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As you know, Space X streams its missions and the speed and altitude data of the vehicle is shown. After stage separation, stage one gains speed and accelerates in its decent trajectory till reentry engine burn. With its engine on, it is obvious that the vehicle decelerates, but after engine burn out, it still loses speed and decelerates until the landing burn. Why does this happen? I expected the vehicle to accelerate after reentry burn out due to gravity.

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    $\begingroup$ It continues to decelerate because of the air friction. $\endgroup$ Commented Jun 27, 2022 at 5:58
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    $\begingroup$ Hint: have you ever seen someone use a parachute? Do they continue to accelerate until they hit the ground at light speed? If not, why not? $\endgroup$ Commented Jun 27, 2022 at 8:59
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    $\begingroup$ In two words: Air Resistance $\endgroup$ Commented Jun 27, 2022 at 11:19
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    $\begingroup$ It would be a waste of fuel to decelerate to slower than terminal velocity before landing. $\endgroup$
    – Wyck
    Commented Jun 27, 2022 at 16:46
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    $\begingroup$ @Wyck Yeah, I think Wyck's point was just that they don't need to burn the engines to get down that slow because the air will do it for them. They just need to get slow enough to not take damage without requiring a bunch of extra complicated/heavy/expensive heat shielding. $\endgroup$
    – reirab
    Commented Jun 28, 2022 at 6:10

2 Answers 2

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The falcon 9 decelerates because of the air friction. One example is when you jump out of a plane, you continue to accelerate until you reach a speed of around 240 km/h. The higher the air density is the slower you will get. With the Falcon 9 rocket it will turn on its engines to avoid burning up because of the high air friction, then it will slow down using aerodynamic drag.

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In two words, air resistance. While the Falcon 9 is coming down, it is constantly "shoving" air particles out of the way. The air particles don't "want" to move, and before they will do so they slow down the vehicle by a tiny bit. Doing this also makes a lot of heat. Actually, the only purpose of the entry burn is only to avoid being lit on fire by the heat. If heat wasn't an issue, then SpaceX would skip the entry burn as it will slow down aerodynamically (which takes no propellent). The entry burn is done because it will overheat before slowing down enough. Of course, the obvious question is, why isn't is already slowing down before that. Well, the further down you get in the atmosphere, the denser that the air is, and therefore there are more air particles. That causes more air particles to have to move, more air particles pushing back, and more deceleration. Since prior to the entry burn it is higher up, the downward pulling force of gravity is stronger than the stopping force of air resistance. One last thing to note is that on most launches, rocket actually continues accelerating for a bit after the entry burn and then starts decelerating again. This is because it is too high or going to slow (or a combination of the two) at entry burn shutdown on certain missions and therefore doesn't have enough drag to slow it down yet.

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    $\begingroup$ Right, very high up, where the reentry burn starts - nearly 60km (nearly 200,000 feet) up - the air is extremely thin, thus rather little aerodynamic drag happens. Per the numbers shown in this answer from a Falcon 9 re-entry burn, the burn stops at around 31 km, at which point the vehicle is still accelerating somewhat from gravity, but aerodynamic drag begins to exceed gravity by 22 km (around 72,000 feet.) During the re-entry burn, the vehicle loses nearly half of its gravitational potential energy and nearly 80% of its kinetic energy. $\endgroup$
    – reirab
    Commented Jun 27, 2022 at 22:05
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    $\begingroup$ Yep, that's why my comment began with "Right." :) I was agreeing with you and just providing some additional details and a link that with yet further details of the reentry profile. I upvoted your answer. $\endgroup$
    – reirab
    Commented Jun 28, 2022 at 5:00
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    $\begingroup$ This answer is so great that it's shadowed solely by my professor's ъ-щ formalism:en.wikipedia.org/wiki/… $\endgroup$
    – Vorac
    Commented Jun 29, 2022 at 11:27

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