The Devil's Sewing Machine: A Brief History of the Interrupter Gear

The interrupter gear, or synchronization gear, stands as one of military history's most pivotal and ingenious inventions. In its simplest form, it was a mechanical linkage, a “brain” of cams, rods, and gears that connected an Aircraft's Engine to its forward-firing Machine Gun. Its purpose was as elegant as it was deadly: to time the gun's firing sequence so that bullets would pass harmlessly between the spinning blades of the Propeller. Before its invention, mounting a machine gun to fire directly forward on a “tractor” aircraft (where the propeller pulls from the front) was suicidal. The pilot was more likely to shred his own propeller than to hit an enemy. The interrupter gear solved this seemingly impossible riddle, transforming the airplane from a fragile machine of observation into a true predator of the skies. It was the device that created the modern fighter plane, the concept of the “dogfight,” and the archetype of the fighter ace. Its brief, brilliant life, from a pre-war patent to its obsolescence in the jet age, is a dramatic story of battlefield necessity, mechanical genius, and the relentless, bloody-minded pace of technological warfare.

When the great powers of Europe stumbled into World War I in the summer of 1914, the Aircraft was a newcomer to the battlefield, a creature of wood, wire, and fabric whose role was uncertain. It was seen primarily as a tool for reconnaissance, a mobile high ground from which to observe enemy trenches and troop movements. Combat, when it occurred, was a clumsy and often personal affair. Pilots and observers took potshots at one another with service revolvers, rifles, and shotguns. Some even carried grenades or small bombs to drop by hand. Stories, perhaps apocryphal, tell of pilots flinging bricks, ropes, and grappling hooks at their adversaries in a desperate attempt to foul an engine or tear a wing. The sky was a vast, new arena for war, but its combatants lacked a proper sword. The problem lay in the fundamental design of the era's most promising aircraft. Two main configurations existed:

• Pusher Aircraft: These planes had their engine and propeller mounted behind the cockpit. This design offered a superb, unobstructed forward field of fire for a machine gun, and early armed aircraft like the British Vickers F.B.5 “Gunbus” used this layout to great effect. However, the pusher design was an aerodynamic dead end. The complex tail boom structure created immense drag, limiting speed and maneuverability.
• Tractor Aircraft: These planes, with the engine and propeller at the front, were the future. The “tractor” configuration was aerodynamically cleaner, allowing for greater speed, a better climb rate, and superior agility—all the qualities a fighting machine would need. But it presented a fatal flaw: the giant, spinning wooden propeller sat directly in front of the pilot, blocking the most logical place to mount a fixed, forward-firing weapon.

The tactical advantage of a fixed, forward-firing Machine Gun was immense. It would allow the pilot to aim not just the gun, but the entire aircraft. The plane itself would become the weapon, pointed directly at its foe. But how could one shoot through a propeller spinning at over 1,200 revolutions per minute without instantly blasting it to splinters? This was the central, lethal riddle of early air combat. The nation that solved it first would not just gain an advantage; it would own the sky.

The idea of timing a gun's fire to a machine's movement was not entirely born in the crucible of war. As early as 1913, the Swiss engineer Franz Schneider, then working for the German LVG company, had patented a rudimentary synchronization device. However, his design was flawed and never progressed beyond the drawing board. The war, with its urgent demand for a solution, would be the true catalyst. The first nation to produce a workable, if brutal, answer was France.

The French engineer Raymond Saulnier had been wrestling with the synchronization problem before the war, developing a mechanical device to interrupt the gun's firing. His initial attempts failed due to the inconsistent firing rates of the Hotchkiss machine gun he was using. Frustrated but undeterred, he conceived of a cruder, more direct solution. If you couldn't guarantee the bullet would miss the propeller, perhaps you could simply protect the propeller from the bullet. He shared this idea with a daring and innovative young pilot, Roland Garros. Together, they developed the “deflector gear.” It was a masterpiece of battlefield engineering, as simple as it was savage. They took the propeller of Garros's Morane-Saulnier Type L parasol monoplane and fitted armored steel wedges to the rear face of the blades, directly in the line of fire. The concept was based on a calculated risk: most bullets fired from the machine gun would pass through the gaps between the blades. The few that did strike a blade (Saulnier calculated around 7 to 10 percent) would hit the steel wedge and be deflected harmlessly away. It was a brute-force solution that placed immense stress on both the propeller and the Engine's crankshaft, but it worked. In April 1915, Roland Garros took his modified aircraft to the skies. The effect was immediate and terrifying. Unsuspecting German observation planes, accustomed to fending off haphazard pistol shots, were suddenly confronted by a French aircraft that could fire a stream of machine gun bullets straight at them. Garros shot down three German aircraft in two weeks, a stunning tally at the time, becoming the world's first true fighter ace. The German High Command was mystified and panicked, and the period became known to the Allies as the “Garros Scourge.” The reign of this steel-wedge terror was, however, destined to be brief. On April 18, 1915, Garros was forced down behind German lines by a simple ground-fire rifle bullet that ruptured his fuel line. He tried to set his plane alight, but the fire failed to take hold, and his aircraft—along with its revolutionary secret—fell intact into German hands.

The captured Morane-Saulnier was a priceless intelligence windfall. The German High Command immediately tasked several engineers with examining and replicating the device. Among them was a charismatic and ambitious 25-year-old Dutch aircraft designer named Anthony Fokker, whose company built planes for the German military. The popular legend, encouraged by Fokker himself, is that he was given the captured propeller, and in a fit of isolated genius, designed and built a vastly superior system from scratch in a mere 48 hours. The truth, as is often the case, is more complex but no less impressive. Fokker and his team, including chief designer Heinrich Lübbe, were ordered to copy the French deflector system. Fokker recognized it for what it was: a crude and dangerous stopgap. The repeated impacts of bullets, even when deflected, would eventually shatter the propeller or damage the engine. He knew there had to be a better way. Crucially, Fokker was likely aware of Schneider's forgotten 1913 patent, which provided the conceptual framework for a true synchronization gear. Instead of deflecting bullets, Fokker's team decided to control the gun itself. They created a system of mechanical linkages that would become the standard for years to come. The device, known as the Stangensteuerung (push-rod control), was a marvel of mechanical clockwork.

To understand the Fokker gear, it is best to imagine a sewing machine. In a sewing machine, a complex system of gears and cams ensures the needle moves down through the fabric at the precise moment the feed mechanism is still, then retracts before the fabric is moved again. It is a perfect symphony of synchronized motion. Fokker applied the same principle to his deadly problem.

• The Cam: First, a small cam wheel was attached to the spinning crankshaft of the aircraft's rotary Engine. This cam had lobes, or bumps, on its surface corresponding to the number of propeller blades (two, in this case).
• The Push-Rod: A long push-rod rested against this cam. Every time the engine completed a rotation, the lobes on the cam would push the rod up and down in a precise, rhythmic motion, perfectly in time with the engine and, by extension, the propeller.
• The Trigger: This push-rod was then connected to the trigger mechanism of a Parabellum MG14 or Spandau LMG 08/15 Machine Gun.

The operation was breathtakingly simple. When the pilot pulled his trigger, it didn't fire the gun directly. Instead, it “engaged” the system, allowing the push-rod to connect with the gun's firing mechanism. From that point on, the Engine itself was firing the gun. The push-rod, driven by the cam, would only trip the trigger when the propeller blades were clear of the muzzle's path. The pilot was no longer just flying a plane; he was aiming a perfectly timed death machine, a “devil's sewing machine” that stitched lines of bullets through the empty spaces in the blur of its own propeller. Fokker installed this system on his new monoplane, the Fokker E.I Eindecker (“one-wing”). The aircraft itself was not remarkable; it was stable and relatively easy to fly, but it was not particularly fast or agile. Armed with the interrupter gear, however, it became the most feared weapon in the world.

In the summer of 1915, the first Fokker Eindeckers arrived at the front. The balance of power in the air shifted almost overnight. Allied reconnaissance planes, which had flown with relative impunity, were now systematically hunted and destroyed. The period that followed, from mid-1915 to early 1916, became known to the British as the “Fokker Scourge.” Allied pilots, flying inferior pusher planes or tractor designs with no effective forward-firing armament, were little more than “Fokker Fodder.” Morale plummeted. The new technology didn't just create kills; it created a new kind of warrior: the fighter ace. Pilots like Oswald Boelcke and Max Immelmann became national heroes in Germany, their scores mounting as they perfected the art of aerial gunnery. They were the first to truly grasp the tactical revolution the interrupter gear had wrought. The aircraft was no longer a platform carrying a gun; the aircraft was the gun. This fundamental shift gave birth to the first formalized doctrines of air-to-air combat. Oswald Boelcke, a brilliant strategist as well as a deadly pilot, codified his experience into a set of rules known as the Dicta Boelcke. These eight rules, which included principles like “attack from the sun” and “always keep your eye on your opponent,” became the foundational text for fighter pilots everywhere. Concepts that are now axioms of air combat—gaining an altitude advantage, attacking from the enemy's blind spot (their “six o'clock”), and firing in short, controlled bursts—were all born from the tactical possibilities unlocked by the Fokker interrupter gear. The device had not only changed the machine; it had changed the entire culture and science of aerial warfare.

The Allies were in a state of desperate crisis. They tried various stopgap measures—mounting machine guns on the top wing of biplanes to fire over the propeller arc, or having observers in two-seaters fire rear-facing guns—but none could match the deadly efficiency of the Eindecker. The only solution was to capture a Fokker and reverse-engineer its secret, or to invent a superior system of their own. They eventually did both. After several Eindeckers fell into Allied hands, British and French engineers quickly developed their own mechanical interrupter gears. The British Vickers-Challenger gear and the French Alkan-Hamy system were essentially copies of the German push-rod mechanism. They worked, but they were often unreliable and difficult to maintain. The true Allied breakthrough came not from a mechanical engineer, but from a brilliant Romanian physicist and inventor named George Constantinesco.

Working for the British, Constantinesco developed a system that was light-years ahead of the clumsy German mechanical rods. His invention, the Constantinesco Synchronization Gear, was hydraulic. Instead of a solid rod being physically pushed by the engine, Constantinesco's gear used a tube filled with a mixture of oil and paraffin.

• A “pulse generator” was connected to the engine's camshaft.
• This generator sent a precise pressure wave—a sonic pulse—through the fluid in the tube.
• This wave traveled at the speed of sound to a “trigger motor” on the machine gun, firing it at the exact right moment.

The Constantinesco gear, or “C.C. Gear,” was a revolutionary improvement.

• Reliability: With fewer moving parts, it was far less prone to breaking or jamming than the mechanical systems.
• Flexibility: The flexible tubing allowed guns to be mounted almost anywhere on the fuselage, and it could easily synchronize two guns at once.
• Performance: Crucially, its operation was independent of engine speed. A mechanical gear's timing could drift as the engine RPM changed, but the speed of the hydraulic pulse was constant, making the C.C. Gear more accurate and allowing for a higher rate of fire.

By 1917, the technological tide had turned. Equipped with the C.C. Gear, new Allied fighters like the British Sopwith Camel and S.E.5a, and the French SPAD S.XIII, could meet and defeat their German counterparts on equal terms. The Fokker Scourge was over, but the world it had created—a world of deadly, synchronized fighter aircraft engaged in swirling dogfights—was now the permanent reality of air warfare. The interrupter gear had become a standard, essential piece of technology for every air force.

The interrupter gear reigned supreme through the 1920s and early 1930s, the “Golden Age of Aviation.” It was refined, perfected, and became a standard feature on the sleek biplane fighters of the interwar years. Yet, like the biplane itself, its days were numbered. The very progress in aircraft design that it had helped to spawn would ultimately render it obsolete. The catalyst for its decline was the rise of the all-metal monoplane. In the 1930s, aircraft like the British Hawker Hurricane and Supermarine Spitfire, and the German Messerschmitt Bf 109, featured advanced cantilever wings. These wings were so strong and stable that they could serve as weapons platforms themselves. Engineers could now mount batteries of machine guns and cannons inside the wings, completely outside the arc of the propeller. This new approach offered several distinct advantages:

• Higher Rate of Fire: With no need to be synchronized with the propeller, wing-mounted guns could fire at their maximum possible rate, dramatically increasing the weight of fire a pilot could bring to bear. A Spitfire could be armed with eight machine guns, all firing uninterruptedly.
• Greater Firepower: It was far easier to fit larger, more powerful weapons like 20mm cannons into a wing than to synchronize them through a propeller hub.
• Simplified Design: It eliminated the complex, maintenance-heavy interrupter gear entirely.

A vigorous debate emerged among aircraft designers. Some still favored nose-mounted guns, arguing that their fire was more accurate since it didn't require “convergence”—angling the wing guns so their fire crossed at a specific distance. This led to hybrid solutions, like the Bf 109, which often featured both synchronized machine guns firing through the propeller and a Motonenkanone, a cannon firing through the hollow propeller hub itself—a feat that still required a form of synchronization. Ultimately, however, the sheer volume of firepower offered by wing-mounted armament won out. The interrupter gear, once the most critical piece of technology on a fighter, was relegated to a secondary role. The final, definitive end came not from a rival technology, but from a complete paradigm shift in propulsion. The development of the Jet Engine during and after World War II eliminated the propeller from fighter aircraft altogether. With no spinning blades to avoid, the problem that the interrupter gear had so brilliantly solved simply ceased to exist. The interrupter gear's life cycle was complete. Born from a deadly battlefield riddle, it had transformed the very nature of war, created a new kind of hero, driven a furious arms race, and pushed aviation technology forward at a breakneck pace. It was a perfect microcosm of military invention—a device that provided a brief but absolute supremacy before the ceaseless cycle of innovation and countermeasures consigned it to the museums of technology, its ghost living on in the tactical DNA of every fighter pilot who followed.