A late C19 mobilisation centre which was modified from the late 1930s for rocketry research, and in the late 1940s for the top-secret development of Britain’s first atomic bomb. Reasons for Designation Fort Halstead, a late C19 mobilisation centre which was modified from the late 1930s for rocketry research, and post-war for the top-secret development of Britain’s atomic bomb, is scheduled for the following principal reasons: * Rarity and form: one of 13 purpose-built mobilisation centres nationally erected in the late C19. Fort Halstead was the largest (and most expensive) built and is one of only four designed for artillery deployment. Also, a highly significant site in terms of mid-late C20 rocketry and atomic bomb research and development; * Survival and diversity: a mobilisation centre which survives in largely intact form. Although a section of late C19 ditch has been infilled, this survives intact as a buried feature. The fort also includes evidence for the site’s later research and development role with limited alteration of some of the casements and magazines for this later use; this modification adds to the fort’s interest; * Documentation: the site has the potential to significantly enhance our understanding of the development and operation of the late C19 mobilisation centres constructed to defend the capital, also to aid our understanding of the nationally significant atomic bomb research and development undertaken here; * Group value: with listed buildings within and immediately outside the Fort representative of the site’s later role as a military research establishment. History Fort Halstead is one of fifteen late C19 mobilisation centres interspersed with entrenchments established to defend London in the event of invasion. The capital was not encircled by these centres but was protected to its north-east, east and south, the anticipated directions of attack, and their construction represents a lack of confidence in the Royal Navy’s ability to protect the country from its enemies at that time. Designed in 1894, Fort Halstead was probably constructed between 1895-7 and was intended to be a nodal point where volunteer forces could collect equipment and ammunition if the need arose. Unusually for a site of this type it also had the provision to mount machine-guns in emplacements. The fort is not shown on the 1896 Ordnance Survey map (although the associated caretakers’ cottages are depicted) and therefore it is not clear whether the fort area had been deliberately left blank for security reasons (a common convention for military structures on early maps) or whether it had yet to be built. In common with many of the other mobilisation centres, Fort Halstead was generally unoccupied although its upkeep and security was ensured by on-site caretakers for whom two cottages were built outside of the fort ditch. Most mobilisation centres also had a contemporary tool store, located outside of the fort, although at Fort Halstead the tool store was not built until 1920. The London Defence Scheme, of which Fort Halstead was a part, was abandoned in 1906. During the First World War the fort was used as a defendable ammunition store forming part of the London anti-invasion stop-line. In 1937, after sixteen years of private ownership, the War Office bought the site to accommodate the Projectile Development Establishment as it provided a remote and contained site for rocket development building on earlier work by the Ballistics Branch at the Royal Arsenal in Woolwich. From the late 1930s the site expanded with a number of buildings constructed inside and outside the fort. After the end of the war, Fort Halstead became the top-secret High Explosives Research headquarters with the task of developing Britain’s atomic bomb (developing the Mark 1 warhead which when assembled in its casing was known as ‘Blue Danube’) and this work was to dominate the work at Fort Halstead. Other structures associated with Blue Danube have been scheduled recently at the former RAF Barnham. Additional structures for this research were built in and around Fort Halstead, all within a secure fenced enclave. As was common to projects of the time different research establishments were responsible for developing different components of weapons systems. Although few records exist it is known that Fort Halstead personnel were responsible for developing both high explosive and electronic detonators for the atomic bomb. Britain exploded her first atomic bomb on the Mont Bello Islands, Australia on 3 October 1952. Atomic weapons research and development continued at Fort Halstead until 1955 when staff transferred to the Atomic Weapons Research Establishment at Aldermaston (Berkshire). Fort Halstead has since continued as a government defence research establishment concentrating on explosives and other research. Details Fort Halstead is located on the crest of a steep chalk escarpment of the North Downs, to the north-west of the village of Dunton Green and overlooking the Darenth Valley. The fort is polygonal in plan and is surrounded by an earth rampart and a deep external ditch with a sloping earth counterscarp and concrete revetted scarp. The ditch is extant for much of the circuit except at the north-west and west of the fort where is has been infilled although survives as a buried feature. Within the base of the ditch to the south is a brick compound built against the concrete revetment and containing a domed structure of unknown function. The original access to the interior parade is from the north-west where two of the eleven angles of the fort form a re-entrant. There is a later additional entrance to the south-west linking the fort interior with the wartime experimental rocket filling area to the south-west of the fort. This entrance is shown on an aerial photograph of 1952 but not on one of 1946. The original entrance is via a north-south causeway over the ditch which is flanked by low concrete walls topped with original metal fence posts and a modern wire mesh fence. Originally the whole fort would have been enclosed within a high steel fence with gates hung on steel girders at the entrance (traces of which survive). Where the roadway cuts through the rampart it is flanked by concrete retaining walls. The rampart is a massively built earthwork, part revetted in concrete, with a parapet, banquette (infantry fire-step) and terreplein (a platform or level surface on which heavy guns are mounted). Along the crest of the rampart are surviving traces of the brick-revetted emplacements for machine-guns positions some with small expense magazines set into the rampart. (Nine appear to be shown on the Bradshaw plan of 1922). At the north-east corner of the fort is an additional structure of Second World War date, possibly a fire watchers post. This is of brick and concrete construction and has a protected entrance. A number of traverses project from the internal face of the rampart, the majority of which are mounded over magazines or casements. There is a linear central traverse which crosses the parade in a broadly north-south axis, centrally to the fort, curving to the north-east at its northern end. There are three magazines cut into traverses to the north-east (Building F3), south-east (Building F5) and south (Building F6). These are concrete built and cellular providing chambers for the storage of ammunition (shells and cartridges). All are provided with safety lamp recesses with glazed and metal grill covers: these recesses ensured that the flame of the lamp was separated from, but lit, the ammunition storage areas. The earth-covered reinforced concrete roofs have an added layer of flint within the earth cover designed as a bursting layer to detonate any enemy shell before it reached the magazine. There are also three sets of casemates, to the west, north and east. All the casemates are concrete built and cellular although with some discreet elements in brick. The northern (Building F2 which was not available for inspection) is cut into the northern end of the central traverse. In plan this has nine casemates running broadly north-south with a covered access corridor to the west. An associated ammo store is cut into the rampart to the north-west across the access passage. The eastern group (Building F4) is cut into the rampart. These casemates were available for inspection and have access steps leading down from the north parallel to an access ramp. To the east of the covered corridor are the four chambers. These have solid planked double doors with timber-framed overlights. The passage is now (2012) covered in a clear plastic corrugated roofing material and there is extensive pipework in the corridor above head height. The western and largest group of casemates (Building F8) comprises 17 chambers with a reinforced concrete roof, and also two ammunition stores for small arms (Buildings F7 and F9) set west of the concrete revetted access passage within the rampart. It was originally earth-covered but this has been removed. Two flights of steps lead from the passage up onto the rampart. Access was prohibited at the time of inspection but it is understood that these casemates have experienced some minor alterations for use from the mid C20 onwards but that their C19 form remains legible. Photographs from c1989 show that the corridor, its roof covering and chamber entrances are of the same form as Building F4. However, many of the entrance overlights had been bricked-up at that time. The same photographs suggest that the internal chamber divisions are at least partly brick built. At least one C19 hydrant survives within the fort. EXTENT OF SCHEDULING The monument excludes all buildings and structures constructed post-1900 (although the ground beneath them is included), the one exception being the Second World War firewatcher’s post on the north ramparts which is included. For the avoidance of doubt Buildings F2, F3, F4, F5, F6, F7, F8 and F9, which are all part of the C19 fort, are included in the scheduling as are any post-1900 modifications to these structures. Former entry on the Register of Scheduled Monuments: A polygonal "place of arms" or assembly point and store built sometime between 1895 and 1897 as part of the defences of London. Consists of a concrete revetted moat, vaulted barracks on the W side of the interior, a magazine on the E. Had the potential to be armed if necessary. Now used by MOD (AERE).