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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. $\endgroup$ Jun 27 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? $\endgroup$ Jun 27 at 8:59
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    $\begingroup$ In two words: Air Resistance $\endgroup$ Jun 27 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
    Jun 27 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
    Jun 28 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 it 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 make friction, which then turns itself to heat. The entry burn is only to avoid being lit on fire by the heat caused by the friction. 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, causing a larger 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.

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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
    Jun 27 at 22:05
  • $\begingroup$ I state that in different terms in my answer @reirab $\endgroup$ Jun 28 at 0:14
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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
    Jun 28 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
    Jun 29 at 11:27

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