Gyrojet - Design

Design

The inherent difference between a conventional firearm and a rocket is that the projectile of a conventional firearm builds up to its maximum speed in the barrel of the firearm, then slows down over its trajectory; the rocket continues to accelerate as long as the fuel burns, then continues its flight like an un-powered bullet. A bullet has maximum kinetic energy at the muzzle; a rocket has maximum kinetic energy immediately after its fuel is expended. The burn time for a Gyrojet rocket has been reported as 1/10 of a second by a Bathroom Reader's Institute book and as 0.12 seconds by "The 'DeathWind' Project."

A firearm's rifled barrel must be manufactured to high precision and be capable of withstanding extremely high pressures; it is subject to significant wear in use. The Gyrojet rocket is fired through a simple straight, smooth-walled tube of no great strength.

Accuracy is increased by spinning the projectile. This is achieved for a bullet by being forced against spiral rifling grooves in the barrel. A rocket does not have enough initial energy to allow stabilization this way. Spin stabilization of the Gyrojet was provided by angling the four tiny rocket ports rather than by forcing the projectile through a rifled barrel. Combustion gases released within the barrel were vented through vent holes in it. Spin stabilization is limited in accuracy as a targeting technique by the accuracy with which one can point the launching tube and the accuracy with which the orientation of the projectile is constrained by the tube. The technique requires the shooter to have a line of sight to his target. For this reason the Gyrojet has been made obsolete by modern miniature inertial guidance equipment which suffers from none of these limitations. Advantages of the Gyrojet design over a hypothetical Inertially guided small arms rocket launcher are that, if ammunition for both were manufactured in sufficient quantity, the Gyrojet ammunition could be less expensive than ammunition incorporating an inertial measurement unit. Gyrojet ammunition is of a smaller caliber than is likely to be produced as a rocket with a current technology IMU, considering the size of highly miniaturized IMU components that were available in 2007 and are now so small that accelerometers and gyros are included in iPads.

The rocket leaves the barrel with low energy, and accelerates until the fuel is exhausted at about 60 feet (18 metres), at which point the rocket has a velocity of about 1250 feet per second (FPS), slightly greater than Mach one, with about 50% more energy than the common .45 ACP round. While test figures vary greatly, testers report that there was a sonic crack from some rounds, but only a hissing sound from others, suggesting that the maximum velocity varied from slightly below to slightly above Mach 1.

In 1965, the manufacturer of the pistol claimed 5-mil accuracy (about 17 MOA, or about 4.5 inches at 25 yards), worse than conventional pistols of the time. However in later tests accuracy was very poor; the difference seems to have been due to a manufacturing flaw in later production runs which partially blocked one of the exhaust ports, creating asymmetrical thrust that caused the projectile to corkscrew through the air.

About 1,000 of the "Rocketeer" model pistols were produced; a few saw service in the Vietnam War, and were featured in a James Bond book and movie You Only Live Twice, as well as one of The Man from U.N.C.L.E. novels. At about the same general size as the Colt M1911, the Gyrojet was considerably lighter at only 22 ounces (625 g) as the structure was mostly made of Zamac, a zinc alloy. The weapon was cocked by sliding forward a lever above the trigger to pull a round into the gun; the lever sprang back when the trigger was pulled. The lever hit the bullet on the nose, driving it into the firing pin. As the round left the chamber, it pulled the lever forward again to recock it. The pistol lacked a removable magazine; rounds had to be pushed down from the open "bolt" and then held in place by quickly sliding a cover over them on the top of the gun. Reloading quickly was impossible.

Tests in 2003 found that the acceleration, rather than being constant, started at a high value and decreased, leading to velocities at close range which were not as low as expected, about 100fps at 1-foot instead of the calculated 20fps. The testers suggest that the (secret) manufacturing process was designed to achieve this effect.

At longer range the velocity increases, so that the projectiles' trajectory does not drop as much as conventional ammunition, simplifying aim at longer ranges.