Grand Slam – largest bomb ever dropped by British forces

The first Grand Slam bomb dropped. It was live! Target Ashley Walk "Sub Pen". In photo River Avon and Sandy Balls seen below, ground position SU 1649 1456 or 50°55'48.04"N - 1°46'0.44"W. 13-03-1945
Author: Gareth Owen

On 13 March 1945 the largest bomb ever dropped by British forces, during WWII, was first tested ‘live’ in the New Forest at Ashley Walk Bombing Range.

The Grand Slam was a 22,000-lb MC high explosive deep-penetration bomb designed by Barns Wallis following on from his creation of the smaller Tallboy (a 12,000-lb MC deep-penetration bomb). Due to limited production and testing time the first test of this bomb used a live Grand Slam.  The test range was Ashley Walk and it was dropped from a modified Avro Lancaster.

The bomb aimer sighted on the Ministry of Home Security Target, known locally as the ‘Sub Pen’, and due to their height and speed released the bomb whilst flying over Fordingbridge.  The bomb then fell eventually hitting and exploding 100 yards from the target.  The crater it made was 140 feet wide and 70 feet deep.

On impact the bomb would penetrate deep into the ground before exploding and could also penetrate reinforced concrete. The successful trail enabled the decision to be made in March 1945 to make 40 Grand Slam bombs and they were dropped on the V2 and V1 launching sites in the Netherlands as well as against viaducts, railway tracks (deep in tunnels) and U-boat Pens.

On this site the Ashley Range Overview page has links to pages about the other targets on the range.

March 2017
Documents from the National Archives added:

AIR 14/2011
“Tallboy” and “Grand Slam” bombs: Trials and development – 1944 Jan-1945 May
Former reference in its original department: IIH/258/1/157 5G/50/12

  1. 1945-03-07 – AIR 14-2011 – Estimated crater size and volume for “GRAND SLAM”
  2. 1945-03-12 – AIR 14-2011 – Grand Slam terminal velocity of 4,580 feet per second. (Note; no year is given on this document, but we suggest 1945)
  3. 1945-04-09 – AIR 14-2011 – Code name “GRAND SLAM” compromised. Grand Slam is to known as the 22,000lb M.C. and the Tallboy as 12,000lb M.C.
Date: 1945
  1. John Murrell

    This may be the largest bomb dropped by British Forces but I believe the Atomic Bombs dropped by the RAF as part of the UK tests had a greater equivalent explosive power. So it depends on how you define largest.

    The Orange Herald Atomic bomb was reported to have an explosive power of 720 to 800 kilotonnes of TNT very much larger than the Grand Slam.

  2. Gareth Owen

    Hi John
    Thanks for the comment. You are correct in that the Atomic Bombs were ‘larger’ than the Grand Slam. Our article here is referring to Second World War history and in this case the Grand Slam was the largest ordinance ever dropped. To help clarify the historical period being spoken about I’ve added “during WWII” in the intro’ paragraph.

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