Tuesday, 15 December 2009

Bomb Sights

The first bombing raids on London, by the Graf Zeppelin in 1917, democratised warfare as never before. The killing was know no longer confined to the battlefield and  soldiers but now included cities and the civilian population. By the nineteen thirties the prospect of city bombing had gathered pace to the extent that Britain's RAF invested greatly in bombers. (Far more so, in fact, than in the fighters  and radar that would eventually win the Battle of Britain.) These new bombers were to be a deterrent force that would make attack unthinkable.  But when the war began it was soon realised that the accuracy with which the bombers could deliver their payload was quite another matter.

At the start of the Second World War, aircraft navigation, especially at night, could be extremely poor. Prague was bombed only once and that was by accident. The pilot who bombed Prague thought he was bombing Dresden, an error of over a hundred miles.

Then, assuming that the target was found, there was the issue of bomb aiming. In the early days this had been left to the pilots judgement. As aircraft altitudes and speeds increased bombing effectively became more difficult. The fundamental problem of air navigation is the difference between the aircraft's motion through the air and its motion over the ground. To bomb accurately the motion over the ground must be known, but the aircraft instruments of the 1930s and 1940s could only determine motion through the air.

The instruments of the time could measure airspeed with reasonable accuracy and heading can be found from a magnetic compass. But, because of wind, speed and direction over ground rarely corresponds with speed and direction through the air. The air is usually in motion, and any calculations regarding distance travelled, based only on speed through the air and direction pointed will be subject in error. In order to correct speed over the ground, and track (the path of the aircraft over the ground) wind speed and direction must be known.

The first generation bombsights attempted to add in a predicted wind speed based on weather forecasts. But the forecast winds were often way out and the vector sights which allowed the bomb aimer to ‘dial in’ the predicted winds were soon found to be inadequate. But aircraft motion over the ground could be determined by observing and tracking a point on the ground.

The Norden bombsight was one of several computing bombsights developed by the USA, Britain and Germany which were capable of deriving the wind speed and direction from observation of the target. The Norden bombsight was probably the most hyped weapon of the Second World War. It was an improvement on the vector sights its true accuracy was nothing like the claims made of it. And, long after its secrets were well know in Germany, crews had to carry out a post mission procedure of locking the bombsight up that was as useful as the ceremonial changing of the guard outside Buckingham Palace. Germany had long had a comparable bomb sight of its own, the Lotfe 7. And this was well known to the Allies from Lotft 7s found in shot down aircraft.

The RAF precision bombing experts, 617 squadron, used a British designed sight, the SABS (top picture), and very effectively. At their best, 617 could deliver precision weapons such as Tallboy to an accuracy of 125 yards. The Tallboy bomb, devised by Barnes Wallis, was used to bomb the Tirpitz and against the Saumur tunnel.

All these bombsights were tachymetric sights -calculating speed (windspeed) through observation.

Initially the bomb aimer set the known vertical separation (altitude over ground) between the bomber and the target into the bomb sight computer. He then had to manually align a telescope, which was part of the bombsight, on to the target. The bombsight's computer, which was getting airspeed and heading, then continuously calculated a pointing angle for the telescope as the target was approached. This pointing angle steered the telescope through a servo mechanism.

In reality this would only work in ideal, no-wind conditions, and in practice, the target drifts across the field of view of the telescope. The bomb aimer has two controls which he must use to 'null out' the drift. These controls set up a 'vector' in opposition to the actual wind vector, (which is the speed AND direction of the wind). With the drift correctly neutralised the telescope will automatically track the target. And the adjustment of the drift controls has produced values for the direction and strength of the wind. With the wind known the bomb release point can be determined with much greater accuracy.

The American and German tachymetric bombsights were designed for use with aircraft with autopilots. They produced a steering signal which was fed into the autopilot to correct the aircraft heading. An automatic bomb release signal was also produced. The British SABS bombsight drove director lights which the pilot had to follow.

In an earlier blog I mentioned how the guidance technology developed in the Second World War was developed for use in missiles and, eventually, the Apollo moon landings. The technique of tracking an object, manually on a bombsight, was, within ten years fully automated. ICBMs used  automatic star tracking to achieve great accuracy. The submarine launched missile system Polaris is so named because of its ability to automatically track the Pole star.

By the time engineers and airmen had achieved great accuracy the development of nuclear weapons seem to have made accuracy obsolete. The reality has turned out a little differently, bomb accuracy continues to be improved with guidance such as GPS and laser now achieving accuracies of better than 13 metres.


  1. Hey, I commented on this last night? Where's my comment?

    Damnit, when will I lean never to write anything sizeable in a web-browser? If I'd written if offline in 'vi', I'd still have the text to repost.

  2. I've only received the one comment for moderation.