The rank system is pretty simple.
You start your career as ensign (when playing the tutorial) and get promoted to lieutenant right after it. A lieutenant that is commanding a ship... well ok... unusual but ok.
You keep that rank for the first 10 ranks...
at rank 10 you will be promoted to lieutenant-commander and get a new ship for free
at rank 20 you will be promoted to commander (also with new ship)
at rank 30 you will be promoted to captain (at this point missions will become more difficult but hey you get a new ship again)
with rank 40 you can call yourself rear admiral (lower half) -> this will be the final time that you can get a new ship for free)
and five ranks later you will be read admiral (upper half)
Reaching lvl50 you also reach the final rank that you can get -> vice admiral)
Now you might say "lvl50, that will take ages"... that is not really true.
If you play the story campaign you can get up to 6000 exp. per mission... and you can at some point do system patrol missions where you get at least 2000exp.
These system patrol missions are so damn simple... you fly to the system (just fly to every system in sirius and regulus) then you take out 5-6 npc squads and the mission is done after 10-15 mins.
So with other words you can reach lvl50 in 1-2 weeks.
Does this mean that you achieved everything?
Well... I would say it this way... a lvl50 char is a noob.
The only thing that this rank really means is that you can buy all ship types now. In pvp you still will be a victim only.
LVL 50 also means that you will not gain any experience anymore. All experience from missions will be transfered to your bridge officers.
Now that could be a problem.
Because the experience you gained till LVL50 is just enough to fill your skill tree to 50%.
It is not possible to lvl up skills anymore.... so you have to choose carefully which skill you level up.
