Looking on Missile Batteries, they may only fire every other round, and the payout for this abominable cost is the low space requirement and a decent damage and range. My thinking is even with modern day technology, Missiles have spectacular guidance systems, which certainly exist in 40k fluff. So, as an alternative to the traditional Missile Battery entry, i add the following ability at an addition 2 space requirement (total 3). They may still only fire every other round.

Firestorm: The Missile Battery's unique method of attacking allows a versatility unseen by anything but fighters. All Missile Batteries may fire in a 180 degree arc, from any weapon mount. Broadsides may fire Forward and Aft in addition to their weapon mount, and Dorsal-Mounted Missiles may fire to any Direction.

Feel free to make use of this yourself or word it more eloquently. My thinking is that this would turn Grand Cruisers previously rendered rather obsolete by their smaller Battlecruiser brethren would be able to use these to devastating effect Fore and Aft, and Transports that would normally spend most of their time running will now be able to at least provide a token defence while being pursued.

Sounds good. Though personally I would leave the space requirement the same,as that's part of what makes it appealing and simply up the SP by a point.

I'd also change it to say that port & starboard mounted Missile batteries may also fire in the Forward arc. Having things fire in the rear illiminartes a lot of ship vulnerabilities, which I don't think they should.

Although modern missile batteries do have amazing guidance, I would be suprised if the enemy ships weren't (as standard) chucking out buckets of ECM and flak to nerf this particular advantage. even so, we reckoned that the given missile batteries are more like rocket batteries, dumb fired and using mass ordanance to hit, so we house ruled a couple of extra rules for missiles.

we let certain missile batteries fire 360 degree to represent homing abilitys, but this cuts the range of the batteries in half to represent the lost fuel and speed on tracking missiles.

For guided missiles, we let them add the detection rating of the ship to hit as better sensors would be key to feeding them targeting info.

Apart from this, we created a warship missile battery which required little power, a lot of space for the missiles, had a large strength and did decent damage at shortish range. One of the Pc's ships is a cruiser with five missile batteries, a lot of armour that they use as a linebreaker.

If you have Battlefleet Koronus, check out the missile battery broadsides on the Chaos flagship, they're fairly impressive and don't have the 'fire every other turn' limitation.

Take a look at the rules for Turbo-Weapon Batteries on page 164 of Into the Storm.

I'd originally thought that this upgrade was interesting but poorly balanced. Removing the penalty for firing at long range from a macrobattery without requiring extra power or space is so useful that if the option is allowed, then it makes no sense for everyone not to have it. And everyone having it changes the nature of space combat drastically.

However... For missile batteries, it actually makes perfect sense that there'd be no penalty for long-range fire due to the limited self-guidance idea described in posts above. Missiles also make sense of why there's be no penalty to hit then suddenly the shots stop, since they have limited fuel. I'd therefore houserule that all Missile Battery weapon components have the Turbo-Weapon Battery upgrade, with the potential to buy the upgrade at higher levels to represent more advanced missiles.

Firing only every other turn, but doing so accurately at very long range makes Missile Batteries worthwhile, and suitably distinct from other macrobattery weapons.

I personaly used the missile batteries as macro weapons, because as I read the missiles are just pointed at the right direction and fired, no real guide system is in there, thats the main difference between torpedos and missiles, the missile is go and forget type, but self propelled so can reach very high speeds and can hit the traget faster as macros, so thats why they can hit a traget from far much better.

and the space is 1 because you dont need that many servitors and crew to operate, only a tube and a place to store the missiles...

I would take away the every other turn rule and they are ok in my opinion