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How Rockets Launch

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1. The law of action and reaction

The basic principle behind rocket propulsion is Newton's third law, the law of action and reaction. When combustion gas is strongly expelled downward, the reaction force pushes the rocket upward.

2. The relationship between thrust and weight

For a rocket to lift off the ground, the thrust generated by its engines must exceed the rocket's total weight, and solving the problem of the rocket becoming heavier as more fuel is loaded is a core challenge in rocket design.

3. Why multi-stage rockets (with stage separation) are used

Leaving a lower stage attached after it has used up its fuel becomes unnecessary dead weight, so it is separated at each stage, allowing the remaining fuel to be used more efficiently as the rocket becomes lighter. South Korea's Nuri rocket is also a multi-stage rocket, consisting of first, second, and third stages.

4. Escape velocity

The minimum speed needed to completely break free of Earth's gravitational pull, about 11.2 km/s measured from the surface.

5. Orbital velocity

To remain in low Earth orbit without falling, a horizontal speed of about 7.9 km/s is needed β€” this is the point at which the speed of falling toward Earth balances with the speed of moving away along Earth's curvature.

6. Liquid-fuel rockets

Fuel and oxidizer are stored in liquid form and their flow is controlled by valves, allowing thrust adjustment and engine re-ignition, so this type is widely used in rockets requiring precise control, such as the Falcon 9 and Nuri.

7. Solid-fuel rockets

Fuel and oxidizer are pre-mixed and solidified in advance; this method has a simple structure and is favorable for long-term storage, but once ignited, thrust is difficult to adjust or stop, so it's mainly used as an auxiliary booster.

8. Fairing separation

Once a rocket clears the atmosphere, the fairing β€” the covering that protected the satellite from air resistance during the early part of the flight β€” is jettisoned, reducing weight for the remainder of the flight.

The secret behind the force that sends massive rockets soaring skyward

A rocket flies into space using nothing more than the fundamental laws of physics, without any special magic. The cumulative reaction force generated by rapidly expelling fuel pushes the rocket up and beyond gravity.

Also explore the Falcon 9, launched using this principle

Even based on the same principle, rocket design and reusability approaches differ significantly between rockets. See our overview of SpaceX and its major achievements to explore what the Falcon 9's reuse technology means.

Frequently Asked Questions

Why are most rockets divided into multiple stages?

Because leaving a lower stage attached after it has used up its fuel becomes unnecessary dead weight that reduces efficiency, so it is separated at each stage, allowing the remaining flight segment to be accelerated more efficiently as the rocket becomes lighter.

What type of fuel does the Nuri rocket use?

Nuri is a liquid-fuel rocket that uses kerosene (refined kerosene) and liquid oxygen, designed with a configuration that clusters multiple engines with adjustable thrust.