Glossary

A–Z of launch terms

A plain-English guide to the terminology used in UK launch coverage — from apogee and NET to scrub, static fire and sun-synchronous orbit.

A

Apogee: The highest point of an orbit — the moment a spacecraft is furthest from the body it is orbiting.

Booster: The first-stage rocket that provides most of the thrust needed to lift a vehicle off the pad and through the lower atmosphere.

Cryogenic propellant: A rocket fuel or oxidiser stored at very low temperatures, such as liquid oxygen or liquid hydrogen, to keep it in liquid form.

Deorbit burn: A controlled engine firing that slows a spacecraft enough for it to re-enter the atmosphere at a chosen time and place.
Downrange: The area along the flight path away from the launch site, where spent stages fall and tracking stations observe the rocket.

Expendable: A launch vehicle or stage that is used once and not recovered, in contrast to a reusable design.

Fairing: The protective nose cone at the top of a rocket that shields the payload during the noisy, high-pressure climb through the atmosphere.
First stage: The lowest section of a rocket, containing the main engines and most of the propellant, which fires from lift-off until stage separation.

GEO (Geostationary orbit): A circular orbit roughly 35,786 km above the equator where a satellite appears to hover over one point on Earth.
Ground segment: The mission control centres, tracking stations and networks on the ground that command a spacecraft and receive its data.

Hold: A planned or unplanned pause in the countdown, either to work through a checklist item or to resolve a technical or weather issue.

Integration: The process of assembling the rocket, connecting the payload and completing the electrical and mechanical checks needed before rollout.

Kick stage: A small upper stage that fires after the main rocket has done its job, precisely placing satellites into their final orbits.

Launch window: The period during which a rocket can lift off and still reach its intended orbit or destination — sometimes a few seconds long, sometimes hours.
LEO (Low Earth orbit): An orbit typically below about 2,000 km altitude, used by most Earth-observation satellites and crewed missions.
Lift-off: The moment a rocket physically leaves the launch pad, once thrust exceeds the vehicle's weight and the hold-down clamps release.

Max-Q: The point during ascent where the rocket experiences the greatest aerodynamic stress, usually about a minute after lift-off.
MECO: Main Engine Cut-Off — the moment the first-stage engines stop firing, shortly before stage separation.

NET (No Earlier Than): A launch date given as the earliest possible date, used when the schedule has not yet been confirmed and is likely to slip later.

Orbital insertion: The engine burn that places a spacecraft into its intended orbit around Earth or another body.

Payload: The cargo carried by a rocket — typically one or more satellites, a spacecraft, or an experiment.
Perigee: The lowest point of an orbit — the moment a spacecraft is closest to the body it is orbiting.
Polar orbit: An orbit that passes close to the north and south poles, so the satellite eventually flies over every part of Earth's surface.
Propellant: The combination of fuel and oxidiser burned in a rocket engine to produce thrust.

Rideshare: A launch that carries multiple satellites from different customers on a single rocket to share the cost.
Rollout: The physical move of a fully assembled rocket from its integration building out to the launch pad.

Scrub: The cancellation of a launch attempt, either before or during the countdown, usually because of weather or a technical problem. A new attempt is scheduled later.
Second stage: The upper section of a rocket that ignites after first-stage separation and carries the payload the rest of the way to orbit.
Spaceport: A licensed site from which rockets are launched. The UK's vertical-launch sites include SaxaVord in Shetland.
SSO (Sun-synchronous orbit): A polar orbit timed so a satellite passes over each location at the same local solar time — very useful for consistent Earth imaging.
Stage separation: The moment a spent rocket stage detaches so the next stage can ignite, saving the mass of an empty tank and unused engines.
Static fire: A ground test in which the rocket's engines are ignited while the vehicle is held down on the pad, used to confirm the engines and systems are working before launch.

T-minus / T-plus: Countdown notation. T-minus counts down towards lift-off (T-minus 10 seconds); T-plus counts up from lift-off (T-plus 30 seconds).
TBC (To Be Confirmed): A schedule note meaning a date has been proposed but is still being confirmed. Often used interchangeably with TBD in launch listings.
TBD (To Be Determined): A schedule note meaning the launch date has not yet been set.
Throttle: The rate at which an engine burns propellant. Modern engines can throttle up or down to manage stress during ascent.
Thrust: The force produced by a rocket engine, measured in kilonewtons (kN). A vehicle can only lift off once total thrust exceeds its weight.
Trajectory: The path a rocket flies from launch to orbit, shaped by the target orbit, safety corridors and downrange geography.

Upper stage: A rocket stage that operates in the vacuum of space, using vacuum-optimised engines to finish placing the payload into orbit.

Vertical launch: A launch profile where the rocket lifts off straight up from a pad, the approach used at SaxaVord.