Tachometric Bombsights
One of the main problems using vector bombsights was the long straight run needed before dropping the bombs. This was needed so the pilot would have enough time to accurately account for the effects of wind, and get the proper flight angle set up with some level of accuracy. If anything changed during the bomb run, especially if the aircraft had to maneuver in order to avoid defences, everything had to be set up again. Additionally, the introduction of monoplane bombers made the adjustment of the angles more difficult, because they were not able to slip-turn as easily as their earlier biplane counterparts. They suffered from an effect known as "Dutch roll" that made the more difficult to turn and tended to "hunt" after levelling. This further reduced the time the bomb aimer had to adjust the path.
One solution to this later problem had already been used for some time, the use of some sort of gimbal system to keep the bombsight pointed roughly downward during maneuvering or being blown around by wind gusts. Experiments as early as the 1920s had demonstrated that this could roughly double the accuracy of bombing. The US carried out an active program in this area, including Estoppey sights mounted to weighted gimbals and Sperry Gyroscope's experiments with US versions of the CSBS mounted to what would today be called an inertial platform. These same developments led to the introduction of the first really useful autopilots, which could be used to directly dial in the required path and have the aircraft fly to that heading with no further input. A variety of bombing systems using one or both of these systems were considered throughout the 1920s and 30s.
During the same period, a separate line of development was leading to the first reliable mechanical computers. These could be used to replace a complex table of numbers with a carefully shaped cam-like device, and the manual calculation though a series of gears or slip wheels. Originally limited to fairly simple calculations consisting of additions and subtractions, by the 1930s they had progressed to the point where they were being used to solve differential equations. For bombsight use, such a calculator would allow the bomb aimer to dial in the basic aircraft parameters - speed, altitude, direction, and known atmospheric conditions - and the bomb sight would automatically calculate the proper aim point in a few moments. Some of the traditional inputs, like airspeed and altitude, could even be taken directly from the aircraft instruments, eliminating operational errors.
Although these developments were well known within the industry, only the US Army Air Corps and US Navy put any concerted effort into development. During the 1920s, the Navy funded development of the Norden bombsight while the Army funded development of the Sperry O-1. Both systems were generally similar; a bomb sight consisting of a small telescope was mounted on a stabilizing platform to keep the sighting head stable. A separate mechanical computer was used to calculate the aim point. The aim point was fed back to the sight, which automatically rotated the telescope to the correct angle to account for drift and aircraft movement, keeping the target still in the view. When the bomb aimer sighted through the telescope, he could see any residual drift and relay this to the pilot, or later, feed that information directly into the autopilot. Simply moving the telescope to keep the target in view had the side effect of fine-tuning the windage calculations continuously, and thereby greatly increasing their accuracy. For a variety of reasons, the Army dropped their interest in the Sperry, and features from the Sperry and Norden bombsights were folded into new models of the Norden. The Norden then equipped almost all US high-level bombers, most notably the B-17 Flying Fortress. In tests, these bombsights were able to generate fantastic accuracy. In practice, however, operational factors seriously upset them, to the point that pinpoint bombing using the Norden was eventually abandoned.
Although the US put the most effort into development of the tachometric concept, they were also being studied elsewhere. In the UK, work on the Automatic Bomb Sight (ABS) had been carried on since the mid-30s in an effort to replace the CSBS. However, the ABS did not include stabilization of the sighting system, nor the Norden's autopilot system. In testing the ABS proved to be too difficult to use, requiring long bomb runs to allow the computer to "solve" the aim point. When the Bomber Command complained that even the CSBS had too long a run-in to the target, efforts to replace it with the ABS ended. For their needs they developed a new vector bombsight, the Mk. XIV. The Mk. XIV featured a stabilizing platform and aiming computer, but worked more like the CSBS in overall functionality - the bomb aimer would set the computer to move the sighting system to the proper angle, but the bombsight did not "track" the target or attempt to correct the aircraft path. The advantage of this system was that it was dramatically faster to use, and could be used even while the aircraft was manoeuvring, only a few seconds of straight-line flying were needed. Facing a lack of production capability, Sperry was contracted to produce the Mk. XIV in the US, calling it the Sperry T-1.
Both the British and Germans would later introduce Norden-like sights of their own. Based at least partially on information about the Norden passed to them through the Duquesne Spy Ring, the Luftwaffe developed the Lotfernrohr 7. The basic mechanism was almost identical to the Norden, but much smaller. In certain applications the Lotfernrohr 7 could be used by a single-crew aircraft, as was the case for the Arado Ar 234, the world's first operational jet bomber. Late in the war the RAF had the need for accurate high-altitude bombing and introduced a stabilized version of the earlier ABS, the hand-built Stabilized Automatic Bomb Sight (SABS). It was produced in such limited numbers that it was at first used only by the famed No. 617 Squadron RAF, The Dambusters.
All of these designs collectively became known as "tachometric sights", "tachometric" referring to the timing mechanisms which counted the rotations of a screw or gear that ran at a specified speed.
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