Blast pen
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A blast pen was a specially-constructed E-shaped double bay at British RAF World War 2 fighter stations, being either 150 feet or 190 feet wide and 80 feet front-to-back, accommodating aircraft for safe-keeping against bomb blasts and splinters during enemy air-attacks.
Although the pens were open to the sky the projecting sidewalls preserved the aircraft from lateral damage, with 12-inch thick, 9-feet high concrete centres, and banked-up earth on either side, forming a roughly triangular section 18-feet wide at their base. The longer spine section behind the parking areas usually encloses a narrow corridor for aircrew and servicing personnel to use as an air raid shelters.
Examples may still be seen at the present Kenley Aerodrome and at North Weald aerodrome in Essex close to "The Squadron" bar, some pens have had their second bay removed over the years, thus becoming U-shaped.
