Ballistics of British High Capacity (HC) bombs

[33k in the air]

Hey all,

Does anyone have or know of any reports or studies comparing the ballistics of the British HC bombs to their GP and MC counterparts?

I ask because the shape of the HC bombs was not particularly aerodynamic and thus it would seem to me more difficult to aim accurately. The 4,000 lb and 8,000 lb HC were flat-ended cylinders with no tail fins of any kind, while the 2,000 lb HC was a long cylinder that had a tail consisting of a cylinder the same diameter as the bomb body which had large slot openings cut into it. The 12,000 lb HC bomb was a blunt-nosed cylinder that at least had a conventionally shaped tail fin assembly.

It would be interesting to see the 4,000 lb HC compared to the 4,000 lb GP and 4,000 lb MC bombs as these latter two had a conventional streamlined bomb shape with tail fins. A comparison of the 2,000 lb HC to the 1,900 lb GP would likewise be interesting.

I presume the aimability of the 12,000 lb HC bomb was not too different from that of the 12,000 lb DP/MC "Tallboy" given the former's more conventional shape.

 

[EwenS]

Hey all,

Does anyone have or know of any reports or studies comparing the ballistics of the British HC bombs to their GP and MC counterparts?

I ask because the shape of the HC bombs was not particularly aerodynamic and thus it would seem to me more difficult to aim accurately. The 4,000 lb and 8,000 lb HC were flat-ended cylinders with no tail fins of any kind, while the 2,000 lb HC was a long cylinder that had a tail consisting of a cylinder the same diameter as the bomb body which had large slot openings cut into it. The 12,000 lb HC bomb was a blunt-nosed cylinder that at least had a conventionally shaped tail fin assembly.

It would be interesting to see the 4,000 lb HC compared to the 4,000 lb GP and 4,000 lb MC bombs as these latter two had a conventional streamlined bomb shape with tail fins. A comparison of the 2,000 lb HC to the 1,900 lb GP would likewise be interesting.

I presume the aimability of the 12,000 lb HC bomb was not too different from that of the 12,000 lb DP/MC "Tallboy" given the former's more conventional shape.

Not true. Both the 4,000lb and 8,000lb bombs were fitted with a cylindrical tail. See below (which had links to official bomb manuals)

Bomb, 4000 lb HC, Mk 1

Description The body is a cylindrical shell which has a hollow, conical nose piece welded to it, parallel sides, and a closing plate bolted to an angle ring. The angle ring is

www.bulletpicker.com

Bomb, 4000 lb HC, Mk 2, Mk 3, Mk 4, Mk 5, Mk 6

Description The body consists of a fabricated steel, cylindrical shell with a domed plate welded to the nose end, parallel sides, and a closing plate bolted to the angle ring,

www.bulletpicker.com

Bomb, 8000 lb HC, Mk 1, Mk 2

Description The bomb consists of two cylindrical sections, each approximately 4 ft. long and 3 ft. 2 in. in diameter, connected together by bolts. The front section has a dome

www.bulletpicker.com

The 12,000lb HC and the Tallboy were completely different designs of weapon. Info at same source as above.

 

[Engineman]

Hey all,

Does anyone have or know of any reports or studies comparing the ballistics of the British HC bombs to their GP and MC counterparts?

I presume the aimability of the 12,000 lb HC bomb was not too different from that of the 12,000 lb DP/MC "Tallboy" given the former's more conventional shape.

Unfortunately I do not have any info on the specifics of aiming these bombs, i.e, the settings and targeting data that were used to aim the specific weapons. However, be assured that free-fall weapons of different size, aerodynamics and Mass will all vary in their flight path. That said, different size, shape and weight bombs can be made to approximate the flightpath of other bombs, generally by aerodynamic modifications. Taking for example more recent weapons, the UK 1000lb bomb could be imitated in freefall with a 28lb practice bomb and the same 1000lb bomb with a high drag retarder for low-level retarded delivery could be imitated with a 3kg hi-drag plastic case practice bomb. So, bomb flight characteristics could be modified, but it is unclear how much these aspects were incorporated in WW2 bombs and aiming tactics. IMO, The streamlined Tallboy and Grandslam bombs will have had big differences to the blunt 4/8/12 thousand pound MC bombs. Also, I believe there is film of the big HC bombs dropping together with the very lightweight incendiary firebombs and it would be interesting to know if there was any correlation between the expected fall of that combination?!

Eng

 

[EwenS]

Unfortunately I do not have any info on the specifics of aiming these bombs, i.e, the settings and targeting data that were used to aim the specific weapons. However, be assured that free-fall weapons of different size, aerodynamics and Mass will all vary in their flight path. That said, different size, shape and weight bombs can be made to approximate the flightpath of other bombs, generally by aerodynamic modifications. Taking for example more recent weapons, the UK 1000lb bomb could be imitated in freefall with a 28lb practice bomb and the same 1000lb bomb with a high drag retarder for low-level retarded delivery could be imitated with a 3kg hi-drag plastic case practice bomb. So, bomb flight characteristics could be modified, but it is unclear how much these aspects were incorporated in WW2 bombs and aiming tactics. IMO, The streamlined Tallboy and Grandslam bombs will have had big differences to the blunt 4/8/12 thousand pound MC bombs. Also, I believe there is film of the big HC bombs dropping together with the very lightweight incendiary firebombs and it would be interesting to know if there was any correlation between the expected fall of that combination?!

Eng

This is one photo showing the dropping of a 4,000lb "Cookie" and incendiaries.

A 4,000-LB "COOKIE" AND A LOAD OF INCENDIARIES DROPPING FROM A LANCASTER OF BOMBER COMMAND DURING ...

www.awm.gov.au

The idea behind such a load was that the Cookie, being a blast weapon, would open up buildings with the incendiaries then landing within them rather than just on the roofs, so maximising fire damage. But with a mass raid by many aircraft, there would be less need for each aircraft load to land together to achieve the desired effect.

Back in 2010 Channel 4 TV in the UK broadcast a four part series presented by Tony Robinson of Time Team fame, titled "Blitz Street". In it were demonstrated, using live explosives, the effects of typical air dropped bombs of the WW2 period on a typical British city street. All the way from incendiaries to V2 warheads. The series is now available on You Tube.


View: https://m.youtube.com/watch?v=L0v2Uq8z7aI

View: https://m.youtube.com/watch?v=IZ963jJBO8o

View: https://m.youtube.com/watch?v=gZjoKpGLoZs

View: https://m.youtube.com/watch?v=PrOCuNezj28

Watch out for the milk bottle in one episode left undamaged by the Bomb blast while much around it suffered!

 

[z42]

The Tallboy and Grandslam additionally had angled fins causing the projectile to spin, that might cause some small change in the average impact point, although the main function would be to reduce the SD.

 

[33k in the air]

Not true. Both the 4,000lb and 8,000lb bombs were fitted with a cylindrical tail. See below (which had links to official bomb manuals)

Bomb, 4000 lb HC, Mk 1

Description The body is a cylindrical shell which has a hollow, conical nose piece welded to it, parallel sides, and a closing plate bolted to an angle ring. The angle ring is

www.bulletpicker.com

Bomb, 4000 lb HC, Mk 2, Mk 3, Mk 4, Mk 5, Mk 6

Description The body consists of a fabricated steel, cylindrical shell with a domed plate welded to the nose end, parallel sides, and a closing plate bolted to the angle ring,

www.bulletpicker.com

Bomb, 8000 lb HC, Mk 1, Mk 2

Description The bomb consists of two cylindrical sections, each approximately 4 ft. long and 3 ft. 2 in. in diameter, connected together by bolts. The front section has a dome

www.bulletpicker.com

In other words, similar to the 2,000 lb HC, so my question still stands. Did such non-conventional tails provide much in the way of aiming ability as compared to the more traditionally shaped 4,000 lb GP and MC bombs?

Harris' Despatch on War Operations 1942-1945 has this passage regarding the 4,000 lb MC bomb which prompted my question (emphasis added):

The bombs were originally intended for low level bombing, a type of operation not normally undertaken by Bomber Command at this period. Small numbers of these bombs were dropped on city targets from high altitude, but accurate photographic assessment of the results was never possible, and, as 4,000-lb HC bombs were in good supply, the use of MC bombs against this type of target did not appear to be justified. During the autumn of 1944, 4,000-lb MC bombs were used by Mosquito aircraft for high altitude night bombing, as the ballistics of these weapons are much superior to those of the 4,000-lb HC bomb; it was also used with success to breach the dykes on Walcheren Island. Partly because only one could be carried by Lancaster aircraft and none by Halifaxes, however, it never became a widely used store in Bomber Command.

"Much superior" ballistics of the MC over the HC according to Harris. It'd be nice to see that quantified in some manner.

 

[33k in the air]

If anyone has access to the archives, I can point you to some reports. The publication AIR 41-81 SD 719 Armament — Volume I — Bombs and Bombing Equipment has this to say about the 2,000 lb HC:

A drum tail was designed during the early months of 1942 by the Armament Department of A. & A.E.E., and ballistic trials gave satisfactory results. 3

The '3' footnote refers to this: A. & A.E.E. Report A.7.10.G.28, June 28, 1942.

There are likely other similar report references for HC bombs in the AIR publication. If I come across any, I'll add them to this thread.

 

[Frank Stewart]

Hey all,

Does anyone have or know of any reports or studies comparing the ballistics of the British HC bombs to their GP and MC counterparts?

I ask because the shape of the HC bombs was not particularly aerodynamic and thus it would seem to me more difficult to aim accurately. The 4,000 lb and 8,000 lb HC were flat-ended cylinders with no tail fins of any kind, while the 2,000 lb HC was a long cylinder that had a tail consisting of a cylinder the same diameter as the bomb body which had large slot openings cut into it. The 12,000 lb HC bomb was a blunt-nosed cylinder that at least had a conventionally shaped tail fin assembly.

It would be interesting to see the 4,000 lb HC compared to the 4,000 lb GP and 4,000 lb MC bombs as these latter two had a conventional streamlined bomb shape with tail fins. A comparison of the 2,000 lb HC to the 1,900 lb GP would likewise be interesting.

I presume the aimability of the 12,000 lb HC bomb was not too different from that of the 12,000 lb DP/MC "Tallboy" given the former's more conventional shape.

Most of the "High Capacity" bombs had slightly domed front ends and flat sheet steel back ends with flat sheet steel rolled to form a cylinder. These bombs did not make any significant fragments and thus their only effect was "Blast" against soft targets. Their bombing accuracy was abysmal, even from the low altitudes they were mostly used. CEP = 1/2 mile from 10,000'! The higher they went, the more exaggerated the dispersion, IE almost one mile from 15,000'!
The tallboy/grand slam streamlined bombs had good aerodynamic properties, but the Lancaster's dropping latches/mechanisms were very un-reliable, and the scatter was huge! Both long and short and side to side! Look at the map of bomb impacts from the Tirpitz raids! One hit and two near misses, IIRC, out of over thirty drops??? Some 1-1/2 miles away from the target from 17-19,000'???
Late in the war, we rigged a couple of B-29s to carry two Tallboys on inboard wing pylons, or one Grand slam/T12? Cloud maker with cut away bomb bay doors. Bombing accuracy Varied from less than 100' or so to over 500' CEP from 20-30K'. After the war, we installed racks for all three types of bombs in the B-36 and dropped two T-12?s from 30K' and 40K' or three to four Grand slams, or Tall boys per mission. Minimum bombing altitude was over 30K' and maximum was over 45K' for the last Tall boy! Accuracy ranged from under 100' to under 600' CEP!

 

[Frank Stewart]

In other words, similar to the 2,000 lb HC, so my question still stands. Did such non-conventional tails provide much in the way of aiming ability as compared to the more traditionally shaped 4,000 lb GP and MC bombs?

Harris' Despatch on War Operations 1942-1945 has this passage regarding the 4,000 lb MC bomb which prompted my question (emphasis added):



"Much superior" ballistics of the MC over the HC according to Harris. It'd be nice to see that quantified in some manner.

The RAF's post war report on bombing effects stated that typical accuracy was 50% inside the Target aria! The "Target Area" being the entire city!

 

[WATU]

I am travelling so don't have access to any hard information. As regards "aimability" the stability moving through the air was one key factor (hence the nose and tail additions) as was good knowledge of the terminal velocity. Considerable time had to be invested (by RAE?) in calculating the ballistics of each bomb type. Much smaller practice bombs could be used in trials and the performance related to other bomb sizes/types if the ballistics were known for both.

 

[Engineman]

I expect that much of the RAF Bomb Aiming and Release Trials would have been done by the A&AEE based at Boscombe Down from Sep '39. We have read so far that there does seem to have been a big spread on the fall of the blunt 4/8/12000lb HC bombs. However, given their widespread use, there must have been a considerable amount of policy on use and aiming these weapons. Furthermore, the large blast effect was employed on specific factory targets and that would require fairly accurate aiming. So, I hope someone knows more of the details? Please tell us!

Eng

 

[EwenS]

Most of the "High Capacity" bombs had slightly domed front ends and flat sheet steel back ends with flat sheet steel rolled to form a cylinder. These bombs did not make any significant fragments and thus their only effect was "Blast" against soft targets. Their bombing accuracy was abysmal, even from the low altitudes they were mostly used. CEP = 1/2 mile from 10,000'! The higher they went, the more exaggerated the dispersion, IE almost one mile from 15,000'!
The tallboy/grand slam streamlined bombs had good aerodynamic properties, but the Lancaster's dropping latches/mechanisms were very un-reliable, and the scatter was huge!

Define "very un-reliable"?

Tallboy & Grand Slam were supported in the bomb-bay by a ML sling made up of a series of interconnected links. The top ends were fixed to the bomb bay roof. The other ends came together in a heavy duty electro-magnetic release unit under the bomb. There were some late releases reported and this unit was investigated but no specific faults were found. It can't have been too much of a problem or it would have been pursued further or changed. There was a manual release cable that ran alongside the electric release wiring.

Incidentally, the USAAF adopted that equipment for dropping the atomic bombs from the B-29s

As for "scatter was huge!", you are aware that the 617 squadron did not approach a target singly, line up and drop their bombs? Their approach to the target was in a "gaggle formation". So the aircraft were spaced laterally, longitudinally & vertically to minimise collisions while allowing them to cross the target in the shortest space of time. In some cases that could be in the order of 1 minute. So some spacing of the bombs was inevitable. 9 squadron adopted a line astern attack for some of its missions otherwise it also used the gaggle formation.

The two squadrons were also using different bomb sights. 617 used SABS while 9 used the Mk.XIV. And there was rivalry between the squadrons as to which wwas the more accurate!

But these bomb weren't designed to cause their effect by a direct hit. A near miss was intended to be adequate by exploding underground and creating a "camouflet", a large hole that would undermine the foundations of the target. One of the best examples of this were the Wizernes V2 launch site where the concrete dome of the site was not hit, but near misses destroyed the foundations in front of it. Another was the Bielefield Viaduct where some of the arches of the viaduct collapsed into the Grand Slam crater.

Camouflet - Wikipedia

en.wikipedia.org

In actual use however it came to be used to penetrate large concrete structures like U boat pens etc and armoured targets like warships.

Both long and short and side to side! Look at the map of bomb impacts from the Tirpitz raids! One hit and two near misses, IIRC, out of over thirty drops??? Some 1-1/2 miles away from the target from 17-19,000'???

The experience from WW2 was that using a level bomber, 4 engined or not, to attempt to hit a ship, stationary or not, and regardless of the size of bomb used, was not as easy as you might think. Near misses, especially from large weapons as Tallboy, could add significantly to the damage caused.

Firstly lets look at the Tirpitz dimensions. Length overall 823 feet. Beam 118 feet. Then consider how high you would be dropping from. 14-18,000 feet. It only takes very slight distractions from weather, smoke, AA fire to cause a miss.

There were 3 Tallboy raids against the Tirpitz.

Operation Paravane 15 September 1944 from bases in USSR. 21 of the 27 aircraft carried Tallboys but only 15 were dropped and another was a hangup that after 3 runs dropped 4 miles away. There were to be 4 waves of 5 Tallboy carriers with each wave flying as a gaggle on slightly converging courses, and aiming to put their bombs into a 750 yard square box with Tirpitz at the centre. The attack was along the length of the Tirpitz. Bombing was from 14-18,000ft. The target was gradually covered in smoke meaning that 3 aircraft followed orders and, not being able to see the target, took their bombs home. The rest were bombing into the smoke using mastheads & gun flashes for aiming points. The flak from ships and shore was described as intense but mostly burst below the bombers, but still some aircraft suffered damage. As a result the bombing was not very accurate and only a single hit on the bow and several near misses were achieved.

However the damage finished Tirpitz as a fighting vessel. As a result she was moved to a position near Tromso, which was within range of bases in Britain if the Lancasters were modified to maximise their range, to act a floating gun battery. Unfortunately all the defences she had in Kaa Fjord (smoke generators, flak batteries) were not moved with her.

Operation Obviate 29 Oct 1944. 20 aircraft from 9 Sdn & 19 from 617 Sqn. Each squadron was to compress its attack into a one minute time window spaced 5 minutes apart. Bombing heights were supposed to be between 13-16,000ft. Unfortunately the weather changed just as the raid went in, bringing with it a changed wind direction and cloud to interfere with sighting the target. Again bombs had to be aimed at gun flashes. 6 took their bombs home in accordance with orders.

Operation Catechism on 12 November 1944. 31 aircraft + a film unit Lancaster were dispatched. 2 did not bomb. Clear weather encountered for the first time. Attack altitude was c16,000 feet. 617 Sqn bombing was descibed as "very concentrated". That of 9 Sqn who were following was affected by smoke from the hits already obtained and the last few fell well away from the target. The attack lasted 3 minutes.

Just by way of comparison, on 28th July 1945, 11 B-24 flying from Okinawa attacked the Japanese carrier Amagi from high altitude. She was moored close to shore near the Kure IJN base. This is what her TROM from the Nihon Kaigun site has to say:-

"- 1200 Eleven B-24s carry out level bombing of AMAGI and KATSURAGI from high altitude. Though none strike directly near-misses land close alongside to port, detonating below the waterline, whose fragments riddled the port shell in places."

Katsuragi was undamaged by this attack.

Late in the war, we rigged a couple of B-29s to carry two Tallboys on inboard wing pylons, or one Grand slam/T12? Cloud maker with cut away bomb bay doors. Bombing accuracy Varied from less than 100' or so to over 500' CEP from 20-30K'. After the war, we installed racks for all three types of bombs in the B-36 and dropped two T-12?s from 30K' and 40K' or three to four Grand slams, or Tall boys per mission. Minimum bombing altitude was over 30K' and maximum was over 45K' for the last Tall boy! Accuracy ranged from under 100' to under 600' CEP!

The first USAAF Tallboy tests were authorised in Feb 1945 at which point a single aircraft 42-63577 was being modified by Bell to carry a single Tallboy in a modified bomb bay with cut-away doors. By the time the final report was produced on 15 June 1945 it was noted that the AAF Proving Ground was testing an aircraft modified to carry both Tallboy & Grand Slam. "should this new installation prove to be functionally reliable, it is believed that this installation would be more desirable than the B-29 modified to carry only the 12,000lb Tall Boy bomb". 3xB-29A modified to carry a single Tallboy or Grand Slam (45-21747, 45-21750, 45-21751) were sent to Britain in March 1946 to participate in Projects Ruby and Harken.

T10 & T14 were the US designations for Tallboy & Grand Slam respectively. The T12 Cloudmaker was a 43,600lb weapon meant to be dropped by the B-36.

The underwing bomb racks for the B-29 were, AIUI, developed by Boeing to carry 4x4,000lb each but were also capable of carrying a Grand Slam. The photos of a test B-29 carrying two Grand Slams dates to the post war period appear on the internet quite frequently. What is unknown is what weight of fuel was being carried by this test aircraft.

Starting in Feb 1945 the VB-13 Tarzn was developed (basically a Tallboy with an aerodynamic shroud, a new tail section and radio guidance in both range & azimuth). Again a single weapon was carried in the modified bomb bay of the B-29 carrier aircraft. The 19th BG began operations in Korea between Dec 1950 & March 1951 with at least 3 B-29 Tarzon carriers. Of the first 10 drops only 1 hit was achieved and it didn't seem to get any better, hence its early withdrawal from service.


So the question really is with these big bombs, are they inaccurate weapons in themselves or are there a multitude of factors related to dropping such weapons from high altitudes that make them next to useless if the objective is to hit a very small target?

 

[EwenS]

I expect that much of the RAF Bomb Aiming and Release Trials would have been done by the A&AEE based at Boscombe Down from Sep '39. We have read so far that there does seem to have been a big spread on the fall of the blunt 4/8/12000lb HC bombs. However, given their widespread use, there must have been a considerable amount of policy on use and aiming these weapons. Furthermore, the large blast effect was employed on specific factory targets and that would require fairly accurate aiming. So, I hope someone knows more of the details? Please tell us!

Eng

The Lancaster Archive details a fair number of the bomb loads carried by that aircraft. Note how each was designed to deal with a particular type of target.

You will note that for attacking factories etc the bomb load was codenamed "ABNORMAL" and consisted of 14x1,000lb MC or GP bombs.

Bomb Loads

Lancaster Archive

www.lancaster-archive.com

4,000lb Cookies were used in conjunction with loads containing a lot of incendiaries. IIRC the RAF found that using a Cookie with its large blast effect was relatively poor at destroying machinery in a factory. It would often blow a factory building apart but leave the heavy indutrial machinery intact. The rubble would be cleared away, a temporary roof fitted and production could restart. It was very good at opening up residential / office buildings to let incendiaries inside to really cause damage. It was also used in loads designed for the NOBALL V1 sites.

 

[wuzak]

The RAF's post war report on bombing effects stated that typical accuracy was 50% inside the Target aria! The "Target Area" being the entire city!

Since most bombing raids by the RAF were aimed at Target Indicators dropped by the Pathfinder Force, it seems unlikely that the target area of any raid was "the entire city".

It must also be noted that the RAF didn't jus bomb cities. They also bombed industrial targets, transport targets and oil facilities.

On oil targets RAF bombing was more effective than the USAAF, mainly because of the larger bombs used. Chief among them was the 4,000lb HC bomb.

 

[33k in the air]

Another ballistics study on the 2,000 lb HC referenced in AIR 41-81 SD 719 — Armament — Volume I — Bombs and Bombing Equipment. The relevant passage:

The ballistic trials were completed in November when four bombs were released singly from a Halifax, each simultaneously with a 250 lb G.P. bomb, from approximately 5,000 feet.[2]

Footnote [2] refers to this: A. & A.E.E. Report A.T.O. G.28, Dec. 6, 1942.

 

[33k in the air]

The Lancaster Archive details a fair number of the bomb loads carried by that aircraft. Note how each was designed to deal with a particular type of target.

You will note that for attacking factories etc the bomb load was codenamed "ABNORMAL" and consisted of 14x1,000lb MC or GP bombs.

Bomb Loads

Lancaster Archive

www.lancaster-archive.com

Bomb loads were far more varied than is shown there. Nor do those graphics capture the progressive increase in ordnance carried -- later in the war the Lancaster carried up to 20 x 500 lbs bombs, quite a bit more than the 14 x 500 lb shown. Another load frequently seen later was 1 x 4,000 lb and 16 x 500 lb bombs.

Check out this post to find an Excel file listing the bomb loads for several RAF squadrons as recorded in the squadron ORBs.

 

[Engineman]

The Lancaster Archive details a fair number of the bomb loads carried by that aircraft. Note how each was designed to deal with a particular type of target.

You will note that for attacking factories etc the bomb load was codenamed "ABNORMAL" and consisted of 14x1,000lb MC or GP bombs.

Bomb Loads

Lancaster Archive

www.lancaster-archive.com

4,000lb Cookies were used in conjunction with loads containing a lot of incendiaries. IIRC the RAF found that using a Cookie with its large blast effect was relatively poor at destroying machinery in a factory. It would often blow a factory building apart but leave the heavy indutrial machinery intact. The rubble would be cleared away, a temporary roof fitted and production could restart. It was very good at opening up residential / office buildings to let incendiaries inside to really cause damage. It was also used in loads designed for the NOBALL V1 sites.

Yes, Thanks. We know that the thin case "blast bombs" were not totally effective against hard targets like reinforced concrete emplacements and some very heavy industrial machinery. That is Weapons Effect Planning. What I am asking about is the aiming of the HC bombs. There must have been laid-down aiming regulations for all these different loads and I am interested in that. Also, when referring to bombing TI's, that is similar to visual aiming. There must have been strict targeting instructions. Beyond that, I am pretty certain that A&AEE will have done loads of work on this. Must be someones specialist subject?

Eng

 

[33k in the air]

Most of the "High Capacity" bombs had slightly domed front ends and flat sheet steel back ends with flat sheet steel rolled to form a cylinder. These bombs did not make any significant fragments and thus their only effect was "Blast" against soft targets. Their bombing accuracy was abysmal, even from the low altitudes they were mostly used. CEP = 1/2 mile from 10,000'! The higher they went, the more exaggerated the dispersion, IE almost one mile from 15,000'!

I suspect the seemingly low ballistic accuracy of the HC bombs was acceptable since they were blast bombs mostly used in conjunction with the 4 lb (and 30 lb) incendiary --- and the 4 lb was not an accurate device as its release from the quick-opening small bomb container saw it scattered widely.

U.S. testing of its quick-opening AN-M7 cluster (128 x 4 lb) from 10,000 feet saw 90% of the bombs falling in a race-track shaped area 700 ft x 2,450 ft in size. That worked out to one bomb every 14,000 sq ft --- hardly the stuff of high accuracy and concentration.

The solution was the aimable cluster (cluster projectile in British parlance). These fell as a complete until reaching a predetermined altitude -- typically 5,000 feet -- at which point it broke open and released its contents. They were shaped like and could be aimed like regular bombs. The result was much better accuracy and concentration.

The AN-M17 aimable cluster (110 x 4 lb) dropped from 20,000 feet and opening at 5,000 feet saw 90% of its bombs falling in an area 420 ft x 630 ft. That was one bomb every 2,300 sq ft, a result six times better than that of the quick-opening AN-M7.

The use of such aimable clusters began in 1944. Some 26% of Bomber Command's 4 lb incendiaries dropped in 1944 were in aimable clusters/cluster projectiles; this increased to 49% in 1945.