Railguns: Newton's Third Law Defied Or Upheld?

Railguns use electromagnetism to fire projectiles at tremendous speeds. They work by passing an electric current through a moveable projectile, which is propelled out of the railgun by the resulting magnetic field. According to Newton's third law of motion, every action must have an equal and opposite reaction. However, this does not seem to be the case for railguns, leading to an interesting debate in scientific circles about whether railguns break Newton's third law.

Railguns use electromagnetism to fire projectiles

Railguns are weapons that use electromagnetism to fire projectiles at tremendous speeds. They consist of two parallel rails, with a moveable projectile that acts as a wire. An electric current passes down one rail, through the projectile, and back up the other rail. The magnetic fields created by the electric current in the rails interact with each other and with the current in the projectile, propelling it out of the railgun at high speed.

The railgun system can be considered a closed system, in which case the law of conservation of momentum applies. This law states that the momentum of the system must remain constant unless acted upon by an external force. When the projectile is fired, it gains momentum, and in order to conserve the system's momentum, the railgun itself would have to move in the opposite direction with equal momentum. However, when a railgun is fired on a planet's surface, it does not move backward. Instead, the force of the planet pushes back against the railgun, cancelling out the momentum of the projectile. This is what we perceive as recoil.

In the absence of an external force, such as when the railgun is fired in outer space, the railgun would indeed move backward in accordance with Newton's third law. This law states that for every action (force) in nature, there is an equal and opposite reaction. In the context of the railgun, the action force is the firing of the projectile, and the reaction force is the backward movement of the railgun, assuming no external forces are at play.

While railguns do not appear to break Newton's third law, there has been debate and ongoing research surrounding the location, magnitude, and cause of recoil forces in railguns. Some claim that the magnitude of the force on the armature is significantly greater than any predicted equal and opposite reaction force on the rails, challenging the notion of conservation of momentum. However, these claims have not been supported by direct experimental observation, and further investigation is needed.

Railguns don't technically recoil

Railguns are linear motor devices that use electromagnetic force to launch high-velocity projectiles. They don't rely on explosives but on the projectile's high kinetic energy to inflict damage.

Railguns use electromagnetic effects to accelerate a sliding projectile called an armature. 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 flow of electrons creates a magnetic field, which, in turn, creates a Lorentz force that accelerates the projectile along the rails and away from the power supply.

The recoil force in railguns acts on the breech closure just as in a chemical firearm. However, the recoil force can be redirected or eliminated. If the railgun is mounted on a spaceship or space station, there is no planet to push back against to resist the recoil. In this case, the spacecraft's momentum will change to cancel out the momentum of the projectile.

In practice, the rails used with most railgun designs are subject to erosion from each launch. This, along with the heat generated from the electricity flowing through the rails, means that current railguns would require frequent replacement of the rails or the use of a heat-resistant material.

Some erroneous work has suggested that the recoil force in railguns can be redirected or eliminated. However, careful theoretical and experimental analysis reveals that the recoil force acts on the breech closure just as in a chemical firearm.

Railguns and Newton's Third Law

Newton's Third Law states that for every action (force) in nature, there is an equal and opposite reaction. In other words, if object A exerts a force on object B, then object B also exerts an equal force on object A. The two resultant forces have the same strength but act in exactly opposite directions.

A railgun uses electromagnetism to fire projectiles at tremendous speeds without using an expanding gas. It consists of two parallel rails and a moveable projectile that acts as a wire. An electric current travels down one wire, across the projectile, and then back down the other rail. The magnetic fields created by the rails interact with the projectile's current, propelling it forward and out of the railgun.

When a railgun is fired, the projectile is launched with significant momentum. According to the law of conservation of momentum, the momentum of an isolated system remains constant. If the railgun were truly isolated, it would move backward with sufficient speed to cancel out the momentum of the projectile, resulting in a net momentum of zero, similar to the recoil of a conventional gun.

However, when a railgun is fired on a planet's surface, it does not fly backward. Instead, the force of recoil is transferred to the gun carriage, tripod, or some other mechanism that is ultimately braced against the Earth's surface. This recoil force is equal and opposite to the momentum of the projectile, in accordance with Newton's Third Law.

The absence of observable recoil in railguns has led some to question the validity of Newton's Third Law and the law of conservation of momentum. However, the recoil force in railguns is not as apparent as in conventional guns due to the absence of expanding gases and the efficient transfer of recoil energy to the mounting structure.

In conclusion, railguns do not break Newton's Third Law. The law of conservation of momentum still applies, and the recoil force is present but may be less noticeable due to the unique characteristics of railgun operation.

Railgun history

The concept of the railgun was first introduced by French inventor André Louis Octave Fauchon-Villeplee, who created a small working model in 1917 with the help of the Société anonyme des accumulateurs Tudor (now Tudor Batteries). During World War I, the French Director of Inventions at the Ministry of Armaments, Jules-Louis Brenton, commissioned Fauchon-Villeplee to develop a 30-mm to 50-mm electric cannon on 25 July 1918, after delegates from the Commission des Inventions witnessed test trials of the working model in 1917. However, the project was abandoned once World War I ended later that year on 11 November 1918. Fauchon-Villeplee filed for a US patent on 1 April 1919, which was issued in July 1922.

In 1923, Russian scientist A. L. Korol'kov criticised Fauchon-Villeplee's design, arguing against some of the claims that Fauchon-Villeplee made about the advantages of his invention. Korol'kov concluded that while the construction of a long-range electric gun was possible, the practical application of Fauchon-Villeplee's railgun was hindered by its enormous electric energy consumption and need for a special electric generator.

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 and a projectile containing 0.5 kg 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, 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.

In 1980, the Ballistic Research Laboratory began a long-term program of theoretical and experimental research on railguns. The work was conducted predominantly at the Aberdeen Proving Ground, and much of the early research drew inspiration from the railgun experiments performed by the Australian National University. Topics of research included plasma dynamics, electromagnetic fields, telemetry, and current and heat transport.

Since 1993, the British and American governments have collaborated on a railgun project at the Dundrennan Weapons Testing Centre that culminated in the 2010 test where BAE Systems fired a 3.2 kg projectile at 18.4-megajoules. In 1994, India's DRDO's Armament Research and Development Establishment developed a railgun with a 240 kJ, low-inductance capacitor bank operating at 5 kV power, able to launch projectiles of 3–3.5 g weight to a velocity of more than 2,000 m/s. In 1995, the Center for Electromagnetics at the University of Texas at Austin designed and developed a rapid-fire railgun launcher called the Cannon-Caliber Electromagnetic Gun. The launcher prototype was later tested at the U.S. Army Research Laboratory, where it demonstrated a breech efficiency over 50 per cent.

The practical implications of recoil

The law of conservation of momentum states that the momentum of a system must not change unless acted on by an outside force. For example, if we have a bomb that is at rest, it has zero momentum. If we light the fuse and it explodes, the momentum of the system must still be zero after the explosion, even though the bomb casing fragments are not at rest. This is because momentum is mass multiplied by velocity, and velocity has a direction. So, if we have two fragments of equal mass and speed moving in opposite directions, the combined momentum of those two parts is zero.

Now, let's consider a railgun. If a railgun was truly isolated, like our bomb in outer space, then when the projectile is fired, the gun would move backward with sufficient speed to cancel out the momentum of the projectile, leaving a net momentum of zero. However, a practical railgun fired on a planet's surface is not an isolated system. It doesn't fly backward because something pushes back very hard to cancel out the momentum of the projectile. This pushback is what we call recoil.

Recoil is the practical implication of the gun trying to fly backward with an equal and opposite momentum to the projectile. It is our way of defining our gun system so that it is not in isolation. We add a gun carriage, tripod, rifleman, or Navy ship to the system, ultimately bracing it against the Earth's surface. Recoil is complicated, and while we can engineer tricks to manage it, we cannot change the law of conservation of momentum.

For example, designers of rifles can lessen and use the recoil by diverting expanding gases to expel the spent round and chamber the next one. However, if the gun system is not to fly backward, something must still push hard against the breech. In the case of a rifle, this would traditionally be the firer's shoulder. Getting this bracing right is crucial because accurate bullets are what kill the enemy, not volume of fire. If the firer can't control the recoil, they can't fire accurately.

In the context of railguns, the recoil force acting on the breech is the same as that of a conventional gun firing a projectile of the same mass and velocity. However, there are ways to negate recoil in railguns. For instance, in the Human Legion books, when describing handheld carbines used in space, the recoil is compensated for as the guns are designed to be used in conjunction with powered armor that knows how to brace and push against the recoil. Additionally, there is a recoil absorption system that transforms some of the recoil shock into heat and then dumps that energy out to another dimension.

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