The Navigation and Bombing System, or NBS, was a navigation system used in the

Royal Air Force The Royal Air Force (RAF) is the Air force, air and space force of the United Kingdom, British Overseas Territories and Crown Dependencies. It was formed towards the end of the World War I, First World War on 1 April 1918, on the merger of t ...

's V-bomber fleet. Primary among its parts was the Navigation and Bombing Computer (NBC), a complex

electromechanical computer A mechanical computer is a computer built from mechanical components such as levers and gears rather than electronic components. The most common examples are adding machines and mechanical counters, which use the turning of gears to increment ...

that combined the functions of

dead reckoning In navigation, dead reckoning is the process of calculating the current position of a moving object by using a previously determined position, or fix, and incorporating estimates of speed, heading (or direction or course), and elapsed time. T ...

navigation calculation with a

bombsight A bombsight is a device used by military aircraft to drop bombs accurately. Bombsights, a feature of combat aircraft since World War I, were first found on purpose-designed bomber aircraft and then moved to fighter-bombers and modern tactica ...

calculator to provide outputs that guided the aircraft and automatically dropped the bombs with accuracy on the order of a few hundred metres on missions over thousands of kilometres. Inputs to the NBS system included late models of the

H2S radar H2S was the first airborne radar system, airborne, Airborne ground surveillance, ground scanning radar system. It was developed for the Royal Air Force's RAF Bomber Command, Bomber Command during World War II to identify targets on the ground f ...

, the True Airspeed Unit, a

gyrocompass A gyrocompass is a type of non-magnetic compass which is based on a fast-spinning disc and the rotation of the Earth (or another planetary body if used elsewhere in the universe) to find geographical Direction (geometry), direction automaticall ...

and the Green Satin radar. A Mk 6 radar altimeter was used for accurate height measurement but was not connected to the NBC. These inputs were used to set the

Ground Speed Ground speed is the horizontal component of the velocity of an aircraft relative to the Earth’s surface, also referred to as "speed over the ground". It is vital for accurate navigation that the pilot has an estimate of the ground speed that wil ...

Unit, which carried out the navigation calculations, which in turn fed the

autopilot An autopilot is a system used to control the path of a vehicle without requiring constant manual control by a human operator. Autopilots do not replace human operators. Instead, the autopilot assists the operator's control of the vehicle, allow ...

system. The NBC did not feed the T4 bombsight computer for visual sighting. The NBC was a electro-mechanical computer developed by EMI in the late 1940s and early 1950s. Besides navigational computation for position it was essential for automatic bombing computation. This was achieved with a calculator, Calc 3. This could calculate the square root of height above target using a square rooting pin wheel. Wheel with numerous pins which was rotated and had a pickoff that moved between centre to edge providing a voltage output. There was a height carriage which, when activated, could make or break a number of switches. Its operation was controlled by a cassette containing a 35mm file which represented the flight profile of different bombs. There were dark or clear areas similar to punch paper templates. The output was the forward throw of a bomb in nautical miles which was be calculated from the time of bomb fall and the groundspeed in knots. Additional factors such as the aircraft change in altitude and the bomb characteristics the time of bomb fall. These factors cause the bomb to trail behind the aircraft, this is the Trail Angle. As it trails behind the aircraft it is also subjected to wind effect for a greater time. The Trail Angle may be split into components along the flight line and across the flight line. The system time from the firing impulse to the bomb release is also factored in. Finally, if dropping a stick of bombs, a stick length of up to 1,400yds may be set. The system then calculates half this length*. This is represented by the formula: FT = Vg (√2h/g ± ḣ/g + τ+ TA) +((h x Tan λ) x Cosδ) + SL/2 FT = forward travel in nautical miles, Vg = groundspeed in knots, h = height above target in feet, g = gravitational constant in feet, ḣ = the rate of change if height in feet per second, τ = Tau, the symbol for the bomb characteristics or drag, TA = the extra time in seconds to allow for system delay, λ = Lambda or trail angle, δ = Drift angle SL = Stick length. *if the bomb aimer set the stick length on the computer at half the computer would be using only a quarter. Assuming a speed of about 420kts or 240yd/sec, and 21 bombs dropped at intervals of 0.3sec, the stick length would be 1440yd. If the value set was 720yd the first bomb would be released 360yd late.

References

* {{cite web , first=Norman , last=Bonnor , url=https://www.raf.mod.uk/rafcms/mediafiles/EEA1B9D1_5056_A318_A88B16BD35A68C31.pdf , work=History of Air Navigation in the RAF , title=Chapter 10: From the 60s to the 80s, The Last Days of Airborne Analog Computing , year=1997 , pages=98–106 , archive-url=https://web.archive.org/web/20170803174115/https://www.raf.mod.uk/rafcms/mediafiles/EEA1B9D1_5056_A318_A88B16BD35A68C31.pdf , archive-date=2017-08-03 , url-status=dead Mechanical computers Aerial bombing