Why does the Donnager use railguns at close quarters and torpedoes at long range?

Question

In the fourth episode of the TV show The Expanse, "CQB", the Martian warship MCRN Donnager is attacked by stealth ships. When the attack begins they fire torpedoes at long ranges and then switch to railguns when they are at close quarters.

Why wouldn't they use railguns at long distances when there is nothing to slow the projectile down in a vacuum? The only countermeasures the ships seemed to have are guns to shoot incoming torpedoes but this would be impossible with railgun slugs.

Also surely it would be better to use the torpedoes at close range as there would be less chance for the enemy to destroy them with their defensive guns?

Answer 1

In The Expanse, the efficient Epstein spaceship drives, combined with steroid cocktails (the "juice") that make humans withstand such maneuvers, are capable of doing high-g maneuvers to evade incoming fire and point defense.

Rail guns fire large slugs at high velocity, but they only shoot in a straight line, and are trivial to evade at long range. Therefore, guided missiles/torpedoes are used at long range instead, making it a fight between the onboard spaceship autopilot computers (or even human pilots) and the torpedoes' guiding computers.

At short range, however, rail guns are more effective as they're harder to evade, travel faster than torpedoes, and pack a larger punch.

The book that the first season of the TV show is based on, Leviathan Wakes, explains this in Chapter Eleven during the corresponding scene:

In the history of the Coalition, no capital ship had ever gotten into a close-quarters battle. But here they were, firing the ship's big cannons, which meant that the range was sufficiently short that a nonguided weapon was viable. Hundreds or even dozens of kilometers, not thousands. Somehow the Belt ships had survived Donnager's torpedo barrage.

Answer 2

Railguns are also more used as a stealth weapon, there were scenes where a railgun was used to take out a ships drive in season 4 that was moving in a straight line that, a railgun can easily hit something like that. In a battle though, Missiles are the primary weapons but medium range, railguns are used as they can track far more easily. At point blank range i.e. a few kilometers ships just use their CIWS/PCDs.

It's quite realistic really, a modern destroyer would work in the same way, it's guided missiles are it's main long range armaments', it's 5 inch gun is its medium rage anti ship weaponry and CIWS and the bushmasters are for close range combat.