When bombs were first dropped from the primitive bombers of WW1 the effect of wind severely affected the accuracy of bombing. The first bombs were small and to be effective, a direct hit became necessary. The only way to nullify the effect of the wind was to bomb up or down the wind direction. For the attacking aircraft the defences could anticipate the direction and thus increasing the danger to the bombers.
Wimperis a graduate engineer working for the Royal Naval Air Service in their research laboratory, devised in 1916 his Course Setting Bombsight (CSBS) that would enable a bomber to attack their target from any direction irrespective of wind direction and speed.
The CSBS was very successful and was eventually fitted to the then giant bombers of the Independent Air Force (IAP) which was being set up to bomb Germany. Due to the bombing of London and other cities by German Gotha aircraft, public opinion had forced retaliation to be undertaken. However, WW1 ended before the I.A.P. could become effective.
The Research Laboratory now operated by the Air Ministry closed in 1932 and from that date all research associated with bomb sights was carried out at Farnborough. There was a large range of sights designed and built at Farnborough from dropping bombs, mines and torpedoes to parachutists and for dive bombing. There was even bomb sights intended for use in the bombing of bomber formations as they approached British cities.
The CSBS or Wimperis sight as it was known overseas was purchased by many foreign air forces including the Americans who eventually had a stock of 11,000 British designed sights some of whom were purchased direct or built in the USA under licence. The Japanese used a sight developed from the CSBS, which was in service use until the early days of the entering into WW2.
Carl Norden, a graduate engineer who emigrated to the USA and worked for a time with the Sperry Gyroscope Company, was requested to study the Wimperis variant with a view to improving it. After a long period of development, the famous Norden Bombsight of WW2 resulted, although in the process, he came to reject the principles of the Wimperis bombsight.
The Americans quickly appreciated the fact that accuracy could be doubled by stabilising the bombsight so that it in remained in one plane and was not disturbed by aircraft vibration and changes of levelling due to turbulence or by aircraft manoeuvres. Although the British too understood this, it was rather longer before they were able to put it into practice.
There were several variations of the CSBS to allow for bombing at low and high altitudes. At first the speeds of the post-war biplane bombers did not materially increase beyond 100 mph. The Vickers Virginia with a top speed of 108 mph remained in front line service until 1937. With the re-armament of the RAF in the mid to late 1930s change was on the way and the biplane bombers slowly gave way to the monoplane. Air speeds greatly increased as well as the requirement for greater bombing heights. The range and weights of the bombs it aimed, also increased.
When the faster monoplane started to supersede the trundling biplane it brought further problems in its wake for the flat turns of the old biplanes were no longer possible with the new monoplanes and accuracy appeared to suffer. A monoplane introduced a degree of bank, which interrupted the apparent smooth flow of the target down the drift wires until the aircraft could return to an even keel. Stabilisation of the bombsight could have corrected this anomaly but it was never applied to the CSBS in service although a system was developed by Farnborough, but not adopted. The rapid rate of change in bomber performance lead in turn to increased development of the CSBS leading to the introduction of Mks Vll in about 1932 and Mk lX in 1939.
The CSBS suited the requirements of the pre-war armament training camps where targets were attacked on a one to one basis and in regime of precision bombing. Under these conditions the CSBS was capable of considerable accuracy; one important problem with the sight remained unrecognised. To achieve the accuracy of which the sight was capable the aircraft had to be flown at a constant speed ‘straight and level’ up to the target. This in wartime, was operationally impractical.
Flying conditions in the armament training camps did not reflect in any way the operational flying conditions to be anticipated in the coming war. Opportunities for night flying were restricted for reasons of cost and little could be done to simulate the effects of anti-aircraft fire. The illuminated cities of Britain and Europe in peacetime gave little practice in navigation for using D.R. plotting over a wartime-darkened Europe.
Two versions each of the MkVll and MklX CSBS were in service use, namely the ‘A’ and the ‘C’. The former was graduated in mph and intended for light bombers with restricted range. while the ‘C’ were graduated in knots and intended for longer range operations and using dead reckoning (D.R.) plotting.
By the mid thirties it was felt that the use of the CSBS should be simplified and work was started on developing the Automatic Bombsight(ABS) at Farnborough perhaps influenced by the rumoured existence of the American Norden sight.
Some information about its specification had been supplied by an officer stationed at RCAF Trenton, who had been allowed to witness trials in the US. The ABS had a very difficult passage through the development stages with frequent changes in requirements and problems in obtaining serviceable test aircraft. Modern high performance aircraft of the time were not easily available for testing the new sight. Much of the early testing was done on a Boulton Paul Overstrand which only the aged prototype was available. At one time testing had to be delayed until the manufacturers of the Overstrand could sufficiently refurbish the airframe to allow a few more hours of flying time. Eventually Blenheim’s and Hampden’s were made available for test purposes.
The ABS lacked one essential element in its design although the need for it had been the subjects of much discussion since its inception. This was stabilisation which Farnborough knew would double the accuracy of the instrument and rumour had it the Norden incorporated.
The bombsight being tachometric like the Norden, required a long run up to the target flying straight and level. No tactical freedom of manoeuvre was possible adding greatly to the stress on the bomber crew due to the ferocity of defences protecting the target. It was possible to anticipate the position of the bomber for some forty seconds before its bombs could be released thus increasing materially, the danger to the aircraft from defensive fire.
A decision was finally made to add stabilisation to the ABS but only when it was in full production with the result production was almost brought to a standstill.
Running in tandem to the ABS was a redesign of the CSBS which used the same principles but bore little resemblance to its predecessor. This was the CSBS MkX. Little testing had been carried out on night operations since it was then thought day light operations would predominate. Five thousand of these sights had been completed by the manufacturers, and were awaiting delivery, when its unsuitability for night operations were recognised and it was abruptly cancelled.
The new four engine bombers were well advanced in production and in view of the problems with the ABS the then successful CSBS MkX was specified for the the new British bombers as well as those ordered from America such as the Liberator and the Baltimore. These aircraft had been ‘plumbed up’ on the production line with necessary services and mountings to receive the new sight. To provide the quantities required for the new aircraft, companies in Canada and Great Britain ceased production of the Mk lX and were re-equipped to manufacture the new MkX.
At this point it has to be remembered that RAF Bomber Command expected that a major part of its operations would be by day. They were certain in their belief that the combined effect of their machine gun based defence systems would be sufficient to defend their bombers against the opposing fighters; providing the aircraft remained in tight formation. They tragically assumed that Britain alone, had ‘radio-location’ (radar) defence systems which would warn of the approach of the RAF bombers.
For a year before war was anticipated, operations had been meticulously planned that the German fleet would be attacked at Wilmshaven and so several raids were mounted in the first months of the war. The results were disastrous with German radar giving sufficient notice of the attack. for the defending German fighters. Much of the cream of Bomber Command aircraft were destroyed together with their gallant crews. The Air Staff criticised the leaders of the attack for permitting poor formation flying but it was apparent the bombers had totally inadequate defences for undertaking day light attacks. The solution was to concentrate upon night operations.
With the cancellation of the MkX sight, remaining stocks of the Mk Vll and Mk lX bombsights were raided to be fitted to the new aircraft where the performance envelope was appropriate. In America the Norden bombsight was in short supply for US operated bombers and a new similar Sperry tachometric bombsight was hurriedly developed. Supplies were sought to replace the MkX for the RAF Liberators and other bombers. The few ABS sights available went to the new Manchester bombers and later to Hampden’s.
Bomber Harris and his aircrews were not slow in voicing their strong dislike of both the Mk lX and the ABS. and called for their early replacement. Their strong views were registered with the Tizard Committee and a founder member of that committee Prof. Blackett volunteered to design a new sight to meet the needs of Bomber Command. He was given facilities at Farnborough and the services of a small team of engineers. The bombsight that resulted was the Mk XlV regarded then as the wonder sight of the day. It was designed to enable the run up to the target flying straight and level to be restricted to a mere ten seconds and enable the pilot to carry out evasive manoeuvres on his approach to the target. It could be used to bomb both on the climb and the glide. The bombsight consisted of a computer cabinet mounted to the left of the Air Bomber and a stabilised sighting head with optical graticule. The sight was one of the first practical uses for a mechanical computer and Babbage would have been proud of it.
This was the bombsight of choice for Bomber Command until the end of the war and beyond. Shortly after its entry into service, its manufacture was subcontracted to the Sperry Gyroscope Company in America who after re-engineering it to meet American standards, arranged for A.C. Spark Plug , Division of General Motors to manufacture in quantity. Known as the ‘T1’ version a total of 23,000 were made for use in the RAF and Commonwealth. Air forces. In some respects, it was a mechanical improvement on the British manufactured sight but was fully compatible with it in every way.
The principal source of inaccuracy was the need to set on the computer the wind speed and direction which under operational conditions, could be often in error.
A T1A version was produced for use with the faster Mosquito and to allow for the greater operating height. T2 and T4 versions were British manufactured developments based upon the T1 bombsight and used on post-war aircraft and to allow for a connection to Green Satin radar systems. The MkXV and MkXVll versions were intended for Coastal Command but never entered production.
For a time the USAAF considered using the T1 on their medium sized bombers but declared it to have a fault and rejected it. The reason remains unknown it was probably unsuitable for the tactics used and the Norden was predominately used.
The ABS continued its development at Farnborough and emerged in August 1943 as the SABS Mk llA tachometric precision bombsight precision sight. The SABS provided an even more complex mechanical computer being able to calculate its own ‘wind’ and to automatically release bombs. These were qualities it shared with the Norden and probably the German Lotfe sight.
Starting in 1941 Barnes Wallis had designed a range of very large bombs, namely the ‘Tallboy’ of 12,000 lbs‘ and ‘Grand Slam’ of 22,000 lbs. These bombs to be effective, had to be dropped within 150 yards of the target from 20,000 feet and the SABS MkllA proved to be the ideal sight for this purpose. A direct hit was not required as it was anticipated that the bomb if landed just short of the target would travel forward under the target before detonating. The resulting explosion would destroy the foundations of the target causing a degree of damage that would take many months to repair.
This sight was mainly fitted to the Lancaster's of 617 squadron and used in their precision bombing of tunnels, V1 and V2 ;launch sites. In company with 9 squadron using ‘Tall boy’ and ‘Grand Slam’ earthquake bombs the German battleship ‘Tirpitz was sunk in 9 minutes of commencement of attack. To achieve such a high level of accuracy required a considerable amount of bombing practice on the bombing range. These attacks were carried out by day or by night.
The accuracy of 617 squadron improved greatly with an average radial error of 170 yards being recorded over the period of June to August 1944 and improved to 125 yards in the period of February 1945 to March 1945. Two other precision bombing squadrons were formed based upon the MkXlV bombsight and in the period of February to March 1945 their average error was 195 yards. It is not surprising that when the Norden was offered to the RAF later in the war it was rejected.
Less than 1,000 SABS bombsights were manufactured and after the war great difficulty was experienced in finding sufficient sights to equip two Lincoln squadrons for precision bombing against Japan. Compare this with the 23,000 T1 sights manufactured in America.
There was in Bomber Command at the time much discussion on the comparative merits of the two bombsights. The SABS although potentially more accurate lacked the degree of tactical freedom afforded by the MkXlV/T1. As a result the MkXlV/T1 was known to Bomber Command as the ‘area’ bombsight of the RAF and the SABS as the ‘precision sight.’
It was a much more complex sight to use and to maintain than the MkXlV/T1 and required more man-hours in manufacture. For the majority of the squadrons in Bomber Command the MkXlV/T1 was still the preferred sight.
A more controversial aspect was how the American, British and German sights compared. The Norden, the SABS and the German Lotfe 7D or H were all tachometric sights used in the sitting position Norden and Lotfe 7D/H both had direct connections into the automatic pilot systems. so that in the run up to the target the bomb-aimer effectively flew the aircraft without the intervention of the pilot. Work on a SABS Mk lll was cancelled in 1943 which may have had this facility.
The SABS Mk llA uses a simpler system in that was connected to an instrument called the Bombing Direction Indicator(BDI) which was mounted on the pilot’s instrument panel. The BDI indicated to the pilot the amount of turn required left or right to bring the sight on to the target.
Using the Norden bombsight, USAAF bombardiers rarely matched the accuracy of those of 617 squadron, or even those of 9 Squadron when using their MkXlV. It could have been the greater proportion of time spent by 617 squadron practicing over the bombing ranges, and a higher standard of Air bomber.
Using tactics devised from pre-war experiments it was standard USAAF practice to fly over hostile territory in large tight formations relying upon the massed machine guns of the formation for defence. Only the bombing leader or his deputy would use their Norden bombsights with the remainder of the formation, dropping their bombs upon sight of the leader’s weapons leaving the aircraft.
It was therefore not surprising that only 31% of American bombs would fall within a radius of 1000ft of the target. Further factors were thought to be due to inaccurate settings on their bombsights and higher than specified manufacturing tolerances.
RAF Aircrew veterans will recall the claims of the time that an American Bombardier using the Norden could drop a bomb into a ‘pickle barrel from six miles up’. This was a myth but is still attached in some measure today, to the Norden bombsight; although oddly enough, not to the equivalent Sperry sight.
Fortunately the Luftwaffe lacked an effective bomber force to demonstrate the effectiveness of their Lotfe sight. Our German friends may be gratified to know that at one time earlier in the war the German Lotfe 7D sight was found to be so good, the suggestion was made to equip a RAF squadron with sights gathered from crashed aircraft. RAE Farnborough strongly objected to the suggestion and nothing more was heard of the proposal.
In contrast to the Americans many RAF aircrew and the British public remain unaware of the existence of the SABS bombsight. No articles dedicated to the sight appeared to have been published and it is rarely mentioned in books.
All bombsights mentioned in this article can still be
found in many aviation museums; but examples of the SABS have
not been preserved.
CSBS Bomb Sight
T1 Bomb Sight
Back of T1 Bomb Computer
SABS Bomb Sight