How Rockets Work: The Physics of Getting to Space

A rocket launch looks like brute force: fire, smoke and a roar you can feel in your chest.

Underneath the spectacle, though, is a surprisingly simple idea that a curious ten-year-old can grasp,

and a few pieces of physics that explain why getting to space is so hard.

Here you will learn what actually pushes a rocket upward, why reaching orbit is mostly about sideways speed,

how engines turn fuel into motion, and why nearly every rocket sheds parts of itself on the way up.

Action and Reaction: The Core Idea

Everything starts with Isaac Newton's third law of motion: for every action there is an equal and opposite reaction.

When a rocket engine blasts hot gas out of its nozzle,

the gas pushes back on the rocket with the same force, and the rocket moves the other way.

You can feel this yourself. Stand on a skateboard and throw a heavy ball forward, and you roll backward.

Let go of an inflated balloon and it zips across the room as air rushes out.

A rocket does the same thing continuously, throwing enormous amounts of mass out the back every second.

This clears up one of the most common misunderstandings about rockets.

They do not push against the air or the ground.

The push comes entirely from the exhaust leaving the engine, which is why rockets work perfectly well in the vacuum of space.

In fact, engines are slightly more efficient there, because no surrounding air pressure pushes back on the exhaust.

Thrust: Throwing Mass Backward, Fast

The force a rocket engine produces is called thrust.

It depends on two things: how much mass the engine throws out each second, and how fast that mass leaves the nozzle.

Doubling either one roughly doubles the push.

Chemical rocket engines expel their exhaust at roughly 2.5 to 4.5 kilometers per second, several times faster than a rifle bullet.

To lift off, a rocket's thrust must be greater than its weight.

A large launcher burns through tons of propellant every second at liftoff, and most of its mass on the pad,

often around 90 percent, is propellant waiting to be burned.

The Real Challenge: Speed, Not Height

Space is not very far away.

The most widely used boundary, called the Karman line, sits about 100 kilometers (62 miles) up, closer than many cities are to each other.

If height were the only goal, reaching space would be relatively easy, and small research rockets do it routinely before falling back down.

Staying in space is a different matter.

To remain in orbit, a spacecraft has to move sideways so fast that as it falls toward Earth,

the planet's curved surface drops away beneath it at the same rate.

Just above the atmosphere, that takes about 7.8 kilometers per second, or roughly 28,000 kilometers per hour (17,500 miles per hour).

Once you add the speed lost fighting gravity and air resistance on the way up,

a rocket needs to deliver a total change in velocity of roughly 9 to 10 kilometers per second to reach low Earth orbit.

That number, not altitude, is what makes rockets so big.

Inside a Rocket Engine

A rocket must carry both its fuel and the oxygen or other oxidizer needed to burn it,

since there is no air to supply it in space.

The two meet in a combustion chamber, burn at extremely high pressure and temperature, and escape through a bell-shaped nozzle.

The nozzle first narrows, then widens, which speeds the hot gas up to several times the speed of sound and turns heat into directed motion.

Liquid-Fueled Engines

Liquid engines store fuel and oxidizer in separate tanks.

Powerful pumps, often driven by small turbines, force them into the chamber at tremendous rates.

Liquid engines can be throttled, shut down and in many cases restarted, which makes them ideal for precise maneuvers.

Common propellant combinations include:

• Refined kerosene and liquid oxygen: dense and relatively easy to handle, a favorite for first stages that need lots of raw thrust.
• Liquid hydrogen and liquid oxygen: the most efficient common chemical combination, but hydrogen must be kept extremely cold and needs very large tanks.
• Liquid methane and liquid oxygen: a middle ground that burns cleanly, which suits engines designed to be reused.
• Hypergolic propellants: pairs that ignite on contact with each other, prized for spacecraft thrusters that must fire reliably after months in space, though they are toxic to handle.

Solid Rocket Motors

Solid motors hold fuel and oxidizer premixed in a rubbery block.

They are simple, can be stored for long periods and deliver huge thrust,

which is why they are often strapped to the sides of big rockets as boosters.

The drawback is that once lit, a solid motor generally burns until it runs out.

Engineers compare engines using a figure called specific impulse, which is essentially a measure of how much push you get from each kilogram of propellant.

Higher exhaust speed means higher specific impulse and a more efficient engine.

The Rocket Equation and Why Rockets Have Stages

In 1903, Konstantin Tsiolkovsky worked out the relationship now called the rocket equation. In plain words,

the total speed a rocket can gain depends on its exhaust speed and on the ratio between its mass when full and its mass when empty.

The catch is that the relationship is logarithmic, so returns diminish quickly.

To go faster, you need more propellant, but that propellant must itself be carried and accelerated, which requires even more propellant.

A single rocket carrying heavy empty tanks and engines all the way to orbit wastes a large share of its energy.

Staging solves this. A multistage rocket is really several rockets stacked on top of each other.

When the first stage runs dry, it drops away, and a smaller, lighter stage fires to keep accelerating.

Each time dead weight is discarded,

the remaining rocket needs far less propellant to gain the same speed. That is why nearly every orbital rocket has two or more stages.

Steering a Rocket

A rocket does not fly straight up for long.

Shortly after clearing the tower, it begins tilting gradually toward the horizon in a maneuver called a gravity turn,

letting gravity help bend its path so it builds the sideways speed an orbit demands.

To steer, most rockets swivel their engines slightly, a technique called gimballing, which angles the thrust and turns the vehicle.

Some use fins or vanes while still in the atmosphere, and small thrusters handle fine pointing in space.

Onboard computers make these corrections many times per second, balancing a tall, slender vehicle on a column of fire.

Once in space, spacecraft can use far gentler engines.

Ion thrusters, for example, accelerate charged atoms with electric fields to extremely high speeds.

Their push is feeble, roughly the weight of a sheet of paper, far too weak to leave the ground,

but over months of continuous firing they can build up impressive speed while sipping propellant.

Frequently Asked Questions

Why do so many rockets launch toward the east?

Earth spins from west to east, and at the equator its surface moves at about 1,670 kilometers per hour (1,040 miles per hour).

Launching eastward adds that speed for free,

which is one reason many launch sites sit near the coast and as close to the equator as practical.

Why can't a rocket just fly straight up and stop in space?

It can reach space that way, but it cannot stay there.

Without enough sideways speed, gravity simply pulls it back down, just as a ball thrown straight up returns to your hand.

Is it harder to land on another world than to launch from one?

It depends on the world. On Earth, the atmosphere helps slow a returning capsule but also heats it fiercely.

On the airless Moon, every bit of speed must be cancelled with engine power, which is why landers carry their own rockets for the descent.

The Bottom Line

A rocket is a machine for throwing mass backward as fast as possible, and Newton's third law does the rest.

Reaching orbit is hard not because space is high,

but because you must reach roughly 28,000 kilometers per hour while carrying your own fuel and oxidizer.

Efficient engines, careful steering and the clever trick of dropping empty stages are what turn that enormous challenge into a routine sight in the sky.