by Chris Woodford. Last updated: July 19, 2014.
Conquering space was probably the greatest technological achievement of the 20th century. Space rockets put men on the Moon, launched satellites that sent back dramatic pictures of Earth, and gave people an understanding of the Universe they could never have gained from telescopes trained on the sky. Since Sputnik, the first satellite, was launched into space in 1957, several thousand successful space missions have now been flown. What are rockets and how do they work? Let's take a closer look!
Photo: An Atlas Centaur rocket launches an Intelsat space satellite in 1991. Picture courtesy of NASA Kennedy Space Center (NASA-KSC).
Who invented space rockets?
Chinese inventors made rockets possible when they invented gunpowder c.700–800CE; the first rockets were actually firework missiles used by the Chinese in 1232CE to defend the city of Kaifeng against a Mongolian invasion. Space rockets owe just as much to US physicist Robert Hutchings Goddard (1882–1945), "the father of modern rocketry," who pioneered many rocket science techniques during the early 20th century. German scientists also played an important role, notably with a rocket-propelled missile called the V-2, which was used to devastating effect in World War II. Intense rivalry between the United States and the Soviet Union saw the Russians putting Sputnik into space in 1957, but American astronauts were the first to land on the Moon in 1969, propelled by a Saturn-V rocket. Today, rockets are still the cheapest way of putting satellites into space. Over half of all commercial satellites are now launched from French Guiana by the European Ariane rocket.
Photo: The father of modern rocketry, Robert Hutchings Goddard, pictured in November 1925 with one of his inventions, the double-acting rocket engine. Goddard first got the idea of traveling into space as a teenager, after climbing a cherry tree in his family's garden: "I imagined how wonderful it would be to make some device which had even the possibility of ascending to Mars..." His many inventions included powering rockets with liquid fuels and constructing rockets with multiple stages—two fundamentally important ideas used in virtually every successful space rocket launched to date. Picture courtesy of Great Images in NASA.
How rockets work
It's a common mistake to think that rockets work by "pushing back against the air"—and it's easy to see that this is a mistake when you remember that there's no air in space to push against. Space is literally that: empty space! How, then, do rockets work?
Like jet airplanes, space rockets work on a principle called action and reaction (another name for Newton's third law of motion). The massive force (action) generated by hot gases firing backward from a rocket's engines produces an equal force (reaction) that pushes the rocket forward through space. Most of the fuel on-board a rocket is used in the first few minutes of the mission to achieve an escape velocity of at least 25,000 mph (7 miles per second or 40,000 km/h)—the speed a rocket must theoretically attain to escape Earth's gravity.
"Escape velocity" suggests a rocket must be going that fast at launch or it won't escape from Earth, but that's a little bit misleading, for several reasons. First, it would be more correct to refer to "escape speed," since the direction of the rocket (which is what the word velocity really implies) isn't all that relevant and will constantly change as the rocket curves up into space. (You can read more about the difference between speed and velocity in our article on motion). Second, escape velocity is really about energy, not velocity or speed. To escape from Earth, a rocket must do work against the force of gravity as it travels over a distance. When we say a rocket has escape velocity, we really mean it has at least enough kinetic energy to escape the pull of Earth's gravity completely. Finally, a rocket doesn't get all its kinetic energy in one big dollop at the start of its voyage: it gets further injections of energy by burning fuel as it goes. Quibbles aside, "escape velocity" is a quick and easy shorthand that helps us understand one basic point: a huge amount of energy is needed to get anything up into space. (You can read a much more detailed explanation in the Wikipedia article on escape velocity.)
Photo: Test firing the Space Shuttle's main engine. Picture courtesy of Great Images in NASA.
How rocket engines work
Like the gunpowder missiles of ancient China, solid-fuel rocket engines are little more than giant fireworks. Although they are very powerful, they cannot be switched off or controlled in any way, so they are typically used only during liftoff. The solid-rocket boosters (SRBs) used on spacecraft like the Space Shuttle are examples of rockets that work this way.
Unlike airplane jet engines, which take in air as they fly through the sky, space rockets have to carry their own oxygen supplies (oxidizers) with them because there is no air in space. Liquid-fuel engines pump liquid hydrogen (the fuel) and liquid oxygen (the oxidizer) into a combustion chamber at the bottom of the rocket, burn the mixture (which is called the propellant, because it propels the rocket), and allow the hot exhaust to escape through a jet nozzle to produce thrust. The oxygen and hydrogen burn at a very high temperature, which makes the engine more efficient and powerful. However, before combustion, both substances are stored at extremely low temperatures to keep them liquid. This ensures more fuel can be stored than if gases were used. The low temperature also cools the nozzle to protect it from the heat generated during liftoff. Unlike solid-fuel engines, liquid-fuel engines can be switched on and off during flight using valves.
Artwork: How a space rocket works: Liquid hydrogen (the fuel) from one tank is mixed with liquid oxygen (the oxidizer) from a separate tank using pumps and valves to control the flow. The oxidizer and fuel mix and burn in the combustion chamber, making a hot blast of exhaust gas that propels the rocket. The payload (the cargo—such as a satellite) occupies a relatively small proportion of the rocket's total volume in the nose-cone at the top.
A typical rocket: the Atlas Centaur
Photo: An early Atlas rocket photographed in 1963. Picture courtesy of Great Images in NASA.
One of the most successful space rockets ever developed is the Atlas, produced by the Lockheed Martin company. Atlas rockets have launched over 100 unmanned space missions, including voyages to the Moon, the Pioneer missions that flew past Jupiter and Venus, and the Voyager space probe that landed on Mars. NASA's first Atlas rocket took off from Cape Canaveral, Florida, on June 11, 1957. The latest version, Atlas V, has been used from late 2001 as a launch vehicle for government and commercial satellites and is expected to remain in use until at least 2020.
One version of Atlas, known as the Atlas Centaur rocket, illustrates the basic idea of how a rocket works very well. It's called the Atlas Centaur because the lower stage (a section of the rocket used for part of the flight) called Atlas is joined to an upper stage called Centaur. The rocket's payload (cargo), typically a spacecraft or satellite, rides on top of the Centaur stage and is protected from heat and vibration by a nose cone called the payload fairing.
How Atlas launches a satellite
The Atlas and Centaur stages power the rocket through different points of its mission. The massive Atlas stage helps the rocket escape Earth's gravity and pushes it into orbit. Later, the smaller Centaur stage puts the payload satellite into orbit before separating and returning to Earth.
- Liftoff: The Atlas stage powers the rocket with a two-chamber booster engine (operational during liftoff only), a sustainer engine (operational from liftoff until all fuel is exhausted), and four solid rocket boosters (SRBs). The Atlas stage contains 343,000 lbs (156,000 kg) of liquid fuel.
- SRBs jettisoned: The solid rocket boosters are used to increase thrust during the first two minutes of the flight and are jettisoned when their fuel supply is exhausted.
- Booster engine jettisoned: The booster engine cuts off and is jettisoned by releasing 10 pneumatic (air-operated) latches.
- Payload fairing jettisoned: spring-operated thrusters jettison the protective payload faring once the rocket has cleared Earth's atmosphere.
- Atlas and Centaur separate: As the rocket near its orbit, the Atlas and Centaur stages separate and the Atlas stage is jettisoned.
- Centaur moves into orbit: Centaur's twin engines give it the precise altitude and velocity it needs to launch the satellite.
- Satellite is launched: Centaur separates from the satellite. The satellite continues in orbit, while Centaur positions itself for a return to Earth.