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Basic question for those who breathe rockets:

The earth's rotation imparts to the rocket a free delta V of roughly 450 m/s if you launch straight eastward.

If you launch westward instead, for a retrograde orbit inclination of 123 deg out of Vandenberg, say, it seems you lose that free delta V.

But is this correct? Are westward launches more expensive in fuel than eastward launches? Is the earth's free delta V normally included in the velocities reported during launch?

(I just want to know if I should add that free delta V to the numbers shown for specific launches in order to compare them to my own calculations.)

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    $\begingroup$ Yes that's correct. en.wikipedia.org/wiki/Near-equatorial_orbit "only launches eastward take advantage of this boost of speed. Westward launches, in fact, are especially difficult from the Equator because of the need to counteract the extra rotational speed. " Don't know about the reporting part so posting as comment. $\endgroup$
    – Organic Marble
    Jan 18, 2021 at 20:12

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The earth's rotation imparts to the rocket a free delta V of roughly 450 m/s if you launch straight eastward.

About 464 m/s for eastward launch from the equator; it's less than that at higher latitudes (proportional to the cosine of the latitude), about 400 m/s from Canaveral, and less again if launching into higher inclinations.

If you launch westward instead, for a retrograde orbit inclination of 123 deg out of Vandenberg, say, it seems you lose that free delta V.

But is this correct? Are westward launches more expensive in fuel than eastward launches?

Yes. The total delta-V budget to eastbound LEO is usually about 9200-9500 m/s, varying with the exact trajectory, so a pure westward retrograde orbit would be about 8.5% more expensive.

Is the earth's free delta V normally included in the velocities reported during launch?

Depends on who's reporting, and why. For information reported to the public during launch, it's more intuitive to start with surface-relative velocity (it would be confusing to say the rocket is moving at 400 meters per second just as it begins to move), but at some point during ascent when atmospheric resistance is no longer as relevant (the atmosphere moving, roughly, at the same speed as the surface) it makes more sense to report velocity in Earth's non-rotating frame instead.

I don't know if e.g. SpaceX blends between reporting surface-relative and non-rotating velocity during ascent, or switches over hard at some point.

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  • $\begingroup$ Thanks, @RusselBorogove! This is super helpful. $\endgroup$
    – user36480
    Jan 19, 2021 at 6:35
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    $\begingroup$ @a1ex I stumbled across this old answer that suggests that SpaceX might use surface-relative velocity in their webcast reporting all the way to orbital insertion. $\endgroup$
    – Russell Borogove
    Jan 26, 2021 at 22:06
  • $\begingroup$ Thanks @RusselBorogove! This is great to know. $\endgroup$
    – user36480
    Jan 27, 2021 at 3:04

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