A cannon 130 meters long, embedded in a limestone hill, with angled booster chambers running down its sides like a steel millipede. This episode explains the physics and the history of the V-3, code-named Hochdruckpumpe and nicknamed Busy Lizzie. We start with why conventional guns hit a pressure ceiling, how the multi-charge principle spreads propulsion along the barrel like timed pushes on a swing, and why a smoothbore barrel and fin-stabilized shells were needed to chase 1,500 meters per second.

The idea was not German. Azel Storrs Lyman patented an accelerating firearm in 1857, and the 1880 Lyman-Haskell gun at Frankford Arsenal failed because leaking gas ignited the side charges early. French engineer Louis-Guillaume Perreaux patented a similar concept, and 1918 French plans drafted in response to the Paris Gun sat in archives until Ge ...