To make D-Day a success, the Allies thought they might have to break through formidable German defences. One unconventional idea was a "giant firework" that would deliver a one-tonne bomb: the Panjandrum.

After conquering much of Western Europe in the first few years of World War II, Nazi Germany then diverted a huge effort into protecting what it had invaded.

Once the United States entered the war on the side of the Allies in late 1941, the threat of invasion from the sea went from a distinct possibility to certainty.

To prevent it, hundreds of thousands of forced labourers – some of them Russian prisoners captured on the Eastern Front – were set to work. They built walls, tank traps and reinforced-concrete emplacements. The fortifications stretched around 5,000km (3,105 miles) from France's border with Spain all the way to the northern tip of Norway.

Adolf Hitler called it the "Atlantic Wall", and there are still many traces of it, littering beaches from the Bay of Biscay to the sub-Arctic fjords.

Allied military planners had many challenges to wrestle with during their long preparations for the liberation of Europe. Seizing a port made the most sense – it would be easier to get vital supplies to the troops on the beachhead by unloading ships more speedily on the docks. But the ports on the English Channel coast had been heavily fortified by German defenders.

A bold plan to temporarily take over one of these ports – Dieppe, in France – in August 1942 showed how difficult a port would be to capture. Thousands of mostly Canadian troops were killed or captured in a botched attempt to push through defences; supporting tanks became bogged down on loose shingle sand and the built-up surroundings gave the defenders plenty of cover from which to fire on the invading forces.

Dieppe, it turned out, had the wrong kind of beach. The French coast had plenty of beaches firm enough to support tanks and other vehicles coming ashore, but these beaches would be overlooked by the Atlantic Wall defences the Germans were quickly building. How could they be breached, with the minimum loss to Allied soldiers? An eccentric plan was born…

Nevil Shute Norway was an accomplished aeronautical engineer who had worked on one of Britain's most high-profile airship designs. The R100 airship had been designed by engineer Barnes Wallis – who would later invent the bouncing bomb of Dambusters fame – for engineering firm Vickers, with funding from the government. Norway later took over as chief engineer when Wallis left to work on other projects.

The R100, intended for long-distance voyages across the British Empire, carried out successful publicity tours as far afield as Canada. It was developed alongside a similar airship designed and built by the UK's Air Ministry, called R101; this design was fatally flawed, and crashed with the loss of 48 lives in northern France while on its maiden flight.

News of the crash flashed around the world, killing Britain's emerging airship industry for good, and Norway drifted into more conventional aircraft designs, including the highly successful Oxford trainer designed by his own aircraft company, Airspeed.

When war broke out, Norway joined the Royal Navy Volunteer Reserve, initially serving on naval ships. But his flair for engineering took him in a different direction: into the navy's secret department for experimental weapons.

The Directorate of Miscellaneous Weapon Development was known informally as the "wheezers and dodgers". They drew bookish, lab-bound talent from the UK's universities and research institutes and challenged them to come up with new weapons that could be used in the war. No idea, however outlandish, was discouraged.

One of the weapons the British armed forces needed was something that could be deployed from a ship and was powerful enough to breach the strong concrete sea wall now in place across much of Europe. From there, Allied forces could hopefully capture the ports needed to sustain a sea invasion from behind, where they would be less well-defended.

Norway and team did the calculations and arrived at a minimum explosive weight to break through the concrete defences: a tonne of high explosives. Placed against the wall, this should be enough to blow a tank-sized hole that will allow invading troops and vehicles to pour through. But to drive it up across a well-defended beach was likely to be too hazardous.

Norway and the naval research team turns to an unlikely inspiration: a firework known as the Catherine wheel. The firework uses part of the energy of the rocket, which is usually pinned to the structure, to spin; it makes a much more impressive display. Enough rockets, the team calculated, could generate enough energy to........

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