In late 1916, Krupp’s technical manager, Professor Rausenberger, approached General Ludendorff with a proposal for an ultra-long-range 21cm gun, firing 100kg shells to a maximum distance of 100km. The project was approved, subject to the condition that the gun should be ready for production in no more than 14 months.
Initial calculations showed that an unprecedented muzzle velocity of 1,500m/s (metres per second) was required in order to meet the required range, which in turn meant an exceptionally long barrel. To speed up development, the gun was based on surplus 35cm naval guns originally intended for the Mackensen-class battlecruisers, which would be fitted with 21cm rifled liners to reach the required range.
In early 1917, with development still in its early stages, the German High Command suddenly requested a 20km increase in range (due to a planned withdrawal to shorten the line of the Western Front). Rausenberger’s team had to perform the calculations all over again, increasing muzzle velocity to 1,610m/s to achieve the phenomenal range of 120km.
This created further problems – in order to reach the necessary muzzle velocity, a barrel length of at least 24m was required, but Krupp’s largest rifling machine could only handle barrels up to 18m long. The design team decided to extend the rifled barrel with a 12m smoothbore tube, which was bolted on to a flange attached to the muzzle. This long, slender barrel had to be supported by an elaborate bracing system, in order to prevent it from drooping under its own weight.
Strengths: extremely long range
Weaknesses: poor shell lethality, low rate of fire, inaccurate at maximum range
The exceptionally high velocity meant that each shot caused considerable bore wear, so each barrel was supplied with 60 numbered 106kg shells of gradually increasing diameter, which had to be used in numerical order, to avoid the risk of an explosion caused by an over-size round jamming in the barrel. (One gun was destroyed in just this way.) After the 60 rounds had been fired, the barrel was sent back to Krupp, bored out to 238mm and issued with a new set of shells.
Three guns were completed and began bombarding Paris from Saint-Gobain Forest near Laon in March 1918, firing at an average rate of one round every 15 minutes, aimed at the Gare du Nord.
Given the extreme range, it was hardly surprising that shells were scattered widely across Paris. Between March and August, when they were forced to withdraw out of range of Paris, the guns fired almost 370 rounds, but only 183 landed within the city, killing 256 Parisians and wounding another 620.
Although the design was an amazing technological achievement for its day, it was little more than a very expensive propaganda weapon. The immense resources devoted to the project could be ill-afforded by 1917/1918, when the Allied blockade was throttling the German war economy, and would have been far more effectively used to increase the number of conventional heavy artillery batteries.
Image: Wikimedia Commons.