Recoil

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Introduction

Recoil, and its management, is one of the central issues concerning ballistic weaponry. Every weapon that has the effect of accelerating a projectile experiences recoil as dictated by the conservation of momentum, one of the fundamental principles of physics and laws of nature. Since this subject is touched upon in many other articles, this page is a "quick and dirty" look at the fundamentals of recoil physics, common recoil management strategies of ballistic weaponry, and speculative notes on the recoil of as-yet-to-be-realized weaponry.

Fundamentals of Recoil Physics

As mentioned previously, recoil is a direct result of conservation of momentum. Momentum is the product of mass and velocity vector of an object. The total change in momentum from an event is called the impulse. The force acting on an object multiplied the time the force is applied gives the impulse and thus the change in momentum (if the force varies over time, the impulse is the integral of the force over time – basically you accumulate small amounts to the impulse over tiny slices of time that are short enough that the force doesn't change much, and add them all up to get the total impulse). For every interaction, if one object exerts a force on another then that other object exerts a force of equal magnitude but opposite direction back on the first. If you push down on a table, the table pushes back up on you. If a magnet pulls on a block of steel, that steel block pulls back on the magnet. Because of this, the impulses the two object impart on each other are also equal in magnitude and opposite in direction. And thus, the net momentum of the system of both objects does not change.

What this means for ballistic weaponry is that for every meter-per-second added to every kilogram of projectile that is thrown down range by the apparatus of kinematic, the apparatus itself (and its bearer) would be propelled in the opposite direction by the same kilogram-meter-per-second, although it is free to choose to pay it in either currency, mass or velocity. This generalized view on recoil is appropriate in so far as it describes the macroscopic consequence of the "power-level" of ballistic weaponry, but it does not aptly describe the process by which this effect is applied, and thus can miss out on important caveats that is of interest to the science fictional author and audiences.

Recoil for Conventional Guns

A conventional gun ignites a flammable substance (usually a granular material called gunpowder) in a rigid metal chamber behind a bullet. The burning of the powder produces hot high pressure gas. The pressure of that gas pushes the bullet down a long tube (the barrel). But the same pressure also pushes out and back on the gun itself from the inside. Because the bullet is pushed down the barrel, the forces on the gun are not balanced and there is a net backward force from the gas pressure pushing back on the back face of the chamber that held the ammunition before firing.

The recoil of a conventional gun is, unfortunately, complicated by the use of a propellant (the powder's combustion gas product) to accelerate the projectile. In addition to the momentum of the projectile itself, the conventional gun has to contend with accelerating both the propellant as they combust and chase the projectile, and after the bullet has left the bore, the further acceleration of the propellant gas itself. It is therefore wise to split the discussion of conventional gun recoil into distinct phases, separated by the moment the projectile exits the muzzle.

Note that while we have a relatively decent understanding of the phenomenon that contributes to recoil in the in-bore phase of projectile acceleration, the outflow phase does not admit simple description. What we discuss here will almost assuredly be a gross simplification, especially for the second phase, and the interested reader is always welcomed to consult up-to-date research, preferably of the computerized-fluid-dynamics coupled simulation kind, for the state of the art, if they were so inclined. However, we are confident that this section will still prove to be valuable for the less technically inclined reader.

Recoil for Electromagnetic Launchers

An electromagnetic launcher is a device that uses electromagnetic fields and electric currents to push or pull a projectile. For a coilgun, the recoil is straightfoward - the fields of the projectile while it is being accelerated push or pull on the currents in the coils the same amount that the fields from the coils push or pull on the currents in the projectile (for a magnetic projectile, these can be the net currents you get by aligning the spins and angular momenta of the electrons around the atoms in the magnet - they're not the sort of currents that can be measured by a multi-meter, but they still physically exist and make and interact with magnetic fields). So for a coilgun, you directly get the impulse on the projectile being the impulse on the gun in an obvious way.

For railguns, the situation is a bit more complicated. The magnetic forces in the railgun push the rails apart, not backward. So the impulse on the projectile is not countered by the impulse on the rails. Instead, the railgun is pushed back where the circuit is closed at the back end of the rails. Where the current completes the circuit in the driving generators and electronics (and power couplings if the generation is kept distant from the rails) is where the recoil is generated, from the same sort of action of the magnetic field on the current that also propels the bullet down the rails.

There is one additional complication. The operation of an electromagnetic gun will produce electromagnetic radiation (in the form of radio and microwaves). This radiation also has momentum. So you don't always have the force exerted by the gun on the bullet being exactly the same as the force exerted by the bullet on the gun, and the tiny bit of excess impulse flies away as the momentum of the emitted electromagnetic radiation. However, in practice this extra momentum is negligible … if it wasn't, there would be so much power and energy in the emitted radiation that the gun would cook the people around it.

Recoil for Lasers

As mentioned for electromagnetic launchers, electromagnetic waves have a small amount of momentum. A laser weapon works by emitting a very high powered beam of electromagnetic waves. So lasers will produce a small amount of recoil. For most purposes, this recoil is completely negligible. The recoil force is the power of the beam divided by the speed of light. The recoil impulse is the energy of the beam divided by the speed of light. You would only get recoil impulses similar to that of a modern firearm if your laser was emitting as much energy per shot as the detonation of several tons of TNT.

Recoil for Particle Beam Weaponries

(WIP)