Glossary — U
3 terms
MISSION
Uncrewed mission
A mission flown without humans aboard.
Example: An uncrewed test flight validated the new vehicle.
LAUNCH
Upper stage
The stage operating after the first stage, delivering the payload to its destination.
Example: The upper stage coasted before the translunar injection.
PROPULSION
Ullage
The empty volume in a propellant tank, or the small settling thrust used before ignition.
Example: Ullage motors settle propellant against the tank outlet before relight.