How do S ranks work?

Do I get an S rank based on in-mission strategy e.g. not getting spotted, no kill bonus ect...

or is it for finishing all parts of the mission including the main parts?

Example in the mission you need to save Kaz, theres capturing the truck driver, not being spotted by the skulls ect...

Solution 1:

There are 2 ways to get an S Rank.

The first is to get 130,000+ points on the mission summary screen. Performing certain mission tasks, rescuing prisoners, and receiving bonuses (such as No Reflex) all give you points. Finishing quickly gives a rather large amount.

The second way is much harder; there is a "no trace" bonus that is earned when Snake leaves no sign he was there. This means no shots fired (would leave shell casings), no thrown magazines (or any other items), and never being seen. (You can fulton things, but that's about it).

But, I had no trouble getting an S Rank on every mission the "regular way". Shoot for the time bonuses -- they are really powerful. "Perfect Stealth, No Kills" is also worth a lot of points.