Kenley's Blast Pens

When the threat of war was imminent in 1939, a lot of hasty building work was carried out at Kenley to make it ready for modern warfare.

It was thought that keeping aircraft in hangars was not a good idea as one direct hit from an enemy bomb could wipe out an entire squadron.

So, to provide a better level of protection, it was decided to “disperse” the aircraft around the airfield. To further increase protection, blast pens – also known as fighter pens or E-pens – were built. The initial building of these was complete and the airfield “ready” by April 1940.

Pen 9 As it Probably Would Have Been in 1941

Originally there were 12 pens at Kenley, of which all except one still exist in various states of completeness. These would provide protection for two squadrons, a maximum of 24 aircraft. Aircraft could also have been positioned around the airfield on various triangular and pentagonal hard standings.

There seems to be no consistent way of numbering the pens, so, in this document, they are referred to as 1 to 11 in a clockwise direction from the Victor Beamish Avenue entrance/hangar area.

This is also convenient as it means number 12 is the one that has been completely demolished. Numbering the other way around would mean that the first one encountered would be number 2.

Pen 2 As it is in 2020

Each pen was the shape of a capital “E” when viewed from above, so forming two “bays” within each pen. Earth-covered revetments surrounded the aircraft on three sides providing some protection not only for the aircraft, but for anyone refuelling, rearming or repairing them. The angled nature of the revetments would direct any nearby blast up and over the bank, but were no protection against a direct hit.

In this document, the component parts of the pen are referred to as “Rear Wall”, “Side Walls” and “Centre Wall”. “Wall” is used to mean the entire construction of brick/concrete/earth. The term “Spine Wall” is often used to refer to the centre wall, although technically, a spine wall runs parallel to the long axis, so should really be the rear wall!

Every pen at Kenley was built to a subtly different specification. For a start, there are two different overall sizes. Pens 1-4 and 9-12 have two bays each of 16.6m wide x 16.5m deep, whereas pens 5-8 have two bays each of 24m wide x 19m deep. Most aircraft at Kenley were fighters, either Hurricanes or Spitfires, but other types were also present and presumably it was intended that pens 5-8 could accommodate larger aircraft if need be.

Pen number 2, also known as the “Tribute” pen, gives the best idea of how they would have looked originally, although the centre wall is a reconstruction from 2017. This pen still has the frontal brickwork in place, whereas most of the others have lost these post-war creating a sloping earth bank to the front.

Pen 2 Viewed from Front

The rear wall contains a shelter which was built from pre-cast concrete sections. There are steel doors at both ends. The left hand doorway provides access not only to the shelter, but also to the rear via a dogleg. The right hand doorway provides access to the shelter only.

Pen 9 Rear Wall Shelter Exposed During Restoration Works 2017

The shelter would have provided protection for personnel during an air raid, but they were also used for storage.

Pen 2 Rear Wall Shelter Interior

Perhaps the most noticeable difference between the pens is when viewed from the rear. The fronts of the pens are all virtually level with the perimeter track, but, mainly due to the topography of the airfield, some of the pens have contour drops behind them.

Pen 2 from Rear Showing Low Brick Wall

This necessitated the construction of larger reinforced concrete walls at the rear as opposed to the low brick wall seen at the rear of pen 2.

Pen 9 from Rear Showing High Concrete Wall

Some even need to have a slope leading up to the rear doorway.

Pen 9 from Rear Showing Ramped Access to Door

The two side walls are constructed from banked earth with a poured concrete “lintel” on top. A concrete retaining wall/sill is present on the inner side, but there is no retaining wall on the outer side.

Pen 7 Side Wall Showing Concrete Lintel During Archaeological Dig 2018

During the archaeological dig in 2018, the construction of pen 7 was investigated and a metal rod was driven through the earth below the lintel and no internal brick or concrete structure was found.

Cross Section through Side Wall

The centre wall was originally just a simple wall, 215mm wide and 2.5m tall and may have had sandbags surrounding it for extra protection. There is some photographic evidence to suggest that the central wall may initially have been constructed entirely from sandbags. The wall originally went all the way up to the rear shelter section with no easy access to the rear between the two bays. The concrete retaining wall/sill sections on the rear and side walls must have been added after this wall was built as you can see a cut-out in this concrete sill in the middle of the rear wall.

Inside Pen 2 Showing Rear Wall Retaining Wall/Sill with Centre Wall Cutout

At a later date, the centre section was modified to the prism-shaped version with earth banks as seen on the pen 2 reconstruction, although the end wall would have been triangular.

It was also shortened at the rear to allow access between the two bays and a small additional simple wall was added at the same time to reduce the effects of a blast in one bay affecting the other. This modification was probably carried out following the famous raid of the airfield on August 18th 1940. The centre walls were certainly in this new configuration by November 1941.

Cross Section through Centre Wall

There are several metal rings set into the gound at various postions inside the pens. These would have been used as anchor points for tying down the aircraft. 

Pen 10 Tie-Down Ring

The thickness of the concrete roof/ceiling above the doorways is another noticeable difference between the pens. For example, on pen 2 it is a 220mm thick rectangular slab whereas on pen 11 it has an additional cast concrete section creating an overall height of 800mm.

Pen 2 Concrete Section Above Doorway

It is thought that all pens would have had the additional concrete sections, but pens 1 and 2 were modified and reduced in height at some later point. This additional height would have allowed for a thicker layer of earth banking above the internal shelter than that on pen 2 today.

Pen 10 Concrete Section Above Doorway

Three of the pens had rectangular and/or circular concrete constructions on the top of the side/rear walls.

Pen 8 Rectangular Concrete Machine Gun Post

These would have housed anti-aircraft machine guns and the swivel mount for one of these can still be clearly seen on pen 8.

Pen 8 Machine Gun Post Mount.

Each pen had a concrete “pathway” to each side which contained a series of metal rings. It is unsure as to the purpose of these, although a fixing for some form of camouflage netting seems to be the current thinking.

There are a series of bolts fitted into the concrete “lintels” along the top of both the side walls and the centre wall, so maybe the camouflague netting was fixed to these. If so, there would have been some form of post or pole fitted to the bolts as the netting would have needed to be much higher to clear any aircraft in the pens. A Hawker Hurricane, for instance, was 3.99m high, whereas blast pen 2 is only 2.3m high. No photographic evidence showing netting has been discovered.

Pen 9 Concrete Strip/Path with Tie-Down Rings

All photographs, diagrams and drawings by Neil Broughton.

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