Why is the apogee of the Falcon 9 SES launch going clear out to 80,000 km? GEO is only 36,000 km or so, it seems like a waste to go so far beyond it.


2 Answers 2


Short answer: to save the satellite propellant.

The launch vehicle upper stage only delivers the satellite to a transfer orbit, typically with a low perigee and apogee near GEO - in this case, the target orbit is 295x80000 km at 20.75$^\circ$ inclination. After that, it's up to the satellite itself to use its onboard propulsion system to get itself into geostationary orbit, which is circular at 35,786 km and as near 0$^\circ$ inclination as possible.

The $\Delta V$ for an impulsive combined maneuver to change altitude and inclination is given by the following equation: $$\Delta V=\sqrt{V_i^2+V_f^2-2 V_i V_f \cos \Delta i}$$ Where $V_i$ is the initial orbit velocity, $V_f$ is the final, and $\Delta i$ is the inclination change. Since the inclination change is what's interesting in this problem, let's just look at a pure inclination change with no change to the orbit's semi-major axis. Note that in the above equation, if $V_i = V_f$, it simplifies to $$\Delta V = 2V_i\sin\left(\frac{\Delta i}{2}\right)$$ So for a given $\Delta i$, the required $\Delta V$ changes linearly with $V_i$. For a circular orbit, $V=\sqrt{\frac{\mu}{r}}$, where $\mu$ is the standard gravitational parameter for the central body (Earth in our case), and $r$ is the circular orbit radius. From this, we see that $V$ decreases as $r$ increases (for non-circular orbits, we get the same effect).

Putting these together: increasing $r$ decreases your velocity $V$, which in turn decreases the amount of $\Delta V$ to reorient your orbital plane. So this means if you do your maneuver(s) to take out your inclination, you stand to save a lot of propellant (thus adding potentially months or years to your life) by doing it at a higher inclination. In this specific case, it's a no-brainer for SES, since the Falcon 9 easily has the performance to inject to the higher orbit.


It actually saves satellite fuel, and for more than just the inclination. But first of all, let's start with the overall delta v budget, from this calculator. Using an inclination of 20.5, and a normal GTO (300km perigee, 35686 apogee) orbit, a calculated delta v is required of 1666 m/s. With no inclination, the change required is 1466 m/s. With the 80,000 km orbit, 20.5 the delta v required is 1414 m/s, and without inclination, 1334 m/s. So, the fuel saved on the spacecraft is:

  • 20.5 inclination- 252 m/s
  • 0 inclination- 52 m/s

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