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Why did Ariane carrying JUICE spacecraft on 14th April 2023, after its initial vertical ascent, flow practically parallel to Eath's surface for some time, before finally departing vertically into deep space?

After the start, the distance from Earth rose and following that dropped a bit before the final departure.

Why didn't it leave our Earth directly?

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    $\begingroup$ All boosters do that. Search this site for "gravity turn". space.stackexchange.com/search?q=gravity+turn $\endgroup$ Commented Apr 15, 2023 at 14:15
  • $\begingroup$ Yes all boosters do that, but why use a gravity turn if you are headed for Jupiter rather than Earth orbit? $\endgroup$
    – Slarty
    Commented Apr 16, 2023 at 2:52
  • $\begingroup$ @Slarty because if you didn't, you would not even make it to LEO. $\endgroup$ Commented Apr 16, 2023 at 11:30
  • $\begingroup$ @Organic Marble I have no doubt that I'm wrong as is evident from the actual trajectory used. But the exact rationale is not clear to me. A spacecraft like JUICE presumably has sufficient energy to reach Earth escape velocity. And I don’t think that LEO is an intermediate requirement for Earth escape its just a matter of getting far enough away fast enough. So it would appear at first impressions and all things being equal (which they are probably not) that flying straight up would involve travelling through less atmosphere than curving around slowly into LEO first? What am I missing? $\endgroup$
    – Slarty
    Commented Apr 16, 2023 at 13:31
  • $\begingroup$ @Slarty if you fly straight up, you are wasting propellant fighting gravity for no good reason, propellant which should be used to accelerate the vehicle. "A 102 second vertical climb costs 1 km/s delta V in gravity loss." It's well explained here, which is the origin of the quoted sentence: space.stackexchange.com/a/8627/6944 $\endgroup$ Commented Apr 16, 2023 at 17:49

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To begin with, a rocket never accelerates vertically. To reach orbit and beyond, the goal is to accelerate tangentially to Earth's surface and gain enough speed to reach orbit. The rocket pitches down a few degrees as soon as it clears the launch tower, it initiates a gravity turn that allows the rocket to gain more and more horizontal speed while raising its altitude above the atmosphere.

Every space mission (to LEO, GTO, GEO, MEO or interplanetary) begins with transit through LEO. The difference between Ariane 5 and other launchers (such as Atlas V and Falcon 9) is that the Ariane 5 upper stage engine cannot be restarted.

For example, if you take a look at the launch of Perseverance on Atlas V you will see that the rocket first injects the payload in LEO, then coasts and finally reignites the upper stage engine to eject the payload on a hyperbolic orbit towards Mars. Ariane V cannot do that, there is no coasting phase between LEO injection and Earth departure burn, there is a continuous burn from launch to escape.

Then to answer the question about the altitude vs downrange graph, Ariane 5 has a weak core stage engine, the Vulcan engine only provides 10% of the thrust at launch. So the boosters give some altitude margin to the core stage in order to give the core stage time to push horizontally while falling. But when the horizontal speed exceeds orbital speed (8 km/s) the upper stage regains altitude, then more speed means more vertical speed because the spacecraft approaches and then exceeds Earth escape velocity (11 km/s). This final vertical speed explains the end of the graph. To finish, the Earth is round so in 3D the trajectory is smoother than the way it is plotted on the altitude vs downrange plot.

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