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The Crew Dragon undocks from ISS but then it spend 18 hours in orbit before deorbit burn.

From there, it takes less than one hour to come back to Earth.

Why it needs 18 hours? Evven if they check all systems, its unlikely they need so much time.

Other capsules (Soyuz etc) also spend that much time?

Related question: What are the main stages of a Crew Dragon descent?

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    $\begingroup$ They aren't aligned with the landing zone when they first undock $\endgroup$ – john doe Aug 4 at 19:43
  • $\begingroup$ Yet the ISS travels fast enough to orbit Earth every 90 minutes. The capsule starts from ISS so it has the same speed. That means it can align in max 90 min $\endgroup$ – Joe Jobs Aug 4 at 19:48
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    $\begingroup$ @Joe Jobs not quite how it works - (though it's true it's orbiting along with the ISS) - the orbits are tilted and the earth rotates under them, so you need multiple orbits to reach a point that wasn't under your orbit at first. $\endgroup$ – ikrase Aug 4 at 19:53
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There are a few necessary activities and schedule considerations that contributed to needing 18 hours between undock and entry.

Sleep

Loading the Crew Dragon with returning (“down-mass”) gear, readying the capsule for undocking and maneuvers, and prepping the astronauts themselves is a full day of work, even for the three U.S. astronauts that were on the ISS. There were likely tests and measurements to be done for this first crewed flight that also slowed down the process. Bob and Doug needed their 8 hours of crew sleep period so they would not be sleep deprived for the entry, descent, and landing.

Undocking maneuvers

The trajectory that the undocking capsule follows, detailed in this Scott Manley video, requires about a full orbit (1.5 hours) of careful maneuvering. This is due to two factors:

  1. The thruster firings must be small and off-axis in order to avoid thruster plumes impinging on the ISS, especially on its solar panels.
  2. From mission rules, at no point can the capsule bring itself into a trajectory that has a chance of hitting the station within the next four orbits.

Longitudinal Phasing

An elliptical orbital plane is fixed in an inertial reference frame, which means the non-inertial rotation of the Earth is independent of the orbit. This means the point where an orbit crosses above the equator moves 360 degrees every sidereal day (a little less than 24 hours). In order to land in a particular location without an obscene amount of fuel use, you must wait for your orbit to phase close to above that point AND plan ahead so you reach your mark to start your descent on time. The phase aligns twice per sidereal day, so the schedule must be planned around that.

Other

This is speculation, but I think Bob and Doug were tasked with more orbital maneuver testing and checkout of the Crew Dragon, which takes some more time. Also, there is some buffer time built into the schedule to handle minor unexpected events.

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  • $\begingroup$ But they can probably take those 8 sleeping hours on ISS right before leaving it, no? $\endgroup$ – Joe Jobs Aug 4 at 20:40
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    $\begingroup$ @JoeJobs No, there is very time-sensitive stuff going on for undocking like heating tanks, priming thrusters, disconnecting power connections, etc. It works well for the astronauts to spend an entire work day on the preparation and undocking in one go. They can’t be stopping in the middle of a heightened-risk procedure to get sleep. $\endgroup$ – CourageousPotato Aug 4 at 20:43
  • $\begingroup$ A good analogy for this: the complexity and dynamics of a crewed ISS undocking rival that of a launch. This is no picnic. $\endgroup$ – CourageousPotato Aug 4 at 20:46
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    $\begingroup$ @JoeJobs They sleep both before and after the undocking. It takes a good 8+ hours to accomplish everything that needs to happen within hours of before/after undocking. $\endgroup$ – CourageousPotato Aug 4 at 20:59
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    $\begingroup$ @JoeJobs Note shuttle undocked, surveyed the TPS, went to sleep, had an on-orbit day, slept, then deorbited. What's the rush? nasa.gov/centers/johnson/pdf/567071main_FLT_PLN_135_F.pdf $\endgroup$ – Organic Marble Aug 4 at 23:17

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