Rocket science glossary
The words that come up when you watch a launch, explained properly. Every entry has the real number and a link to where it came from.
Apogeealso: highest point of an orbit
Apogee is the point in an orbit where a spacecraft is furthest from Earth. Orbits are usually ovals rather than circles, so there is a far point and a near point.
Delta-valso: Δv
Delta-v is how much a rocket can change its speed — the real currency of spaceflight. Reaching low Earth orbit costs roughly 9,400 m/s once losses are included.
Entry burn
An entry burn is a short engine firing that a returning booster makes into the oncoming air. It slows the booster, but its main job is to shield it from re-entry heating.
Escape velocity
Escape velocity is the speed something needs to leave a planet for good without any further push. From Earth’s surface it is about 11.2 km per second — roughly 40,270 km/h.
Free fall
Free fall is moving under gravity alone, with nothing holding you up. Anything in free fall feels weightless — including astronauts, who are falling around the Earth rather than towards it.
G-forcealso: g-load, gs
G-force describes how heavy something feels compared with its normal weight. Sitting still is 1 g. Accelerating hard in a rocket pushes that to several g.
Gravity turnalso: pitchover
A gravity turn is the slow tilt a rocket makes after liftoff, letting gravity curve its path sideways instead of steering hard with its engines.
Grid finalso: waffle fin
A grid fin is a flat, waffle-shaped steering surface that folds out of a rocket booster on the way back down. Falcon 9 has four, machined from solid titanium.
Hypergolic
Hypergolic propellants catch fire the instant they touch each other. No spark or igniter is needed, which makes them extremely reliable for an engine that must work first time.
Kármán line
The Kármán line sits 100 km above sea level and is the most widely used boundary of space. NASA and the US Air Force use 80 km instead, so there is no single agreed answer.
Landing burnalso: hoverslam, suicide burn
The landing burn is the final engine firing that brings a booster from fast to stopped, right at the ground. It is also called a hoverslam, because the rocket cannot hover.
LOXalso: liquid oxygen
LOX is liquid oxygen — ordinary oxygen chilled until it becomes a pale blue liquid at about −183 °C. Rockets carry it because fire needs oxygen and space has none.
Max Qalso: maximum dynamic pressure
Max Q is the moment during a launch when the air pushes back on the rocket harder than at any other time. On a Falcon 9 it happens about 70 seconds after liftoff, around 13 km up.
MECOalso: main engine cutoff
MECO stands for Main Engine Cutoff — the moment the first stage shuts down its engines. On a Falcon 9 it happens at about 2 minutes 26 seconds, roughly 68 km up.
Microgravityalso: weightlessness, zero gravity
Microgravity is the floating sensation felt in orbit. It is caused by free fall, not by an absence of gravity — at the space station gravity is still about 88% as strong as on the ground.
Octaweb
The octaweb is the structure at the base of a Falcon 9 that holds its nine engines — eight in a ring around one in the centre.
Orbit
An orbit is falling around something instead of into it. Go sideways fast enough and the ground curves away as quickly as you fall, so you never land.
Payload fairingalso: nose cone
The payload fairing is the hollow nose cone that protects a rocket’s cargo from the air during launch. Falcon 9’s is 13.1 m tall and splits in half at about T+2:56.
Perigeealso: lowest point of an orbit
Perigee is the point in an orbit where a spacecraft is closest to Earth. It is where the spacecraft is moving fastest, and where the thin upper atmosphere drags on it most.
Reentryalso: re-entry, atmospheric entry
Reentry is the return of a spacecraft into the atmosphere. The heat comes from the air being violently compressed in front of the vehicle, not from friction.
Regenerative cooling
Regenerative cooling runs the rocket’s own cold fuel through channels in the engine wall before burning it — cooling the engine and warming the fuel at the same time.
RP-1also: rocket propellant 1, rocket kerosene
RP-1 is a highly refined kerosene used as rocket fuel, burned with liquid oxygen. It is close to jet fuel but cleaner, with strict limits on sulphur so it does not clog the engine.
Specific impulsealso: Isp
Specific impulse measures how efficiently a rocket engine uses propellant — its miles per gallon. A Merlin engine manages 282 seconds at sea level and 311 in vacuum.
Stage separationalso: staging
Stage separation is the moment a rocket drops the part that has run out of fuel. An empty tank is dead weight, so letting it go makes everything above it easier to accelerate.
Thrust
Thrust is the push an engine produces. One Merlin engine makes 845 kN at sea level — enough on its own to lift about 86 cars — and a Falcon 9 has nine of them.