upperstage

An upper stage is the section of a multistage rocket that operates after the first stage has expended its propellant and separated. Its primary role is to place the payload into the intended orbit, perform orbital maneuvers, or interplanetary trajectory corrections. Upper stages operate in vacuum and are designed to maximize delta-v while minimizing mass and atmospheric drag.

Design and capabilities: A typical upper stage consists of one or more propulsion engines, propellant tanks, a thrust and attitude control system, and guidance, navigation, and control sensors, as well as pressurization hardware and propulsion feed systems. Many upper stages feature restartable engines for multiple burns, enabling apogee raises, plane changes, and precise deployments. They may use cryogenic propellants (LOX/LH2) or storable hypergolic propellants, depending on mission requirements. Power is usually provided by onboard batteries; some stages carry small solar arrays for long-range missions.

Performance and mission use: Upper stages perform orbital insertions after separation, raise or adjust apogee and/or perigee, and execute plane changes and rendezvous maneuvers. They are often used for deploying satellites in GEO orbits, launching deep-space probes, or performing interplanetary transfers. In multi-payload missions, the upper stage may release satellites in sequence.

Examples: The Centaur upper stage, using LH2/LOX with the RL10 engine, is a well-known restartable upper stage that has flown on Atlas and Delta vehicles. Other upper stages include the Delta Cryogenic Upper Stage and the Fregat upper stage, which use storable propellants and multiple restarts.