Answer to an old question about WWII bombing.

Factual Questions
1

I can’t find the thread where this arose. Someone asked how the fact that the bomb fell behind the bomber because of drag was accounted for.

I was talking to a high school pal today and I happened to remember this question. He was bombardier so I asked him.

For each mission the bombardier entered a bunch constants into the bomb sight. Such things as altitude, airspeed, type of bomb, etc. The type of bomb was important because the lag, or trail, of the bomb behind the dropping aircraft was different for different bombs.

The bomb sight then computed how much the lag would be as a function of the altitude and type of bomb.

The sight also compensated for head-tail wind and cross wind at the bomber’s altitude. The optics of the sight were driven in a direction opposite the planes direction at an adjustable rate. You put the range cross hair on the target and then adjusted the rate until the cross hair tracked the target. At the same time the azimuth cross hair was lined up with the target and when it drifted off the bombsight was swiveled until the target tracked right down the crosshair. Swiveling the sight turned the plane via the autopilot so that by the time of release the bomsight had computed the correct release point for the wind, altitude and bomb aerodynamics.

All of this was done within 60-90 seconds.

So if your were the one that asked the question, here is your answer.

2

A related question: how accurate was WW2 bombing, really? What percentage of bombs hit within, say, 400 yards of the target?

I’ve heard it said that the secrecy surrounding the Norden bombsight had more to do with convincing the allied crews that they were an effective part of the war than it did with keeping a secret from the enemy.

3

While the bombsight was probably not as accurate as proclaimed, the large number of distant misses were the results of a number of different situations.

In order to provide defensive mutual support, the planes flew in “boxes” of several planes flying near each other. However, between the size of the planes (B-17: >103’ wingspan, B-24: 110’ wingspan) plus the need to fly far enough apart to avoid smashing together while fighting turbulence and jinking to avoid flak, the planes were a bit spread out as they passed over the target. For this reason, they adopted a habit of dropping their bombs together, based on the call of the bombardier of the lead plane. One presumes that the lead bombardier selected a target close to the center of the flight path, so that planes to the left and right would not be too far astray.

However, with the turbulence and the vagaries of formation flying, a lot of bombs landed farther from the targets than the USAAF would probably have liked to see. in addition, there was really no way to guarantee the winds between the planes at 23,000 feet and the ground. You might get a reading on the wind direction and speed at the location of the bombers and you might get a reading on the ground, but there is no way to guarantee that somewhere in the 3 1/2 to 5 miles between the bomber and the target there is not a crosswind blowing at an unknown strength in an unknown direction.

Then, the bombsight required a lot of skill, so a less than perfect bombardier or a guy having a bad day, could also wind up farther from the target than desired. Then, for subsequent waves of bombers, the smoke and dust from the fires and explosions from the first wave often obscured the target and, until radar was introduced in the last months of the war, everyone following the lead wave had to guess where they were aiming. (And I am not sure how well the radar worked as the first wave changed the landscapewith their bombs.)

4

Early on, during the Battle of Britain, this Wiki article claims that the accuracy of a bomb was a “city” and that the Brits considered anything within five miles of the target, a hit!

Later on, in 1944 and 1945, no more than 30 percent or so of allied bombs fell within 500 feet of target.

5

The THE UNITED STATES STRATEGIC BOMBING SURVEY: Summary Report is on-line. (The detail report may also be on-line somwehere; I did not look for it.)

It has been a while since I read through the reports, so I do not recall whether accuracy is included in the summary, but the detail report extensively examined issues of accuracy.

6

It is all very circular.

Despite what the bomber people were trying to do, they in fact did something rather different. They wanted to win the war through air bombardment, they ended up destroying the German Air Force.

All these bombers flew over German. The Germans pretty well had to provide flak and fighters and stuff to defend themselves. All this expensive stuff soaked up production, manpower and resources. Even if the bombs did no good at all, the diversion of stuff from the Eastern Front was pretty darn critical.

Next, the Americans eventually fielded long-range fighters to take on the German ones. This further increased attrition amongst German air crews. A large part of the Luftwaffe was wasted defending Germany. Imagine if the German Air Force had been over the Eastern Front, or over Normandy.

As the war progressed, accuracy of bombing increased. Still, even the inaccuracy had positive effects. The German economy was not fully understood and hitting things more or less at random sometimes got lucky. Also even if we could not hit the widget factory, we could smash the widget workers’ housing, their families and their transport. All of this meant fewer widgets were produced.

So, bombing did not work they way it was intended, but it is very hard to see how we could have won without it.

7

The linked site provides ways to look for the information in the complete USSBS.

8

[QUOTE=Xema]
A related question: how accurate was WW2 bombing, really? What percentage of bombs hit within, say, 400 yards of the target?

I’ve heard it said that the secrecy surrounding the Norden bombsight had more to do with convincing the allied crews that they were an effective part of the war than it did with keeping a secret from the enemy.
[/QUOTE]
Given ideal conditions the sight would put the bombs pretty close to the target. Both of these cites http://www.squadron13.com/B17/Norden.htm and Norden bombsight - Wikipedia claim that under ideal condition it could hit within a 100 ft. circle from 21000 ft. Since accuracies are never quoted like that I think that means that the Circular Error Probable (CEP) was 100 ft. In other words 50% of the bombs would fall inside the circle.

The cites also disagree about with me and my pal about the bomb aerodynamics being part of the input. I’m quite sure they were otherwise there would be a much bigger CEP than 100 ft. It wouldn’t be any harder to make a ballistics table for bombs than for artillery. That is you would drop a bunch of bombs from each altitude, see where the center of the cluster was and assign the distance from that center to the aim point as the trail for that bomb at that altitude. This would then be entered into the sight as a constant offset for that bomb. For altidudes between those tested use interpolation.

In actual practice a shotgun approach was used. The formation consisted of flights of six airplace in two three plane V’s one behind and below the other. Three of these flights in a V made up a box and two boxes made up a group. Sometimes, but not all that often, a third box was added.

Bombing was done by flights when the weather was clear and by boxes or groups otherwise. The lead plane would do the aiming for all planes in the flight, box or group who would drop on the leader. The first cite states that only 50% of the bombs in WWII landed within 1/4 mile of the target. This seems pessimistic but I suppose if you include all the times when the bombardier misidentified the target and that sort of thing if might not be too bad. I do know that we regularly managed to get at least some bomb directly on targets such as highway and railway bridges and road junctions from 12-14000 ft.

I don’t think the secrecy was for the purposes of holding up morale. The bombardiers and everyone else knew exactly how well we were doing. we had planes with cameras that recorded the actual strike and recon photos were taken to asses results. Pilots and bombardiers were debriefed on results and critiques were ongoing. We pretty well knew what the results were.

Of course photos can be deceptive. We once bombed a bridge in Koblenz and the strike photo was in the Army paper Stars and Strips to show the ground troops what good support they were getting. The bridge was surrounded by a huge water fountain and looked like a gonner for sure in the photo. Unfortunately, although the bombing was as accurate as can be the bombs all passed through the bridge trusses or went just over or just short and the bridge was relatively undamaged.

9

[QUOTE=David Simmons]
Given ideal conditions the sight would put the bombs pretty close to the target. Both of these cites http://www.squadron13.com/B17/Norden.htm and Norden bombsight - Wikipedia claim that under ideal condition it could hit within a 100 ft. circle from 21000 ft. Since accuracies are never quoted like that I think that means that the Circular Error Probable (CEP) was 100 ft. In other words 50% of the bombs would fall inside the circle.

[/QUOTE]

I have to disagree with you on this, David (despite your first-hand experience!). The second link of my earlier post states that no more than 30% fell within 500 feet of target, and Tom’s second link, the Strategic Bombing Summary, states:

10

[QUOTE=KarlGauss]
I have to disagree with you on this, David (despite your first-hand experience!). The second link of my earlier post states that no more than 30% fell within 500 feet of target, and Tom’s second link, the Strategic Bombing Summary, states:
[/QUOTE]
The quoted number wasn’t the accuracy achieved in actual practice. It was the results on a bombing range, in good weather, with a long bomb run, highly experienced pilots and bombardiers, the bombsight-autopilot link tuned to perfection and little turbulence. They were trying to find out the best that the sight could be under absolutely ideal conditions.

Such conditions were never achieved. For example, even in ideal weather you couldn’t make a 5 minute bomb run so as to get the sight to give you the ideal solution. There wouldn’t have been anyone left to drop bombs.

11

[QUOTE=tomndebb]
While the bombsight was probably not as accurate as proclaimed, the large number of distant misses were the results of a number of different situations.

In order to provide defensive mutual support, the planes flew in “boxes” of several planes flying near each other. However, between the size of the planes (B-17: >103’ wingspan, B-24: 110’ wingspan) plus the need to fly far enough apart to avoid smashing together while fighting turbulence and jinking to avoid flak, the planes were a bit spread out as they passed over the target. For this reason, they adopted a habit of dropping their bombs together, based on the call of the bombardier of the lead plane. One presumes that the lead bombardier selected a target close to the center of the flight path, so that planes to the left and right would not be too far astray.

However, with the turbulence and the vagaries of formation flying, a lot of bombs landed farther from the targets than the USAAF would probably have liked to see. in addition, there was really no way to guarantee the winds between the planes at 23,000 feet and the ground. You might get a reading on the wind direction and speed at the location of the bombers and you might get a reading on the ground, but there is no way to guarantee that somewhere in the 3 1/2 to 5 miles between the bomber and the target there is not a crosswind blowing at an unknown strength in an unknown direction.

Then, the bombsight required a lot of skill, so a less than perfect bombardier or a guy having a bad day, could also wind up farther from the target than desired. Then, for subsequent waves of bombers, the smoke and dust from the fires and explosions from the first wave often obscured the target and, until radar was introduced in the last months of the war, everyone following the lead wave had to guess where they were aiming. (And I am not sure how well the radar worked as the first wave changed the landscapewith their bombs.)
[/QUOTE]
I recall reading about an incident that happened on a B-17 raid over Belgium. The lead bombardier chose a large park surrounded by apartment buildings as his calibration point; problem was, he forgot to unclutch his bombsight (or whatever he was supposed to do), so his bombs dropped when the crosshairs aligned. And the other planes dropped with his. Result: one demolished park and a number of demolished apartments.

The bombardier no doubt considered parting company with the aircraft over the English Channel, but resolved to take his medicine. Just as well — after the commanding general read the proverbial riot act, he revealed the kicker: it seems that the Germans had appropriated the park as a parade ground, and the housing around it as barracks. German losses were high, civilian losses negligible, and the whole fiasco was viewed on the Continent as a sterling example of Allied precision bombing.

12

Wouldn’t the error zone be more elliptical in shape than circular, anyway? Depending on the type of target you’re aiming for, that could make a huge difference, and a circular approximation to the ellipse probably won’t be too great.

13

Specialist bombing teams (such as 617 squadron) had very high levels of accuracy, but they used the SABS bombsight. During one raid (once they were used to the sight) they dropped bombs on flares laid by the Pathfinder squadron. All their bombs were within 500 yards of the flares, which were off target by a considerable margin. This lead to the use of a Mosquito and eventually a Mustang as the marker plane.

The accuracy was considered high enough to have a couple of goes at the Tirpitz with 5 ton Tallboys, including 1 direct hit. Of course, Tallboys and Grand Slams (10 tons) were ground penetrators who did not rely on accuracy but on subsurface shockwaves - a near miss was considered better than a direct hit.

Interestingly, the USAF have recently redeveloped a 20 ton ground penetrating gravity bomb based on the Grand Slam. I wonder where they would be useful :wink:

Si

14

[QUOTE=David Simmons]
The lead plane would do the aiming for all planes in the flight, box or group who would drop on the leader.
[/QUOTE]

What’d the leader do to deserve that? :wink:

15

As an additional note, I believe that if the planes went off-target in the way (that is, they got lost a bit due to weather or night or flak) they looked for anything nice and blew the stuffing out of it. They needed to lose the weight for the trip home.

16

If anyone’s interested in getting some idea what it was like to try and bomb Deutsche Waffen und Munition Fabrik-Hersteller plants during WWII, you need look no further than the game B-17 Flying Fortress II: The Mighty Eighth.

You get to control every aspect of a B-17G Flying Fortress, including the Norden Bomb-sight. Having played the game extensively, I’m in awe of the fact the RAF and the USAAF managed to hit anything at all- it’s surprisingly difficult to put the bombs anywhere near the target, never mind avoiding Flak and the Luftwaffe on the trip out and back… :eek:

17

During the Battle of Britain it was the Germans trying to hit things with their bombs and they were all too accurate for comfort. In the autumn of 1940 they switched to night bombing of cities and - using pathfinders and radio bombing aids - they were still good enough to destroy large areas of some British cities.

It was in 1941 when the RAF was ramping up its night bombing attacks on Germany - theoretically aiming for key military and industrial target such as factories and rail yards - that they realised they were totally failing to hit the targets and you get the less than 33% of bombs within five miles figure. Mind you you can hardly call this a problem with bomb aiming - that’s a problem with navigation!

It was following this that Bomber Command attacks switch to area bombing of industrial cities and “dehousing” the workforce, in truth attacking the civilian population. (I am not criticising - it was probably the only attacking option open to Britain at the time and to second guess the people there at the time is ridiculous. Even worse is to criticise the actual bomber crews.) They then started to introduce navigation aids - Gee, Oboe, H2S etc - and deploy specialist pathfinders and by 1944 RAF night attacks could be remarkably accurate.

One perpetual problem was “creep”. The tendency was always for the point of attack to move back up the track of the bomber stream as each successive aircraft - with each crew eager to turn for home as quickly as possible - dropped its load slightly early.