How Rockets Work, and Why Reusable Rockets Matter

How rockets work: action and reaction

Stand on a skateboard and throw a heavy ball forwards, and you roll backwards. That everyday picture holds the whole secret of a rocket: every action has an equal and opposite reaction. A rocket throws a huge amount of hot gas out of its engine nozzle in one direction, so the rocket is pushed the other way.

A common misunderstanding is that a rocket pushes against the air or the ground. It needs neither, because the push comes from the gas it throws out. That is why rockets work perfectly well in the vacuum of space, where there is no air at all.

Thrust, fuel and why rockets are mostly propellant

Thrust is the push an engine produces. Most rockets make it by burning a fuel with an oxidiser, a substance that supplies the oxygen needed for burning. A jet engine takes its oxygen from the surrounding air, but there is no air to breathe in space, so a rocket carries its own. Together, the fuel and oxidiser are called propellant.

Popular pairings include liquid hydrogen, refined kerosene or liquid methane, each burned with liquid oxygen. Some rockets use solid propellant instead, packed into a strong casing.

Here is the surprising part. A rocket must carry the propellant needed to lift the rest of its propellant, so most of its weight on the launch pad is propellant. The structure, the engines and the payload, meaning the satellite or crew capsule at the top, make up only a small share, which is why engineers work so hard to make every part lighter and every engine more efficient.

Why rockets have stages

Imagine a long hike with a large water bottle that you are never allowed to put down, even once it is empty. A single rocket body would face the same problem: long after its tanks were nearly empty, it would still be hauling all that heavy structure. The answer is staging.

A staged rocket is really two or more rockets stacked on top of each other. The first stage, often called the booster, does the heavy lifting through the thickest part of the atmosphere. When its propellant is used up, it separates and the next stage lights its own engine, now pushing a much lighter vehicle.

Each stage can be tuned for its job. First-stage engines work best in thick air near the ground, while upper-stage engines often have larger nozzles suited to the near vacuum high above. Shedding weight in steps is what lets the final stage reach orbital speed.

Reaching orbital speed: it is about going sideways fast

Many people picture a rocket going straight up. Going up is the easier part, since a commonly used edge of space sits about 100 km above the ground. The real challenge is going sideways fast enough to stay up, which is why a rocket tips over gently soon after lift-off and spends most of its climb speeding up along the horizon.

An orbit is really a never-ending fall. Throw a ball and it curves back to the ground; throw it faster and it lands further away. Near Earth, at about 7.8 km/s, the ground curves away beneath a falling object as fast as the object drops, so it keeps falling around the planet instead of into it.

The International Space Station, about 400 km up, travels at roughly 28,000 km/h and circles Earth in about 92 minutes, around 16 times a day. To break free of Earth's pull altogether and head for the Moon, Mars and beyond, the speed needed from the surface is about 11.2 km/s. Try different heights in the orbit calculator to see how speed and orbit time change, and read satellites and orbits explained to see what each kind of orbit is used for.

Coming home: re-entry and heat shields

Coming back to Earth means losing all that speed. Rather than carrying even more propellant to slow down, most spacecraft use the atmosphere as a brake. As a capsule dips into the air at orbital speed, the air in front of it is squeezed so hard that it becomes extremely hot, and that glowing layer does much of the slowing.

A heat shield protects the spacecraft during this phase. Some shields slowly char and wear away, carrying heat with them, while others use tiles that insulate the craft. A broad, blunt shape also helps by pushing a cushion of hot air ahead of the craft.

Once the craft has slowed, parachutes or engines take over for a gentle finish. Some capsules float down under parachutes onto land or the sea, and some rocket stages now use their own engines to slow down and touch down upright, which leads to one of the most exciting changes in spaceflight.

Reusable boosters: rockets that land and fly again

For most of the history of spaceflight, from the first artificial satellite in 1957 onwards, rockets flew once. Each booster finished its job and was not used again, a little like building a new aircraft for every flight.

Reusable boosters change that picture. After separating from the upper stage, a reusable first stage turns around, relights its engines to steer and slow down, and heads back through the atmosphere guided by small fins. Its engines fire once more near the ground so that it settles gently onto landing legs, upright on a pad or a floating platform at sea, ready to be serviced and flown again.

SpaceX pioneered landing orbital-class boosters upright and flying them again, and has helped make reuse a familiar part of reaching orbit. Its work has encouraged engineers around the world to design rockets, engines, upper stages and capsules with reuse in mind, so that preparing for the next flight becomes more like servicing an aircraft than building a new one.

Why reuse changes what is possible

When the most expensive parts of a rocket can fly again, each trip to orbit can cost less and flights can happen more often. Lower-cost, more frequent access to orbit opens room for ideas that once seemed out of reach.

Reuse also changes how founders work: when a ride to orbit is easier to arrange, a team can test an idea in space, learn from it and fly an improved version sooner. The future of investing in space explains how this shift shapes the thinking of long-term investors, and the space economy explorer maps the layers built on top of launch.

• More frequent flights, so satellites and experiments spend less time waiting for a ride.
• Lower-cost access to orbit, so smaller teams, universities and young companies can take part.
• Faster learning, because engineers can study hardware that has flown and come back.
• Room for new ideas, from Earth observation for farming, forests and weather to research in microgravity.

Nicole Junkermann's perspective

Nicole Junkermann, an entrepreneur and venture capital investor who presents the AI Overview podcast, has long argued that patient capital matters for deep technology. Rockets are a vivid example: the core ideas are simple to state, but turning them into hardware that lands and flies again takes years of careful engineering and steady support.

In her view, reusable rockets matter most as infrastructure: dependable, repeatable access to orbit that others can build on with confidence. She tends to ask what becomes possible once a hard problem is solved reliably, and who gets to build next. Read more in her perspective on the future of investing in space, or see how technologies mature with the technology readiness level explorer.