The V-3 “supergun” was meant to win the war for Germany. In 1943, for the first time since World War II began, Hitler was on the back foot. Allied bombs were devastating German cities and the Fuhrer was rattled. His proposed V-3 cannon would be the biggest gun the world had seen.

The V-3 was built in a truly enormous bunker buried deep in a chalk hill in northern France. Millions of tonnes of rock were excavated by hand and among the workers were hundreds of slave labourers. In its original conception, 25 barrels were to point at London – about 100 miles away – delivering up to one bomb per minute and to create an environment of fear that would turn the course of the war back in Hitler’s favour. And it was a doomed secret “drone” mission to destroy the V-3 that led to the death of Joe Kennedy Junior, a pilot and older brother of the future US president, John F. Kennedy.

The workings of the German supergun remain something of a mystery, because so much of it was destroyed and so few photographs and documents have survived. We estimated that a projectile reaching London must achieve speeds in excess of 1,500 metres per second. Each barrel of Hitler’s gun was 130 metres long, inclined at 50 degrees and we worked out that this is the perfect angle to reach London - they seem to have got it right.

The projectile was to be accelerated by means of a sequence of charges along the barrel. Precise timing of these additional charges was very important and it has been supposed that this was done electrically. In our research for a TV documentary on Channel Four, PBS Nova and National Geographic International, we found no evidence for this. Photographs from the time have no sign of electrical wiring or triggering devices and our own experiments suggest that the charges were triggered by the heat of the advancing gas behind the projectile. ...