I know that rocket launches are extremely complex. From watching a few, I get the sense that there are some things that happen in a standard way in all rocket launches, maybe depending on propellants, booster combinations, and payloads they are more variable than I think. I feel like if I understood what those steps are about, at least a little, it would give me a sense of how the rocket works.
For instance, loading of propellants happens at a certain time, why then? I can only find two run-downs of pre-launch sequences, one for Falcon and one for the Space Shuttle. Falcon:
T-0:02:00 Align Flight Computers to Self Alignment T-0:02:00 Range Verification T-0:02:00 Flight Control to Self Alignment T-0:01:35 Helium Loading Termination T-0:01:30 Final Engine Chilldown, Pre-Valves/Bleeders Open T-0:01:20 Engine Purge T-0:01:00 Flight Computer to start-up T-0:01:00 Pad Deck Water Deluge System Activation T-0:00:50 First Stage Thrust Vector Actuator Test T-0:00:40 Propellant Tank Pressurization T-0:00:25 All Tanks at Flight Pressure T-0:00:15 Arm Pyrotechnics T-0:00:10 Latest VC Abort T-0:00:03 Merlin Engine Ignition T-0:00:00 LIFTOFF
T-9 minutes and counting Start automatic ground launch sequencer Retract orbiter access arm (T-7 minutes, 30 seconds) Start auxiliary power units (T-5 minutes, 0 seconds) Arm solid rocket booster range safety safe and arm devices (T-5 minutes, 0 seconds) Start orbiter aerosurface profile test, followed by main engine gimbal profile test (T-3 minutes, 55 seconds) Retract gaseous oxygen vent arm, or "beanie cap" (T-2 minutes, 55 seconds) Crew members close and lock their visors (T-2 minutes, 0 seconds) Orbiter transfers from ground to internal power (T-50 seconds) Ground launch sequencer is go for auto sequence start (T-31 seconds) Activate launch pad sound suppression system (T-16 seconds) Activate main engine hydrogen burnoff system (T-10 seconds) Main engine start (T-6.6 seconds)
These imply a raft of questions but it's a bit overwhelming. Isn't there a general scheme to things to make it easier to grapple with? An overview, and from there more specific questions can be asked? For an uncrewed launch vehicle, what is the basic outline of the things that need to be done before launch, and the basic reasons?
Edit: The pre-launch checklists included are mostly as examples, a general explanation from maybe once a rocket is on the pad would be great.