The first stage gets the attention at lift off, but the upper stage decides where a satellite actually ends up. It completes the climb to orbit, shapes the final orbit and releases the payloads. Kick stages add further flexibility on top. For customers, the capabilities of the upper stage are often more important than the raw lift capacity of the rocket.
What the upper stage does
After the lower stages burn out and separate, the upper stage continues accelerating in near vacuum. Its engines are optimised for this environment, with large nozzles and high efficiency. Once it reaches the planned orbit, it orients itself and releases the payload at the right time and with the right attitude.
Restartable engines
An upper stage that can restart its engine several times can perform complex missions: it can coast to a higher point of the orbit, circularise there, deploy satellites into different orbits and finally lower its own orbit for disposal. Examples of restartable upper stages include the Centaur stage used on Atlas V and Vulcan, which is powered by RL10 engines, and the upper stage of Ariane 6 with its Vinci engine.
Kick stages
A kick stage is a small additional stage that sits on top of the upper stage and carries the payload. It provides precise final orbit insertion and can deploy several satellites into slightly different orbits. Rocket Lab's Electron, for example, uses a kick stage for this purpose. Orbital transfer vehicles take the same idea further and operate as independent spacecraft after separation.
Disposal of the upper stage
Spent upper stages are a significant source of space debris. Modern debris mitigation rules therefore require that stages are deorbited or moved to disposal orbits. Many upper stages now perform a final burn after payload separation to reenter the atmosphere in a controlled way or to shorten their lifetime in orbit.
What this means for customers
- A restartable upper stage allows more precise orbits and more complex rideshare missions.
- The number of burns and the coasting time affect how long the satellite stays attached, which matters for power and thermal design.
- Customers should ask how the upper stage will be disposed of, since this is part of the mission's debris compliance.
Related reading
- Rocket Staging Explained: Why Rockets Drop Parts on the Way to Orbit
- Space Tugs Explained: How Orbital Transfer Vehicles Deliver Satellites
- How Are Satellites Launched Into Space? From the Launch Pad to Orbit
Sources
Cover image: The GOES-R weather satellite being lifted for mating with its Atlas V rocket, 2016. Credit: NASA (KSC-20161109-PH_DAC01_0017), public domain.