SR West Country and Battle of Britain classes
The SR West Country and Battle of Britain classes, collectively known as Light Pacifics or informally as Spam Cans, or "flat tops", are air-smoothed "Pacific" type steam locomotives designed for the Southern Railway by its Chief Mechanical Engineer Oliver Bulleid. Incorporating a number of new developments in British steam locomotive technology, they were amongst the first British designs to use welding in the construction process, and to use steel fireboxes, which meant that components could be more easily constructed under wartime austerity and post-war economy.
They were designed to be lighter in weight than their sister locomotives, the Merchant Navy class, to permit use on a wider variety of routes, including the south-west of England and the Kent coast. They were a mixed-traffic design, being equally adept at hauling passenger and freight trains, and were used on all types of services, frequently far below their capabilities. A total of 110 locomotives were constructed between 1945 and 1951, named after West Country resorts or Royal Air Force and other subjects associated with the Battle of Britain.
Due to problems with some of the new features, such as the Bulleid chain-driven valve gear, 60 locomotives were rebuilt by British Railways during the late 1950s. The results were similar to the rebuilt Merchant Navy class. The classes operated until July 1967, when all the last steam locomotives on the Southern Region were withdrawn. Although most were scrapped, 20 locomotives are preserved on heritage railways in Britain.
Background
The financial success enjoyed by the Southern Railway during the 1930s was based on the completion of its London suburban electrification scheme in 1929 and the subsequent electrification of the main lines to Brighton and the Sussex Coast and to Guildford and Portsmouth. Despite electrification plans, the Southern Railway's less heavily used lines in the West Country beyond Salisbury did not merit the cost. Lines in Devon and Cornwall were meandering, heavily graded, and although heavy with summer holiday traffic were lightly used during the winter months. The seasonality of railway traffic meant that the West Country branches were worked by the ageing T9 class 4-4-0 and the versatile N class 2-6-0, which could be better utilised on mixed-traffic services elsewhere. As a result, an order was placed with Brighton railway works in April 1941 for 20 passenger locomotives of a type to be determined.During 1943, Bulleid began planning for the post-war locomotive requirements of the railway and identified the need for a stop-gap steam locomotive design for those main lines in South East England scheduled for electrification, had the Second World War not taken place. Although the new Merchant Navy class was available for the heaviest Continental expresses, the resumption of frequent passenger services over poorly maintained infrastructure, following the war, would require a lighter locomotive with wider route availability.
At the same time, there would be a continuing need for fast freight locomotives, capable of operating on both electrified and non-electrified routes, without impeding the intensive use of the system by passenger trains. Suburban electrification used electric multiple units, which had no equivalent freight design. Although Bulleid built two prototype electric locomotives in 1941, these were, as yet, unproven, and freight haulage would be undertaken by steam traction for the foreseeable future.
Design
The detailed design work for the new mixed-traffic locomotives was undertaken at Brighton railway works where they were scheduled to be constructed. The earliest drawings were for a moderately sized 2-6-0 with similarities to the London and North Eastern Railway K4 class, which Bulleid had helped design for the West Highland Line when he was Nigel Gresley's assistant. However, such a design would have been inadequate for the Kent Coast lines, which required a powerful 2-6-2 or 4-6-0 class. It is not clear why the design was subsequently enlarged to become a smaller version of the Merchant Navy class 4-6-2 as the likely traffic requirement did not warrant such lavish provision, but the incorporation of components from that class enabled standardisation during wartime production difficulties.Weight reduction and reduced loading gauge
In order to improve on the route availability of the Merchant Navy class with its 21-ton axle loading, the weight was reduced by 5 tons. This allowed the design to operate on routes where the Maunsell 2-6-0s were the largest permitted and came mainly from several changes:- reduced overall length
- smaller boiler
- more fabricated assemblies
- smaller tender
Bulleid's features
Based on the mechanical experience gained from the Merchant Navy locomotives, Bulleid incorporated his chain-driven valve gear into what became the new design. This now-infamous component was unique in British locomotive design. It gained notoriety because it was difficult to access when things went wrong and, in tandem with the fast-moving Bulleid steam reverser, could cause irregular valve movements. The entire system was in a sealed oil bath, another unique design, that provided constant lubrication to the moving parts.The locomotive also carried a similar "air-smoothed" casing to the Merchant Navy class. This was not regarded as streamlining by Bulleid, a fact demonstrated by the flat front end. Authorities differ as to the purpose of the casing. According to Creer it was intended to be an aid in cleaning the locomotive with carriage washers to reduce labour requirements during the post-war period, whereas Bradley asserts that the intention was to lift the steam and exhaust gases away from the cab. As with the Merchant Navies, the class soon gained the nickname "Spam Cans", due to the resemblance to the distinctive tin cans in which "SPAM" was sold.
The smokebox was an integral part of the air-smoothed casing, being a sheet metal fabrication to the same profile as the firebox that acted as a former to maintain the shape of the casing. In between, the casing was supported by channel-section steel crinolines attached to the frames. The smokebox housed the five-nozzle Lemaître blastpipe arranged in a circle within a large-diameter chimney.
As with the Merchant Navy class, electric lighting was provided on both locomotive and footplate, powered by a steam-powered generator below the footplate. The gauges had fluorescent markings and were illuminated by ultra-violet light. This enabled clearer night-time vision of the boiler steam pressure gauge and the brake pipe vacuum pressure gauge whilst eliminating dazzle, making it easier for the crew to see signals along the track. Close attention was paid to the ergonomics of the cab, which was designed with the controls required for operation grouped according to the needs of both driver and fireman, thus promoting safe operation. As an aid to the fireman, a treadle used steam pressure to open the firehole doors, where the coal is shovelled into the firebox. The footplate was entirely enclosed, improving crew working conditions in winter. Other refinements and innovations used on the Merchant Navy class included steam-powered clasp brakes and the unusual Bulleid Firth Brown wheels.
Frames, boiler, cylinders
Compared with the Merchant Navy class, shorter overall length led to shorter frames and reduced the wheelbase to.The boiler was also shorter and of smaller diameter at the smokebox end, but retaining the operating pressure.
The inner and outer Belpaire firebox was also smaller than the Merchant Navy class also constructed using welded steel.
The cylinders were smaller at.
Tender
Bulleid designed a reduced capacity tender based upon the Merchant Navy version. It could carry water and of coal on a six-wheel underframe. It retained the BFB wheels and streamlining panels, or "raves", that gave the top of the tender a similar cross-sectional outline to carriages. As with the Merchant Navy class, the water tank was of welded sheet construction to save weight, and the tender was fitted with vacuum braking equipment of a clasp-type similar to that on the locomotive. Four train-brake vacuum reservoirs of cylindrical construction were grouped on the tank top, behind the coal space.Construction
The first batch of twenty locomotives was ordered in April 1941, although the changes in design to the Light Pacific arrangement meant that production was delayed until late 1944. Due to wartime contract work at Brighton works, the boilers were built under contract at the North British Locomotive Company. Before the first of the class had been delivered, the order was increased to thirty, with a second batch of ten ordered in September 1944. Deliveries from Brighton works began in May 1945 with prototype No. 21C101 Exeter, and proceeded at the rate of about two locomotives per month. The class was gradually run in on the Central Section until October 1945, when they were successfully trialled on Plymouth and Kentish services. By the time the first fifteen had entered traffic a further order of fifteen was placed, with these entering service between June and October 1946. From this batch onwards, traction was improved by the addition of steam sanding to the front driving wheel, with covers added to protect the motion from sand falling from the filler pipes.A third batch of 25 was ordered and designated the Battle of Britain class. These were identical to the West Country class and the new designation was purely concerned with giving the locomotives names that befitted their intended allocation to the Eastern Section. By the time of the nationalisation of British Railways in January 1948, 70 Light Pacifics had been built at Brighton Works, with a fourth batch of twenty on order. There was a delay in production during the first three months of British Railways control but the last twenty ordered by the Southern Railway entered traffic between April 1948 and February 1949.
In March 1949, British Railways ordered a final 20 from Brighton works despite a pressing need for smaller tank locomotives. This imbalance was rectified by building forty-one examples of the LMS Fairburn 2-6-4T for the Southern Region. Also at this time Brighton works staff were embroiled in the difficulties associated with Bulleid's experimental and problematic Leader class. As a result, Brighton sought assistance from the other Southern Region works to complete this final order. Ashford Works cut the frames and constructed the tenders, and Eastleigh Works constructed six of the final batch of locomotives.
The completion of the final steam locomotive, No. 34110 66 Squadron, in January 1951 was delayed for several months pending consideration of proposals from British Railways management for a major modification to a standard two-cylinder design without the chain-driven valve gear, but the locomotive entered service as Bulleid intended.