What differentiates the Radchaai warship classes?

Question

What is difference between the Radchaai warship classes in the Imperial Radch series?

As far as I am able to tell, Justice-class vessels are troop carriers, and Sword-class vessels seem to be specialized for space superiority. What about Mercies?

Answer 1

The ships in the Radchaai military don't map especially well to anything in our contemporary ocean-going navies, which came out of a specific history shaped first by the Washington Naval Treaty and then by Cold War innovations like ballistic missile submarines (as well as by the use in land conflicts of carrier-based aviation, helicopter-borne marine forces, naval gunfire and, more recently, sea-launched cruise missiles).

We don't really know what environment the Radchaai fleet evolved in. We don't know where a ship like Sphene that fought in the early conflicts over Anaander Mianaai's authority fits in against contemporary Radchaai ship types, or even if the Justice / Sword / Mercy distinction goes back that far, or evolved later during Anaander's expansion. We don't know what sort of space-based resistance a system like Athoek or Valskaay could mount against an annexation, or whether, after the initial more-or-less civil war that established Raadch space as we know it, Anaander conquered other multi-system nations or empires (on the scale of the Gerentate, for instance), and what their fleets would have been like.

What we do know, about where things stand now:

Swords are the fastest, and most heavily armed.

1. my companion ships — the smaller, faster Swords and Mercies, and most numerous at that time, the Justices, troop carriers like me. [AJ]

2. Swords were faster, and better armed, and if Mercy of Kalr was gone, I would make Seivarden’s warning good, if I possibly could. [AS]

But not so much faster and more heavily armed that Mercy of Kalr can't contemplate taking on Sword of Atagaris:

1. Sword of Atagaris appeared, less than five hundred kilometers from where we had just been. Before it was even fully out of its gate, it began transmitting. “Unknown ship, identify yourself or be destroyed.”

   “I’d like to see it try,” said Ship, but only to me. [AS]

(See also this answer and the accompanying graphic, which suggests a Sword is about 50% larger than a Mercy, based on crew capacity.)

Nonetheless Swords and their crews consider themselves the cream of the fleet, and behave accordingly.

1. “We’ve been wondering if Fleet Captain Breq isn’t going to send Captain Hetnys here and take Sword of Atagaris for herself. It is the faster and the better armed of the two, after all.” [AS]

2. The Swords were so arrogant, so sure they were better than the Mercies and the Justices. [AS]

A Justice is a troop carrier, slower (as we saw above) than a Mercy, but more heavily armed …

1. Mercy of Kalr was armed, though not as heavily as a Sword or even a Justice would be. [AM]

… as well as carrying thousands of troops:

1. I had once been a ship. An AI controlling an enormous troop carrier and thousands of ancillaries, human bodies, part of myself. [AS]

The Mercies, then, are the smallest and most lightly armed, used for patrol duties and, apparently, to carry supplies:

1. …a ship entered the system, came out of gate space only a few hundred kilometers from the patrol ship Mercy of Sarrse. [AJ]

2. [Seivarden] found herself in the sick bay of a patrol ship. … “Some dingy little Mercy, with a shabby, provincial captain.” [AJ]

3. Maybe if the Mercy of Kalrs had ever seen combat…. Hadn’t spent so long on a Mercy, ferrying supplies or running long, dull patrols. [AJ]

Answer 2

I think of the Swords as being like modern cruisers and the Mercies like destroyers. There seems to be no equivalent to the carrier in Radch space.

I've been asked to elaborate... From Wikipedia:

In the later 20th century, the obsolescence of the battleship left the cruiser as the largest and most powerful surface combatant (excluding aircraft carriers).

The books describe Swords unequivocally as the most powerful warships. Since modern destroyers are less powerful, I equate Mercies with them...

Another way to look at it would be to go back to WWI classes instead of modern... Then Swords would be equivalent battleships and Mercies equivalent to cruisers. This would make a lot of sense because we have to go back to WWI before carriers existed to make better comparisons.

And of course Justice class ships are troop carries as is explicitly mentioned several times in the series.