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Fort Halstead: Building F11, Sevenoaks

Description: Fort Halstead: Building F11

Grade: II
Date Listed: 21 March 2013
Building ID: 1412292

OS Grid Reference: TQ4986259145
OS Grid Coordinates: 549862, 159145
Latitude/Longitude: 51.3117, 0.1488

Locality: Sevenoaks
Local Authority: Sevenoaks District Council
County: Kent
Postcode: TN14 7BP

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Listing Text


Experimental Filling Shed (Building F11), Fort Halstead, 1938.

Reason for Listing

Building F11 at Fort Halstead, an experimental filling shed of 1938, is listed at Grade II for the following principal reasons:
* Rarity and early date: this is the earliest surviving purpose-designed building associated with rocketry research and development nationally. Most buildings associated with this area of research are of post-war date rather than pre-war as here;
* Design and form: the building’s function is legible through its form, the internal vertical bays to accommodate the filling of 9 foot rocket casings survive particularly well;
* Historic interest: F11 is significant as part of the early research programmes at Fort Halstead under the Projectile Development Establishment and, given the specialist nature of this work, is of national interest.


Fort Halstead is one of fifteen late C19 mobilisation centres established to defend London in the event of invasion. Designed in 1894, it 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.

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 first atomic bomb (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. Additional structures for this research were built in and around the fort, 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.

Designed in February 1938 and erected in the same year. F11 is probably the earliest surviving building in the new research establishment and it was designed to allow the filling of cordite rocket motors. It was later dubbed ‘Poole’s Folly’ as there are questions as to whether or how much it was used for its intended function. However, this is the earliest surviving purpose-built rocket-related building in England.


The Fort Halstead site is located on a prominent hilltop, which is part of the North Downs, to the north-west of the village of Dunton Green. The site includes both the late C19 Mobilisation Centre ('Fort Halstead’) in the south of the site and a large number of buildings associated with the research facility to the north and north-east of the fort. Building F11 lies within the fort.

A two-storey concrete-framed building encased in buff brick laid in English bond. F11 is L-shaped in plan and has a sloping concrete framed roof. Its principal elevation faces east which has an external metal stair leading to a first floor covered walkway. There are two pairs of double doors to the ground floor and two pedestrian doors to the first floor, all with glazed lights. The fenestration comprises metal framed casements. Its rear (west) wall has four further external doors (two to the ground floor and two to the upper floor but now blocked) possibly all emergency exits, plus further metal framed casements. The south elevation has paired replaced casements to both floors. The north elevation is blind. Over-head cable gantries extend north from the building.

The interior was not inspected but it is known from English Heritage’s Research Department report that internally the southern half of the building is occupied by brick-built vertical filling bays (to accommodate rocket casing for the assembly of a 9 foot rocket).

Listed building text is © Crown Copyright. Reproduced under licence.