Two (actually four) questions on damage control:
1. What does it do and does it need spares to do it? It is clear that it fixes stuff but I'm not sure about the mechanics mainly.
It repairs systems on the ship, except for armour, and it requires double the number of spares that would be needed to fix a maintenance failure. The percentage chance of repair is equal to:
((Length of Increment / System Cost) * Damage Control Rating)/10
Examples:
If you needed to repair an engine that costs 30, the increment was 60 seconds and the damage control rating was 1, the chance would be: ((60/30)*1)/10 = 0.2%
If you needed to repair a sensor that costs 50, the increment was 300 seconds and the damage control rating was 2, the chance would be: ((300/50)*2)/10 = 1.2%
If the damage control rating was 10 in both cases, the chances would be 20% and 12%. So the major effect of the damage control rating is the speed at which systems can be repaired. Each engineering system adds one the damage control rating. Each damage control system adds 10 to the rating, improved damage control adds 20, etc.. A high damage control rating is useful for a warship because it allows repairs to be made fairly quickly.
2. Is it worth putting it on a PDC and if you do do you need to add in spares?
Yes and yes.
Is there any way to select which type of armour you want to use, other than the best stuff you have? I looked but didn't see anything other than the button at the bottom and that gives you the best you have.
No, at the moment you can only choose the best armour.
When building facilities, for example a fabrication factory, you have cost: 120 and duranium: 120. Is my thinking that cost is in Aurora Monetary Units (AMUs) and that the build time is 120 given by the sum of the minerals? So if I have build capacity of 2400 per year it would take 1/20 of a year (3 5-day periods) to complete?
Cost is in Wealth and the build time is based on the cost. While installations usually have mineral cost = wealth cost so it makes no difference, with refits the cost may be higher than the amount of minerals required. If you have a build capacity of 2400 per year then each 5 day increment will provide 5/360 (days in Aurora year) x 2400 = 33.33, so it will take 4 increments to build a 120 cost factory.
Can you use mixed shields on a ship?
No, just one shield type. This is avoid design and gameplay headaches where different shield types have different recharge rates.
Just as a side note: a 1000 kg missile moving at 12,000 km/s has the kinetic energy equivelent of 170 kTon of HE, or is about the equivelent of 11 Hiroshima scale nuclear weapons. For the most part detonation of the weapons warhead is not required in planetary attacks, so less radiation but the dust would still be an issue.
The radiation effects of bombardment are as much for gameplay reasons as anything else. I wanted to avoid the Starfire situation of wiping out the population with a saturation bombardment on Monday then shipping in your own colonists to the still perfectly habitable world on Tuesday.
Steve