Secrets of the Mystery Gun that Shelled Paris (Jun, 1930)
Secrets of the Mystery Gun that Shelled Paris
By COL. HENRY W. MILLER
Chief Artillery Engineer, A.E.F.
The secrets of the Paris Gun! For the first time in any magazine, Modern Mechanics here reveals the inside facts concerning the most startling and closely-guarded mystery of the World war—the official story of the giant German guns which, in 1918, dropped shells on Paris from a distance of 75 miles, a feat so incredible that artillery experts refused to believe it possible, thinking for a time that the shells were bombs dropped by high-flying aircraft. After the war the guns were destroyed and all information concerning them locked in secret archives. It was declared high treason, punishable by death, for anyone who possessed vital information concerning the guns ever to divulge it. Nevertheless, Col. Miller, author of this article and of the gripping book, “The Paris Gun,” obtained military pictures and technical secrets from confidential German sources which has enabled him to reveal to Modern Mechanics’ readers the astonishing story of the longest range guns the world has ever known.
ON THE morning of Saturday, March 23, 1918, as all the world knows, a supposed air bomb dropped into a Paris street and, when the fragments had cooled enough to be picked up} it was discovered they were marked with the lands and grooves of rifling, something no air bomb ever possessed.
By the time five or six more had crashed down, at intervals averaging about 15 minutes, the French artillery department had definitely decided Germany had accomplished the impossible and was shelling Paris with a gun that must be at least seventy, and probably seventy-five miles
away. What that meant may be visualized by recalling that the longest land bombardment in previous history occurred in 1915 when the Germans shelled Dunkirk from a distance of 23V-J miles with a 15-inch naval gun mounted on a railway truck.
Twenty-five shells dropped into or near Paris on that March Saturday, killing sixteen and wounding twenty-nine people.
The long range gun, or rather guns, for there were seven of them built, in all, and six were actually used at the front, remained one of the war’s greatest mysteries for years after the armistice, guarded by the death penalty for anyone caught revealing it. No gun was ever found in place, but we did find an almost complete emplacement in France, abandoned when the Germans retreated too fast to destroy it, and we did find an almost complete gun in Belgium, and the measurements of the two coincided. Long after the armistice 1 learned that our artillery destroyed the railroad junction at Soissons just two hours too late to catch the last of the guns as it was retreating.
But, while the guns remained a mystery, their location did not, for within two or three hours after the first shell fell, artillery experts had figured out where the monsters must be located. An aerial photograph of that district, taken on the previous March 6th, showed two railway spurs which must have been used to place them, and within a short time sound-ranging apparatus at the front had confirmed the diagnosis by identifying the sound as they fired. All that was done before dusk that Saturday, despite elaborate camouflage arrangements that even included trees with six-inch trunks set in slots in the railway line at 30-foot intervals to hide even the tracks to the gun from aerial observers.
Now that the rest of the story can be told, consider the guns themselves: There was a barrel 120 feet in length, approximately twice as long as the biggest guns built to that time—so long, in fact, that the end had to be supported in the air to keep it from bending down and being shot off by its own shell. In fact, that very thing happened to the first of the guns tested at the German proving ground, for the barrel bent a full inch under its own weight.
Next they fired a shell 75 to 80 miles or more, over a total trajectory ranging from 90 to nearly 100 miles.
To do that the shell was shot 24 miles above the earth, higher than any man-made thing, save possibly a small sounding balloon, had ever penetrated. At that extreme height the shell traveled through what was almost a vacuum, at a temperature of far more than 100 degrees below zero.
The shell, traveling at an average speed of 30 miles a minute—or sixty times as fast as the usual legal rate for automobiles — took three minutes to complete its aerial flight of 90 miles. It remained away from the earth so long, in fact, that the old world revolved on in space while the projectile was away, so the gunners had to aim a half mile east of the target in order that the target might be there when the shell arrived to hit it.
And, finally, the Paris gun, as I have called it, was the first ever built that contained within itself a device that told where I he shell had landed—in fact, the gunners could tell where the shell hit before it had actually hit there.
And to that might be added one other thing—the fact that the speed with which the Allied artillery chiefs located the guns and took steps to silence them defeated their main object, that of stampeding the people and scaring them half to death by the mystery of the thing. For the mystery, as has been said, didn’t last through the first morning.
The long range guns, it might be well to explain, should not be called “Big Berthas.” The “Big Bertha,” named for a fancied resemblance to the matronly figure of Frau Bertha Krupp, was a short, squat, wide barreled seige mortar, a seventeen -inch piece with which the Germans reduced the stone and steel fortifications of Liege in the first weeks of the war.
The long range guns were the conception of Dr. Von Eberhardt, a German physicist, who, early in 1916, with his chief, Dr. Rausenberger, convinced General Ludendorff that a cannon with a sixty-mile range could be built. Work was started, but at the end of the year there came a sud- den wire from the front ordering the range to be increased to seventy-five miles. The German high command had decided to retreat the next spring from the Somme to the Hindenburg line, and a sixty-mile gun would no longer be within range of Paris. To build the giants the Krupp factory took 15-inch, 45 calibre naval rifles, with railroad mounts, reduced the bore to 8.26 inches and just about doubled the length by shrinking into it the end of a 98-foot long rifled tube. That was the plan for the original 60-mile gun, and it had been built when the order came to increase the range to 75 miles.
No one had ever succeeded in joining two rifled tubes together and keeping them in alignment, and there was no boring mill in Germany big enough to bore a barrel twenty feet longer—the extra length needed to increase the range. For every millimeter added to a gun’s length the projectile will go three meters farther, and it figured out that twenty more feet of barrel would turn the trick. So the Germans decided the rifling in the 98-foot tube was sufficient to start the shell revolving and keep it from wobbling in the air, and, therefore, the extra twenty feet could be smooth bore tube, and that is how they did the job.
By the summer of 1917 the work was far enough along, it was believed, to order the emplacements built for the first three guns. They were to be placed in what was known as the Laon corner, a salient which enclosed the forest of St. Gobain, and was the nearest approach of the Hindenburg line to Paris.
The guns had been built, but then a hitch developed, for every shell tried was a complete failure. It was not until the following January that a shell was designed which proved saisfactory, and it was this shell that was used on Paris in March.
Ordinary cannon projectiles are smooth steel cylinders fitted with copper “driving bands.” As the shell passes through the rifled barrel the soft copper is engraved in grooves by the rifling, imparting the twisting motion which keeps the projectile from wobbling while in flight. The copper bands also act as piston rings to keep the gas from escaping from behind the shell.
But with a pressure twice as great as anything used theretofore the copper bands were sheared off the long range shells. Finally the German engineers created shells with rifling grooved in the steel walls, and copper bands behind the rifles to keep the gases in place. The shells were screwed into the barrel rifles at the breech and when fired, the rifles, with 4-degree pitch, started the projectiles spinning at 107 revolutions a second when they left the rifled portion of the barrel.
The shell had an enormously thick base and lower side walls to withstand the tremendous pressure of a million pounds. It was fitted with two fuses, to lessen the chance of failure to explode — and in fact not a single one did fail. The load was eighteen pounds of T.N.T. After it was shoved up and twisted into place in the rifled barrel two silk bags and one brass cartridge case of powder were inserted in the I breech, a total I of 431 pounds of powder for the initial charge in a new gun.
The firing table for the gun was one of the weirdest wonders ever conceived in bal listics. It took into consideration the state of the gun bore, the wind direction, velocity, barometric pressure, and even the compass bearing of the target, to allow for the turning of the world. The gun was fired at only one elevation—55 degrees—and the range “was corrected by changing the powder •charge. Another addition had to be made to each powder charge to compensate for the wearing of the barrel during the previous shot. The powder chamber of a gun is bored larger than the rifled portion of the barrel, and where the two meet the chamber is tapered down to barrel size in what is called the pressure cone. Each time the gun is fired tiny bits of the cone are •eroded, so each following shell slips a little farther up the barrel, and thus shortens the useful barrel length.
How the Gun Wore Out Cone wear of a fraction of a calibre up to one calibre is usually considered sufficient to warrant sending the gun back to be rebored. (A calibre is the diameter of the shell, thus a 6-inch gun of 50 calibres length has a 25-foot barrel.) But in the German guns cone wear progressed up to six feet of barrel length before they were discarded. This excessive wear explains the fact that shells fired at Paris the first day traveled ^n average of three miles farther than the ones fired the second day.
The muzzle velocity—practically a mile a second—was the highest ever reached in a big gun. The shell started out at 5500 feet a second; had dropped to 3300 feet by the time it had climbed to twelve miles, and was down to 2200 feet at the height of its trajectory, 24 miles in the air. But more than three-fourths of the total trajectory was up in the rarefied air, where resistance was practically nil, which explains the extreme range attained. When gravity began to pull the shell nose down toward earth the velocity climbed again to 3300 feet, but as it continued to fall it actually lost speed, due to air resistance, and was not traveling more than 2450 feet a second when it hit Paris. The initial energy when the shell left the muzzle, by the way, reached the enormous total of 8 billion foot pounds. (9,000,000 h.p.w.) I mentioned some paragraphs back that the gun contained a device to tell the gunners where the shell hit, even before it had hit. This was a pair of pressure recorders, set into the breech, to measure the gas pressure at the moment of discharge. German powder varied so much in quality the last year of the war that the muzzle velocity differed as much as 100 yards for various discharges, so the gunners never knew until after the shell was on its way where it was going. But as soon as they had the pressure figure they could tell from their tables where it would arrive.
Despite all these handicaps it is interesting that of the 25 shells fired the first day, 12 landed within a two mile circle. Two, however, were more than eight miles apart, both falling outside of the city walls on opposite sides of Paris.
¦ To take up the shock of the recoil when a million pounds of pressure started pushing a shell up the 120-foot steel tube required an enormous emplacement. In the later installations a pit eight feet deep and forty feet across was dug. On the bottom heavy “I” beams, arranged in a radial pattern, were laid, and on these a doughnut shaped steel caisson was assembled, with manholes in the tops through which it could be packed tight with sand bags. The turn table, key plates and the mount for the railroad gun carriage were built up on these, with a track down the center for the railway car. On either side additional railroad tracks carried a gantry crane of 175 tons capacity to handle the gun barrel.
When all was ready the railway trucks of the gantry crane were unloaded from their car on ramps provided for the purpose, and on them the gantry legs were set up and bolted down. Each gantry leg was topped by a jig crane, and with these the cross beam of the gantry was hoisted into place. Next the gun carriage, on its own wheels, was rolled onto the revolving turntable-like center section of the mount, a turn table that revolved on 96 steel balls 8 inches in diameter. Four powerful hydraulic jacks lifted up the carriage while the railway trucks were detached and rolled out of the way, then lowered it into place and it was secured to the turn table.
The gantry then picked up the 150 ton gun and swung it into place over the carriage. It took two weeks or more to prepare the emplacement and mount the gun.
Because of the complicated firing data, and the natural dispersement of the shots due to the extreme range, gun wear, wind drift, poor powder and other causes, only one target was ever used. That was the geographical center of Paris, a spot at the east end of the Palais de Louvre.
It is interesting that within an hour and a half after the first shell fell Allied officers had not only calculated the location of the gun but estimated the target on which it was laid probably was the “zero point” in Paris, the exact center of the Place in front of the Cathedral of Notre Dame, on the Isle de la Cite, in the Seine. The distance between the geographical center and the zero point is about one-half mile.
The location of the gun and the probable target were both calculated by charting the first seven shell bursts and drawing a line along the center on which they seemed to he falling. It is a curious thing that a number of shells fired from the same gun at the same target usually will be so equally spaced that if you draw a rectangle around all the bursts, and divide it with eight vertical and eight horizontal lines, 25 per cent of the shells will be found to have fallen within the first spaces on either side of the center lines, either up and down or across; 16 per cent in the next spaces; seven in die next and two per cent in the outer ones.
After the war when it became possible to talk to German officers, view the ground and consult German records we found just how effective the counter activity of the Allied artillery and aerial observers had been.
Five German long range guns were used at the St. Gobain woods position. One gun had burst while being fired and killed its own crew. Two positions, within the first week, had been shelled until they could no longer be occupied. A third was practically untenable when orders came to abandon it. The fifth gun fired just 64 shots before it was worn out. In five weeks 5,000 French heavy caliber shells fell on and around the gun emplacements. Toward the end they dropped as fast as 100 shells an hour. And in that five weeks the five German guns had fired just 183 shells in and around Paris!