The underlying rationale of the mechano-pneumatic machine gun is the fact that the “fuel-weight equivalent” of providing the necessary kinetic energy for projectile firing is lighter than the mass of the equivalent smokeless propellent, reducing logistic and transport burdens. In other words, the mechanical energy required to compress air to the same pressure than the smokeless powder in terms of fuel expended in a diesel engine represents considerably less weight than hauling gunpowder. This clever “cheat” allows attack helicopters and weight sensitive weapon carriers to either increase range or carry far more ammunition than otherwise possible. The simple reason is that the air acts as a medium between mechanical force, the compressor is driven by a compact diesel engine, and the projectile, it is an energy deliver medium, not a source, it merely converts mechanical energy into potential energy (pressurized gas) back to mechanical energy. The source of the projectile’s acceleration energy is nothing other than the expansion of highly pressurized gas generated by combusting or deflagrating a quantity of solid propellent, typically nitrocellulose and nitroglycerin, petroleum jelly, and acetone. The combustion within a fixed volume causes a rapid pressure rise, yielding a very hot gas volume desirous to equilibrate to the atmospheric pressure at the end of the barrel. From the first handheld cannons in the 14th century, the “Bâton à feu”, which fired metal pellets and even rocks, to the modern assault rifle, still rely on using combustion to generate the gas pressure, a cumbersome and thermally challenging method which imposes a number of mechanical limitations, especially with respect to firing speed. What’s more, since the smokeless double-base powder is a solid that is often configured in a pellet form to maximize surface-to-volume in order to heighten the combustion velocity, it can only be stored in some cartridge or container that is mechanically fastened to the projectile, this occupies considerable volume beyond the mere volume of the relatively small projectile. Out of the roughly 3.07 cm3 of volume for the entire 5.57×45 NATO round, the bullet occupies only 0.247 cm3. This means for the same volume, a cartridge-ammunition-less projectile launcher will be able to store 12.4 times more ammunition in a given volume storage container, whether on the launcher itself, the user’s plate carrier, or a vehicle/aircraft. However, it must be said that for all oxidation reactions known, laminar flame speed increases with temperature and decreases with pressure, so the high-pressure environment created by its combustion serves to actually inhibit further burning. The fundamental premise behind the direct-pneumatic fired projectile launcher is that the mechanical energy needed to compress the gas to the same pressure as that which would be achieved through combustion is the same. The question then emerges as to whether the mechanical energy to compress gas represents a smaller mass of chemical fuel to drive the compressor than the equivalent amount of smokeless powder plus shells. The elimination of the shell and gunpowder yields a huge reduction in installed mass. The density of nitrogen at 400 K and 5000 bar is 0.4319 kg/liter. Each standard 5.56mm NATO cartridge contains 1.85 cm3 of gas volume at maximum chamber pressure, so if this is filled with 5,000 bar gas, we consume 0.22 cubic meters of gas at a density of 431.9 kg/m3, or 95 kg. With a compressor with a mechanical efficiency of 85%, 4 stages, an intercooler with an outlet temperature of 40 C, and an inter-stage pressure drop of 0.25 bar, the power consumption is 0.60 kWh/kg or 57 kWh for an hour of non-stop firing at 2000 rounds per minute. The energy required to compress gas is logarithmic with pressure since the gas rises in density faster than pressure, resulting in less volume reduction with increasing pressure. A high-speed synchronous motor rotates a shaft without riffling within the main pressure-bearing carbon fiber barrel, thereby reducing blowby (combustion gasses leak past these grooves wasting energy) and increasing projectile velocity, conventional rifling employs grooves which act as a passageway for expanding gases, this is called “rotating inner-barrel projectile twist method without rifling”. The principal advantage of a pneumatic energy delivery medium from mechanical shaft power delivered by compact back-pack carried adiabatic diesel engines is that no thermal energy is imparted into the barrel as with conventional solid propellants which possess high flame temperatures. This affords longer barrel life, reduced wear, no risk of warpage, increased rates of fire, and no need for barrel cooling or barrel swapping as with the infamous MG-42. Because the highly compressed gas is injected into the chamber at moderate temperatures, the machine gun can fire all day long without stopping, ideal for fixed installations such as gun turrets on ground vehicles or aircraft. Another advantage of the mechano-pneumatic “projectile launcher” is the reduced acoustic signature. Since there is no combustion of propellent, there is no rapid release of pressure and the accompanying sound waves it generates, there is only a smooth flow of already-pressurized nitrogen flowing through the barrel producing a hissing sound but not the loud bang or pop associated with the deflagration of gunpowder.
Each 5.56×39 brass shell weighs approximately 7.4 grams, the volume of gunpowder is 1.85 cm3, with a density of 0.95 g/cm3, and 1.75 grams of gunpowder is stored in each shell. The lead bullet weighs only 3.9 grams. Smokeless powder is 58% nitroglycerine, 37% nitrocellulose, and 3% mineral jelly, it has considerably greater gaseous density than regular air.
Since Nitrocellulose is 64% oxygen and 16% carbon, nitrocellulose is 28% carbon and 58% oxygen, the average gas density is around 1.75 g/cm3, 43% greater than air, due to a higher carbon and oxygen content.
This means the same mass of air required to accelerate the projectile to the same velocity is only 1.23 grams. At a rate of fire of 1500 rounds per minute, the mechano-pneumatic machine gun consumes 110 kg of air, using 68 kWh of mechanical power, or only 10.3 kg of fuel. In contrast, using conventional 5.56×39 NATO shells, the total mass is 823 kg per hour of firing at 1500 rounds per minute. Volumetrically speaking, this is also a considerable saving in space, 263 liters for an hour of firing.
Nitrocellulose: C24H36N8O38
Carbon: 27.6%
Nitrogen: 10.727%
Oxygen: 58.2%
Hydrogen: 3.47%
Average gas density: 1.76 kg/m3
Nitroglycerin: C3H5N3O9
Carbon: 15.86%
Hydrogen: 2.21%
Nitrogen: 18.5%
Oxygen: 63.4%
Average gas density: 1.70 kg/m3
CHRISTOPHE De RIVALS MAZERES 