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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?

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    Presumably railguns (straight-line projectiles) are trivially easy to evade at long range. Missiles which can change direction would be harder to dodge. – Valorum Nov 16 '16 at 0:09
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    @Valorum This is absolutely the reason, and I'd suggest you write it up in an answer. Space battles in The Expanse rely heavily on onboard computers doing high-G maneuvers to evade projectiles, so it would indeed be trivial to evade railgun slugs at long distance. – tobiasvl Nov 16 '16 at 10:32
  • Or perhaps I'll do it myself, I found a relevant passage in Leviathan Wakes – tobiasvl Nov 16 '16 at 10:42
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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.

  • Thats a good answer, thank you. I thought it could be that but didn't take into account how far away the ships are and how fast they can move. – MattG Nov 16 '16 at 12:14
  • Well answered. You have my +1 for finding a solid quote. – Valorum Nov 16 '16 at 15:00

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