Simply put, it shows the length of time it would have taken for an attacking bomber to fly from the coast to London (in blue) -- call it the crossing time -- and the time it would take taken for a defending fighter to climb high enough to intercept (in red) -- call it the intercept time. And how these changed over time, obviously. As can be seen, the fighters generally had enough time to climb high enough to intercept the bombers before they got to London, but the margin decreased over time, from 15 or so minutes during the First World War, to less than 5 in the Second.
But all this is not straightforward so I'll explain further. To begin with, the data is slightly dodgy. It's mostly drawn from the same source as this, which is fine as far as it goes. But that means that I'm showing how long it would have taken British bombers to penetrate from the coast to London, which was not really a great worry. Having said that, it's probably reasonable to assume that the performance of British bombers was roughly in line with those used by Continental air forces. (And the RAF's own air defence exercises had to make this assumption, too, because borrowing somebody else's air force for a day wasn't feasible.) One day I'll create a dataset for European aircraft ...
How are the numbers derived? First, the bombers (blue). This is just the distance from the coast to London divided by each bomber's maximum speed (which is not necessarily realistic). Why the coast? Because it was only when the incoming raiders crossed the coast that they could be detected by ground observers, and fighters dispatched to intercept them. 1 What is the distance from the coast to London? Well, obviously it varies, depending on which direction the enemy came from (and some writers expressed fears that they would fly up the Thames Estuary and avoid detection). Looking at a map, 50 miles seems like a reasonable approximation.
Next, the fighters (red). The time it takes for a fighter to climb meet the bombers is the height of the raid divided by the climb rate of each fighter. This climb rate is a bit of a problem. I don't a good source for this number and had to plunder Wikipedia. That's bad enough in itself, but it's worse because the data is inconsistent. Sometimes -- when it's not missing -- it's expressed in feet per minute, and sometimes in the number of minutes to reach a given height. Obviously one can be turned into the other, but actually both are only approximations, and I've had to extrapolate and interpolate from these to get a usable number. 2 What height would the bombers be at? Well, that varied -- it was higher on average during the Second World War than in the First because aircraft were more capable, and also because bombers tried to climb higher to escape the fighters. I've assumed that this height was 10000 ft in the 1910s, 15000 ft in the 1920s, 20000 ft in the 1930s, and 25000 ft in the 1940s. 3 I just plucked these numbers out of the air, more or less, but they seem to work well in terms of keeping the red and blue trends in touch with each other. If anything they are probably underestimates.
Some other points. Firstly, the fighters would generally have to move horizontally to intercept the bombers, as well as vertically. This plot says nothing about that. But given the edge fighters had in speed and the location of their aerodromes, they should be able to cover that distance while climbing. Secondly, the data points are for the year each aircraft entered into RAF service. But since they remained in service for several year, at least, the data points should really be horizontal lines. 4
Thirdly, I'm assuming a perfect command, control, communications and intelligence system. Fighter Command (and its predecessors) was good, but it still took a finite but non-zero amount of time for sightings to be reported, sifted, collated and reported, and then for squadrons to be allocated, given orders, and take off. Also there was a chance that raids might not be observed, that squadrons could be given the wrong vector, that the enemy could be missed in cloud -- so the greater the gap between the red data points and the blue ones the better. The more inefficient Fighter Command, the narrower the margin for error. 5
Now we can show what difference radar made. The Chain Home system came into operation in 1939 and had an effective range of 120 miles. What this means here is that instead of only having to cross 50 miles from the coast to London after being detected by the observers on the coast, the bombers now had to cross 170 miles after being detected. As the above plot shows, this pushed up the crossing time dramatically: from 1939, the defenders could generally expect to have around 40 minutes' warning of any raids. The margin for error increased dramatically, from only 5 minutes or less, to more than half an hour, which is far better. In theory, the defending fighter squadrons would now have plenty of time to get in position before the enemy arrived. Of course, that's not the whole battle, but it's a good start!
Lastly, here's a counterfactual which I've long wondered about. Between 1933 and 1935, the Air Ministry put a fair amount of effort into researching the feasibility of using acoustic mirrors as a comprehensive early warning system. The acoustic mirrors were, mostly, concrete hemispheric dishes for focusing sound, which had been used as early as 1916. The biggest ones, at Dungeness in Kent and Maghtab in Malta, were 200 feet long curved walls. Land was actually purchased along the Thames Estuary for the beginnings of a national acoustic mirror system, but work never started because radar came along. But if it hadn't, then in 1940 Fighter Command might have relied upon a network of these acoustic mirrors all along the coast. 6 How useful would they have been?
The experimental mirrors had a maximum detection range of 22 miles (on very windy days it was a lot less). I'll be generous and call it 25 miles, which is then added to the 50 miles from the coast to London for a total distance of 75 miles. The Thames Estuary acoustic mirrors probably would have come online in 1936, and so again I'll be generous, and assume that London at least would have a working early warning system from that year.
Taking all this into account, the results can be seen above. And sadly the acoustic mirrors wouldn't have made much difference -- a margin of only about 10 minutes, not much improved on the 5 minutes with no warning system. Of course, even a few minutes' extra warning was worth having, but the Air Ministry was right to terminate development of the acoustic mirror network in order to concentrate on the far more promising radar.
John Ferris has argued against the idea that 'Air defence in Britain began during 1934 and only because radar was developed', and that the importance of the C3I system -- ultimately a legacy of the First World War -- has been underestimated by historians: it was 'ideally preadapted to radar'. 7 And he's right. Even without effective early warning, as long as the enemy bombers could be intercepted and shot down on their way back home, air defence could still work by inflicting prohibitive casualties. Except, that is, when the casualties from bombing were predicted to be massive, and then a failure to stop the bomber getting through would have devastating consequences. Radar was part of the antidote to the fear of the knock-out blow. Or rather it could have been, if it hadn't remained secret until 1941 ...
(Just to repeat: the data and assumptions underlying these plots are on the dubious side, and are not fit for any purpose, probably including this one!)
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- I'm neglecting radar, obviously, but see below. I'm also neglecting the fact that sound detectors, of the type that had been developed during the First World War, had a range of about 5 miles. But see even further below. Distant patrol aircraft were also used as a kind of picket line.
- What I really need are curves showing climbing time vs. height because the higher an aeroplane flies, the harder it is to climb in the thin air. I assume these are available somewhere, but digging them up is too much work for a quick and dirty plot like this!
- Fighters got a lot better at climbing very rapidly by the late 1940s, but as that happens I'm shifting the goalposts ever higher, as it were, and so the above graph is understating the rate of climb of fighters.
- E.g., the two red triangles in the late 1930s are the Hurricane and Spitfire, which between them were the RAF's primary interceptors throughout the war. This plot makes it look like there wasn't anything able to catch raiders in 1940, which was not the case!
- I could model this inefficiency by adding a fixed number of minutes to the climb time of the fighters -- call it the response time -- but I don't know what a reasonable number is and it might vary a fair bit. For instance, in 1918 LADA (London Air Defence Area) had a response time of 2.5 to 5 minutes, according to John Ferris, Fighter defence before Fighter Command: the rise of strategic air defence in Great Britain, 1917-1934, Journal of Military History 63 (1999), 853 (JSTOR). But it presumably rose after LADA was dismantled after the war. David Zimmerman, Britain's Shield: Radar and the Defeat of the Luftwaffe (Stroud: Sutton, 2001), 25, seems to suggest that 5 minutes was the time it took in 1933 just to transmit observations to ADGB (Air Defence of Great Britain) HQ, but that's for the big acoustic mirrors which probably required more computation than normal acoustic detectors. So, pending more comprehensive figures, I'll just leave the response time out of it.
- See ibid., chapter 2, for more on the acoustic mirror research of the 1930s.
- Ferris, ibid., 845, 884.