Rocket launch glossary: every term explained

New to watching launches entirely? Start with the how-to-watch primer, then keep this glossary open in a tab. When a real countdown is running, the live schedule shows the current target in your own timezone so you can follow along with these terms in real time.

Countdown & scheduling

NET (no earlier than)

The earliest date or time a launch could happen — not a promise, just a floor. You'll see "NET" a lot for missions still working through hardware, paperwork, or weather; the real time firms up as launch day approaches.

T-minus / T-plus

The launch clock. "T" is the moment of liftoff: T-minus counts down toward it (T-minus 10 seconds), and T-plus counts up after it (T-plus 2 minutes into flight). Milestones are almost always announced against this clock rather than the wall clock.

Hold

A pause in the countdown. Some holds are planned — built-in stops that let the team catch up or wait out a condition — and some are unplanned, called when something needs checking. A hold isn't a cancellation; the clock can pick back up if there's still time in the window.

Scrub

Calling off the launch attempt for the day. A scrub can come from weather, a technical issue, a boat in the range, or simply running out of time in the window. It's rescheduled for a later date, often the next day. A scrub isn't a failure — it's the team refusing to fly until everything is right.

Go/no-go poll

The roll call near the end of the countdown where the launch director asks each station — weather, propulsion, range, and the rest — to declare "go" or "no-go." A single "no-go" can stop the clock. It's the human checklist that clears a rocket to fly.

Launch window

The span of time during which a launch can happen and still reach its target. Some windows are hours long; others are instantaneous — a single second — because the destination, like the space station or a specific orbit, is only lined up briefly. A short window is why some missions scrub the moment anything goes sideways.

Static fire

A pre-launch test where the rocket's engines are briefly ignited while it stays clamped to the pad. It lets the team wring out the engines and ground systems without going anywhere — a dress rehearsal for the loudest part of the day.

Wet dress rehearsal

A full countdown run-through with real propellant loaded (that's the "wet" part), stopping just short of engine ignition. It shakes out the fueling process and timeline so launch day holds fewer surprises.

Ascent milestones

Liftoff

The instant the rocket clears the pad and starts climbing. It's a hair after engine ignition — the engines light, build to full thrust, the clamps release, and only then does the vehicle actually rise. "Liftoff" is when the ground lets go.

Max Q

The moment of maximum aerodynamic pressure on the rocket — the point of greatest stress from pushing through thick lower atmosphere at rising speed. Engines are often throttled down slightly through Max Q, then back up once the air thins. It usually happens in the first minute or two of flight.

MECO (main engine cutoff)

When the first-stage (booster) engines shut down, near the end of that stage's burn. MECO is the cue that stage separation is seconds away. On a webcast you'll hear "MECO" called, then the booster falls silent as it's about to let go.

Stage separation

The point where a spent stage detaches so a fresh, lighter stage can take over. Dropping dead weight — an empty tank and used engines — lets the remaining rocket accelerate far more efficiently toward orbit.

Hot-staging

A separation technique where the upper stage lights its engine while still attached to the booster, then pushes off, rather than coasting apart first. It squeezes out a bit more performance and produces the dramatic ring of flame you may see between the stages on some vehicles.

SECO (second-engine cutoff)

When the upper stage's engine shuts down after reaching the target orbit or trajectory. SECO is often the moment the payload is effectively "in space" and coasting; a mission may have more than one, labeled SECO-1, SECO-2, and so on, if the engine relights later.

Fairing jettison

Dropping the nose cone — the "fairing" — that shields the payload during the loud, buffeting climb through the atmosphere. Once the rocket is high enough that the air is too thin to matter, the fairing splits and falls away, shedding weight. Some are recovered and reused.

TLI (trans-lunar injection)

The engine burn that flings a spacecraft out of Earth orbit and onto a path to the Moon. You'll hear TLI on lunar missions; it's the go-fast moment that trades a parking orbit for a lunar trajectory.

Coming back down

RTLS (return to launch site)

A booster landing profile where the first stage flies itself back to a landing pad near where it took off, instead of heading downrange. It's dramatic to watch — a sonic-boom double-crack often reaches spectators right around touchdown — and it's used when the mission has enough performance margin to spare.

ASDS / droneship

An Autonomous Spaceport Drone Ship — the ocean landing platform a booster touches down on when it doesn't have the fuel to fly all the way back to shore. Stationed downrange under the flight path, it catches boosters far out at sea and ferries them home.

Booster catch

Instead of landing on legs, some boosters aim to be caught in mid-air by arms on the launch tower. It's a demanding maneuver that removes the weight of landing legs from the rocket and is meant to speed up reuse. Expect nail-biting on any flight that attempts one.

Expendable vs reusable

An expendable rocket flies once and is gone — stages fall into the ocean and aren't recovered. A reusable one is designed to land, be refurbished, and fly again. Sometimes a booster is flown expendable on purpose, when a heavy or high-energy mission needs every last bit of propellant for the payload.

Orbits & destinations

LEO

Low Earth Orbit — the busy band of space a few hundred miles up where the space station, most Starlink satellites, and countless others live. It's the easiest orbit to reach, so a huge share of launches are headed there.

GTO

Geostationary Transfer Orbit — a stretched, egg-shaped orbit used to hand a satellite off toward geostationary orbit some 22,000 miles up, where it can hover over one spot on Earth. Communications and weather satellites often ride to GTO, then use their own engine to circularize.

SSO / polar

A near-polar orbit that carries a satellite roughly over the poles, so the whole planet rotates underneath it over time. Sun-synchronous orbit (SSO) is a special polar orbit that keeps the lighting consistent — favored by Earth-imaging satellites that want every pass at the same local time.

Apogee & perigee

The high and low points of an orbit around Earth. Apogee is the farthest the spacecraft gets from the planet; perigee is the closest. A perfectly circular orbit has the two nearly equal; a transfer orbit deliberately does not.

Words you'll hear

LOX

Liquid oxygen — the oxidizer that lets a rocket's fuel burn where there's no air. It's kept so cold it's a liquid, which is why you'll see white vapor venting off a fueled rocket on the pad; that's the LOX boiling off and being topped up until the last moment.

Hypergolic

A pair of propellants that ignite the instant they touch, with no spark needed. They're toxic and finicky to handle but wonderfully reliable, which is why they're favored for spacecraft thrusters and some upper stages that must light on demand.

Anomaly

The measured, understated word for "something went wrong." It can mean anything from a minor off-nominal reading to a serious failure. Teams say "anomaly" because it's precise and neutral while they figure out what actually happened.

Nominal

Everything is going exactly as planned. It's the word you most want to hear on a webcast — "trajectory is nominal," "engines are nominal" — meaning the numbers are landing right where they're supposed to. Nominal is the sound of a good day.

Payload

Whatever the rocket is actually there to deliver — a satellite, a crew capsule, a cargo ship, a science probe. Everything else on the vehicle exists to get the payload where it's going.

Cryogenic

Describes propellants chilled to extreme cold to keep them liquid, like liquid oxygen and liquid hydrogen. Cryogenic fuels are powerful but demanding — they must be loaded shortly before launch and constantly topped up as they boil away, which shapes the whole countdown timeline.

Once these click, a webcast turns from noise into a story you can follow beat by beat. Try it on a real one: watch the next Starship test flight for hot-staging and Max Q, an Artemis mission for TLI, or a Starlink launch for a textbook MECO and droneship landing — all pinned to the current target on the live schedule.