I became aware of Operation Grapple in July 1956. There wasn’t a big Top Secret stamp on it. Everyone in the country had an idea that Britain was trying to become a nuclear power. I was a 22-year-old co-pilot in the RAF, just happy to be chosen for the mission.

My main role was to monitor the flight instruments to make sure the captain was flying correctly. We practised dummy drops for months, flying Valiant bombers from Wittering airbase on the Cambridgeshire/Northamptonshire border to a range over Orford Ness.

After almost a year perfecting the drills with 1321 flight and, later, No 49 Squadron, we left for Christmas Island. Our drop proper came on 31 May 1957. The target was Malden, a tiny uninhabited island in the Pacific. After 50 minutes of flying we reached the rendezvous and the bomb aimer, one of five crew, went to lie down on a mattress in the well in the belly of the fuselage; from that position, he looked straight ahead through the bomb sight at the aiming point. We then made a practice run – up to the release point, then a sharp turn to get away.

When the order was given to proceed with the live run, we put metal shutters over all the windows in the cockpit so the violent light from the explosion wouldn’t affect our eyesight; the bomb aimer had to keep his shutter open until the drop, though. I could only see through a small piece of welder’s glass in one of them. Despite being 45,000ft above the Pacific, we were in total darkness, the captain, Sqn Ldr David Roberts, and I flew purely by instruments.

The bomb aimer took the safety latch off the weapon and said, “Steady, steady, steady”, to get us to hold our heading. As we crossed the release point, he pressed the button and announced, “Bomb’s gone.” We felt some elation, and the aircraft noticeably lifted as the 12,500lb bomb – codenamed Orange Herald – fell away. But almost immediately came the escape manoeuvre: Roberts rolled the aircraft into a steeply banked turn to get away from the upward rush of the exploding nuclear bomb. If we had carried on in a straight line, the fireball would have come up under the aircraft and that would have been it. The trouble was the accelerometer wasn’t showing any G-force at all. I told the captain “More G!” but as he pulled back on the stick we started to judder; the aircraft was stalling. We had been turning, we just hadn’t felt the added G. We realised the accelerometer wasn’t working, and the overcorrection sent the bomb aimer – who was making for his seat – crashing back into the well.

We knew that if we didn’t turn, we wouldn’t escape the explosion. Thankfully the captain regained control, the bomb aimer scrambled to his seat and, 53 seconds after the moment of release, the weapon exploded. Complete darkness turned to unfiltered daylight. It was as if there was no dark glass in the small aperture; you could see the sky and the land below as clear as anything. Three minutes later, we felt a biggish bump as the shockwave hit the aircraft, then a smaller one a second later. We saw the sky lit up; 10,000ft above our eye level was a writhing molten mass. I was in a state of awe.

At 85, I’m one of three aircrew still alive from Operation Grapple, and to my knowledge I’m the only person to drop two nuclear bombs. For my second flight, the captain was Sqn Ldr Barney Millett, and instead of targeting an atoll like Malden, the drop on 8 November was just off Christmas Island itself. Same procedure, this time no faulty instruments, though this was a thermonuclear bomb – a hydrogen bomb yielding 1.8 megatonnes of TNT – more than twice the power of the previous one.

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We were checked with a Geiger counter when we got back: it wasn’t mentioned that we were in major danger, so no one was in a state of jitters. To date, I haven’t had medical issues resulting from Grapple, but I know that people on the ground who watched the explosions claim to have been affected. I think the services took the precautions they believed sufficient, but I don’t want to decry the people who say they have fallen ill.

As for the ethics, my feeling was, it was a weapon of war. Whether it’s a 1,000lb bomb or a 10,000lb bomb, they’re not nice things to have, but they’re essential to defend our country. This was at the height of the cold war. The Soviet Union was a powerful nation; we assumed it would use nuclear weapons if necessary, just as we would have done.

• As told to Nick Thompson.

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