See this thread for the forum post: http://forum.kerbalspaceprogram.com/threads/72036-Poseidon-STS-Stock-Space-Shuttle!-(Includes-cargo-bay!)

Stats:
Cargo Size: 2.5 Meter
Cargo Capacity: 10 tons
Crew Capacity: 3
Boosters: Solid/Liquid Fuel Hybrid Rocket
OMS: 2 909 Engines

Information:
Explaining everything about this craft is rather difficult, so I will just tell you the basics on how to fly it: First the engines, each booster includes 6 Rockomax SRBs and 1 Skipper engine. The external fuel tank has 1 Skipper and 2 Aerospikes. The external fuel tank engines will need to be manually controlled during the flight. The Poseidon Orbiter has 4 LV-T45 engines, the upper 2 have a tweaked thrust of 77%, this thrust setting is meant for an empty cargo bay, so I highly recommend increasing this when lifting a heavy payload. During launch, let the SAS do the flying, it is very important to watch the pitch indicator. As the rocket becomes unbalanced (as it drains fuel), the SAS will compensate with pitch input. Basically, when the pitch indicator nears full pitch, the SAS is basically saying: "I can't hold it much longer, do something!" This is when you need to do something with the engines (throttle down, toggle off/on, etc), the action groups will be covered in the launch instructions.

Now some info on the cargo bay: The doors work by undocking from the shuttle, but they are held in place by some structural things, so hey shouldn't float away (hopefully). Once undocked, you can switch focus to them ([/]), and then open/close them with the yaw controls. Since they are now separate crafts, DO NOT timewarp, exit to the KSC Menu, focus on a craft over 2.5 Km away, etc. Close them first. When closing the doors, simply rotate them back to the docking ports. Note that there are docking ports in BOTH the front and back of each door. Sometimes the back ports may not dock, if this happens, try opening and closing them again. Leaving the door's SAS off seems to help with this.

Action Groups:
1: Toggle OMS thrusters
2: Toggle orbiter main engines
3: Undock cargo bay doors (needs to be pressed twice to fully undock)
4: Toggle external fuel tank aerospikes
5: Toggle upper 2 orbiter main engines (not usually used, only for emergencies)
Abort: What abort?

Launch Instructions:
1) Set upper 2 LV-T45s to appropriate thrust setting.
2) Make sure payload is secure.
3) Throttle up, when throttle reaches 50%, press "space" to active main engines.
4) At 100% throttle, press "space" again to launch!
5) At 200 m, do a roll program (roll shuttle so "up" is east/whatever direction you're launching in).
6) When the shuttle starts to pitch up too much, tweak the ET's (external fuel tank) Skipper to a lower thrust.
7) When the SRB's cut out, QUICKLY throttle the skipper to full, and press "4" to active the aerospikes.
8) At 10Km, begin gravity turn, reach 45 degrees by 15Km.
9) Note that when the SRB's liquid fuel is drained, the booster skippers will NOT cut out, instead, drop the boosters when the uppermost fuel tank of the ET begins to drain.
10) As the ET drain it's fuel, throttle down the skipper to compensate, yes, this will eventually reach 0% thrust.
11) When the apoapsis is 30 Km, pitch to 50 degrees (from vertical).
12) When the apoapsis is 40 Km, pitch to 60 degrees.
13) When the apoapsis is 55 Km, pitch to 85 degrees.
14) When the periapsis is about to become 20 Km, jettison the external fuel tank (if you haven't already).
15) Press "1"to toggle OMS, now powered by liquid fuel and oxidizer.
16) Orbit! Do whatever you want now.

Landing:
1) Deorbit on the opposite of Kerbin from KSC.
2) During re-entry, maneuver the shuttle to land at KSC
3) Once the flames stop, pitch down to 45 degrees (A VERRRRY steep descent path) (Blame KSP's terrible aerodynamic model for that). Basically, dive toward the runway.
4) If night, turn on the landing lights.
5) About 100 meters above ground, PULL UP! If this is done correctly, you should touch down at about 55 m/s, and not crash. If done incorrectly, expect lots of explosions.

Poseidon STS Space Shuttle