In space, there has always been some debate on which would be the dominant form of gunnery. However starting with mankind's expansion into the Solar system in the mid-twenty-first century, and culminating with the Colonial Wars that ancient debate has now been answered. Energy weapons have largely replaced old-style mass driver weapons as the main guns of starships. Even alien races such as the Krall and the Cathamari don’t employ such weapons on their ships, and favor energy-based weaponry instead. While some science fictions would have you believe that such weapons would remain effective in space, and could even dominate, the reality is they aren’t such dominant weapons. They do have their advantages, but their conns as well.

One major advantage kinetic weapons have over an energy projectile is that they do not suffer from dissipation. This means that once fired a round could travel pretty much indefinitely. This only affects their maximum range, however, and in no way impacts their effective range. Still against a large, slow-moving target, this means you can shoot from practically anywhere and still expect the round to hit. Especially if you can accurately predict where the round will land. Against smaller more agile targets, however, it means little.

This brings us to one of the first conns. Kinetic rounds are actually fairly slow by space standards, and ships are quite fast. Engagements often occur in ranges in the tens of thousands of kilometers as well. This means it can be quite difficult to score hits reliably against a moving target such as a ship. Anything more than ten seconds travel from the firing ships is generally considered to be outside the effective range of a weapon, and hit chances are largely negligible even with computer aid.

More to consider is that the rounds for a kinetic weapon take up physical space. In addition, to be effective against a starship, these rounds have to be fairly large, and a ship would also need to carry a fair number of them. That means they end up being rather bulky weapons without even considering the weapon itself. Modern kinetic weapons employ electromagnetic coils to accelerate projectiles to c-fractional velocities, these coils draw power from a series of capacitor banks which draw power from a primary reactor. This grants significantly more power to the weapon than any chemical propellant, but it also makes it larger. So in the end these weapons are fairly bulky.

Not to mention they are also expensive to operate. Each round fired has to be manufactured, and there is a cost associated with that. The energy cost of firing the round however is actually negligible, and the same is true of firing an energy weapon. Where a single round from a mass driver can cost upwards of several thousand credits and takes up a fair amount of physical space, while an energy round is only a few dozen credits and takes no space. That doesn’t even factor in the logistical considerations or the fact that an energy weapon simply has higher endurance during a battle.

Those weren’t the only nails in the coffin of mass drivers. The other nails have to deal with advancing defensive systems. As ships advanced, they started mounting ever-increasing defenses against kinetic impacts. Structural fields, optimized armor schemes, and in the case of alien vessels even deflector screens. These mounting defenses made it increasingly harder for kinetic weapons to actually have a major impact without them being prohibitively large and/or fast. Worse these every evolving defenses require elaborate penetrator mechnisms to deal with. A fact that greatly increases the individual cost of each round. As a result of all these most kinetic weapons are largely useless against ships, even when inside their honestly short effective range. Those that aren’t are often prohibitively expensive. Energy weapons on the other hand can penetrate these defenses much more easily and more cheaply. Not to mention they are much easier to maintain on the logistical side of the equation.

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Energy weapons on the other hand can be fired as long as a ship has fuel, with the main limiters being how quickly the ship’s reactors can recharge the capacitor cells, and how quickly ship systems can dissipate heat. Properly maintained an energy weapon can fire virtually indefinitely. In addition, they can also fire faster while delivering similar or greater firepower when compared to kinetic rounds. While they lack the range, they do make up for it with a superior velocity, and consequently a larger effective range envelope. Since they don’t require bulky magazines or mechanical reloading systems they are also smaller even after factoring in the needed cooling systems. Even better the capacitor systems for them aren’t much larger than those already used for kinetic weapons. The result is lower logistical strain and footprint for these weapons. Not to mention a quantifiable increase in firepower.

Of course, energy weapons haven’t completely replaced projectile-based weapons. A projectile can travel much further than any energy blast. As such many ships carry a complement of torpedoes and missiles to supplement their primary energy weapons. Unlike kinetic rounds, these projectiles are guided, mount their own engines, and carry a destructive warhead. They can hit agile targets from well outside the main gun ranges of most warships, and deliver the highest destructive force of any known weapon, which justifies the high logistical cost of these weapons. Make no mistake they are expensive weapons, more expensive than kinetic weapons, but unlike kinetic weapons, they have not been rendered obsolete and impotent by the advent of energy weaponry. Although that isn’t to say there aren’t certain niches where a kinetic weapon might prove advantageous, orbital bombardment is one where their supreme range becomes inherently beneficial for example. However, in such a role, it is often easier to make use of readily available space debris and a tractor beam to accelerate that debris towards a target. There is little reason to waste valuable space actually carrying rounds, and a dedicated kinetic weapon for that purpose.

In conclusion, kinetic weapons fell out of favor due to diminishing effectiveness, space concerns, logistical concerns, and economics. While missiles and torpedoes remained effective due to the use of warheads, and guided engines, allowing them to continue to inflict considerable damage. Especially since warhead technology was able to improve and keep pace against improving defenses whereas pure kinetic rounds were not. Energy weapons ended up rapidly filling the gap left behind by kinetic weapons falling out of favor. Well at least in space, on the ground they still have a few niche uses, even if they have been largely supplanted by directed energy weapons.