On launch, the space shuttle does not directly inject into its final orbit. rather, main-engine shutdown occurs at the apogee of an elliptical orbit at 100 km altitude. the perigee of this orbit is only 30 km altitude, to ensure that the discarded external tank will safely burn up over open ocean. after the external tank is jettisoned, the orbiter uses its orbital maneuvering system (oms) to maneuver into another elliptical orbit with a perigee at 100 km altitude, and an apogee at 250 km. this is called the oms-1 burn. at the new apogee the shuttle performs the oms-2 burn to circularize the orbit at the mission altitude of 250 .