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In SpaceX launch on May 1st, 2017, we get to see the full flight path of the first stage rocket of the falcon 9 from launch to land:

It accelerates to around 1685m/s at an altitude of 68.5km (minute 14:21 in the video), then separates and starts decelerating still going up. It reaches a maximum altitude of 165km before starting to fall back down (at around 16:35 in the video).

My question is - at the maximum altitude, why is the speed ~300m/s and not 0? Is the speed calculated relative to something else other than the earth's ground?

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    $\begingroup$ It's moving 300m/s horizontally. To go into orbit, a spacecraft needs to get out of the atmosphere and achieve about 7800m/s horizontal velocity. While most of the speed is gained by the second stage, the first stage does contribute. The 1685 m/s at separation is diagonally upward, not straight up. $\endgroup$
    – Russell Borogove
    May 1, 2017 at 14:17
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    $\begingroup$ Man, they're really getting good at those landings. $\endgroup$
    – Jason C
    May 2, 2017 at 5:55

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Two possibilities: The Earth rotates at about 465 m/s at the equator. This video's measurements could include that speed, though I don't believe so as it starts from 0 m/s.

Most likely it's because the rocket is traveling in a horizontal arc during its travel.

Rockets trying to reach orbit need to reach a horizontal speed of around 7.8 km/s in addition to the height. In fact, the vast majority of a rocket's energy to get into an orbit is horizontal movement, as reaching the height necessary to get into space is comparatively easy.

By the time the rocket first stage passes its apogee it still has significant horizontal velocity to burn off, which is why it's still traveling at 300 m/s~ at that time.

Here's a visual example of the flight profile (not to scale) of the Falcon 9 Drone Ship landing Falcon 9 Launch to Drone Landing (Source: Spaceflight101.com)

And an example of the profile with a return to launch site landing Falcon 9 Launch to Return Landing (Source: Space.StackExchange.com post)

Some further interesting information, include a GTO Profile (One where the rocket first stage does not return to a landing site) can be found in the SpaceX Falcon 9 User's Guide on the SpaceX website.

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  • $\begingroup$ @JDługosz thanks for fixing up my crummy grammar =) $\endgroup$
    – Sarah Bailey
    May 2, 2017 at 0:21
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    $\begingroup$ Click on the «edited some time ago» text to review who did what. Sarah Bailey paitiently copyedited the post — I just fixed a homonym mistake and improved formatting to reach the 6 char minimum. Oh, I see the first colon was a fix, too. $\endgroup$
    – JDługosz
    May 2, 2017 at 0:29
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The velocity is calculated with reference to a sort of 'Earth's extended atmosphere'. Imagine yourself flying in a small plane, with a handheld GPS. That instrument gives your speed with respect to the 'Earth's absolute space'. If you fly horizontally, the GPS will give a horizontal velocity, and if you dive vertically, you'll also get the (vertical, in this case) velocity. And if the plane climbs at 45º to the horizontal, you'll get the speed along that trajectory, too...

With this rocket, it's the same thing...

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