Operation Grapple Y - 1958

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Grapple Y was conducted in the spring of 1958 following the extremely successful Grapple X trial. The planning for this operation was done in super-secrecy, only a few informed individuals knew of the proposed post-Grapple trials. The theoretical design of the Grapple Y device was essentially complete by Grapple X and in September 1957 early lines of development were being outlined.

Several test designs were proposed for Grapple Y:

1.Double bomb with a spherical Dick secondary,
2.Device similar to Grapple X Round A but with improved compression
3.Technologically complex device, abandoning separate combined fissile and fusile stages in favor of shells made of intimate mixtures of these materials. These shells would consist of micro-sized particles of uranium-235, uranium-233, and lithium deuteride.

A February 1958 deadline was set and the final decision would have to be by November 1 on the test device. It was determined that the third mixture device would not be ready by the proposed deadline and would be tested in a possible summer or fall 1958 trial. Meanwhile, calculations were being made on different versions of a layered Dick component. These included 5-shell stage, cylindrical stage using using lithium-7 deuteride, and a non-cylindrical stage that used more lithium-7 deuteride at the expense of uranium-235. Eventually, the non-cylindrical device was chosen for the Grapple Y trial.

Calculations on a five-layered secondary could not be completed by the February deadline so it was decided to use a three-layered component, similar in design to the Round A bomb. This device would use greater amounts of lithium deuteride. A test of the five-layer system would have to be postponed until later trials.

A primary objective of Grapple Y was not only to produce a larger yield but also to improve the efficiency of the thermonuclear device. Great efforts were made to rely less on the fast fission of the uranium present in the Dick and rely an a large yield from the lithium deuteride. Accurate diagnostic measurements would be made to determine what exact nature the reactions inside the bomb. Efficiency would be measured by using radiochemical samples. However, the presence of uranium-238 in both stages would present difficulties in discriminating between the yield of each. It had been prosed to use thorium to substitute the natural uranium tamper. Meanwhile, staff shortages were plaguing the development process.


Restricted bomb loading site at Christmas Island

Early estimates on yield ranged from 2 to 3 megatons. Current firing procedures at Christmas Island were based on 3 megaton maximum yield and would need to be altered for Grapple Y. A restricted yield on the Grapple Y test was rejected in favor of producing a yield as high as possible for the design. The ground zero for the test would have to moved out 10 miles for the test. This presented a problem as the diagnostic instruments would not collect as accurate data on a more distant shot. Collecting accurate diagnostic data was a critical objective of the Grapple Y trial.

While development of the test device was underway at Aldermaston, preparations were being made at the test site. Extensive measurement and monitoring programs were put in place, both for weapon diagnostics and safety purposes. Great effort would be made in determining the gamma radiation output from the device, as well as the effects of the explosion on the drop aircraft.

Facilities for for measuring radioflash from the explosion had been set up on Fanning Island, along with a mobile caravan and cooler unit, air-cooled hut, three generators and a Land Rover for the electronics measurements group. Air, water, and fish sampling would be made in Honolulu, Fiji, Samoa and Raratonga, Kwajalein Atoll, Canton, Penrhyn, Malden and Fanning Islands. Equipment used for air sampling of fallout consisted of motor-driven air filters, 'sticky papers', and rain-water bottles.

Considering the high predicted yield of the test, modified Canberra samplers equipped with Napier Double Scorpion rocket engines would be needed to reach the high altitudes the atomic cloud would reach. It was planned that one hour after shot time the conventional Canberras would obtain samples from the cloud at 50,000 feet, followed by two rocket-assisted Canberras taking samples at 60,000 feet two hours later. However, an accident to another Scorpion-powered Canberra at another location made Air Vice Marshal Grandy unwilling to use them at Christmas for Grapple Y until the exact cause of the accident was found. The air sampling missions for this trial would be conducted only by conventional Canberras.


The Test

Code Name: Grapple Y
Time and Date: 19:05 April 28, 1958 (GMT)
Location: Christmas Island
Height: 7200 Feet
Type: Air Drop
Predicted Yield: 2700 - 5000 kilotons
Actual Yield: 3000 kilotons

The period between January and February 1958 marked a busy time for the preparations team. Meteorological monitoring flights began in February, by mid-March the Joint Operations Center at Christmas Island was up and running. Practice bombing began at Christmas at the end of March. Meanwhile, the Grapple Y Tom primary was on its way to Christmas by sea. A back-up Tom would be flown to Christmas Island on April 20, in case faults were discovered as had been during Grapple X. Two days later the Dick secondary would be delivered.

The safety parameters of the personnel on Christmas was revised for Grapple Y. Thousands of anti-flash hoods, gloves and white boiler suits, nicknamed 'Zook' suits, were provided for the observers. Special attention was given to the protection against blast given the damage caused by Grapple X. Additional venting was given to existing buildings and despite the preparations, some damage was expected and would have to be repaired at a later time.

Throughout April a program of fly-overs, HE practice drops, and operational rehearsal was conducted. The live drop was scheduled for April 28 depending on weather conditions. Task Force Headquarters issued safety instructions on April 21:


All personnel on Christmas Island were issued anit-flash gear for the test

“After the weapon has exploded and the thermal flash has passed, it will be followed approximately one to two minutes later by a loud bang and blast wave. The blast wave will cause loose material, rubbish, etc. to be blown before it and it may cause some damage to buildings.”

To prepare for this, loose items were secured for the shot, glass windows and doors were to be removed and stowed, and all electrical equipment not essential for the test would be switched off. Tall cupboards would be laid on their sides and secured, telephone handsets had to be tied to the receiver, vehicles parked with the rear end facing ground zero and all windows left open, pets would be placed in suitable, humane containers and be the responsibility of their respective owners. A large effort was made to round up and secure stray animals for the shot.

On shot day the Valiant drop plane, piloted by Squadron Leader Bob Bates, took off from Christmas Island. Cloud cover during the morning resulted in a 1.5 hour delay prior to take off. The weapon would be dropped from an altitude of 46,000 feet with the assistance of a T.4. Bombsight and the moment of release being automatically delayed to allow the weapon to overshoot the visual target by 1.5 miles. The bomb itself was fused to explode some 53.5 seconds after release at an approximate altitude of 8,500 feet.

Once the drop plane reached the bombing altitude, Squadron Leader Bates determined the conditions were suitable for a visual drop, although cloud cover obscured several places on the island. The plane conducted an initial practice bombing run before being directed to proceed with the live drop. The bomb was dropped at the correct instant of release and the aircraft then carried out a normal escape maneuver. The bomb exploded at 19:05 on April 28, 1958 (GMT). No damage was reported by the captain from the effects of the flash and blast and was cleared back to base some 5 minutes after the weapon exploded. It was determined the bomb missed its intended zero point by 245 yards.


Group Captain Kenneth Hubbard, who witnessed all five Grapple explosions to date, described the explosion:

“Had we been standing without protection, there is no doubt we should all have been blown off our feet ... the majestic build-up of the mushroom shaped cloud ... towering to a great height. The fireball appeared as a huge red and orange cauldron of fantastic energy, which gave the impression of revolving. As it did so it emerged at its apex into a stream of orange-coloured cloud mass moving upwards all the time, and as it ascended the colour changed to white. Then somewhere in the region of 50,000 feet it curved and fanned outwards from its center making a cap similar to the top of a mushroom. All the time this fantastic formation moved upwards, progressively increasing the spread at the top until it stabilized, then the edges of the actual mushroom shape partially drifted from the main structure as upper winds began to carry these white whiffs downwind.”


At 3 megatons the test was a complete success. Grapple Y remains the largest nuclear test conducted by Britain. Immediately after the test weather conditions deteriorated and any further delay would have pushed the shot back at least one week. Cloud cover resulted in poor photography of the explosion in some areas. No damage was reported to personnel or to buildings indicating the effectiveness of the pre-shot preparations.

Grapple Y Video
This video shows several views of the Grapple Y explosion. The first scene shows personnel, dressed in white anti-flash uniforms, with their backs turned to the immense initial burst of energy. Following this is footage filmed from the Valiant drop plane as it flew away from the explosion along the intended escape route. Two cameras mounted on the tail of the bomber automatically filmed the explosion. Cloud cover over Christmas Island obscured much of the footage from the ground as seen in the last few sequences. Click on the thumbnail image to the left to watch the video.

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Footage source: Britain's Cold War Superweapons
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Grapple Y Aftermath

The Grapple Y explosion had demonstrated Britain's ability to design not only large but also efficient thermonuclear weapons. The device featured an enhanced thermonuclear yield and was a highly successful hydrogen bomb. However, Britain's thermonuclear capability was still not perfected. A “1 ton/1 megaton' warhead immune to intense radiation from defensive nuclear missiles was still on the drawing boards. Design principles that did not make the Grapple Y February deadline remained unproven. Further nuclear tests would be necessary, Grapple Z was to follow later that year.

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