Here's the solution, courtesy of PlayStationTrophies.org:
To get the Rocket Launcher you need to earn an A-Rank in the storyline. The requirements for an A-Rank are pretty strict; you'll need an intimate knowledge of the game on top of skill and speed. You will earn this trophy after the credits during the score screen and only if you have an A-rank.
A-Rank Requirements
here are various factors that must be met for you to earn an A-Rank. The big one is how long it took you to complete the game and various smaller factors as well. Each requirement earns you so many "points." To earn an A-Rank you must meet a certain number of points at the end of the game.
Time Score
04:30:00 or below 8250 points
04:30:01 - 05:00:00 7550 points
05:00:01 - 05:30:00 7000 points
05:30:01 - 06:00:00 6450 points
06:00:01 - 07:30:00 5500 points
07:30:01 - 15:00:00 5000 points
15:00:01 - 20:00:00 2500 points
> 20:00:00 2000 points
*The clock is ALWAYS running, there is no way to stop it.
Special Score
1) Did you give the Hemostatic Bottle to the prison guard?
Yes = +250
No = -1000
2) The speed it took you to save Steve from the Gold Luger Trap.
Fastest = +400
Average = 0
Slow = -1000
*There is no known numeric value, clearing the 4 zombies from the hall outside the room helps with this though.
3) Map discovery
100% = +100
Anything less = 0
4) First aid spray
No use = +1800
If used = 0
5) Typewriter saves
No save = 0
First time = -1000
Every time after = -50
*The free save after killing the Tyrant and before landing in Antarctica does not count.
6) Retry
No Retry = 0
First time = -1000
Every time after = -50
Total Score
10000-10800 = A rank
7000-9950 = B rank
4500-6950 = C rank
2500-4450 = D rank
2450 or below = E rank
As you can see, there are many things you must do to be sure you make it.
PS: This one is quite hard to do...