I have a possibly dumba** question - how come there is no leeway in the launch countdown, like there was in the majority of the STS launches? A good example was the final STS launch (135), where the countdown paused for just over 2 minutes at T-31 seconds while they worked an issue.
Is it because the Dragon has less ability to maneuver once in space to make up for lost time?
The 'critical' moment in the launch windows is at T-35 minutes. If the liquid propellant is loaded into the rocket, the way I'm reading the information is that the ignition and liftoff must occur at ZERO. The Falcon system is not designed to hold past zero, or go early, after the propellant is loaded.
The Space Shuttle had two SRB that could sit loaded and ready to fire for a long, long time. Procedure was to bring the Shuttle liquid fueled engine to full power, and if they were not happy with the power output, they could abort the launch AFTER main engine start. However, once the SRBs were fired, it was going to launch. Period.
The reason for the foam sprayed onto the exterior of the Space Shuttle External Tank was to prolong the amount of time the liquid fuel would be viable. That also added a lot of weight that had to be lifted off the pad. Heck, just the white paint for the first to Shuttle launches was responsible for 600 lbs of liftoff weight. Everyone was happy with the decision to not paint the foam. The foam had a design weight of 4,832 lbs according to the information on the Wikipedia page.
My understanding - which may be wrong - is that the Space Shuttle liquid fuel LOX and Hydrogen were loaded long before the crew boarded.
EDIT - Found it - the Space Shuttle liquid fuel loading started at T -5 hours 15 minutes. The reason for all the foam, and weight of the foam, was to extend the time for the fuel to remain viable after it was loaded. Here is a description of the fuel flow of the Space Shuttle.https://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/prop/flowsequence.html
Not all who wander are lost.