Effectiveness of Bombs

[wuzak]

In May 1943 the Armament Research Department released a report into tests done with different explosives in the 4000lb HC Cookie, the 1000lb MC bomb, and with different exploders systems in the 500lb MC bomb.

In tests with the 4000lb HC bomb it was calculated that the mean "maximum radius for total demolition" was 126ft. This result was from discarding "outliers" - two results were ignored because they were low - one of which had 200lb less explosive filling. The other result was discarded because it was much higher - 155ft. The explosive in that case was a 60/40 mix of RDX/TNT.

Of the other 11 tested, 7 were 60/40 Amatol, two of which were "fine grain", from various manufacturers. Two were 50/50 Amatol and two were Amatex 9. It was cocluded that the fine grain Amatol gave no advantage, and that occasional low yields could be expected.

Also calculated was the "maximum radius for visible damage". The mean for the tests, excluding the two high and one low results, was 259ft. For RDX/TNT it was 325ft.

For the MC bombs of 1000lb and 500lb only the "maximum radius for visible damage" was calculated, since the calculation for "maximum radius for total demolition" was considered unreliable at those ranges.

For the 1000lb MC bomb (equivalent to US 1000lb GP bomb, ~50% charge by weight) 4 tests were done with 60/40 Amatol, 2 with 60/40 Amatol fine grain, 2 with Amatex 9 and 2 with 60/40 RDX/TNT. Two of the 60/40 Amatol results were excluded because they were low, while the RDX/TNT results were excluded because they were higher. The average of the remainder was 70ft.

All the 500lb MC bombs (equivalent to US 500lb GP bomb, ~50% charge by weight) 6 tests were conducted, all with 60/40 Amatol, divided equally between two methods of igniting.

It was found that the method of igniting mattered little in the 500lb bomb, an average of 40ft for one method and 39ft for the other.

In terms of "maximm radius for visible damage" the 1000lb MC bomb was 27% as effective as the 4000lb HC bomb, and the 500lb MC was 15% as effective.

In terms of area, the 1000lb MC is 7.3% as effective and the 500lb MC 2.3% as effective. In terms of volume the numbers are 2% and 0.4% respectively.

In terms of filling weight, the 100lb bomb has approximately 16% and the 500lb bomb approximately 7% of the explosives as the 4000lb HC bomb. This is very approximate as the weight of explosive varied with the type.

Note that the RAF still used their GP bombs well into the war. These had a lower charge % of around 25-30%, so would have been far less effective than the same size in an MC bomb.

 

[Kryten]

GP bombs are designed to cause large numbers of splinters however, the thicker the casing therefore the lighter the bursting charge the more and larger splinters produced, the HC bombs were basically demolition weapons, so I suppose it's a case of what your dropping them on?

 

[wuzak]

Yes, the intended target would determine which type of bomb would be used.

Consideration as to whether the bomb would penetrate structures to do required damage to the contents inside (like machine tools in a factory) or if merely blowing the roof off is required.

In the case of the RAF they used mainly 500lb and 1000lb MC and GP bombs (British GP bombs were older and less effective).They also used 4000lb bombs of both HC and MC types. They didn't have a 2000lb MC bomb, and didn't much use the pre-war 1900lb GP bomb.

To my mind the splinters are most effective against soft targets - ie humans. For industrial targets I woudl think the blast bomb just as effective, if not more so, than the GP/MC types.

109 Squadron Operational Record Books show that in 3 raids on the synthetic oil plant in late June/Early July 1944 different bomb loads were carried. The first raid (25/26 June) saw a load of 4000lb MC bombs (for those not carrying TIs). The second raid (30 June/1 July) was with 4000lb HC Cookies (when noted and not carrying TIs). The third on 1/2 July was with 6 x 500lb MC bombs.

139 Squadron notes in their ORB for July 4/5 that they took part in raids against oil refineries at Scholven, and that they were part of a total force of 24 Mosquitoes, 12 of which carried 4000lb bombs.

They attacked Scholven again the next night. The total force was 34, 28 of which carried 4000lb bombs. And again on 6/7 July, with 31 of 34 carrying 4000lb bombs.

Although not stated, I believe all these were 4000lb HC bombs.

The conclusion I would draw is that the 4000lb HC bomb was indeed very suitable for industrial targets.

 

[Kryten]

Didn't the loadout evolve to comprise a mix of HC and Incendiary?

I seem to recall that was considered the most effective for industrial targets?

 

[wuzak]

Didn't the loadout evolve to comprise a mix of HC and Incendiary?

I seem to recall that was considered the most effective for industrial targets?

You could be right on that, though I have no evidence either way.

I was wondering if incendiaries, or even napalm, would be effective in damaging heavy machine tools in ways that HE could not.

 

[Shortround6]

Fire would have a pretty good impact. It would be a bit unpredictable but fire is certainly can destroy most/all electrical wiring and motors. depending on heat and duration it can affect the hardness of the sliding surfaces (and gears) and distort/warp the frame work of the machine themselves. I don't know about war time but in peace time it wasn't uncommon to allow cast frames for large machine tools to 'age' for a number of months before machining/finishing the sliding surfaces (ways) to insure long term accuracy.
In some cases all that might be needed to 'repair' a large machine tool might be a new motor and wiring, and to scrap/true up the ways.

ways are the polished/ground surfaces. Trouble is that in any one factory there might be fewer than a dozen men who can really do that kind of work and if there are dozen of machines to repair? Faster than making new ones but but it is going to take time.

 

[stona]

During early 1943 much work was undertaken by various researchers into the effectiveness of various bombs and bomb loads. The Central Interpretation Unit of Bomber Command published a report suggesting that the 30lb incendiary was more effective than the 4lb incendiary. This was disputed by the ORS and Dickins told Saundby that there was little to choose between the two but added that

"if anything positive emerges at all, it favours the 4lb bomb and in view of the fact that a small bomb container carries a 50% greater weight of 4lb bombs than the 30 lb bombs, there is little doubt that the former is preferred."

The ORS was now to undertake much research, the results of which, published in August 1943, would influence Bomber Command loads for the rest of the war.
Acre for acre destroyed, incendiaries were noticeably more efficient than high explosive bombs.
The 8000lb HC bomb could destroy 1.68 acres per ton. The 4000lb HC and both the 4 and 30lb incendiaries could destroy 3.2 acres per ton. Least effective was the 1000lb GP bomb only destroyed 0.56 acres per ton.
The effectiveness of incendiaries was dependent on three factors. The target must be relatively intact, bombing must be concentrated and there must be enough high explosives to be a threat to fire fighters and anti aircraft gunners.
When all this was factored in the best loads for each aircraft were calculated, this for an area raid.

For the Lancaster: 1 x 4000lb HC plus 17 1/3 SBCs of incendiaries. This gave a load of 10,000lbs and a corresponding 'damage factor' of 9.98 acres. This was less bomb weight than the 6 x 2000lb GP bombs the aircraft could carry, but this load gave a damage factor of only 6.25 acres.

For the Halifax the differences were less obvious. The best load was 15 SBCs, 5490 lbs and with a damage factor of 6.30 acres. The need for some high explosive meant that a preferred load was 3 x 1000lb MC bombs plus 12 SBCs for a total load of 7449 lbs and a damage factor of 6.12 acres.

The Stirling was seen simply as an incendiary carrier.

Fortunately all this research and calculation broadly supported the rule of thumb adopted earlier, that a roughly 70% incendiary load was right for devastating Germany's cities. The ORS was keen to point out that as more and more cities targetted would already have been damaged by fire this optimum percentage of incendiaries might change.

It is also not difficult to see from the figures above why Harris was so keen on the Lancaster and not the Halifax. Acreage destroyed was his prime objective and the Lancaster did it significantly better.

Cheers

Steve

 

[gjs238]

How did this compare to the loads used for B-29 "fire bombing" of Japan?

 

[stona]

I don't know but as the US Air Forces in Europe bombed more and more through cloud and in poor conditions, using aids such as H2X, the percentages of incendiaries in their loads crept up. They were not precision bombing at all but carrying out what the 8th AF euphemistically called 'area type raids'.
Cheers
Steve

 

[GrauGeist]

How did this compare to the loads used for B-29 "fire bombing" of Japan?

The initial bombing raids were using a conventional HE type load, dropped from high altitudes. However, because of the difficult air currents and cloud cover, they were not very effective. When they lowered their attack altitudes to improve accuracy, the B-29 groups were mauled by AA and fighters.

The decision to switch to incendiary was due to the fact that the industrial centers were crowded by residential structures that were very flammable. This would ensure that the military target would be damaged or destroyed and disrupt and demoralize the civilian work force. The incendiary attacks also mean they could operate at night as well as not needing a high level of accuracy to still successfully contact their target(s).

The bombs typically used, were the E-46 cluster-bomb which contained 36 M-69 napalm bomblets. The E-46 weighed 500 pounds (230kg). Also used to a lesser degree, was the M-47, which contained a blend of jelled gasoline and white phosphorous, weighing 100 pounds (45kg).