Muzzle Velocity and Chamber Compression
Professor von Eberhardt had been hard at work on the super gun. He had calculated that in the near vacuum commencing 12 miles above the Earth, air resistance would be reduced to almost zero. Maximum range could be achieved at an angle of incidence of 45 degrees. To compensate for initial degradation of the nascent launch angle, firing had to be done at an angle of 50 degrees. To produce the ability to reach the required 12-mile height, the muzzle velocity had to be 1 mile per second. The shell would have to leave the muzzle 1/50th of a second after ignition and with a rotational speed of 100 revolutions per minute.
To achieve the required muzzle velocity, the propellant charge had to be considerable. In addition, the powder had to be of the highest quality, with no deviations between batches acceptable. The charge was divided into three components. The lower charge of 154 pounds of powder was encased in a brass cylinder called a Hulsenkartuche, which gave support to the two forward charges. Above the brass cylinder was the Vorkartuche (main charge) of 165 pounds supporting the Zusatzkartuche(auxiliary charge). Both of the upper charges were encased in silken bags. The total charge was nominally set at 450 pounds, including the auxiliary charge of 112 pounds. The auxiliary charge could be adjusted by adding or subtracting powder to maintain proper chamber pressure. The gun itself could be fired at only one elevation. Range adjustments were made by increasing or reducing the charge in the Zusatzkartuche.
With all the scientific calculations and engineering problems engendered by this project, the research of Coriolis was reviewed by Professor von Eberhardt, and an additional adjustment made. Having been advised that the projected firing site was in the forest of Crépy-en-Laonnois, near Laon, and the target Paris, he calculated the distance between the two as 67.6 miles. The firing vector was close enough to a north-south axis, bringing the Coriolis Effect into play. Coriolis and von Eberhardt knew that Laon and Paris were traveling at different speeds. Although each rotated once in 24 hours, Laon was farther from the equator than Paris and thus moving somewhat slower in miles per hour. A point on the equator travels at 1041.66 mph.
Eberhardt estimated a rotational speed of 567.126 mph at Paris on the 49th parallel, and 555.55 mph at Laon on the 48th parallel. An adjustment of 11.576 mph, or .003215 miles per second, had to be provided for in the laying of the gun.
The final calculations were assembled. To achieve the required muzzle velocity, a chamber pressure of 59,000 pounds per square inch had to be reached. Flight time was predicted at 176 seconds. This called for an easterly correction of 0.5659 miles or roughly 995.984 yards to compensate for the differing rotational speeds of gun and target. On March 23, 1918 everything was ready to go.
The Shelling of Paris
The first shell burst without warning at 7:20 am at #6 Quai de Seine. With no hostile aircraft evident, the explosion was a mystery. At 7:40 am, a second explosion rumbled the Boulevard de Strasbourg a few feet from the Metro (subway system) in front of the Gare d’Este. This hit killed 8 persons and injured 13.
The mysterious explosions continued with a hit at 8:05 am on Rue du Chateau Landon. The next hit at 8:17 on #5 Rue Charles Cinq. Number five landed at 8:35 at #24 Rue des Ardennes. By 2:45 pm 25 shells had landed. Experts gathered shell fragments and found some with steel more than 2 inches thick. Also found were fragments of copper, which were the obduration bands, displaying engraved rifling grooves. At 4 pm, authorities announced that Paris was being shelled by artillery of 24.0 cm caliber (actual size was 21.0 cm).
At the time of the first firing, ordinary German artillery went into action in order to mask the sound of the “Paris Gun” from the very adept French sound-ranging system. Nevertheless the French soon determined that the monster cannon was coming from along a line connecting Paris to Laon. With the source predictably located in a forest, but with no precise location, spotting aircraft were immediately detailed to find the gun’s hideout. The size of the weapon made it obvious that this piece of artillery was practically stationary.
The Germans were busy, too. The first firing had developed a chamber pressure of only 53,800 psi, which required an adjustment to the propellant charge. Initial calculations had predicted a target distance of 67.1 miles, but this figure had been based on a straight line moving through the Earth. The actual distance along the curved surface was 67.6 miles. Adjustments were made.
The actual distance traveled by the projectile was 92 miles in a flight time of 176 seconds. The shell reached a height of 12 miles in 25 seconds with a velocity of 3,000 feet per second (fps). Degradation of the muzzle velocity was caused by the friction of the atmosphere and gravity so that at the apogee of the trajectory, which was at 24 miles into space, the shell was moving at a speed of 2,250 fps as it “went over the top.” Coming down on the reflective angle, speed increased, and at a height of 12 miles the projectile was again moving at 3,000 fps. In the final leg of its journey the projectile reached a terminal velocity of 3,075 mph. Its approach was soundless because it was traveling at more than the speed of sound, Mach 3.5 actually. Its travel sound would not arrive until well after the explosion.
The French government hastily implemented countermeasures. Within 12 hours of the first hit, a battery of 12-inch railway guns was sent to Vailly, the nearest point to the firing site suitable for counter-battery fire. But not surprisingly, French aircraft reported a heavy smoke screen over the entire Laon corner to mask activity on the ground.
The Germans resumed firing the next day, Palm Sunday. They continued on Monday and then stopped for three days. They took it up again on Good Friday. At 4:30 pm, in Ste. Gervais Church opposite the Hotel de Ville several hundred were at prayer. A direct hit on one of the stone pillars supporting the vault of the nave brought the structure down onto the worshipers. Eighty-eight persons were killed, including 67 women and two children. One hour later, a subsequent shell landed in a nearby cemetery.
By April 6, 1918, counter-battery fire was beginning to approach the first “Paris Gun.” In addition, wear was becoming evident. After 48 rounds the gun was declared worn out and in need of being rebuilt. A second gun went on line, but by May 1, after firing 64 rounds, it, too, was declared worn to the point of necessitating repair. In addition, the French artillery had landed over 3,000 shells in increasingly close proximity to the firing site and it was only a matter of time before a direct hit would register.
A third gun was installed and inspected by the Kaiser. After a lunch at the site and after expressing his satisfaction, he ordered firing to commence the following day. It did, all through that Sunday and part of Monday. Then a hidden flaw surfaced. The gun exploded and killed most of its crew.
On May 27, the Germans installed a newer, larger gun at Beaumont, about nine miles closer to Paris. This gun proved more durable and fired 104 shells before being declared in need of repair. Then a French offensive threatened it and it was withdrawn.
By July 15 the Beaumont gun was back in the fray after being rebored and refurbished. After firing only 14 rounds, it had to be withdrawn again from the advancing French. Resited again, it shelled Paris from August 5 to 12 with 64 rounds. Again it had to be withdrawn in the face of being overrun.
The Big Guns Had a Negligible Impact on the War
In all, 367 shells struck in or about Paris. But no vital installations were hit. The attractive target was not as vulnerable as Ludendorff anticipated.
For one, the guns were incapable of the accuracy necessary to service individual targets. Spotting aircraft could not be used owing to communication limitations and the presence of large numbers of French aircraft in the area. Even if they were employed, adjustments for pinpoint accuracy were simply beyond the ability of the gun crews.
Dispersion of hits had been very wide but shells had predominantly landed on a northeast/southwest line mainly in the 7th, 10th, and 19th Arrondissements, which were mostly residential areas. Rail concentrations, armaments plants, and government centers, the targets that Ludendorff had thirsted for, were not damaged.
Total casualties reached 250 killed and 620 injured, mainly civilians. The actual effect on Parisian morale and the French war effort was insignificant. Considering the investment in men, materiel, and exertion, the “Paris Guns” could not be considered compensatory. That autumn, the Allies drove the Germans back toward Germany until the Germans called for an armistice.