The component description says "It can only fire every other turn". What does this mean?

After each turn in which the battery fired - it can't fire next turn, only on the second turn - etc.

Turn 1: you fire it. Turn 2: It has a "cooldown", and/or the crews reload ordnance. Turn 3: you fire it. Turn 4: reload it. Etc. Like several other more complicated, keep track of ammo" weapons, this one can take along time, in between shots, to be readied to shoot again.

It doesn't require to keep track of ammo, however. Just another macro-battery.

But it's so feeble it's useless. Mars Pattern Macrocannons inflict greater damage with better Crit threshold and shoot twice as often. Lathe Grav-culverin Broadside has options for better range or damage, thus more flexibility as well. And everything fires twice as often. And if used to suppress shields for lance (in which case its own damage doesn't matter), battery working only half the time makes the lance also useless half the time. So why anyone even makes it and anyone buys it?

Since the whole point of a missile battery is great volume of fire, maybe it would make more sense with either Warhead Swarm rule from Rak'Gol guns (which is similar to Storm) or better a bonus to hit, with much the same effect on moderate DoS, but less hits on very high rolls (like Twin-linked) if it has somewhat dubious grouping? It would be comparable with the weakest cannons in average damage output (2x hits, but per 2 turns), still have issues from giving the enemies a guaranteed "blind spot" every other turn and force predictable "shoot one turn, maneuver one turn" tactics, but it would hit harder when it hits well. Also, it would be better than guns at suppressing shields for lance, but still only 1:2 turns.

It is of comparable strength to Mars Pattern Macrocannons (1 less damage, crit rating higher by 1, capable of doing 2 more hits). Having extended reload does drop performance, and as such it doesn't have a place on capships.
Anything smaller, however, and it has a good quality of being less power-reliant and, most importantly, requiring only 1 space on the ship. Capitals, merchants and heavy escorts do have enough of space - however, many ships are smaller than that, and an enterprising Rogue Trader might have to choose between installing Mars Pattern Macrocannons or these, the difference being Jovian Missile Batteries allow to save 2 space to install, say, a Cargo Hold&Lighter Bay.

It is not a kill-all weapon of destruction, but it is cheap and allows shipwrights to install more of other stuff inside the hull.

On ‎12‎/‎29‎/‎2018 at 3:56 PM, Dwergar said:

It is of comparable strength to Mars Pattern Macrocannons (1 less damage, crit rating higher by 1, capable of doing 2 more hits). Having extended reload does drop performance, and as such it doesn't have a place on capships.
Anything smaller, however, and it has a good quality of being less power-reliant and, most importantly, requiring only 1 space on the ship. Capitals, merchants and heavy escorts do have enough of space - however, many ships are smaller than that, and an enterprising Rogue Trader might have to choose between installing Mars Pattern Macrocannons or these, the difference being Jovian Missile Batteries allow to save 2 space to install, say, a Cargo Hold&Lighter Bay.

It is not a kill-all weapon of destruction, but it is cheap and allows shipwrights to install more of other stuff inside the hull.

This. Something like a Vagabond-class can pack dorsal and prow missiles in the same space as a single battery of Mars- or Thunderstrike-pattern cannons, and be able to lump out a volley big enough to make a wolfpack raider go " maybe not this one, then ...." without eating into the space needed to function as a trading ship.

High strength is probably more important than damage-per-hit for a PC ship, because between the various wonderbuddies pulling Lock On Target, Exceptional Leader, Put Your Back Into It, and a Master Of Ordnance's abilities, plus spent fate points, squeezing every potential hit out of a volley isn't actually that hard.