# Why is the Orbital Velocity after SECO of Crew Dragon with 27K km/h lower than of ISS?

Crew Dragon after SECO: Orbital Vfelocity at 27,000km/h at 200km altitude (roughly 9-10min after liftoff). ISS Orbital Velocity at ca. 28,000 km/h at 400km altitude.iss_altitude,iss speed

However from the equation $$v = \sqrt{\frac{GM}r}$$, I expect Crew Dragon to actually rotate faster than the ISS around earth?! What is wrong with my thinking?

• sources for you figures? Most likely a non-circular orbit, with further orbital rendezvous manoeuvrers to follow. – user20636 Dec 24 '20 at 10:10
• 1) If your source was the video, that might still be ground speed relative to the rotating Earth's surface, after all, it starts at zero, not 465 $\sin(lat)$ m/s right? 2) The vis-viva equation is $$v=\sqrt{GM \left(\frac{2}{r}-\frac{1}{a} \right)}$$ which works for elliptical orbits (and I think hyperbolic ones as well if you are careful). Your equation is for circular. Use the complete equation and solve for $a$ and you may discover that Crew Dragon was in an elliptical orbit that will carry it to or past the ISS' altitude. It's just a guess – uhoh Dec 24 '20 at 10:21
• Min/Max apogee 197km/202km (source: Commentator Crew Demo in Livestream Youtube Video) that orbit is elliptical, but always roughly 200km below ISS. – Randy Welt Dec 24 '20 at 10:22
• Possible duplicate of 2nd stage speed - with respect to what? (SpaceX webcast of Orbcomm OG2 deployment). The answer makes it clear that @uhoh is correct: The velocities are Earth-centered Earth-fixed rather than Earth-centered inertial. – David Hammen Dec 24 '20 at 16:03
• @DavidHammen thanks for finding that, this sounded familliar. Wow five years ago already! – uhoh Dec 24 '20 at 17:25

## 1 Answer

Thank you to David Hammen:

The Velocity of Spacex's Livestream telemetry data is described by ECEF coordinates (earth-centered, earth-fixed). ECEF Wikipedia

The equation for orbital velocity used above is valid only for ECI coordinates.

ECEF coordinates are used preferrably in Satellite navigation, because it offers precise values without having to choose a specific ellipsoid. James R. Clynch: Earth Coordinates.PDF