Final letdown, translation hover and landing on the lunar surface from 1800 m above the surface was performed by the terminal landing module. Engine thrust could be throttled down to 1546 kgf. The engine was presumed to be based on a reasonable development of any one of three on-going engine programs for Apollo of the time: Aerojet General's 3600 kgf thrust Transtage engine; Reaction Motors' Model TD-294 4500 kgf thrust LEM research engine; or the 4500 kgf thrust engine under investigation by Rocketdyne for possible use in the LEM. The engine in the terminal landing module was ablatively cooled, throttleable, and had an expansion ratio of 20:1.