Purpose of this blog

Dmitry Yudo aka Overlord, jack of all trades
David Lister aka Listy, Freelancer and Volunteer

Sunday, November 21, 2021

Firestarter

 What I find interesting, and I’m not sure if I’ve said it before, is how Britain studied the effects of German bombing conducted during the Blitz on contemporary life and structures. They quickly discovered the Germans were dropping too small bombs. This meant that even when the Germans did land a stick of bombs in the right place, it had minimal effect and the infrastructure was soon back in operation. This was quickly seen as a worry, as the British bombing efforts were all but identical to the German ones. It was quite sensible to assume that their bombing efforts were as effective on the Axis as the Germans were against the UK. Thus, the British went away and worked out what would work, so that when they started bombing the Germans, they did it properly. As Bomber Harris said, ‘They sowed the wind, and now they are going to reap the whirlwind’. This was very literally true. This study work, of course, covered incendiary bombs. 

Testing of German 1kg incendiary bombs. Top picture is at 15 seconds after ignition, bottom is at 45 seconds.

‘Beat Firebomb Fitz’ was a famous poster from the Blitz and featured the German B1E 1kg incendiary bomb. It has become a slightly iconic poster of the time. The principle behind it was that a small detonating charge would trigger a thermite warhead filler. This in turn would cause the body of the bomb, which was made from magnesium, to catch fire. Magnesium is very difficult to extinguish, indeed using water on a magnesium fire is a very bad idea. But despite this, Britain did not burn… but Germany would. 

 

The main weapon for the RAF for starting fires was the humble 4-pounder incendiary bomb. Now despite what I said earlier, this was in service from the start of the war. It worked on an identical principle to the German weapon, containing an igniting element and a magnesium body. But today very little is written of it, especially when compared to ‘Firebomb Fritz’s’ reputation. I would suggest that this in part is down to looks. The German weapon looks like a bomb, whereas the British 4-pounder weapon is a hexagon some 21 inches long, and about 1.67inches wide, and barely looks like a piece of ordinance. 

German civil defence personnel holding a 4-pounder to give an idea of the size.
 

In August 1940 it was realised that these incendiary bombs were using a colossal amount of magnesium. So, a study was begun to find parts of the bomb that could be changed from that precious and scarce metal to much more common (and cheap) cast iron. Even a small saving would pay big rewards due to the large number of bombs being constructed. This simplified bomb became the Mk.III weapon (the Mk.II was just alterations to the internal arrangements). At the same time the supply issues meant the British started looking at a bomb that was 2in shorter than standard. This would save about 0.25lbs per bomb, with the new bomb using 0.75lbs of magnesium.

During 1941 two major factors happened, first the Bomb was to be produced in the US. Changes to the design to allow improved manufacturer became the Mk.IV bomb, which was subsequently mass produced. The other change was in how the bomb was tested. 

Colour shot of a surviving 4-pounder Mk.IV at the IWM.
 

In October the task of assessing how incendiary bombs would work was passed to the Road Research Laboratory. Their solution was to build a German style house roof, including attic floor. They managed to obtain copies of German house construction styles and copied it to the letter, even going so far as to obtain supplies of German roofing tiles (Bibeschwanz and Ludovici tiles, in case you’re wondering). The mechanics of incendiary bombs suggest that the ideal effect of a bomb is to penetrate the roof tiles, then the attic floor boards before igniting in the structure below. So, this was tested by the expedient of the RRL converting a 2-inch mortar to fire the bombs at the test target. Different impact velocities could be obtained due to an alterable gas check, made from brass, at the base of the mortar. To keep the hexagonal bomb straight in the round tube several guide lugs were added at the muzzle end. One point in these tests were that similar US tests, held at the Standard Oil Company, were getting very different results. There was an investigation, and despite the comparison identifying several different factors between the two test targets (such as tile overlap, rafter spacing and the like) and these being standardised to the British model the divergence of results continued.

In the middle of 1942, the shortened version of the 4-pounder bomb which had begun development in 1940 finally made it to testing. Those experiments showed that the incendiary effect was not lowered by the reduction in magnesium. In the middle of 1942, another suggestion was for the bomb design to have a spring-loaded pop-out tail, so when loaded into a bomber it would only be 8in long, with the tail deploying when the bomb was dropped. Both bombs were tested against the RRL’s German roof target, and one critical point was discovered. As the bombs were lighter their impact energy was lower. Thus in turn they failed to penetrate the target. This lack of effect doomed both projects, with them being cancelled in 1943.

So far, we’ve just focused on the 4-pounder’s incendiary effect. However, from before the war it was recognised that one way to improve the effectiveness of the bomb was to incorporate explosives with a time delay to hamper firefighting efforts. Thus in 1939, after a few months of development starting in 1938, ‘Type E’ bombs were produced. For those following such things the nomenclature was the ‘type’ of bomb would come after the Mk number, so for example ‘4-pounder, Mk.IV.E’. These incorporated a small gunpowder charge in the body of the bomb and would detonate between 1 minute and 56 seconds to 4 minutes after impact. It produced a fairly paltry explosive effect but was deemed acceptable.

The Germans did something very similar with some of their incendiary bombs, so the British learned and in December 1940 started looking at using a high explosive charge. For this weapon they added a canister at the rear of the tail with a small HE charge (later models would have a nose filled with explosive). These would be ‘Type X’. The tail-based container was also switchable to chemical weapons if needed, with the idea that this would prove much more effective at deterring fire fighters. What followed was a lot of bureaucratic backwards and forwards between the Ordnance Board and other departments within the government. The problems of supply of the explosive, and construction were causing a bit of a circular development, with one party changing the design to fix a flaw and causing follow on problems. This continued until 1941, when it was decided to see if such an addition had a useful effect. 

Lancaster loaded with a bomb-load codenamed 'Usual'. This consisted of a 4,000lb 'cookie'. The idea was to create massive blast that would damage the roofs of German cities. The damage would weaken the roof structure so that when the 4-pounders impacted they would have a easier time penetrating into the structures.
 

Thus, London Fire Brigade was consulted on the effectiveness of such bombs, as they had encountered them in the shape of the German weapons dropped in the Blitz. The effectiveness of the bombs against both personnel and the chance to break hoses was considered.

LFB* raised several objections to the weapon. The foremost being that during a heavy night raid the crews were so busy that the presence of such bombs would not affect how they worked. It was also pointed out that such bombs would be almost invisible in the blackout conditions. Equally it was advised that if they did start to suffer several casualties, orders would be issued to stay clear of any suspected weapons for a period of time to give it a chance to detonate. As to the idea of breaking hoses, the hoses were put through such rough treatment during the course of normal operations that a few additional incidences of damage would be hardly noticed. With this information in hand, it was recommended to the Air Ministry that the idea be dropped. However, the RAF stated the requirement remained. They had a strategy of having 50% Type X bombs, and 50% normal weapons per bomb load. This would be continued until German fire crews were warned to stay away from the bombs for a period. At which point the loads could be switched to nearly 100% normal incendiaries, in the hope the Germans would stand back allowing these to develop into decent fires. 

Loading 4-pounders into a bomb bay one at a time would have taken huge number of man hours. So the bombs were loaded into 'Small Bomb Containers'. Here we see three partition versions, but they also came in two and four versions. Each partition can take twenty 4-pounders. For more detail.
 

In February 1942 the Air Ministry suddenly demanded 2,000 Type X bombs by 1st of March for a planned operation. These were manufactured with a 4.5-minute delay on the explosive. On the 28th of March 1942 234 bombers attacked the Port of L├╝beck. 25,000 incendiaries were dropped and created the first Allied firestorm of the war. More were to follow, and most were powered by the humble looking grey hexagon.

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 Image credits:

www.berlinluftterror.com