I just created a Modelica Model of a rocket landing in vacuum. Now I attempt to verify it against reality. However, I have a hard time to figure out some plausible initial parameters. Apollo 11's Eagle had a Periapsis of 15km. But what was its initial speed?

I assume that everyone does one deorbit burn and later attempts to get as close to a suicide burn as reasonable. So I modeled my experiment after a falling mass under retrograde thrust in a cartesian coordinate system. Obviosuly, this model has a hard time when it comes to very long landing approaches (since the cartesian coordinates to not take into account the curvature of the target body), so lowering the periapsis to 0 is not an option right now (although probably the most efficient approach). I arbitrarily choose an initial surface velocity of 1km/s which gave me a landing at 140km.

So what is a plausible surface speed after the deorbit burn?

  • 4
    $\begingroup$ Start from the end: Model a launch from the Lunar surface, to the orbit. Of course you'll have to tweak fuel usage to be negative ;) Once you achieve the orbit, you have your "initial" parameters, just replay the whole thing backwards. $\endgroup$
    – SF.
    Jun 24, 2016 at 15:15

1 Answer 1


This document provides the Apollo 11 LM velocity at the start of powered descent as 5564.9 ft/s.


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