T-minus. Time before liftoff. A countdown is not simply a clock running down — it can be held, which freezes it while a problem is worked, and resumed. Mission time after launch is written T+.
Propellant load. Filling the vehicle with liquid oxygen and liquid methane, typically starting under an hour before launch. Both are cryogenic — cold enough that the tanks visibly frost over, which is the frost line you see climbing the vehicle.
Densified propellant. Cryogens chilled below their boiling point so more mass fits in the same tank. The catch is that they warm up: a long hold lets temperatures drift out of limits, and out-of-limits propellant is a genuine reason to scrub rather than a paperwork technicality.
Commit criteria. The pre-agreed limits — winds, temperatures, system states — that must be satisfied to proceed. The point of writing them down in advance is to stop anyone from talking themselves into a launch in the moment.
GO/NO-GO poll. The flight director asks each station in turn whether it is ready. Any single NO-GO stops the count. It is deliberately not a vote or an average.
Hold. Pausing the count to work an issue. A built-in hold is planned; an unplanned hold means something needs attention. Holding is cheap, and holding is usually the right instinct.
Scrub. Standing down for the day. Expensive and disappointing, and vastly preferable to the alternative.
Ignition sequence start. Engines light a few seconds before release, so the flight computer can confirm healthy thrust before letting the vehicle go. If the count is going to abort safely, this is where it happens.
Liftoff / tower clear. Release, then the moment the vehicle has risen past the tower. Between the two there is essentially no abort option.
Max-Q. Maximum dynamic pressure — the point of greatest aerodynamic stress, usually around a minute in. It is a product of speed and air density, so it occurs when the vehicle is going fast and still low. Vehicles often throttle down through it.
MECO. Main Engine Cutoff — the first stage finishing its burn. On a vehicle that stages hot, most engines shut down while a few keep running.
Hot-staging. Igniting the upper stage before the lower stage has let go, firing through a vented interstage ring. It avoids the coasting instant where propellant sloshes away from the intakes, at the cost of a ring that has to survive being blasted.
Ullage. The gas space above the liquid in a tank — and, as a verb, settling propellant toward the intakes before a relight so the engines get liquid rather than bubbles. Hot-staging exists largely to avoid needing it.
SECO. Second Engine Cutoff — the upper stage finishing its burn and arriving on its target trajectory.
Boostback burn. The first stage firing retrograde to cancel downrange velocity and head back toward the launch site. Modern profiles stage this burn — full cluster, then fewer engines, then fewer still — so the last of it is a fine trim rather than a shove.
Entry burn. A short burn before the thickest air, slowing the vehicle enough that aerodynamic heating and structural loads stay survivable.
Grid fins. Lattice control surfaces near the top of the booster. They steer by moving the aerodynamic centre of pressure relative to the centre of mass — a lattice gives a lot of control authority for its weight and stows flat.
Landing burn. The final deceleration. Started late and deliberately short — "suicide burn" is the informal name — because burning early means hovering, and hovering wastes propellant the vehicle does not have.
Bellyflop. The upper stage falling flat, belly-down, using its whole body as an airbrake with flaps trimming attitude. It is a controlled aerodynamic descent, not a tumble, and it sheds nearly all horizontal velocity before the landing flip.
The flip. Rotating from belly-down to engines-down in the last seconds of flight, immediately before the landing burn. Short, violent, and unforgiving of a bad starting altitude.
Divert. Giving up on the landing site and putting the vehicle down offshore. Loses the hardware, saves the pad. Frequently the correct call.
Splashdown. A water landing. For a test programme these are real objectives — a soft touchdown over water proves the landing sequence without risking the tower.
FTS. Flight Termination System — explosive charges that destroy the vehicle if it leaves its safety corridor. Modern systems are autonomous, triggering on onboard logic rather than a ground command. Using it correctly is not a failure of the job; it is the job.
Range. The organisation responsible for everything outside the vehicle: keeping ships, aircraft and people clear of where debris could land.
Abort. Ending the flight deliberately. On the pad it is safe. In flight, for a vehicle with no crew escape system, it means terminating the vehicle.
Loss of vehicle / loss of pad. The two outcomes worth ranking. Losing hardware is expensive; losing the ground infrastructure costs the programme months, which is why an offshore divert is nearly always cheaper than a hopeful catch.
Anomaly. Anything not behaving as expected. Graded by severity — a minor anomaly may clear on its own, a major one usually will not, and telling them apart quickly is most of the skill.
Launch mount. The structure the vehicle sits on, holding it down through ignition and releasing at commit.
Flame trench / deluge. Channels that redirect exhaust away from the pad, and an enormous volume of water dumped through the deck and trench at ignition. The water is not primarily for cooling — it is there to absorb acoustic energy that would otherwise reflect off the ground and shake the vehicle apart.
Quick disconnect (QD). Umbilical connections carrying propellant, power and data, designed to detach cleanly at launch.
Chopsticks. The tower's two movable arms — used to stack the vehicle, and on a return flight to catch it. See how the catch works.
Related reading: how the chopstick catch works covers the recovery profile in depth, and the Flight Manual walks the countdown minute by minute.