 Inside the shell, a large spherical hydrazine tank made up most of the volume, topped by two smaller spherical nitrogen tanks and a 90 N injection rocket to slow the spacecraft down to go into lunar orbit, which was designed to be capable of firing twice during the mission.Attached to the bottom of the sphere was a 90 N vernier rocket for mid-course propulsion and lunar orbit maneuvers which could be fired four times.This space engine was designed and built under contract with NASA by the Space Technology Laboratories (STL) of TRW.