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At the end of The Expanse Season 6, Episode 3, Bobby Draper and James Holden pull some sort of maneuver which allows them to hit Inaros' ship, the Pella. I couldn't quite follow exactly what happened. It seems like they wanted to use the railgun to get the Pella to dodge in an anticipated direction, while firing their point-defense cannons at it, I guess.

But:

  • Can the PDCs reach that far? edit: Ok, yes, they will technically not slow down, I mean is it realistic to assume that a ship, any ship, will not have already changed its course even slightly in the time it takes the PDCs to reach it?
  • What about the incoming torpedoes? How were they also destroyed, while the PDCs are supposedly targeting the Pella?
  • Weren't their PDCs out-of-order at that point?

2 Answers 2

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Can the PDCs reach that far?

PDCs fire solid physical projectiles (40mm rounds in this case) in a microgravity vacuum. Once fired, they continue on the same trajectory and at the same velocity until they hit something, be it an incoming torpedo, the ship that fired them, some rock behind that ship, or the upper atmosphere of one of the gas giants on the other side of the solar system months, years, or millennia later. For more detail, I'd refer you to newton's first law of motion.

What about the incoming torpedoes? How were they also destroyed, while the PDCs are supposedly targeting the Pella?

The Pella didn't fire another salvo of torpedoes during that last exchange, it was the Roci that let off four more torpedoes to make sure the Pella and it's own PDCs were fully occupied and not paying attention to the burst of incoming PDC fire, so when they dodged the railgun round, they didn't notice they were dodging right into a face full of high velocity armour piercing rounds.

Weren't their PDCs out-of-order at that point?

Two were down, the other four were still online.

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  • I guess instead of "can the PDCs reach so far" I should ask "can the PDC rounds cover the distance so quickly that the enemy ship would not have surely changed its course already"? But otherwise +1.
    – einpoklum
    Dec 26, 2021 at 21:29
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    @einpoklum It's all about the timing. The PDC burst was manually timed so that the rounds would arrive right when and where the Pella would dodge in response to the subsequent railgun shot. Indeed, since when they were fired they were on a trajectory that would pass harmlessly to port, they would have been discounted as a threat by the targeting computer, with the 4 converging torpedoes flagged as the priority targets.
    – Kris
    Dec 26, 2021 at 21:53
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    The novel was pretty good at explaining the plan and the conflicting priorities presented to the Pella's systems.
    – Michael
    Dec 26, 2021 at 22:54
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PDCs fire kinetic slugs in vacuum. The slugs will keep moving until they hit something. See Newton's 1st law.

The PDCs were not all out of order. Rocinante has at least six PDC mounts, each capable of independent fire control.

Bobbie: Four more torpedoes incoming. We can't keep this up forever. We're damaged! Port PDCs are misfiring!

Amos: Copy that on the PDCs.

Clarissa: Where are you going?

Amos: I gotta fix 'em.

Clarissa: In the middle of the fight?

Amos: That's the job, Peaches.

From Gizmodo:

And speaking of you-know-who, the Pella has missile lock on the Rocinante. Filip takes the shot, but fortunately the Roci’s able to defend itself. Still, as Holden aptly surmises, “We’re out of our weight class here.” It’s gonna take some tricky maneuvering from here on out, though the Roci catches a break when the two ships traveling with the Pella duck out of the fight. Filip fires again, and the Roci suddenly needs some urgent repairs. “In the middle of the fight?” Clarissa says in disbelief. “That’s the job, Peaches,” Amos replies, and they scramble off to take care of business, even as the Roci flips and tumbles. Bobbie aims, fires, and notices the Pella keeps dodging the same direction every time. Her next shot doesn’t miss.

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