 1994 doc provided all of the information I needed, including line drawings and dimensions for the munition in question.  It also, interestingly, included information about fuzing options, and noted that the standard fuze for the item was the "British No. 721". 

That seemed a little odd; as a former gunner, I was familiar with the M-series fuzes in all their infinite variety (point-detonating, base-detonating, variable time, proximity, MTSQ – Mechanical Time Super-Quick – and so forth), but I’d never heard of a “No. 721”.  Nor was it simply a matter of adding an “M” designator; the “M721” has a lot of different meanings (including the rocket body and tail cone assembly for the M-72 disposable rocket launcher, and the illuminating shell used with the 60 mm infantry mortar – which itself uses an M776 mechanical time fuze), but there is  no “M721 fuze” on record.

I eventually found a summary document on British fuze labelling from the 1850s to the end of the Second World War – which, perhaps not surprisingly, is when labelling standards were rectified and we all started using NATO stock numbers and US labelling conventions.  According to that summary,
"A direction was given in 1887 to number all fuzes. Initially some attempt was made to apply order to this system. The numbers 1 to 19 were for percussion fuzes and numbers 20 and 21 were for rocket fuzes. Numbers 22 to 50 were to be for time fuzes and 51 onwards was to be for time and percussion fuzes."
So far so good; the munition I’d been interested in was, in fact, a rocket, and its fuze number did end with “21”.  But whence came the “7”?  Reading on, I discovered that – again, not surprisingly – nobody designing the numbering system in 1887 had made allowance for the sudden explosion (no pun intended) in the variety and types of fuzes that came into use during the wars of the 20th Century.  The end result was an impenetrable mish-mash of disorganized numbering as producers and logisticians struggled to come to grips with the vast array of new weapons, and the fuzes that went with them:
"Percussion fuzes numbered from 1 to 19 then jumped to 44.  Combustion time fuzes started at 50 then went back to 25 as some numbers were left over.  Mechanical time fuzes began at 200.  Some combustion time and percussion fuzes were converted to time only. Adding 100 to the original number of the fuze indicated this. But you had to know the original number of the fuze if this was to mean anything to you. Fractional numbers were introduced to indicate that the original numbered time fuze had had a percussion fuze added underneath as a gaine. Eg. 80/44.  Original 500 numbered fuzes were supposed to be for naval service only. The brits ruined this by using many of these fuzes in land systems. The 600 numbers were allocated to the demolition and booby trap stores.
The 700 series were for the rockets or proximity fuzes. RAF bomb fuzes were given the same numbers as the Army. Endless confusion here. 1941 the RAF were given the block of numbers from 844 to 999. Unfortunately the Army also used some of these numbers. Some early fuzes were renumbered to bring the fuze into line with the new block of numbers. eg 46 became 846."
Fascinating stuff, right?  The immediate point, of course, was that the 700 block had been reserved for rockets and proximity fuzes.  So again, the No. 721 was the right nomenclature for the right fuze.  Mystery solved.

But on the Intertubes, every mystery you solve generates a new one.  In the course of my research, I had come across another peculiar notation regarding the principle uses of different fuzes.  The “British Tail Pistol No. 58”, it seemed, had been used extensively in conjunction with a weapon that had a very odd name.

In pommy explosives nomenclature, “pistol” is more or less interchangeable with “fuze”, although technically speaking it refers only to the device designed to actuate the ignition chain, through the detonators and booster charges all the way to initiation of the main charge.  A “tail pistol”, of course, is a fuze designed to be inserted in the base of a munition, rather than the nose or ogive.  This has always been one of the commonest fuzing preferences for projectiles with certain specific purposes – e.g., the 90 mm M691 HESH-T “high explosive squash head” projectile that was developed for the LAV Assault Gun variant, which is designed to splat up against a target and be initiated by a base-detonating fuze, creating a shock-wave that spalls metal fragments off the inner armour of a target.  For aircraft bombs, though, tail fuzing has always been the preferred option, for a variety of reasons including (but not limited to) ease of access; the use of a precise number of propeller rotations to serve a mechanical arming or timed detonation function; the fact that bad things tend to happen to the front end of a multi-thousand-pound air-dropped projectile when it hits the ground at a thousand feet per second; and the possibility that the front of the bomb might be compromised by inclusion of a fuze well. 

This latter reason, it turns out, was the one that applied to the weapon whose peculiar name caught my attention.  It was called the “Disney Bomb”. 

The Disney Bomb? Seriously, how can you not be intrigued by that?

It took some trawling through newsgroups and military modelling websites to figure out what this was all about.  Wikipedia provided some assistance, although I had to take a shower and some antibiotics afterwards.  According to the Wiki page for “Bunker Busters”, the Disney Bomb was a rocket-assisted air-dropped munition designed to penetrate hardened concrete structures, like the U-Boat pens that the RAF and USAAF had spent so much time trying to destroy:
"Thought up by Royal Navy Captain Edward Terrell, it had a streamlined hardened case and weighed some 4,500 lb (2 tonnes). The bomb was dropped from 20,000 ft (~6,000 m). At 5,000 ft (~1,500 m) a barometric fuze fired the rocket in the tail to give it a velocity at impact of up to 2,400 ft/second (730 m/s). It was first used by the 92nd Bomb Group on 10 February 1945 on S-boat pens at IJmuiden, Netherlands, one bomb under each wing of 9 B-17 Flying Fortress. On that occasion a single direct hit was scored. A total of 158 "Disney Bombs" were used operationally by the end of hostilities in Europe."
I was also able to find, on one of the modelling websites, a picture of the device:

Figure 1 - The Disney bomb, grounded and mounted.  The fins are at the rear of the weapon; the thicker part of the bomb houses the rocket motors.  And that guy in the centre looks an awful lot like he's wearing a Royal Navy uniform.  Could it be the "Captain Edward Terrel, RN" referred to in the Wikipedia article?
(Source: allegedly T.Burakowski and A.Sala, Rakiety i pociski kierowane, 1960, np).

While short on detail, this gave me something more to go on.  I found a reference to a Polish book published in 1960 by T.Burakowski and A.Sala, entitled Rakiety i pociski kierowane, or "Rockets and guided missiles"; and another to “USSTAF Armament Memorandum No. 3-133, dated 28 January 1945”, which doesn’t appear to exist on the Web.  A big help was an off-hand annotation in a comment on a modelling blog which provided a link to an old issue of Flight magazine.  The page, dated May 30th 1946, did not use the “Disney” designation – but it did refer to a 4500-pound rocket-assisted bomb carried by the B-17 Flying Fortress.  It also noted that “An official film of ‘Project Ruby’ shows that the missile resembles the German ‘P.C.’ series carried by Ju-87s and -88s, though the rocket motor, detachable from the warhead, appears even longer.”

“Project Ruby” was the ticket; it sounded familiar.  A few minutes on Google led me through a different set of links, including some historical notations on the history of RAF Mildenhall and a few references to Eglin Field in Florida, before I finally hit the motherlode: the full project report on “Ruby”, titled “Comparative Test of the Effectiveness of Large Bombs against Reinforced Concrete Structures (Anglo-American Bomb Tests Project “Ruby”)”, published at Eglin Field by the USAAF Air Proving Ground Command, on 31 October 1946.

At 12 MB the report took a while to download – but the result was 310 pages of awesomeness, most of it consisting of photographs, schematics, test data, and analysis of the test results.  Fascinating stuff, and a first-class example of relevant, scientific operational research.  Tragically, the photographs are almost illegible; the .pdf copy appears to have been scanned from an old and oft-photocopied version of the original document.  It’s almost impossible to see anything – a real disappointment, as there are hundreds of pictures which would make any military historian salivate.

Most interesting, though, were the line diagrams, including schematics of - you guessed it - the Disney Bomb.

Figure 2 - The Disney Bomb
(Source: the Project "Ruby" report, p. 82)

To get a better idea of how the bomb was loaded and dropped, check out this (very brief) History Channel video that I managed to locate on Youtube.  The best part is that, according to the video, the bomb's nickname was "The Disney Swish".  Sounds like a dance craze.

Project “Ruby”, of course, was carried out in the immediate post-war period with wartime munitions and methods - the good old days, when "post-war nation-building" included "using the enemy's country as a test range for miscellaneous ordnance".  The object of the project was “To compare the performance of British and American bombs of standard and special design when used against reinforces [sic] concrete.”  During the project, “particular attention” would be given to assessing “penetration, strength of cases, insensitivity of exploder system, reliability of pistols and fuzes, [and] insensitivity of main fillings.” (Project "Ruby" Report, p. 6).  I’ll spare you all the details of the introduction, but basically the Project, over the course of a few months, used USAF B-29s and B-17s, and RAF Lancasters to drop hundreds of different bombs on two very special targets: the submarine assembly plant at Farge, Germany, on the Weser River; and the German submarine pens that had been built on the uninhabited island of Helgoland, about 80 km northwest of Bremerhaven in the North Sea.

The Farge target was magnificent; a reinforced concrete structure 1400’ long by 318’ wide.  It was unfinished at the end of the war.  The Germans had been improving the ceiling; about 60% of the original ceiling (14’9” of reinforced concrete) remained; 40% had been improved to 23’ of reinforced concrete.  Just stop and think for a moment about what 23’ of reinforced concrete means (for those of you with some knowledge of construction, according to the project report, building the Farge plant took 650,000 yards of concrete.  The report also contains detailed schematics of how the plant was intended to operate, assembling pieces of Type XXI boats delivered by barge and rail, and releasing them into the Weser through a concealed dock). 

Figure 3 - The Submarine Assembly Plant at Farge, Germany
(Source: the Project "Ruby" report, p. 82)

It was an ideal site for testing deep penetration bombs.  There was only one problem; the village of Farge was not far away, and there were houses and an electrical power plant within 500 yards of the target.  That meant that Farge could only be used for non-explosive tests.  Explosive stability tests were carried out at Helgoland, where the U-Boat shelter was 506’ long, 310’ wide, and had a 10’ reinforced concrete roof.

The project tested 6 different weapons.  The smallest used was the 2000-pound M103 special purpose bomb; after that, they got bigger.  The 4500-pound Disney Bomb was next; then the 12,000-pound Tallboy and 22,000-pound Grand Slam bombs, both of WWII-fame.  The tests also trialled a new variant of the Grand Slam called the Amazon.  Also weighing in at 22,000 pounds, the Amazon had a higher weight-to-diameter ratio, traditionally one of the key characteristics of penetrating bombs (the other being a high length-to-diameter ratio).  Finally, the project also trialled a 1650-pound scale model of a new design based on the Disney Bomb, with the highest length-to-diameter ratio of any of the weapons used in the test. 

Figure 4 - The Project "Ruby" bombs
(Source: the Project "Ruby" Report, p. 81)

Here’s how the different weapons stacked up in numerical terms:

Table 1 - Bombs used in the Project "Ruby" tests (from the project report, p. 81)

Length Overall
M103 SAP
2000 lb
1650 Model
1650 lb
4500 lb
12,000 lb
Grand Slam
22,000 lb
22,000 lb
*Length is for the warhead section only; the tail sections add negligible weight
**C/W is weight of explosive fill as a proportion of overall weight

As you might expect, the next several dozen pages provide test results and data for the bomb trials.  128 bombs were dropped at Farge, and 133 at Helgoland.  Most of the bombs used were, it turns out, Disney bombs.  This was due to the requirement to test them both with, and without, rocket assist.  And as expected, in the pre-laser guidance/GPS era, "on target" was something of a fluid concept.

Figure 5 - Impact records for Project "Ruby" tests, Farge Submarine Assembly Plant
(Project "Ruby" Report, p. 109)

The rocket assist system was probably the most fascinating aspect of the Disney.  It was achieved by adding a long tail section to the bomb body and stuffing it with 19 3” rockets.  Dropped from 20,000’ at an airspeed of 220 mph, the Disney, without rocket assist, had an impact velocity of about 1150’ per second.  The tail section of the bomb carried an M111A2 mechanical time fuze, which allowed 15,000’ of free-fall before igniting the rockets.  The rockets burned for three seconds, increasing the impact velocity to 1450’ per second.  This represents a very significant gain in kinetic energy, which is one of the key determinants of a bomb’s penetration capacity:

Table 2: Comparison of unassisted/rocket-assisted Disney bomb kinetic energies (calculated)

Kinetic Energy (Mj)
Without Rocket Assist
With Rocket Assist


Let’s fast-forward to the end.  The trials demonstrated that without rocket assist, the Disney ("Swish!") bomb was capable of penetrating an average of 10’5” into reinforced concrete, with a standard deviation of 8”.  With rocket assist, however, the same bomb was able to penetrate an average of 13’10” of concrete, with a standard deviation of 14”.  One of the latter bombs fell on the thinner (14’9”-thick) roof section of the Farge target, penetrating it, then penetrating the 3’ thick concrete floor and burying itself completely in the sand underneath the facility.

I wish some of the photos from the report were of decent quality, but they're just awful.  However, in one of life's little ironies, a few years ago a Disney bomb - still fuzed and armed, but unexploded - was discovered stuck in the roof of the Eperlecque blochaus, an infamous relict of WWII located about 30 km southeast of Calais.  The French video about the discovery and the bomb's subsequent disarming and removal provides a pretty good idea of the penetrating capabilities of this weapon. 

(Eperlecque, incidentally, was infamous because it was one of the launch sites for V1 flying bombs. That's relevant to this discussion, in an oddly tangential way, for reasons that I will explain later.)

The failure of the Disney bomb to explode when it struck the Eperlecque bunker tells us a lot about the stability of the filler - which in turn confirms the explosive sensitivity tests at Helgoland in 1946.  All of the explosive mixtures trialled proved to be relatively resistant to shock; in only a few cases were low-order detonations observed.  The mixtures trialled included 70/30 Shellite (picric acid/dinitrophenol); 60/20/16 RDX/Aluminum Powder/Wax; 68/20/12 RDX/Aluminum Powder/Wax; TNT; 20/60/20 RDX/TNT/Aluminum Powder mixed with wax and carbon black; and Picratol (52% Dunnite, or ammonium picrate, and 48% TNT).

The Ruby tests also revealed serious flaws in all of the weapons.  None of the bombs were capable of penetrating the thickest (23’) section of the Farge roof.  The Amazon had a maximum penetration of 15’10”, which was lower than the 16’4” maximum penetration expected from the Disney bomb.  At maximum striking velocity the dreaded Grand Slam could only penetrate 7’8” of concrete; the American (forged) Tallboy penetrated 5’8”; the British (cast) Tallboy, 5’7”; and the M103 AP bomb, 6’0”.  The 1650-pound model bomb only managed to penetrate 4’4” of concrete.  All of the bombs had problems withstanding side impact after perforation of a concrete slab; most of the tests resulted in serious deformation or fracture of the bomb bodies.  The most serious problem with the Disney bomb was the unreliability of its rockets; during live testing, they failed more often than not to ignite.

Well, what was the verdict?  The report concluded that:
“While the Disney bomb is dimensioned properly for good penetration, it also needs modification to prevent break-up on side impact.  The stud holes in the case contribute to break-up on side impact.  The bomb also needs modification to increase the reliability of the rocket assist.  Redesign of the arming wire system to reduce the lengths of the arming wires would eliminate some rocket failures, but improvement in the firing system is also needed to ensure complete rocket action from all rocket tubes.  The explosive charge of the Disney is not large enough to cause material damage to a massive concrete target.” (Project "Ruby" report, pp. 11-12)

The project report recommended a variety of fixes.  One was exploring bombs with smaller diameters, more pointed noses, and greater case strengths than the Amazon, but without materially reducing the weight of the explosive filler (in other words, make everything bigger and stronger and heavier but make the whole thing smaller and lighter – a common result of military weapon tests).  Options for increasing case strength included multiple layer or laminated walls, the use of special alloys, and the addition of internal ribs or corrugations.  The project also recommended finding ways to increase the angle of incidence, ideally to the perpendicular, in order to avoid stresses from non-normal incidence, limit uncertainties in bomb behaviour (one Disney made an 8’ crater in the Farge roof, then bounced out and was found 75’ away), and minimize the distance to be penetrated.  Finally, the report recommended identifying and using the most powerful explosive fillers available.

“Inclosure 14” is an interesting addendum to the report.  It details the physics calculations for bomb penetration, comparing the standard formulae used in UK and US research, and comparing their predictions to the results achieved in the Project Ruby tests.  The report notes that predictions of penetration for the unassisted Disney bombs were a maximum of 12’6” and an average of 10’6”.  As the average of the trial results for unassisted Disney bombs was penetration of 10’5”, this is a stellar example of a predictive mathematical model being confirmed by observed results – the hallmark, in short, of good, solid science.

A note on research material.  Based on the Project Ruby final report, the Wikipedia article on Disney bombs, and bunker busters in general, is chock-full of significant errors (e.g., it was a mechanical time fuze, not a "barometric fuze", that initiated the rockets; there were 19 individual rocket motors, not one unitary one; and the impact velocity was 1450 fps, not 2400 fps).  Bottom line - you can use Wikipedia to look up sources, but anybody who actually cites it when doing serious work needs to have their head examined.

At the end of all of this, one thing – the question that started the whole chain of research – still eluded me.  Why the hell was it called the “Disney Bomb”?  One commenter on the modelling site suggests that it comes from the WWII patriotic cartoons distributed by Walt Disney, at least one of which shows Donald Duck suffering various explosive mishaps in a Nazi munitions plant.  Another commenter suggested that it might be based on a WWII propaganda film called “Victory Through Airpower”, which tended to be shown in conjunction with WWII-era Disney movies, and which seems to show rocket-assisted bombs being dropped on Germany.

I'm thinking that the Disney Bomb probably got its name from RAF Norton Disney in Lincolnshire, which opened in 1939 (it opened as RAF Swinderby, but was renamed in 1940), and which served as a WWII-era munitions FFD (Forward Filling Depot) for the RAF that specialized in, amongst other things, doing final assembly and storage of large bombs – including, interestingly, chemical bombs.  A lot of British mustard bombs were moved there in 1955, shortly before Norton Disney closed in 1958.  In 1998, the year after the Chemical Weapons Convention entered into force, this large dump of mustard gas bombs was uncovered and slated for destruction. 

There’s not much left at RAF Norton Disney these days – a few concrete walls, and some odd, rusty metal trash.  It’s just another of the many derelict places left over from a bygone era.

Figure 6 - RAF Norton Disney as it looks today

But there's still a little more.  According to the Michelin Guide for Disneyland Paris, Walt Disney's great-grandfather, Arundel Elias Disney of Ireland, was descended from one Robert D'Isigny, a companion of William the Conqueror who went north with the duke in 1066.  D'Isigny settled in Lincolnshire, in a town called Norton, which later came to be known as Norton Disney, and which - nine hundred years later - was the origin of Walt's surname.

So when all is said and done, there just might be a connection, however remote, between Walt Disney and the Disney Bomb.  It's not surprising, really; after all, Disney was connected to a lot of folks, at least one of whom had some small knowledge of rocketry, as well as connection to places like the V1 launch site at Eperlecque.

For example, this fellow: a German ex-pat with a promising future who knew a thing or two about rockets, named Werner von Braun.

And th-th-th-that's all, folks!



P.S. Yes, I know Porky Pig is a Warner Brothers character.  I just couldn't resist it.