Vertical spinning tunnel

The vertical spinning tunnel, built between 1948 and 1955, is a Grade II listed structure. It is constructed from pre-formed steel plates and has a circular plan. The tunnel is attached on its south-west side to a large rectangular building that previously housed offices and laboratories, while a square former machine hall adjoins it on the south-east side. However, these two buildings are not included in the listing.

The exterior of the tunnel features a vertical steel pressure-shell shaped like a cylinder, approximately 24.4 meters high and 13.7 meters in diameter. It is divided into sections by regularly spaced, full-height ribs. A cylindrical column on the south side serves as the lift shaft to the motor room, which also contains an emergency staircase. Two beams extend from the top of the structure, providing support for an external hoist used to lift heavy electrical equipment to the motor room, which is surrounded by a wide horizontal band of vertical metal-framed windows.

Inside, the working section of the tunnel is about 9 meters high and has a circular cross-section with a diameter of 4.6 meters, effectively creating a tube within a capsule. There is a catch net below the working section to safely land models as the tunnel speed decreases, and another net above to prevent any models from reaching the fan. The original electric motor, which was mounted above the tunnel and accessed via a metal open tread staircase, has been replaced in the early 21st century, although some original laminated teak blades of the fan remain on site. Between the outer wall of the tunnel and the working section are several small rooms, including an observation room, which are sealed with pressure-tight doors similar to those found in submarines.