Railgun

For railroad artillery, see Railway gun. For other uses, see Rail-gun.

See also: Coilgun

Naval Surface Warfare Center test firing in January 2008

A railgun is an electromagnetic projectile launcher based on similar principles to the homopolar motor. A railgun uses a pair of parallel conductors, or rails, along which a sliding armature is accelerated by the electromagnetic effects of a current that flows down one rail, into the armature and then back along the other rail.

Railguns are being researched as a weapon that would use neither explosives nor propellant, but rather rely on electromagnetic forces to achieve a very high kinetic energy of a projectile. While explosive-powered military guns cannot readily achieve a muzzle velocity of more than about 2 km/s, railguns can readily exceed 3 km/s, and thus far exceed conventionally delivered munitions in range and destructive force. The absence of explosive propellants or warheads to store and handle, as well as the low cost of projectiles compared to conventional weaponry come as additional advantages.

In addition to military applications, NASA has proposed to use a railgun from a high-altitude aircraft to fire a small payload into orbit; however, the extreme g-forces involved would necessarily restrict the usage to only the sturdiest of payloads.

Basics

File:3MpUJGm.jpg

Schematic diagram of a railgun

In its simplest (and most commonly used) form, the railgun differs from a traditional electric motor in that no use is made of additional field windings (or permanent magnets). This basic configuration is formed by a single loop of current and thus requires high currents (e.g. of order one million amperes) to produce sufficient accelerations (and muzzle velocities). A relatively common variant of this configuration is the augmented railgun in which the driving current is channelled through additional pairs of parallel conductors, arranged to increase ("augment") the magnetic field experienced by the moving armature. These arrangements reduce the current required for a given acceleration. In electric motor terminology, augmented railguns are usually series-wound configurations.

The armature may be an integral part of the projectile, but it may also be configured to accelerate a separate, electrically isolated or non-conducting projectile. Solid, metallic sliding conductors are often the preferred form of railgun armature but "plasma" or "hybrid" armatures can also be used. A plasma armature is formed by an arc of ionised gas that is used to push a solid, non-conducting payload in a similar manner to the propellant gas pressure in a conventional gun. A hybrid armature uses a pair of "plasma" contacts to interface a metallic armature to the gun rails. Solid armatures may also "transition" into hybrid armatures, typically after a particular velocity threshold is exceeded.

A railgun requires a pulsed, direct current power supply. For potential military applications, railguns are usually of interest because they can achieve much greater muzzle velocities than guns powered by conventional chemical propellants. Increased muzzle velocities can convey the benefits of increased firing ranges while, in terms of target effects, increased terminal velocities can allow the use of kinetic energy rounds as replacements for explosive shells. Therefore, typical military railgun designs aim for muzzle velocities in the range of 2000–3500 m/s with muzzle energies of 5–50 MJ. For comparison, 50MJ is equivalent to the kinetic energy of a school bus weighing 5 metric tons, travelling at 509 km/h (316 mph). For single loop railguns, these mission requirements require launch currents of a few million amperes, so a typical railgun power supply might be designed to deliver a launch current of 5 MA for a few milliseconds. As the magnetic field strengths required for such launches will typically be approximately 10 tesla, most contemporary railgun designs are effectively "air-cored", i.e. they do not use ferromagnetic materials such as iron to enhance the magnetic flux.

It may be noted that railgun velocities generally fall within the range of those achievable by two-stage light-gas guns; however, the latter are generally only considered to be suitable for laboratory use while railguns are judged to offer some potential prospects for development as military weapons. Another light gas gun, the Combustion Light Gas Gun in a 155 mm prototype form was projected to achieve 2500 m/s with a 70 caliber barrel. In some hypervelocity research projects, projectiles are "pre-injected" into railguns, to avoid the need for a standing start, and both two-stage light-gas guns and conventional powder guns have been used for this role. In principle, if railgun power supply technology can be developed to provide compact, reliable and lightweight units, then the total system volume and mass needed to accommodate such a power supply and its primary fuel can become less than the required total volume and mass for a mission equivalent quantity of conventional propellants and explosive ammunition. Such a development would then convey a further military advantage in that the elimination of explosives from any military weapons platform will decrease its vulnerability to enemy fire.

History

German railgun diagrams

In 1918, French inventor Louis Octave Fauchon-Villeplee invented an electric cannon which is an early form of railgun. He filed for a US patent on 1 April 1919, which was issued in July 1922 as patent no. 1,421,435 "Electric Apparatus for Propelling Projectiles". In his device, two parallel busbars are connected by the wings of a projectile, and the whole apparatus surrounded by a magnetic field. By passing current through busbars and projectile, a force is induced which propels the projectile along the bus-bars and into flight.

In 1944, during World War II, Joachim Hänsler of Germany's Ordnance Office proposed the first theoretically viable railgun. By late 1944, the theory behind his electric anti-aircraft gun had been worked out sufficiently to allow the Luftwaffe's Flak Command to issue a specification, which demanded a muzzle velocity of 2,000 m/s (6,600 ft/s) and a projectile containing 0.5 kg (1.1 lb) of explosive. The guns were to be mounted in batteries of six firing twelve rounds per minute, and it was to fit existing 12.8 cm FlaK 40 mounts. It was never built. When details were discovered after the war it aroused much interest and a more detailed study was done, culminating with a 1947 report which concluded that it was theoretically feasible, but that each gun would need enough power to illuminate half of Chicago.

During 1950, Sir Mark Oliphant, an Australian physicist and first director of the Research School of Physical Sciences at the new Australian National University, initiated the design and construction of the world's largest (500 megajoule) homopolar generator. This machine was operational from 1962 and was later used to power a large-scale railgun that was used as a scientific experiment.

Late into the first decade of the 2000s, the U.S. Navy tested a railgun that accelerates a 3.2 kg (7 pound) projectile to hypersonic velocities of approximately 2.4 kilometres per second (8,600 km/h), about Mach 7. They gave the project the Latin motto "Velocitas Eradico", Latin for "I, [who am] speed, eradicate" (in the vernacular usage, "Speed Kills").

Design

Theory

A railgun consists of two parallel metal rails (hence the name) connected to an electrical power supply. When a conductive projectile is inserted between the rails (at the end connected to the power supply), it completes the circuit. Electrons flow from the negative terminal of the power supply up the negative rail, across the projectile, and down the positive rail, back to the power supply.

This current makes the railgun behave as an electromagnet, creating a magnetic field inside the loop formed by the length of the rails up to the position of the armature. In accordance with the right-hand rule, the magnetic field circulates around each conductor. Since the current is in the opposite direction along each rail, the net magnetic field between the rails (B) is directed at right angles to the plane formed by the central axes of the rails and the armature. In combination with the current (I) in the armature, this produces a Lorentz force which accelerates the projectile along the rails, away from the power supply. There are also Lorentz forces acting on the rails and attempting to push them apart, but since the rails are mounted firmly, they cannot move.

By definition, if a current of one ampere flows in a pair of infinitely long parallel conductors that are separated by a distance of one metre, then the magnitude of the force on each metre of those conductors will be exactly 0.2 micro-newtons. Furthermore, in general, the force will be proportional to the square of the magnitude of the current and inversely proportional to the distance between the conductors. It also follows that, for railguns with projectile masses of a few kg and barrel lengths of a few m, very large currents will be required to accelerate projectiles to velocities of the order of 1000 m/s.

A very large power supply, providing on the order of one million amperes of current, will create a tremendous force on the projectile, accelerating it to a speed of many kilometres per second (km/s). 20 km/s has been achieved with small projectiles explosively injected into the railgun. Although these speeds are possible, the heat generated from the propulsion of the object is enough to erode the rails rapidly. Under high-use conditions, current railguns would require frequent replacement of the rails, or to use a heat-resistant material that would be conductive enough to produce the same effect. At this time it is generally acknowledged that it will take major breakthroughs in material science and related disciplines to produce high-powered railguns capable of firing more than a few shots from a single set of rails. The barrel must withstand these conditions for up to several rounds per minute for thousands of shots without failure or significant degradation. These parameters are well beyond the state of the art in materials science.

Mathematical formula

The magnitude of the force vector can be determined from a form of the Biot–Savart law and a result of the Lorentz force. It can be derived mathematically in terms of the permeability constant ($\mu _{0}$), the radius of the rails (which are assumed to be circular in cross section) ($r$), the distance between the centrepoints of the rails ($d$) and the current in amps through the system ($I$) as follows:

It can be shown from the Biot-Savart law that at one end of a semi-infinite current-carrying wire, the magnetic field at a given perpendicular distance ($s$) from the end of the wire is given by:

$\mathbf {B} (s)={\frac {\mu _{0}I}{4\pi s}}{\hat {\phi }}$

Note this is if the wire runs from the location of the armature e.g. from x = 0 back to $x=-\infty$ and $s$ is measured relative to the axis of the wire.

So, if the armature connects the ends of two such semi-infinite wires separated by a distance, $d$, a fairly good approximation assuming the length of the wires is much larger than $d$, the total field from both wires at any point on the armature, or any point in the plane between the two wires is:

$B(s)={\frac {\mu _{0}I}{4\pi }}\left({\frac {1}{s}}+{\frac {1}{d-s}}\right){\hat {z}}$

Where $s$ is the perpendicular distance from the point on the armature to the axis of one of the wires.

Note that ${\hat {\phi }}$ between the rails is ${\hat {z}}$ assuming the rails are lying in the xy plane and run from x = 0 back to $x=-\infty$ as suggested above.

To obtain an approximate expression for the force on the railgun armature, we start by again assuming that the railgun rails can be modeled as a pair of semi-infinite conductors. This allows us to use the above expression for the magnetic field on the armature in the Lorentz Force Law,

$\mathbf {F} =I\int \mathrm {d} {\boldsymbol {\ell }}\times \mathbf {B}$

Inserting the expression for the magnetic field into the Lorentz force law and setting the bounds of integration to $r$ and $d-r$, assuming the armature is a bar between and perpendicular to the rails making $\mathrm {d} {\boldsymbol {\ell }}$ point in the ${\hat {y}}$, we find the force on the armature is

$\mathbf {F} =I\int _{r}^{d-r}\mathrm {d} {\boldsymbol {\ell }}\times {\frac {\mu _{0}I}{4\pi }}\left({\frac {1}{s}}+{\frac {1}{d-s}}\right){\hat {z}}={\frac {\mu _{0}I^{2}}{2\pi }}\ln {\left({\frac {d-r}{r}}\right)}{\hat {x}}$

The formula is based on the assumption that the distance ($l$) between the point where the force ($F$) is measured and the beginning of the rails is greater than the separation of the rails ($d$) by a factor of about 3 or 4 ($l>3d$). Some other simplifying assumptions have also been made; to describe the force more accurately, the geometry of the rails and the projectile must be considered.

With most practical railgun geometries, it is not easy to produce an electromagnetic expression for the railgun force that is both simple and reasonably accurate. Instead, most practical railgun analyses actually used a lumped circuit model to describe the relationship between the driving current and the railgun force. In these models the voltage across the railgun breech is given by:

$V=IR+{\frac {{\text{d}}(LI)}{{\text{d}}t}}$

Then the barrel resistance and inductance are assumed to vary linearly with the projectile position, so that

$R=R'x$

$L=L'x$

from which

$V=I(R'x+L'v)+L'x{\frac {{\text{d}}I}{{\text{d}}t}}$

If the driving current is held constant, there is a power flow equal to $I^{2}L'v$ which represents the electromagnetic work done. In this simple model, exactly half of this is assumed to be needed to establish the magnetic field along the barrel, i.e. as the length of the current loop increases. The other half represents the power flow into the kinetic energy of the projectile. Since power can be expressed as force times speed, this gives the standard result that the force on the railgun armature is given by:

$F={\frac {L'I^{2}}{2}}$

This simple equation shows that high accelerations will require very high currents. For an ideal square bore railgun, the value of $L'$ would be about 0.6 microHenries per metre ($\mu$.H/m) but most practical railgun barrels exhibit lower values of $L'$ than this.

Since the lumped circuit model describes the railgun force in terms of fairly normal circuit equations, it becomes possible to specify a simple time domain model of a railgun.

Ignoring friction and air drag, the projectile acceleration is given by:

${\frac {{\text{d}}v}{{\text{d}}t}}={\frac {L'I^{2}}{2m}}$

where m is the projectile mass. The motion along the barrel is given by:

${\frac {{\text{d}}x}{{\text{d}}t}}=v$

and the above voltage and current terms can be placed into appropriate circuit equations to determine the time variation of current and voltage.

It can also be noted that the textbook formula for the high frequency inductance per unit length of a pair of parallel round wires, of radius r and axial separation d is:

$L'={\frac {\mu _{0}}{\pi }}\ln {\left({\frac {d-r}{r}}\right)}$

so the lumped parameter model also predicts the force for this case as:

$F={\frac {L'I^{2}}{2}}={\frac {\mu _{0}I^{2}}{2\pi }}\ln {\left({\frac {d-r}{r}}\right)}$.

With practical railgun geometries, much more accurate two or three dimensional models of the rail and armature current distributions (and the associated forces) can be computed, e.g. by using finite element methods to solve formulations based on either the scalar magnetic potential or the magnetic vector potential.

Design considerations

The power supply must be able to deliver large currents, sustained and controlled over a useful amount of time. The most important gauge of power supply effectiveness is the energy it can deliver. As of December 2010, the greatest known energy used to propel a projectile from a railgun was 33 megajoules. The most common forms of power supplies used in railguns are capacitors and compulsators which are slowly charged from other continuous energy sources.

The rails need to withstand enormous repulsive forces during shooting, and these forces will tend to push them apart and away from the projectile. As rail/projectile clearances increase, arcing develops, which causes rapid vaporization and extensive damage to the rail surfaces and the insulator surfaces. This limited some early research railguns to one shot per service interval.

The inductance and resistance of the rails and power supply limit the efficiency of a railgun design. Currently different rail shapes and railgun configurations are being tested, most notably by the United States Navy, the Institute for Advanced Technology at the University of Texas at Austin, and BAE Systems.

Materials used

The rails and projectiles must be built from strong conductive materials; the rails need to survive the violence of an accelerating projectile, and heating due to the large currents and friction involved. Some erroneous work has suggested that the recoil force in railguns can be redirected or eliminated; careful theoretical and experimental analysis reveals that the recoil force acts on the breech closure just as in a chemical firearm. The rails also repel themselves via a sideways force caused by the rails being pushed by the magnetic field, just as the projectile is. The rails need to survive this without bending and must be very securely mounted. Currently published material suggests that major advances in material science must be made before rails can be developed that allow railguns to fire more than a few full-power shots before replacement of the rails is required.

Heat dissipation

In current designs massive amounts of heat are created by the electricity flowing through the rails, as well as by the friction of the projectile leaving the device. The heat created by this friction itself can cause thermal expansion of the rails and projectile, further increasing the frictional heat. This causes three main problems: melting of equipment, decreased safety of personnel, and detection by enemy forces due to increased infrared signature. As briefly discussed above, the stresses involved in firing this sort of device require an extremely heat-resistant material. Otherwise the rails, barrel, and all equipment attached would melt or be irreparably damaged.

In practice the rails are, with most designs of railgun, subject to erosion due to each launch; in addition, projectiles can be subject to some degree of ablation, and this can limit railgun life, in some cases severely.

Applications

Railguns have a number of potential practical applications, primarily for the military. However, there are other theoretical applications currently being researched.

Launch or launch assist of spacecraft

Main article: Mass driver

See also: Space gun

Electrodynamic assistance to launch rockets has been studied. Space applications of this technology would likely involve specially formed electromagnetic coils and superconducting magnets. Composite materials would likely be used for this application.

For space launches from Earth, relatively short acceleration distances (less than a few km) would require very strong acceleration forces, higher than humans can tolerate. Other designs include a longer helical (spiral) track, or a large ring design whereby a space vehicle would circle the ring numerous times, gradually gaining speed, before being released into a launch corridor leading skyward.

Parameters of McNab's launcher	Value	Units
Vehicle Mass	1000	kg
Muzzle Velocity	7500	m/s
Muzzle Energy	35	GJ
Launcher length	1600	m
Maximum acceleration	19500	m/s2
Maximum acceleration	1988	gn
Launch time	0.43	s
Current density	6.8	MA/m

In 2003, Ian McNab outlined a plan to turn this idea into a realized technology. Because of strong acceleration, this system would launch only sturdy materials, such as food, water, and – most importantly – fuel. Under ideal circumstances (equator, mountain, heading east) the system would cost $528/kg, compared with $5,000/kg on the conventional rocket. The McNab's railgun could make approximately 2000 launches per year, for a total of maximum 500 tons launched per year. Because the launch track would be 1.6 km long, power will be supplied by a distributed network of 100 rotating machines (compulsator) spread along the track. Each machine would have a 3.3-ton carbon fibre rotor spinning at high speeds. A machine can recharge in a matter of hours using 10 MW power. This machine could be supplied by a dedicated generator. The total launch package would weigh almost 1.4 tons. Payload per launch in these conditions is over 400 kg. There would be a peak operating magnetic field of 5 T—half of this coming from the rails, and the other half from augmenting magnets. This halves the required current through the rails, which reduces the power fourfold.

Weaponry

Drawings of electric gun projectiles

Railguns are being researched as weapons with projectiles that do not contain explosives or propellants, but are given extremely high velocities: 2,500 m/s (8,200 ft/s) (approximately Mach 7 at sea level) or more. For comparison, the M16 rifle has a muzzle speed of 930 m/s (3,050 ft/s), and the 16"/50 caliber Mark 7 gun that armed World War II American battleships has a muzzle speed of 760 m/s (2,490 ft/s)), which because of its much greater mass generated a muzzle energy of 360 MJ and a downrange kinetic impact of energy of over 160 MJ. By firing smaller projectiles at extremely high velocities, railguns can yield kinetic energy impacts equal or superior to the destructive energy of 5" Naval guns, but with much greater range. This decreases ammunition size and weight, allowing more ammunition to be carried and eliminating the hazards of carrying explosives or propellants in a tank or naval weapons platform. Also, by firing at greater velocities, railguns have greater range, less time to target, and at shorter ranges less wind drift, bypassing the physical limitations of conventional firearms: "the limits of gas expansion prohibit launching an unassisted projectile to velocities greater than about 1.5 km/s and ranges of more than 50 miles [80 km] from a practical conventional gun system." Current railgun technologies necessitate a long and heavy barrel, but a railgun's ballistics far outperform conventional cannons of equal barrel lengths. Railguns can also deliver area of effect damage by detonating a bursting charge in the projectile which unleashes a swarm of smaller projectiles over a large area.

Assuming that the many technical challenges facing fieldable railguns are overcome, including tough ones like railgun projectile guidance and rail endurance, the increased launch velocities of railguns will provide advantages over more conventional guns for a variety of offensive and defensive scenarios. Railguns have the potential to be used against both surface and airborne targets.

Many critics of weaponized railgun systems claim operating them with a suitable exit velocity and rate of fire would consume too much power, though this would likely not be a problem for nuclear-powered systems such as on large warships or submarines.

The first weaponized railgun planned for production, the General Atomics Blitzer system, began full system testing in September 2010. The weapon launches a streamlined discarding sabot round designed by Boeing's Phantom Works at 1,600 m/s (5,200 ft/s) (approximately Mach 5) with accelerations exceeding 60,000 g_n. During one of the tests, the projectile was able to travel an additional 7 kilometres (4.3 mi) downrange after penetrating a 1⁄8 inch (3.2 mm) thick steel plate. The company hopes to have an integrated demo of the system by 2016 followed by production by 2019, pending funding. Thus far, the project is self-funded.

In October 2013, General Atomics unveiled a land based version of the Blitzer railgun. A company official claimed the gun could be ready for production in "two to three years".

Railguns are being examined for use as anti-aircraft weapons to intercept air threats, particularly anti-ship cruise missiles, in addition to land bombardment. A supersonic sea-skimming anti-ship missile can appear over the horizon 20 miles from a warship, leaving a very short reaction time for a ship to intercept it. Even if conventional defense systems react fast enough, they are expensive and only a limited number of large interceptors can be carried. A railgun projectile can reach several times the speed of sound faster than a missile, because of this, it can hit a target, such as a cruise missile, much faster and further away from the ship. Projectiles are also cheaper and smaller, allowing for many more to be carried. The speed, cost, and numerical advantages of railgun systems may allow them to replace several different systems in the current layered defense approach. A railgun projectile without the ability to change course can hit fast-moving missiles at a maximum range of 30 nmi (35 mi; 56 km). As is the case with the Phalanx CIWS, unguided railgun rounds will require multiple/many shots to bring down maneuvering supersonic anti-ship missiles, with the odds of hitting the missile improving dramatically the closer it gets. The Navy plans for railguns to be able to intercept endo-atmospheric ballistic missiles, stealthy air threats, supersonic missiles, and swarming surface threats; a prototype system for supporting interception tasks is to be ready by 2018, and operational by 2025. This timeframe suggests the weapons are planned to be installed on the Navy's next-generation surface combatants, expected to start construction by 2028.

Tests

Diagram showing the cross-section of a linear motor cannon

Full-scale models have been built and fired, including a 90 mm (3.5 in) bore, 9 MJ kinetic energy gun developed by the US DARPA. Rail and insulator wear problems still need to be solved before railguns can start to replace conventional weapons. Probably the oldest consistently successful system was built by the UK's Defence Research Agency at Dundrennan Range in Kirkcudbright, Scotland. This system was established in 1993 and has been operated for over 10 years. Using its associated flight range for internal, intermediate, external and terminal ballistics, it achieved several mass and velocity records.

The Yugoslavian Military Technology Institute developed, within a project named EDO-0, a railgun with 7 kJ kinetic energy, in 1985. In 1987 a successor was created, project EDO-1, that used projectile with a mass of 0.7 kg (1.5 lb) and achieved speeds of 3,000 m/s (9,800 ft/s), and with a mass of 1.1 kg (2.4 lb) reached speeds of 2,400 m/s (7,900 ft/s). It used a track length of 0.7 m (2.3 ft). According to those working on it, with other modifications it was able to achieve a speed of 4,500 m/s (14,800 ft/s). The aim was to achieve projectile speed of 7,000 m/s (23,000 ft/s). At the time, it was considered a military secret.

China is now one of the major players in electromagnetic launchers; in 2012 it hosted the 16th International Symposium on Electromagnetic Launch Technology (EML 2012) at Beijing. Satellite imagery in late 2010 suggested that tests were being conducted at an armor and artillery range near Baotou, in the Inner Mongolia Autonomous Region.

U.S. military tests

The United States military is funding railgun experiments. At the University of Texas at Austin Center for Electromechanics, military railguns capable of delivering tungsten armor-piercing bullets with kinetic energies of nine megajoules have been developed. 9 MJ is enough energy to deliver 2 kg (4.4 lb) of projectile at 3 km/s (1.9 mi/s)—at that velocity a rod of tungsten or another dense metal could easily penetrate a tank, and potentially pass through it.

U.S. Navy tests

The United States Naval Surface Warfare Center Dahlgren Division demonstrated an 8 MJ railgun firing 3.2 kg (7.1 lb) projectiles in October 2006 as a prototype of a 64 MJ weapon to be deployed aboard Navy warships. The main problem the U.S. Navy has had with implementing a railgun cannon system is that the guns wear out due to the immense pressures, stresses and heat that are generated by the millions of amperes of current necessary to fire projectiles with megajoules of energy. Such weapons, while not nearly as powerful as a cruise missile like a BGM-109 Tomahawk cruise missile that will deliver 3000 MJ of destructive energy to a target, will theoretically allow the Navy to deliver more granular firepower at a fraction of the cost of a missile, and will be much harder to shoot down versus future defensive systems. For context another relevant comparison is the Rheinmetall 120mm gun used on main battle tanks will generate 9 MJ of muzzle energy. An MK 8 round fired from the 16" guns of an Iowa Class battleship at 2500 fps (762 m/s) has 360 MJ of kinetic energy at the muzzle.

Since then, BAE Systems has delivered a 32 MJ prototype (muzzle energy) to the U.S. Navy. The same amount of energy is released by the detonation of 4.8 kg (11 lb) of C4.

On January 31, 2008 the U.S. Navy tested a railgun that fired a projectile at 10.64 MJ with a muzzle velocity of 2,520 m/s (8,270 ft/s). The power was provided by a new 9-megajoule prototype capacitor bank using solid-state switches and high-energy-density capacitors delivered in 2007 and an older 32-MJ pulse power system from the US Army’s Green Farm Electric Gun Research and Development Facility developed in the late 1980s that was previously refurbished by General Atomics Electromagnetic Systems (EMS) Division. It is expected to be ready between 2020 to 2025.

A test of a railgun took place on December 10, 2010, by the US Navy at the Naval Surface Warfare Center Dahlgren Division. During the test, the Office of Naval Research set a world record by conducting a 33 MJ shot from the railgun, which was built by BAE Systems.

A test took place in February, 2012, at the Naval Surface Warfare Center Dahlgren Division. While similar in energy to the aforementioned test, the railgun used is considerably more compact, with a more conventional looking barrel. A General Atomics-built prototype was delivered for testing in October 2012.

External videos
Official 33MJ video
Additional footage
February 2012 test

The U.S. Navy plans to integrate a railgun that has a range of over 160 km (100 mi) onto a ship by 2016. This weapon, while having a form factor more typical of a naval gun will utilize components largely in common with those developed and demonstrated at Dahlgren. The hyper-velocity rounds weigh 10 kg (23 lb), are 18 in (460 mm), and are fired at Mach 7.

A future goal is to develop projectiles that are self-guided - a necessary requirement to hit distant targets or intercepting missiles. When the guided rounds are developed, the Navy is projecting each round to cost about $25,000, though it must be noted that developing guided projectiles for guns has a history of doubling or tripling initial cost estimates. Some HPV projectiles developed by the Navy have command guidance, but the accuracy of the command guidance is not known, nor even if it can survive a full power shot.

Currently the only US Navy ships that can produce enough electrical power to get desired performance are the Zumwalt-class destroyers; they can generate 78 megawatts of power, more than is necessary to power a railgun. Engineers are working to derive technologies developed for the DDG-1000 series ships into a battery system so other warships can operate a railgun. Most current destroyers can spare only nine megawatts of additional electricity, while it would require 25 megawatts to propel a projectile to the desired maximum range (i.e. to launch 32MJ projectiles at a rate of 10 shots per minute). Even if current ships, such as the Arleigh Burke-class destroyer, can be upgraded with enough electrical power to operate a railgun, the space taken up on the ships by the integration of an additional weapon system may force the removal of existing weapon systems to make room available. The first shipboard tests will be from a railgun installed on an Expeditionary Fast Transport. Though ships of that class are non-combatants, they were chosen for their available cargo and topside space and schedule flexibility. They will not be permanently installed on the EFT, and the Navy has yet to decide which ship classes will receive a fully operational railgun. Single shot tests will be held in 2016, followed by an autoloader in 2018. According to the Navy "current research is focused on a rep-rate capability of multiple rounds per minute which entails development of a tactical prototype gun barrel and pulsed power systems incorporating advanced cooling techniques. Components are designed to transition directly into prototype systems now being conceptualized." So as of March 2014 multiple rep railgun were in the conceptual stage and have a ways to go before reaching the prototype stage.

Though the 23 lb projectiles have no explosives, their Mach 7 velocity gives them 32 megajoules of energy, but impact kinetic energy downrange will typically be 50 percent or less of the muzzle energy. The Navy is looking into other uses for railguns, besides land bombardment, such as air defense; with the right targeting systems, projectiles could intercept aircraft, cruise missiles, and even ballistic missiles. The Navy is also developing directed energy weapons for air defense use, but it will be years before they will be effective.

The railgun will be part of a Navy fleet that envisions future offensive and defensive capabilities being provided in layers: lasers to provide close range defense, railguns to provide medium range attack and defense, and cruise missiles to provide long-range attack; though railguns will cover targets up to 100 miles away that previously needed a missile.

The Navy may eventually enhance railgun technology to enable it to fire at a range of 200 nmi (230 mi; 370 km) and impact with 64 megajoules of energy. One shot would require 6 million amps of current, so it will take a long time to develop capacitors that can generate enough energy and strong enough gun materials.

Outstanding Issues in Fielding Railgun Weapons

Major technological and operational hurdles must be overcome before railguns can be deployed:

1) Railgun durability: To date railgun demonstrations, while impressive, have not demonstrated an ability to fire multiple full power shots from the same set of rails. The Navy has claimed hundreds of shots from the same set of rails. In a March 2014 statement to the Intelligence, Emerging Threats and Capabilities Subcommittee of the House Armed Services Committee, Chief of Naval Research Admiral Matthew Klunder stated, "Barrel life has increased from tens of shots to over 400, with a program path to achieve 1000 shots." However, the Office of Naval Research (ONR) will not confirm that the 400 shots are full-power shots. Further there is nothing published to indicate there are any high megajoule class railguns with the capability of firing hundreds of full-power shots while staying within the strict operational parameters necessary to fire railgun shots accurately and safely. As noted in an article by Globalsecurity.org: railguns should be able to fire 6 rounds per minute with a rail life of about 3000 rounds. Given launch acceleration of up to 60,000 g's, massive pressures and mega amps of current, railgun rails are quickly destroyed and getting to the endurance to fire hundreds of full-power rounds, to say nothing of thousands of rounds, could require breakthroughs in materials science that cannot be scheduled and could be decades in coming.

Until the capability of firing at least hundreds of rounds of full power shots from the same set of rails is demonstrated, railguns as fieldable weapons remain an interesting idea with a lot of potential.

2) Railgun Projectile Guidance: While the Navy has made reference to having successfully integrated "command guidance" into railgun projectiles there is no published documentation of having successfully tested such a capability. Command guidance, as opposed to self-guidance, involves direct control of the railgun projectile by the launching authority using such technologies as radio or wire, etc. With the kind of speeds obtained by railgun projectiles and the ranges possible, command guidance will be of limited use. Further, no details have been given as to effectiveness of the command guidance or if it will actually work with full power railgun shots. Until the Navy releases more information, command guidance effectiveness is an unknown.

3) Self-guided Railgun Projectiles: A future capability critical to fielding a real railgun weapon is developing a robust guidance package that will allow the railgun to fire at distant targets or to hit incoming missiles. Developing such a package is a real challenge. The Navy's RFP Navy SBIR 2012.1 - Topic N121-102 for developing such a package gives a good overview of just how challenging railgun projectile guidance is:

"The package must fit within the mass (< 2 kg), diameter (< 40 mm outer diameter), and volume (200 cm³) constraints of the projectile and do so without altering the center of gravity. It should also be able to survive accelerations of at least 20,000 g (threshold) / 40,000 g (objective) in all axes, high electromagnetic fields (E > 5,000 V/m, B > 2 T), and surface temperatures of > 800 deg C. The package should be able to operate in the presence of any plasma that may form in the bore or at the muzzle exit and must also be radiation hardened due to exo-atmospheric flight. Total power consumption must be less than 8 watts (threshold) / 5 watts (objective) and the battery life must be at least 5 minutes (from initial launch) to enable operation during the entire engagement. In order to be affordable, the production cost per projectile must be as low as possible, with a goal of less than $1,000 per unit."

While the specifications mention 40,000 g's, actual railgun launches can reach 60,000 g's so this contract is to develop a base capability that would be appropriate for launches of less than full power pulling less than 40,000 g's.

On June 22, 2015, General Atomics’ Electromagnetic Systems announced that projectiles with on-board electronics survived the whole railgun launch environment and performed their intended functions in four consecutive tests on June 9 and 10 June at the U.S. Army’s Dugway Proving Ground in Utah. The on-board electronics successfully measured in-bore accelerations and projectile dynamics, for several kilometers downrange, with the integral data link continuing to operate after the projectiles impacted the desert floor, which is essential for precision guidance.

Trigger for inertial confinement fusion

Railguns may also be miniaturized for inertial confinement nuclear fusion.

• Fusion is triggered by very high temperature and pressure at the core.
  • Current technology calls for multiple lasers, usually over 100, to concurrently strike a fuel pellet, creating a symmetrical compressive pressure.
  • Railguns may be able to trigger fusion by firing energetic plasma from multiple directions. The process developed involves four key steps.
    • Plasma is pumped into a chamber.
    • When the pressure is great enough, a diaphragm will rupture, sending gas down the rail.
    • Shortly afterwards, a sufficient voltage is applied to the rails, creating a conduction path of ionized gas.
    • This plasma is accelerated down the rail, eventually being ejected at a large velocity.
• The rails and dimensions are on the order of centimetres.

See also

• Coilgun
• Homopolar generator
• Kinetic energy penetrator
• Light-gas gun
• List of caseless firearms
• List of electromagnetic projectile devices in fiction
• Mass driver
• Non-rocket spacelaunch
• Project Babylon
• Ram accelerator
• Space gun
• USNS Trenton (T-EPF-5), first ship to mount a railgun.

External links

• NRL Railgun Demonstration Video US Naval Research Laboratory, July 2010
• Archived 2009-11-22 at the Wayback Machine Jane's Defence Weekly, 20 July 2006
• Electromagnetic Railgun Popular Science Article, June 2004
• Video of Navy railgun test firing, Navy Electromagnetic Launch Facility, Test Shot #1, 2 October 2006. Source: Fredericksburg.com, accessed 30 January 2007
• World’s Most Powerful Rail Gun Delivered to Navy, 14 November 2007