Strategic Long Range Cannon

Congress directed the Army to stop funding the long-range cannon in its fiscal 2022 appropriations act, and “based on that direction, the Secretary of the Army decided to terminate the [SLRC] project this year,” Ellen Lovett, Army spokesperson said in a 20 May 2022 statement to Defense News. The Army still has four other long-range fires programs set to reach operational Army units in 2023: Extended Range Cannon Artillery (ERCA), the Long-Range Hypersonic Weapon (LRHW), the Mid-Range anti-ship Missile (MRC) and the Precision Strike Missile (PrSM).

For strategic fires, the Army is investing in hypersonics research, along with experiments on a strategic long-range cannon, which conceivably could go upwards of 1,800 km. US Army’s Strategic Long-Range Cannon (SLRC) is intended to shoot out to 1,000 miles or more. The limited literature on this gun is long on heavy breathing and short on details.

The Strategic Long Range Cannon, a long-term development project for a gun with a range of over 1,500 kilometers or more, should not be confused with the unrelated Long Range Cannon, a near-term [in under three years] development project for a gun with a range of at least 70 kilometers. The Long Range Land Attack Projectile, a round specially designed for the Zumwalt destroyers and then promptly cancelled over cost concerns, had a range of about 85 miles. Even the 16-inch guns of the Iowa-class battleships had a range of only 23 miles.

The artwork associated with this program is remarkably sparse. One artist's concept revealed in February 2020 suggested that the gun would resemble the 280mm M65 "Atomic Annie" Atomic Cannon. This artwork also indicated that the SLRC projectile would strongly resemble conventional artillery projectiles, which probably it would not, so there is no particular reason to give much credence to the depiction of the gun itself, though something is better than nothing.

In any event, the Strategic Long Range Cannon would undoubtedly be by far the largest artillery piece in the American arsenal, and mobility [hence survivability] would have to be an important consideration in assessing the system's operational effectiveness. For the past two centuries, counter-battery fire - artillery shooting at artillery - has remained a fundamental battlefield tactic. In recent decades, highly-mobile self propelled artillery platforms have perfected "shoot and scoot" tactics in which the artillery platforms displace from the original firing location before counter-battery fire can home in on the original firing emplacement. Such displacement tactics would probably not be an option for the big SLRC platform, which might require other approaches, such as active defense, to maintain battlefield survivability. The greater mobility of artillery rocket launchers - not having heavy and expensive barrel and recoil mechanisms, is one reason that rockets have found favor over guns for longer range fires.

Left unsaid is where such guns might be deployed, and the targets against which they might be employed. Presumably Poland, the Baltics, and South Korea would be candidate fielding sites, though Poland would probably be a much easer sell than South Korea. And it might be presumed that the targets would be in Russia and China, though it is far easier to describe operational objectives against Russian targets than against Chinese. Such “strategic deep fires” would see land-based artillery used against targets presently reserved for airstrikes and cruise missiles — but the Army see this as synergy, not redundancy.

The Strategic Long Range Cannon is an S&T program independent of the other listed programs that follow. The Strategic Long Range Cannon will provide a surface to surface strategic fires capability to complement a Long Range Hypersonic Weapon to deliver a projectile at strategic ranges. Initial ballistic testing began in FY19, with full technology demonstration in FY23 and first unit equipped occurring in FY25.

The U.S. Army appears to have revealed the first image of the newest Strategic Long Range Cannon in a set of presentation material revealed during US-UK Modernization Demonstration Event. The Linkedin post was shared by the U.S. Army CCDC Army Research Laboratory, said that the U.S. Army hosted a US-UK Modernization Demonstration Event Feb. 20 at Aberdeen Proving Ground, Maryland, to identify capability collaboration to the British Army. “Officials from Army Futures Command, U.S. Army Combat Capabilities Development Command, its centers and lab, briefed interoperability to minimize risks of #modernization divergence,” said in the post. Also was posted some photos on which can replace the posters with information about the long-range cannon that can shoot a projectile 1,000 nautical miles. The FY20 Budget included funding for strategic long-range cannon with a range that will exceed 1,000 miles.

The Strategic Long Range Cannon (SLRC) is comprised of a weapon, prime mover and trailer, projectile and propelling charge capable of delivering massed fires at strategic ranges for multi-domain operations. The SLRC field battery will be included 4 special platforms with cannons and heavy equipment transporters. Also added that the crew of super cannon is 8 personnel per platform.

It should not be imagined that the SLRC is simply an enlarged version of a regular cannon. It should not be thought that this is a successor to the Paris Gun or Gerald Bull's big guns, in which a large cannon fires a passive projectile some great distance. Rather, the Strategic Long Range Cannon fires a self-propelled projectile, providing the initial boost needed to get the projectile up to multi-Mach speeds where the on-board propulsion system can initiate self-sustained flight. This booster function in other systems in typically performed by a solid-rocket motor booster. Whether the substitution of a large gun for a small rocket motor is a net gain has yet to be established.

• Boat tails, are a mechanical way to help reduce the aerodynamic drag at the projectile base. Cannons typically fire simple projectiles. The rear or base end of projectiles are usually either flat or tapered. When firing a projectile at a low angle and a low speed (i.e., short distances), the projectile needs base drag to fly correctly and to prevent it from tumbling out of control. Such a projectile might have a tapered base known as a boat tail.
• A base bleed is another way to reduce base drag. A base bleed is a gas generator that operates similar to a rocket motor. The base bleed fills the aerodynamic void created behind the projectile with combustion gases, thereby reducing the base drag. The combustion gases of the base bleed produce, in effect, a very long boat tail. By reducing the base drag, the base bleed allows the projectile to fly further. For long distances, then, the base bleed is preferred.
• A rocket assisted projectile can considerably increase the range of a gun used to fire projectiles, or other ammunition rounds, by the use of a rocket motor to impart propulsive forces. Such a rocket motor is usually rigidly affixed to the projectile, and the gases evolved from the ignited propellant in the rocket motor provide augmenting propulsion efficiency. Ignition of the rocket motor can be accomplished after the missile leaves the gun barrel. The range and acceleration potential of these rocket assisted projectiles is rather limited due to the small amount of fuel which can be carried and the relatively low specific impulse which can be produced by a rocket motor in comparison to an air breathing ramjet or scramjet propulsion cycle.
• An ramjet projectile typically comprise a centralized tubular design with an internal mid-section constituting a combustion chamber annularly lined with solid fuel for effecting thrust augmentation, or with injectors for liquid propellant. The margin of superiority of ramjet missiles to rockets increases rapidly as missile speed increases. To provide equivalent performance to a Mach 7 supersonic combustion ramjet vehicle at sea level, a rocket would have to have roughly three times as much weight. Such solid fuel ramjet tubular projectiles are typically shot from a cannon, utilizing a sabot to prevent the exploding gases within the cannon's barrel from escaping through the bore of the ramjet tubular projectile.

A cannon-fired missile typically operates in a series of steps. A first launching charge provides the pressure required to eject the missile from a gun barrel in a desired direction. After this discharge step, the missile initiates a propulsion system, such as a propulsion force comes from an engine contained in the missile body. After the missile is launched, the missile may travels for a distance as a ballistic projectile without any self-propelled force. At some time after firing, the missile may fire the engine to initiate self-propelled flight.

Engines used in missile design include rocket engines, scram-jet [supersonic combustion ram jet], ram-jet, gas turbine engines, and pulse jet engines, among others. Operation of a gas turbine or other air-breathing engine may require that the missile provide those systems typical of scram-jet operation, including for example, an air flow system from the exterior region of the missile to an engine inlet. Thus, designs for cannon-fired missiles often call for openings in the missile body that allow air to be pulled from the exterior of the missile and into the turbine engine section. In typical turbine engine operation, the air (or a portion of the air) that is pulled into the scram-jet engine section is then compressed, mixed with fuel, ignited, and discharged through a nozzle section to propel the missile.

A scramjet system launched from a light gas gun for scramjet propulsion testing and experiments in a closed test chamber was documented in 1968 by H. H. King and O. P. Prachar in the Air Force Aero Propulsion Laboratory Technical Report AFAPL-TR-68-9. This study represents the only known attempt to launch a scramjet-shaped projectile from a gun barrel, and was conducted to investigate issues pertaining to launch and acceptable free flight of an annular combustor scramjet model. The scramjet model was too small to include a fuel system, and was therefore limited to unfueled launches to verify structural integrity and aerodynamic stability.

A missile structure, and particularly those missile structures associated with cannon-fired missiles, may be subjected to high G forces during launching, including set back and balloting forces. Additionally, post-launching actions, such as air guide deployment and engine start-up, may further stress the missile structure. During flight missiles may also encounter the general turbulence and stresses associated with projectile flight. However, openings in the missile skin, such as an opening to allow air flow from the exterior of the missile to the interior of the missile, may present points of weakness in the missile structure. Space and weight are often important factors in turbine engine design. This minimal engine weight then allows, among other advantages, for the range of the missile to be extended.

For Mach numbers above 5, the main advantage of scramjet propulsion is that supersonic velocities within the combustion chamber are accompanied by lower static temperatures, pressures, and reduced total pressure losses. By reducing combustion product dissociation reduced temperatures increase combustion efficiency, reduced pressures decrease loads on engine structure, and reduced total pressure losses (entropy gains) increase the flow energy available for thrust production.

Greater thrust and acceleration capability can be achieved by increasing missile diameter, whereas greater range can be achieved by increasing missile length and fuel capacity for a given diameter; trade-offs between length and diameter within a given weight or volume limitation can be made to satisfy the most important mission specification as required.

The projectile includes a forebody having an air compression surface, an engine assembly disposed in a mid-region of the body and including an encompassing cowl disposed about the circumference of the body mid-region, and a nozzle section disposed to the rear of the engine assembly. The projectile assembly also preferably includes a sabot or container-like shell which encircles the rearward portion of the nozzle section, and protects that region of the projectile from the high pressure gases generated by the gun in which the projectile sits until it is fired from the gun. The cowl leading edge portion is configured to cooperate with the external surface of the projectile to capture the air which has been compressed by, and is leaving, the compression surface of the projectile body. The facing surfaces of the cowl and the body are configured to define the region between an internal inlet, isolator and combustor in which takes place further compression of the air, introduction of fuel, and expansion of the combusted air-and-fuel products. The rear end portion of the cowl is configured to direct the exiting combusted air-and-fuel mixture over the nozzle section of the projectile.

A large number of parameters impact the specific impulse (Isp, or thrust per pound of propellant) performance of ramjet and scramjet systems. They include, but are not limited to, the forebody and inlet contraction ratios, the inlet efficiency, the fuel mixing efficiency, the combustion efficiency, and the nozzle efficiency.

The purpose of the inlet is to capture a desired quantity of air flow and deliver it to the combustor at a desired pressure and Mach number with a minimum of entropy producing losses. The technology and parameters necessary to successfully design and operate an efficient supersonic inlet are well-known but difficult to capture in a single design. The mass flow captured by the inlet compared to the drag of the vehicle must be sufficiently large that a net thrust can be expected across the entire Mach number range of operation for achievable values of ramjet or scramjet Isp performance.

The isolator (also known as a constant area diffuser) is located between the inlet and the combustor entrance, and is necessary to adjust flow static pressure from that of the inlet exit to the higher combustor pressure downstream during ramjet and early scramjet ("dual-mode") operation. When combustor pressure rise is large and inlet Mach numbers low, as in ramjet operation, boundary layer separation in the combustor can lead to inlet interaction and engine unstart. An isolator permits a shock train to develop between the inlet and combustor with a near normal shock static pressure rise without any upstream inlet interaction. The length of the isolator is a critical design consideration in carrying out this function.

The combustor provides the physical domain for injecting a liquid or gaseous fuel into high velocity air and mixing the fuel and air for combustion. The fluid and chemical phenomena present in the combustor are extremely complex and include the effects of laminar and turbulent mixing of fuel injection jets with boundary layers and core flows, and the finite rate chemical kinetics of the exothermic combustion reactions. Fuel ignition and flameholding are also important issues. Some of the typical design parameters are the fuel injection geometry, the mixing enhancement devices, and the length of the combustor required to achieve the high mixing and combustion efficiencies necessary for high Isp performance across the Mach number range of interest. Fuel injection location and mixing rate (i.e., distribution of heat release) is also important for controlling if and where flow choking (Mach 1) occurs in the combustor. Fuel is generally injected aft in ramjet mode, both fore and aft in dual-mode (combined supersonic and subsonic combustion), and forward in scramjet mode.

The nozzle or expansion system is critical to the performance of the projectile engine because it produces thrust by accelerating the high static pressure flow exiting the combustor to lower pressure and higher velocity (i.e., high momentum). Typically composed of internal and external nozzles, the objective is to expand the high pressure flow to the lowest pressure possible using a shape that minimizes the combination of friction losses, chemical recombination losses, and flow divergence (angularity) losses.

The ratio of the nozzle expansion area to the inlet capture area and the ratio of inlet mass flow to nonflowpath drag are critical figures of merit in designing a system which produces a flowpath thrust that exceeds the nonflowpath drag and therefore produces acceleration.