Patent Application
20160363403
Every rifle barrel currently made is constructed on the assumption that it is necessary to maintain a tight fit between the projectile and the bore of the barrel along its entire length, in order to minimize blowby and transfer the maximum amount of energy to the projectile. That assumption is valid only for subsonic projectiles; it is invalid when the projectile attains sonic speed. Failure to properly account for supersonic effects limits the performance which can be achieved. Consequently, a typical high powered rifle will transfer only about 10% of the energy released by the burning propellant into the projectile.
This invention is for a rifle barrel that is designed to utilize the phenomenon of supersonic fluid flow to achieve better performance. The construction of the barrel is divided into a subsonic region and a supersonic region. The subsonic region is identical to a conventional rifled barrel, and is a relatively short portion of the total length. The supersonic region is the novelty claimed in this patent.
When the projectile enters the supersonic region, it no longer physically contacts the bore of the barrel. The projectile is then aerodynamically supported and guided down the barrel by the boundary layer. This drastically reduces the friction force on the projectile, eliminates wear of this portion of the barrel, and enables the usage of new materials and/or surface treatments to reduce heat transfer into the barrel. Additionally the end of the barrel is designed to eliminate the hemispherical shock wave at the muzzle and produce a supersonic jet of gas in front of the projectile. The supersonic jet will usually be underexpanded, and consequently will form a diamond pattern of oblique shock waves which will have minimal effect on the stability of the projectile. This invention can be applied to every type of gun, launcher, or device which propels a projectile at supersonic velocity.
The profile of the barrel is generally as shown in Sketch A and Sketch B. The barrel is manufactured with standard machining processes whereby the bore of the subsonic section is drilled and rifled in the normal way. The supersonic section is drilled to a slightly larger diameter, as calculated according to the ballistic characteristics of the gun, and the diverging section is bored, drilled or reamed to achieve the desired shape.
As a projectile accelerates down the barrel it pushes the air in front of it out the muzzle. If the bore has a constant cross sectional area along its length, and the flow is subsonic, then the velocity of the air exiting the muzzle will be nearly identical to the velocity of the projectile. The kinetic energy of the air is dissipated when it exits, known as exit loss, but this is minimal and of little importance. However, when sonic speed is reached, the air speed exiting the muzzle is limited to sonic speed, which is less than the speed of the projectile. Consequently the air pressure increases in front of the projectile and impedes its acceleration. Also, a strong shock wave forms at the muzzle, normal to the flow of air, causing the energy in the air to be mostly converted to heat.
In this invention, the end of the barrel is designed as a special diverging nozzle. At subsonic projectile speeds, the kinetic energy of the air in front of the projectile is isentropically converted to pressure. Because the pressure outside of the barrel is constant at atmospheric pressure, the air pressure inside the barrel is reduced to sub-atmospheric pressure. This effect causes a significant portion of the air in front of the projectile to be evacuated before sonic speed is achieved. When sonic speed is reached, then the pressure in front of the projectile rises just as it does in a conventional design. However, because the pressure is lower when sonic speed is reached with this invention, the pressure rises to a lower level than it otherwise would. The net result of this is that less work is done on the exiting air, which essentially gets wasted and slightly more energy is retained in the projectile. However, the energy in the exiting air is not completely wasted, because it accelerates the air in front of the projectile and reduces its drag to a small extent. The table below shows how the pressure at the throat, the place at which the diverging section begins, decreases as the projectile accelerates toward Mach 1. The air pressure in the region between the nose of the projectile and the throat will be nearly the same as at the throat.
As the projectile accelerates through the subsonic portion of the barrel it behaves identically as in a conventional design. But when it enters the supersonic section, its behavior will be controlled by fluid dynamic phenomenon instead of physical contact with the bore. There is a very small clearance between the projectile and the bore, in which exists the fluid boundary layer. For the high pressure gas behind the projectile to escape through this clearance space it would have to travel through the boundary layer at supersonic speed relative to the surface of the bore. This does not happen. The supersonic portion begins with a constant diameter section through which the projectile accelerates while being guided, supported and stabilized by the boundary layer. As the projectile travels through this aerodynamically stabilized region, it is largely immune to vibration and slight deflection of the barrel which could affect its accuracy. This means that the use of very heavy barrels, bedding the barrel in the stock, and other measures to eliminate vibration will be largely unnecessary.
Gas lubricated bearings are widely applied to support light loads with extremely low friction. The supersonic region of the barrel relies on the same aerodynamic principals, and the film thickness, boundary layer effects, and other factors which dictate the performance are calculated using the well established procedures for gas lubricated bearings.
The projectile then enters the diverging section. As the clearance between the projectile and bore gradually increases, the boundary layer is less effective at preventing blowby. A portion of the high pressure gas now moves past the projectile. It should be noted that while this gas is moving supersonically with respect to the bore, it is moving subsonically with respect to the projectile. The space between the projectile and bore effectively forms a converging diverging nozzle, which causes the leakage gas to accelerate to a high Mach number. The flow of leakage gas gradually increases as the projectile moves through the diverging section. When the projectile finally exits the muzzle, it is contained within a supersonic jet of gas which is moving forward faster than itself. Aerodynamic drag then continues to accelerate the projectile for some distance downstream of the muzzle. Eventually the gas jet around the projectile dissipates and the projectile crosses a weak shock into a supersonic regime.
This is contrasted to a conventional barrel design. When a supersonic projectile exits the muzzle of a conventional barrel it must penetrate the strong shock wave. At nearly the same time, the high pressure gas contained behind the projectile is instantly released when the projectile exits. The gas expands uncontrollably, and any slight imperfection at the muzzle causes asymmetic forces on the tail of the projectile, tending to disturb its stability. This is a well known effect and great care is taken to preserve circularity of the muzzle bore. After exiting the muzzle the bullet travels some distance before it regains stability, all the while travelling in a supersonic regime with comparatively high drag.
The straight, aerodynamically guided zone enables the projectile to accelerate to higher velocities because friction is drastically reduced, and wear is eliminated so that alternative materials can be used. While the special diverging section recovers velocity energy from the escaping air, controls the expansion of the high pressure propulsion gas, prevents formation of a normal shock wave, and produces a supersonic jet ahead of the projectile.
The device shown in Figure A is an add-on to a short rifled barrel. In this configuration the non-contact section is removable and provides several significant benefits. The heavy, expensive, wear resistant alloy material is limited to a comparatively short section. An existing firearm, perhaps one that has a bent barrel or damaged muzzle, can be remachined and retrofitted with this device. Or this can be fitted to the end of an existing barrel to increase the velocity and energy of the projectile. Alternatively, a firearm can be manufactured specifically to take advantage of this. High powered rifles need a barrel which is long enough to provide for optimal expansion of the combustion gas to achieve the desired projectile velocity. These long barrels make those firearms expensive to manufacture and difficult to carry. Using this device, the contact portion of the barrel will be roughly ½ the normal length, and the non-contact portion roughly ½. A rifle barrel which would ordinarily be 26″ long can now be reduced to a 13″ fixed length with a 13″ detachable length. This will result in greatly reduced manufacturing cost, improved carrying ability, and when the non-contact section is made of a material such as titanium or aircraft-grade aluminum, significantly reduced weight. All or a portion of the non-contact section can be detached while transporting the rifle and then reattached before shooting. Or the detachable end section may be left off for short range, rapid shooting to give good maneuverability, and attached for long range, precision shooting.
The device shown in Figure B is a special section which has been integrally formed into a single piece barrel. This configuration is useful where portability or carryability, is not a concern, but there may be a desire to reduce weight, cost, or angular moment of inertia. The heavy wear resistant alloy is needed in the short contact zone, while a lighter section can be used in the non-contact zone. The contact zone is limited to the length required for the projectile to attain supersonic speed. In the supersonic, non-contact zone, the material is selected for the best strength, cost, or other properties. The rifling in the contact zone is faster, in other words it makes more revolutions per length, so that the projectile achieves the correct angular momentum before entering the non-contact zone. This device is shown with a thermal barrier coating (TBC) in the non-contact zone. The TBC reduces transmission of heat from the gas into the barrel, which keeps more energy available in the gas and also reduces the barrel temperature.
