The V3 London gun was designed as revenge weapon by Germany and designed to be permanently aimed at London 100 miles away. If fully built this would have been the longest gun in history. With a barrel over 440 feet long, that could hurl projectiles at speeds in excess of Mach 4.
source.image: Blue Paw Print
The Germans were built a huge underground complex in Northern France that was designed to house 25 of these guns to ensure a continuous bombardment of London. The gun used multiple propellant stages placed along the barrel’s length in order to provide an additional boost. These were ignited by the hot gases that propelled the projectile as it passed them. Watch the video from Blue Paw Print:
Solid-fuel rocket boosters were used instead of explosive charges because of their greater suitability and ease of use. These were arranged in symmetrical pairs along the length of the barrel, angled to project their thrust against the base of the projectile as it passed. This layout spawned the German codename Tausendfüßler (“millipede”).
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The barrel and side chambers were designed as identical sections to simplify production and allow damaged sections to be replaced. The entire gun would use multiple such sections bolted together. The smoothbore gun fired a fin-stabilized shell that depended upon aerodynamic forces rather than gyroscopic forces to prevent tumbling (as distinct from conventional rifled weapons which cause the projectile to spin).
