Як працює ракетний двигун: Третій закон Ньютона, сопло Лаваля та ступінчастість

The Basics of Rocket Propulsion

Rocket propulsion is based on Newton’s Third Law of Motion – for every action, there is an equal and opposite reaction. A rocket engine burns fuel to create incredibly hot, pressurised gas, and that gas rushes out of a nozzle at the bottom of the rocket — this is the ‘action’. The reaction to this ‘action’ is that the rocket is propelled in the opposite direction.

The design of the nozzle – known as a Laval nozzle – is crucial for efficient operation. It’s shaped to accelerate the exhaust gases to extremely high speeds, maximizing thrust. A well-designed Laval nozzle converts thermal energy into kinetic energy, ensuring the highest possible velocity of the exhaust stream.

Rockets don't need to push against air; the reaction occurs between the rocket and its own escaping exhaust. This is why rockets can operate perfectly in a vacuum, such as space, where there’s no atmosphere to contend with. The higher the speed of the exhaust gases, the greater the thrust produced by the rocket engine.

Why Rockets Need So Much Fuel

To escape Earth’s gravity a rocket needs to reach approximately 11.2 km/s – that's roughly 40,000 km/h. The problem is that the heavier the rocket, the more fuel it requires; however, more fuel increases its weight, necessitating even greater amounts of fuel. Konstantin Tsiolkovsky described this ‘tyranny of the rocket equation’ in 1903. Consequently, a typical rocket contains approximately 90% fuel by mass at launch. The Saturn V weighed 2,970,000 kg on the pad; yet, the Apollo capsule that actually reached the Moon and back was only about 28,800 kg – roughly 1% of the total.

Staging: shedding dead weight mid-flight

Engineers get around the fuel problem with staging: instead of one giant rocket, several smaller rockets are stacked on top of each other. The first stage fires and lifts the whole vehicle; once its fuel is spent it is jettisoned, instantly making the rocket lighter. The second stage then fires without having to drag along the heavy, empty first stage, making it far more efficient — and a third stage, if present, pushes the payload the rest of the way into orbit or beyond. It's the same logic as throwing away an empty water bottle before running a race. SpaceX's Falcon 9 takes this further, landing its first stage back on a drone ship so it can be reused, dramatically cutting launch costs.

The de Laval nozzle

The nozzle shape itself matters enormously. A de Laval nozzle narrows to a throat and then flares back out, accelerating hot combustion gas from subsonic to supersonic speed as it expands through the diverging section. Chamber pressure, throat area and the nozzle’s area ratio (exit area ÷ throat area) together set how efficiently thermal energy converts into thrust, and how well the exhaust plume matches ambient pressure at a given altitude — a nozzle tuned for sea level is under-expanded in vacuum, and vice versa. Real engines like the Saturn V (35,100 kN of thrust) and SpaceX’s Starship (~74,000 kN) push enormous mass flow through nozzles engineered around exactly this trade-off.

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