What are the main steps in the lead-up to a rocket launch?

Question

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

Shuttle:

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.

Answer 1

I can explain the STS entries. I'm gonna go off my own knowledge here but if I get ambitious I'll come back with cites. 'll try to make it past tense because no more STS but I'll probably slip up on that.

Start automatic ground launch sequencer The GLS was the software that automatically commanded the final phase of the countdown.

Retract orbiter access arm (T-7 minutes, 30 seconds) The orbiter access arm was the swing arm that reached from the launch tower to the crew hatch. It's where the 'white room' was. If there was an emergency after this point that required the crew to evacuate the Orbiter, it could swing back very quickly.

Start auxiliary power units (T-5 minutes, 0 seconds) The pilot starts the 3 hydrazine fueled APUs that provide hydraulic power to the Orbiter. It's done as late as possible to save fuel. They must be up and running by T-4 minutes when hydraulic pressure is needed to move a valve in the shuttle main engines, when they enter Purge Sequence 4.

Arm solid rocket booster range safety safe and arm devices (T-5 minutes, 0 seconds) Mechanical devices that break the firing chain in the SRB destruct devices are rotated to the non-safe position.

Start orbiter aerosurface profile test, followed by main engine gimbal profile test (T-3 minutes, 55 seconds) Now that hydraulic power is available, the elevons and body flap are exercised, and then the SSME gimbals.

Retract gaseous oxygen vent arm, or "beanie cap" (T-2 minutes, 55 seconds) This is another arm from the launch tower to the tip of the ET. It carries a duct that lets any GOX vented from the ET LOX tank be released away from the vehicle.

Crew members close and lock their visors (T-2 minutes, 0 seconds) This seals the launch and entry suits in case of a bird strike during first stage busting the windows. It is done as late as possible in the count because the suits vent O2 into the cabin in this configuration and you don't want the O2 concentration getting too high.

Orbiter transfers from ground to internal power (T-50 seconds) The voltage on the power feeds from the mobile launcher to the orbiter is dropped so the orbiter fuel cells pick up all the power loads.

Ground launch sequencer is go for auto sequence start (T-31 seconds) If all is well, the onboard launch sequencer in the orbiter is started to control the last few seconds of the count.

Activate launch pad sound suppression system (T-16 seconds) The 'rain birds' start gushing water onto the pad to prevent damage from the acoustic power blasted out by the SRBs (changed after STS-1 when a strut was bent in the fwd RCS because of this)

Activate main engine hydrogen burnoff system (T-10 seconds) The sparklers (ROFIs) ignite to burn up excess Gh2 vented from the engines.

Main engine start (T-6.6 seconds) Those big old water vapor generators start going.

The good old Space Shuttle News Reference is a great starting point to read about this. My link points to the countdown sections. It's old (dated before we started flying again after the 1st accident) but still has lots of great info. Hit the links in the document for background on a lot of the stuff I talk about.

Answer 2

Generally speaking, a rocket launch has these steps:

1. Integration: stacking the rocket stages and the payload together.
2. Rollout: moving the rocket to the launch pad.
3. Fueling.
4. Pre-launch checks.
5. Starting up the rocket: switching on its internal power sources, starting the rocket's control systems etc.
6. Starting the engines and checking if they work properly.
7. Launch.

You want to do as much of the work as possible while the rocket is still in a hangar and easy to work on, but some steps are unavoidable on the launch pad.

Fueling for instance: you want to pump fuel and oxidizer into the rocket as late as possible, to reduce the risk. So for liquid-fuel rockets this is done on the launch pad. For solid boosters you can't do that, so you have to fuel those at an earlier stage.
When fueling has finished, you have to disconnect the fuel lines (umbilicals) from the rocket and stow them so they won't be burned by the rocket. There may be other steps to secure the launch tower at this stage.

Then you want to make sure all the internal systems work properly. Some checks can't be done beforehand (e.g. because this is the first time the rocket's been fueled). Other checks are on general principle: better to test 10 times than to lose a payload.

Until now, the rocket has been receiving power from the launch pad. This must be switched to an internal power source (batteries, generator, fuel cell). Then you have to confirm again that everything works on internal power.

The last step is to start the engines, holding down the rocket for a few seconds so you can make sure the engines work properly. Then release the clamps and launch.

This is only a general overview: each step is split into hundreds of checkpoints and actions, depending on the rocket type. The Shuttle had a much more complicated launch procedure than the Falcon, for instance.