STEP 1
Octaweb + 9 Merlin engines
Nine Merlin engines bolted into a steel frame called the Octaweb — the muscle that lifts everything else.
Why it exists: One huge engine would be simpler. Nine smaller ones mean the rocket can lose an engine and still finish the mission — and the same nine can throttle down to just one to land the booster gently.
Without it: Nothing happens at all. No engines, no launch — the rocket just sits on the pad.
For engineers: Clustering buys engine-out redundancy and lets one small engine design produce 7,607 kN — unit cost falls with volume. It also delivers the throttle range landing demands: nine engines at 100% for liftoff, one at ~40% for the hover-slam — an effective turndown near 23:1 that no single large engine could reach.
Made of: Inconel superalloy, stainless steel and aluminium. They burn rocket-grade kerosene (RP-1) with liquid oxygen.
All nine together push with about 7,607 kilonewtons at sea level — and they drink propellant so fast the tanks above would empty in about two and a half minutes.
STEP 2
Stage 1 tanks
The tall white body of the rocket — really just two giant tanks, one of kerosene and one of liquid oxygen.
Why it exists: A rocket is mostly fuel. To push itself upward it has to throw mass out of the back, very fast, for a very long time — so almost everything you see is tank.
Without it: The engines have nothing to burn. They would fire for a heartbeat and die.
For engineers: Tsiolkovsky: Δv = Isp·g₀·ln(m₀/mf). Only mass ratio sits inside a logarithm you can keep pushing, so structure is shaved until the tank is pressure-stabilised — it needs internal pressure to avoid buckling under its own stack. Sub-cooling the LOX to ~66 K densifies it, packing more oxidiser into the same volume: free Δv for the price of a colder ground system.
Made of: Aluminium-lithium alloy, joined by friction stir welding — a spinning tool that stirs the metal together rather than melting it.
Empty, stage 1 weighs about 25.6 tonnes. Full, it carries about 411 tonnes of propellant — the fuel is roughly sixteen times heavier than the rocket holding it.
STEP 3
Landing legs
Four legs folded flat against the base of the booster.
Why it exists: They exist so the rocket can be used again. Before them, every booster ever built was thrown away after a single flight — like scrapping an aeroplane after one trip.
Without it: The booster flies a perfect landing, touches the pad… and falls straight over.
For engineers: Recovery hardware is a payload tax: legs, fins and landing propellant together cost roughly 30–40% of expendable LEO performance. That trade only closes at high flight rate with cheap refurbishment — which is precisely why reuse was a net loss for the Shuttle and a net win for Falcon 9.
Made of: Carbon fibre and aluminium honeycomb — strong enough to catch a 25-tonne booster, light enough to carry all the way to space and back.
They stay folded for the entire flight and snap open just seconds before touchdown.
STEP 4
Grid fins
Four waffle-shaped fins near the top of the booster.
Why it exists: Falling back from space, the booster is moving faster than sound through very thin air. Ordinary fins would be useless or torn away — a grid fin bites that thin airflow and steers the booster home.
Without it: The booster tumbles all the way down and misses the pad completely.
For engineers: Each lattice cell acts as a small wing, giving high control effectiveness at low hinge moment. Crucially, centre-of-pressure travel through the transonic barrier is small, so the control loop stays stable where planar fins go non-linear. Titanium replaced aluminium after the originals ablated and caught fire — titanium survives uncooled and flies again with no refurbishment.
Made of: Cast titanium. The first ones were aluminium — and caught fire during re-entry.
Each fin survives re-entry with no heat shield at all, and they fly again and again without being replaced.
STEP 5
Interstage
The black tube that joins stage 1 to stage 2.
Why it exists: It carries the entire weight of the upper stage and payload while the rocket climbs — and then lets go at exactly the right instant. It is a structure whose whole job ends in letting go.
Without it: Stage 2 has nothing to stand on, and no clean way to separate when its turn comes.
For engineers: It carries axial compression plus bending through max-Q, then separates with minimal tip-off rate — any angular error at sep eats directly into the upper stage's attitude control budget. Pneumatic pushers instead of pyrotechnic bolts means no shock, no debris, and hardware that survives to fly again; pyros never do.
Made of: Carbon fibre composite over an aluminium honeycomb core.
Pneumatic pushers shove the two stages apart instead of explosive bolts — gentler, and it means the booster survives to fly again.
STEP 6
Stage 2 + Merlin Vacuum
A second, smaller rocket riding on top — one engine, with an enormous bell-shaped nozzle.
Why it exists: Stage 1 only gets you high and fast. Stage 2 does the hard part: going sideways fast enough that you keep missing the Earth as you fall. That is what an orbit really is.
Without it: You reach space — and fall straight back down. Getting to space is easy; staying there is the hard part.
For engineers: Vacuum optimisation: thrust = ṁ·ve + (pe − pa)·Ae. As ambient pressure → 0 you want the largest expansion ratio you can carry, hence the enormous bell — fire it at sea level and flow separation would tear it apart. Staging is the mass-ratio fix: ln(m₀/mf) is bounded per stage, so you reset the ratio by throwing structure away.
Made of: The same aluminium-lithium tanks; the nozzle is a niobium alloy that glows red-hot as it fires.
The nozzle is gigantic because there is no air in space to squeeze the exhaust inward. That same nozzle would be torn apart if you fired it at sea level.
STEP 7
Avionics — the flight computers
The rocket's brain: computers, sensors and radios tucked inside the upper stage.
Why it exists: Nobody flies a rocket with a joystick. It flies itself, correcting its aim thousands of times a second — far faster than any human could react.
Without it: The engines light and the rocket immediately cartwheels. A rocket is naturally unstable — like balancing a pencil upright on your fingertip.
For engineers: Thrust-vector control on a body whose centre of mass sits ahead of the thrust application point is an inverted pendulum: open-loop unstable, demanding closed-loop TVC at high rate. Redundancy is by voting — three cores compare and a dissenter is outvoted — which buys radiation and random-fault tolerance from cheap commodity parts instead of costly rad-hardened ones.
Made of: Triple-redundant flight computers — three of everything, constantly voting on every decision.
If one computer disagrees with the other two, it is outvoted and ignored.
STEP 8
Payload
The reason the rocket exists — the satellite, cargo or spacecraft it is carrying.
Why it exists: Every tank, engine and bolt on this pad exists only to move this one object to orbit. Everything else is a delivery van that destroys itself on the way.
Without it: You have built a very expensive firework.
For engineers: Payload fraction is the whole economic argument: it's the small residual left after the rocket equation takes its cut. Recovery hardware and landing propellant come straight out of this number — which is exactly why return-to-launch-site missions fly lighter payloads than droneship ones, and droneship lighter than expendable.
Made of: Anything: a communications satellite, a science telescope, cargo for the space station — or people.
The payload is a tiny slice of the whole: a Falcon 9 lifts about 22.8 tonnes to low orbit out of roughly 549 tonnes on the pad — about 4%.
STEP 9
Payload fairing
The nose cone — two shells that clamshell shut around the payload.
Why it exists: Punching through the thick lower atmosphere faster than sound would tear a bare satellite apart. Once the air is gone the shield is just dead weight — so the rocket throws it away.
Without it: The air rips the payload off before you ever leave the atmosphere.
For engineers: It shields against dynamic pressure and aerothermal flux and gives the stack a defined aerodynamic shape the control system can model. Jettison timing is a genuine optimisation: drop early and you save ~1.9 t of dead mass in the insertion burn, but risk exceeding the payload's free-molecular heating limit; drop late and you pay that mass all the way to orbit.
Made of: Carbon fibre over an aluminium honeycomb core.
It is 5.2 m across — wider than the rocket itself — and splits in half about three minutes into flight. SpaceX has caught falling halves in nets to fly them again.
