Site Records

Site Name: RAF St. Twynnells 'TWY' GCI R6 ROTOR Radar Station

St. Twynnells

RSG site visit 26th October 2002

[Source: Nick Catford]

Note: A comprehensive image gallery of this site is available here.

In the summer of 1940, a west coast Chain Home (CH) radar station called RAF Warren was established at SR92459757, in July of the same year a Chain Home (CHL) Low radar station with a duplicate was built at SR92149727 and called RAF St. Twynnells, this is 1/4 mile south west of RAF Warren. In 1941 a brick combined transmitter and receiver block replaced the earlier CHL huts at St. Twynnells.

Both Warren and St Twynnells were involved in trials which took place in December 1941 to test the ability of IFF Mk III to respond in various kinds of radar station. A total of eight ground radar installations, seven aircraft and a sloop demonstrated the value of IFF Mk III to British and American observers and universal adoption was recommended.

For further information on CH and CHL radars see:

In the 1950's when the ROTOR radar project was set up, a GCI station was constructed at SR94189741 in an R6 two level surface bunker. This was also called RAF St. Twynnells and was just over a mile from RAF Warren. The target date for completion was 18th September 1953.

Photo: The R6 block and associated buildings viewed from Merrion Camp
Photo: The R6 block and associated buildings viewed from Merrion Camp
Photo by Nick Catford

PRO File AIR20/10699 states that on 17.6.52 Treleaver and St. Twynnells (No's 15 and 16 in the GCI stations listed) were the only two stations without installation and completion dates. It also confirms that economies already made at GCI stations in anticipation of the introduction of Stage 1A radar (Type 80) resulted in the deletion in 1952 of Type14 Mk 8, Type 14 Mk 9 and Type 7 search radars that had been planned. A Type 80 Mk3 (control version) was to be fitted by November 1953. However by 30th June 1953 St. Twynnells had been deleted from the outline plan 'Operation ROTOR 2 and 3'. The fitting out of St. Twynnells had been moved to last in the GCI station priority lists and AIR2/10984 Appendix A shows a control 1A RADAR (Type 80) that was planned for this site was deleted before fitting.

Type 13
AMES Type 11
As the only search radar ever to be installed at St. Twynnells was the mobile Type 11 Mk 7, it is debatable whether the station ever became fully operational . The Type 80 Mk3 modulator building was built and still stands in an adjacent field on the opposite side of the nearby road. Radars installed at St. Twynnells were 1 Mobile Type 11 Mk 7, 2 Type 13 Mk 6 and 3 Type 13 Mk 7.
The full compliment in a fully operational R6 was 80 people per watch. There were only two watches with no cover overnight. The intercept cabins were the most crowded with 9 people in each comprising 1 controller, 1 supervisor, 2 PPI readers, 1 DR navigator, 2 height readers, 1 azicator (azimuth reader) and 1 recorder. The Fighter Marshall had 1 assistant and 2 fixer plotters for liaison with triangulation personnel at the Sector Operations Centre (SOC). The Chief Controller had 1 assistant and an Anti Aircraft (AA) liaison officer. The operations Room had 20 plotters and supervisory personnel, 8 tote fighter screen personnel and 1 teller. There was no allocation for visitors anywhere in the bunker.
AMES type 13
AMES Type 13

After closure, the site reverted back to the local farmer and the majority of buildings associated with the station remain largely intact. Only the standard RAF style guardhouse (slightly shorter than the standard ROTOR guardhouse) is in separate ownership. Standing alongside the road it has been converted into a house and externally remains largely unaltered except for the veranda, which has been modified to form an extra room.

The standby generator house stands at one end of the operations block and is now an empty shell, at the other end is the transformer building which still retains its rusting transformer. One radar plinth stands in the field along with a small building with some electrical equipment, this may have been for the mobile Type 11 radar but this has not been confirmed. Closer to the road there is an underground Royal Observer Corps post, this has been disused since 1991.

On the opposite side of the road there are four more radar plinths and the Type 80 Mk3 modulator building. The disused radio station in the field to the north of the modulator building is not believed to be anything to do with the site.

St. Twynnells was one of 5 R6 bunkers built, the others being at Hack Green (later converted into an RGHQ and now a museum), Hope Cove (later converted into an RGHQ which closed in 1993 and now used for storage), Langtoft (a scrap dealers store) and Treleaver (largely derelict with some farm storage). Of the three R6 bunkers that were not put to later government uses St. Twynnells is by far the best preserved. Although all the radar equipment has been removed much of the plant remains intact as do some of the original signs and situation boards. Unusually for a derelict ROTOR building, all the flimsy internal partition walls are intact and in good condition, many retaining their original paint. There is some standing water on the lower floor but this is 'below the floorboards' and rarely more than an inch in depth. Most of the surviving equipment, plant and artefacts are on this floor.

Although the building was open for a number of years it is now securely locked and visits are discouraged. No use has ever been found for the building and the owner is considering applying for an EC grant to demolish it.


Members of Subterranea Britannica visited the site on 26th October 2002. There were no restrictions on photography and we were given unlimited time to explore the bunker. A comprehensive image gallery of this site is available here

We entered via the emergency exit located at the top of a short flight of stairs covered by a porch protruding from the eastern end of the blockhouse. At the top of the stairs are the original steel blast doors which open onto the upper six foot wide east - west spine corridor. Before going through the blast doors there is a dogleg on the right into the main air intake room with ventilation trunking running from the main intake high on the south facing wall. There are two small fans mounted on the wall.

Upper floor plan
Upper floor plan
Drawn by Dan McKenzie

Key to floor plan

  • 1 = Guard Room
  • 2 = GPO Store
  • 3 = Chief Controller's Office
  • 4 = Operations Office
  • 5 = Unknown
  • 6 = Technical Officer
  • 7 = RAF Cloakroom
  • 8 = Kitchen
  • 9 = Chief Controller's Cabin
  • 10 = WRAF Officers Rest Room & WC
  • 11 = Unknown

Once in the main corridor, the four foot wide back stairs down to the lower level are on the left. Beyond this the 'domestic' rooms are on the left consisting of RAF and WRAF rest rooms and toilets. All the rooms are empty with some vandalism as many of the WC's and hand basins are smashed. There is a small kitchen between the RAF and WRAF rest rooms with a serving hatch into each of these rooms. The kitchen still has a tiled wall, sink, cupboard and a food preparation table and serving counter along two walls. In the RAF rest room the ventilation trunking which runs through each room just below the ceiling turns sharply downwards through the floor to the lower level where it feeds the apparatus fan in the plant room below.

Beyond the domestic rooms is the GPO power room which again is empty apart from an old piano and organ. As the GPO equipment would have been powered by lead acid batteries there are Darlington acid proof tiles laid on the floor. Beyond this is the GPO store and the main stairway (also 4 feet wide) down to the lower level. The winch that was fixed to the ceiling above the stairwell has gone. Beyond the stairs a short corridor to the left leads to the PBX room which is again empty. When in use there would have been 240 speech circuits 4 teleprinter circuits. The main corridor then turns to the right through the second set of blast doors to the main entrance at the bottom of another flight of stairs in another protruding porch. There is a small guard room at the bottom of the stairs.

On the right hand side of the spine corridor the first room is the officers rest room. Beyond this are three rooms with windows looking down into the well of the two level operations room. The three rooms are 'Intercept Cabin No. 4', 'Chief Controllers Cabin' and the 'Fighter Marshall's Room'. All the floorboards have been removed from these rooms but the glass windows are surprisingly unbroken. Although fully equipped Intercept Cabin 4 would not have been used unless we were at war.

Photo: Flight Marshall's room with windows overlooking the Operations Room
Photo: Flight Marshall's room with windows overlooking the Operations Room
Photo by Nick Catford

Beyond these, a short corridor on the right leads into the 'Synthetic Trainers Room' There would have been three film trainers where film of an intercept could be fed back into the training apparatus. There are four small offices accessed from both sides of this corridor. Beyond this on the right is the 'Track Telling Room' again the floorboards have been removed although the under floor cable trunking remains in place.

The final room on the right had no raised floor and is accessed down a short flight of wooden steps. On original drawings, this room is named as the or 'Combined Filter Plot' (C.F.P). Two Kelvin Hughes projectors would have been mounted in the room below, but as no Type 80 radar and associated equipment was ever installed at St. Twynnells it is unclear what the room was used for.

There is a five foot high cableway below the spine corridor, it was possible to lift up a trapdoor in the floor and climb down into this. The cable hangers are still in place along both sides of the wall although the cables have all been removed. A number of truncated cables still run through the wall into this area.

The lower level.
The lower level.
Floor plan drawn by Dan McKenzie

From the bottom of the back stairs the first room entered from the left of the lower spine corridor is the air conditioning plant and switchgear room. This is subdivided into a number of small rooms with brick partitions, there is also a raised area accessed by a ladder (removed) from the floor of the plant room and through a narrow door in the back into the corridor.

The plant room is accessed through double doors leading onto six wide concrete steps. Two compressors were mounted to the left of the steps with two compressed air cylinders above them, all that remains now are the two concrete plinths where they were mounted. At the back of the room within its own room the main air conditioning fan is still in place and behind it in a separate room a large bank of filters standing floor - ceiling. The ventilation trunking runs up to another fan above.

The control cabinets and switchgear would have been to the right of the entrance steps but this has all been removed. Opposite the steps is the raised area with the control equipment and pumps for the Baudelot heat exchangers (these circulated water over cold refidgerent [Methyl Choride!] lines from the A/C compressors, chilled water being pumped from here to a header tank then, on demand, into various in-line air chiller units in the apparatus air supplies). There is a control box with five circular dials on it. The heat exchanger unit consisting of coiled metal pipes is mounted behind the pumps. Water would have flowed by gravity over the outside of the pipes. A narrow raised walkway runs alongside with two narrow doorways into the apparatus fan room; the fan is still in place. The ventilation trunking feeds into the fan from the upper floor.

There are a number of signs still in place on the pumps and fans and one on the water tank for the heat exchangers which says 'check level of water in Baudelot tank weekly this should not be lower then 2 feet below the overflow when all pumps are static'. Water levels were critical in this type of A/C plant and closely monitored.

Beyond the plant room is the radar generator room with two large concrete motor beds which mounted motor-alternator 'motors' (50/500hz rotary converters). Their purpose was to take unregulated (regulated supplies from the main supply transformer being reserved for more critical uses, such as radar consoles and equipment) electric supply and convert it into 'radar compatible power' (high frequency and high voltage), the motor-alternators have been removed. Beyond this is the Radar Room and then the GPO apparatus room both of which are now empty.

On the right hand side of the spine corridor the first room entered is 'Intercept Cabin No. 1' with it's glass window still intact looking into the 'Operations Room'. There is a large wall board on the back wall consisting of a notice board with blackboards on either side with painted headings. The left hand board is headed 'Weather' with sub headings: 'Date', 'Time', 'Height', 'Wind speed', 'Wind Direction','Temp', 'Tropopause Height' and 'Contrail Height'. The right hand board has columns headed 'Base', 'COD', 'Call sign' and 'Frequency GCI'

The next room on the right is 'Intercept Control Cabin No 2' which still has its glass window looking into the 'Ops' room. The 'Projector Room' is next, this always had a glassless window with a counter and wooden steps down into the operations room at one side. Images could be projected onto the wall of the 'Ops' room wall from here. The final room with a window into the 'Ops' room is 'Intercept Cabin No. 3'

Photo: The Operations Room
Photo: The Operations Room
Photo by Nick Catford

Beyond this a short corridor leads into the large 'Operation Room' itself which was originally entered down a flight of three steps. This is the largest room in the bunker spanning both floors. 28.5 feet high with windows looking into it from both levels. The floor is damp but there is no standing water. The wooden tote board frame is still in place along the left hand wall although the tote slats have been removed. The pre-printed tote panels would have been slotted into place from behind. The wording above the totes says 'Mission Tote' and 'AA States - Frequencies' There are wall boards on either side of the tote, the board on the right has the heading 'General Weather' and below it 'Airfield State' with 16 columns. The boards were reached by a wooden staircase with a hand rail leading to a raised walkway behind the tote..

Part of the sloping wooden framework that supported one of the large plotting tables is still in place on the floor. There were two circular sloping tables 8 - 9 feet in diameter. One was the 'Fighter Table' and the other the 'General Situation Table'.

Beyond the 'Ops' room entrance corridor the next room on the right is the 'L' shaped radar office with a radar workshop filling in the 'L'. The floorboards have been removed but the under floor metal cable trunking is still in place with various connection sockets and distribution boxes. At one point the trunking runs up the wall with all the cables still in place.

The final room on the right hand side is the 'Utilisation Room' which would have housed the the Kelvin Hughes projectors. The metal beams to support the projection table above and associated air conditioning trunking are in place at ceiling level but the projectors were never installed. This was also the VHF radio room.

Returning to the other end of the lower corridor three concrete steps lead down into a lobby with three further rooms one of which has a further three wooden steps up again. This is the voltage regulator room which still has a large floor standing electrical control cabinet. At the back of the room there's a strange device that resembles a robot but is, in fact, the main power intake.

A septic tank and filter beds at the bottom of the field complete the structures on this site.

Those taking part in the visit were Nick Catford, Keith Ward, Bob Jenner, Robin Ware, Robin Cherry, Dominic Jackson, Dave Mansell and Dave McKeever

A comprehensive image gallery of this site is available here

[Source: Nick Catford]


  • Bob Jenner
  • Ian Brown
  • PRO Files AIR20/10699 and AIR2/10984 Appendix A

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Last updated 14th November 2002

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