 Orbital periods vary according to the sum of apoapsis and periapsis, and for the CSM were about two hours.The LM began its landing sequence with a Descent Orbit Insertion (DOI) burn to lower their periapsis to about , chosen to avoid hitting lunar mountains reaching heights of .After the second landing mission, the procedure was changed on Apollo 14 to save more of the LM fuel for its powered descent, by using the CSM's fuel to perform the DOI burn, and later raising its periapsis back to a circular orbit after the LM had made its landing.