How rockets work: propulsion systems and launch technology explained

Every orbital launch, from a Falcon 9 lifting a crew to Starship test flights, relies on the same basic physics first described by Isaac Newton: for every action there is an equal and opposite reaction. A rocket engine burns propellant and expels the resulting hot gas out of a nozzle at high speed; the gas pushes the rocket in the opposite direction. According to NASA's Glenn Research Center, this action-reaction pair exists between the rocket and its own exhaust, which is why rockets work in the vacuum of space and do not need air or any other medium to push against.

The physics: thrust and specific impulse

Engineers describe an engine's performance with two numbers: thrust and specific impulse. Thrust is the force produced, driven mainly by how much mass flows out of the nozzle and how fast it exits. Specific impulse (Isp), measured in seconds, describes how efficiently an engine converts propellant into thrust — NASA Glenn defines it as thrust divided by the weight flow rate of propellant, so an engine with a higher Isp produces more thrust from the same amount of propellant, or needs less propellant for the same thrust.

That single figure lets engineers size an engine and compare very different propulsion systems on equal terms, which is why Isp appears constantly in rocket specification sheets.

Chemical propulsion: liquid, solid and hybrid

Almost all vehicles that leave Earth's surface use chemical propulsion, where a fuel and an oxidizer react to produce hot, expanding gas. There are three main configurations. Liquid-propellant engines store fuel and oxidizer separately and pump them into a combustion chamber, which allows the engine to be throttled, shut down and restarted — a flexibility used by both government and commercial launch providers. Solid-propellant motors mix fuel and oxidizer into a single solid grain packed into the rocket casing; they are structurally simple and inexpensive but, once ignited, burn until the propellant is spent. Hybrid systems combine a solid fuel with a liquid or gaseous oxidizer, sitting between the two approaches.

SpaceX's Raptor engine, used on the Starship and Super Heavy vehicles, illustrates a newer approach within liquid propulsion: a 'full-flow staged combustion' design in which all of the fuel and all of the oxidizer pass through two separate preburners before reaching the main combustion chamber, according to aerospace outlet Everyday Astronaut. Raptor burns liquid methane with liquid oxygen — a combination SpaceX chose partly because methane burns without leaving the soot deposits that complicate reuse of kerosene-fuelled engines, and partly because methane can, in principle, be manufactured on Mars from local resources. The outlet reports Raptor produces roughly 2 meganewtons of thrust with a vacuum specific impulse of around 330 to 350 seconds.

Why launch vehicles are built in stages

Reaching orbit from Earth's surface requires reaching an altitude above roughly 150 kilometres while travelling at more than 7.9 kilometres per second, according to the European Space Agency's launcher guide. A single engine design rarely does this efficiently across the whole flight, so most orbital rockets are split into stages that are discarded once their propellant is spent, shedding dead weight so the remaining stages accelerate the lighter vehicle more easily.

ESA's description of a typical Ariane-class flight shows how this plays out in practice: boosters and the main stage fire together during the first roughly two minutes of vertical ascent, the main stage continues burning for about twelve minutes while the boosters have already separated and fallen into the sea near 60 kilometres altitude, and an upper stage then fires for a final period, reaching the mission's peak velocity of around 8 kilometres per second before releasing its payload. After its job is done, a spent upper stage is typically moved to a 'graveyard orbit' so it does not become a collision hazard for active satellites.

Beyond launch: electric and ion propulsion

Once a spacecraft is already in orbit or deep space, chemical propulsion is not always the most efficient choice for long journeys. Many probes instead use electric propulsion, which ionizes a propellant — usually the gas xenon — and accelerates the resulting charged particles out of the thruster using electric fields powered by solar panels. NASA's Jet Propulsion Laboratory describes the thrust from these systems as extremely gentle: in the case of the Psyche asteroid-belt mission, the pressure produced by its ion thrusters is comparable to holding three quarters in your hand, even at full power.

That tiny thrust is not enough to lift anything off Earth, but in the frictionless environment of space it can be applied continuously for months, slowly building up speed. JPL reports that Psyche's solar-electric thrusters have already pushed the spacecraft to roughly 84,000 miles per hour and will eventually bring it to about 124,000 mph — illustrating why ion propulsion is reserved for in-space cruise rather than launch, where chemical engines' far higher thrust is required to overcome gravity and atmospheric drag in a matter of minutes.