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From answers to How much time it takes for the capsules to descend from ISS back down to Earth? I understand it takes 19 hours from the undocking from ISS until it touches the land.
But what are the main stages of the descent and how much time it takes for every such stage?
We'll ignore the 18 hours spent in orbit between Endeavour's undocking and the actual beginning of descent; I don't know what the crew were doing during that period, but it wasn't really part of the return-to-Earth process.
I'm basing this answer on the times given Demo-2 return timeline posted on the NASA ISS blog on August 2, 2020, which as of this writing is the only crewed Crew Dragon reentry which has been flown. We can assume most Crew Dragon descents will be generally similar, though the exact time, altitude, and speed details may change.
We begin in orbit at around 400km altitude, flying horizontally at about 7700 m/s, circling the Earth every 93 minutes. If we don't do anything we will continue to circle the Earth for days or weeks before the traces of atmosphere up here slow us down enough to fall to Earth.
Crew Dragon is made up of two parts: the conical reentry capsule, pressurized, where the crew sits, and the cylindrical trunk, behind the capsule, which includes solar panels and heat radiators. The trunk is expendable; the heat shield which protects the spacecraft is on the crew capsule.
57 minutes before splashdown: Trunk separation
The first step in the reentry process is trunk separation -- we get rid of the trunk section of the spacecraft, because the aerodynamics of the trunk would cause the spacecraft to orient nose-forward when it enters the atmosphere, which is not the way we want to go. At this point, having detached the solar panels, the spacecraft is on battery power, and we're on a short time limit.
52 minutes before splashdown: Deorbit burn begins
In order to reenter the atmosphere, we have to slow down. We only need to cut about 100 m/s off our orbital velocity; this will cause our orbit to become elliptical, and the lowest part of the orbit will dip low enough into the atmosphere that air resistance will cause the spacecraft to slow further. To slow down we fire the small Draco rockets for about 12 minutes; firings of rocket engines are referred to as "burns", so this is the "deorbit burn".
40 minutes before splashdown: Deorbit burn complete
We've now slowed down enough that we are guaranteed to re-enter the atmosphere and land on this orbit, but it's going to be another half hour or so before re-entry really begins.
37 minutes before splashdown: Nosecone closes
I don't know what's up with the nosecone, exactly, but it closes up here. For some strange reason the timeline refers to "deploying" it rather than closing it. We have another open question about the nosecone.
16 minutes before splashdown: Crew Dragon maneuvers to attitude for re-entry
The heat shield is at the back of the crew capsule. Once the capsule is into the atmosphere, its center of gravity is positioned so that the capsule will remain heat-shield forward, tilted at a particular angle which gives the capsule just the right amount of lift to make the ride reasonably comfortable, but we need to make sure we're in that attitude before the aerodynamic forces kick in. We fire the small Draco thrusters at this point to turn the spacecraft.
13-15 minutes before splashdown: Entry
I don't know the exact timing of atmospheric entry for Crew Dragon, but for Apollo capsules it was about 15 minutes before splashdown. Apollo defined “entry” as the point at which air resistance was creating 0.05g of deceleration on the spacecraft, which was reached at around 120km altitude. Up to this point the spacecraft is still moving at over 7000 m/s. The g-force builds up rapidly from here, because the slowing spacecraft loses altitude faster, dropping into denser air, which produces more air resistance, which slows it more rapidly, and so on. The rapid compression of the air in front of the capsule creates a lot of heat; the blunt shape of the heat shield forces the superheated air to the sides to reduce heating of the skin of the spacecraft. The superheated air ionizes, creating electromagnetic interference which "blacks out" radio communications for a period of about 6 minutes.
4 minutes before splashdown: Drogue parachutes deploy
At this point, air resistance has bled off almost all the capsule's orbital speed; it's only going 150 m/s (350 mph), at an altitude of about 5.5km. Small parachutes called "drogues" are released here; they slow the capsule a little bit and help the main parachutes to release cleanly.
3 minutes before splashdown: Main parachutes deploy
The drogues have slowed the capsule down to about 53 m/s (119 mph), which is slow enough that the main chutes won’t be ripped apart. At 1.8km altitude, the 4 large parachutes begin to deploy.
Splashdown
Three minutes after that, the capsule hits the water at a relatively gentle 7 m/s (15 mph).
