[David Brinza]

Originally Posted by EricH

The main engines are ONLY used during launch. After launch, they shut down for the duration.

However, there are those two pods right above the three main engines. Those have smaller, liquid-fueled engines and are used for larger orbital maneuvers. There's a technical name for them, but I can't remember it offhand. There are also the various steering engines at various places around the shuttle.

Eric's referring to the Orbital Maneuvering System (OMS) engines, which are used to adjust the Space Shuttle's orbit after launch and to perform the re-entry burn.

The replacement launch vehicle for the Shuttle being developed for the Constellation program, Ares-1 has a first stage that is only a solid rocket. Originally, the Ares-1 second stage was going to use the Space Shuttle Main Engine (SSME), but concerns on lighting this engine in the air rather than on the ground drove NASA to use the J-2X engine. The J-2X is derived from the design of the J-2 engine that was used on Saturn V rockets that launched the Apollo missions to the moon.