V-3 cannon
The V-3 (German: Vergeltungswaffe 3, "Retribution Weapon 3") was a German World War II large-calibre gun working on the multi-charge principle whereby secondary propellant charges are fired to add velocity to a projectile.
The weapon was planned to be used to bombard London from two large bunkers in the Pas-de-Calais region of northern France, but they were rendered unusable by Allied bombing raids before completion. Two similar guns were used to bombard Luxembourg from December 1944 to February 1945.
The V-3 was also known as the Hochdruckpumpe ("High Pressure Pump," HDP for short), which was a code name intended to hide the real purpose of the project.