by Chris Woodford. Last updated: September 8, 2015.
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 NASA on the Commons.
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
Photo: Test firing the Space Shuttle's main engine. Picture courtesy of NASA on the Commons.
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 closer look at a scientific rocket
It's not rocket science, even when it is! Rockets might be super complex, but if you think about
them carefully, you'll find the bits inside are arranged in a very logical way that soon makes sense.
To see what I mean, let's explore one of Robert Hutchings Goddard's very early rocket designs in a bit more detail.
This artwork comes from a patent that Goddard filed in 1914, for a rocket that could rise to high altitudes and take
photos. Remember that this was back in the early 20th century, long before satellites had entered space or astronauts
had plodded over the moon.
Goddard's clever idea here was to put a rocket inside a rocket, which is a bit like the modern idea of a rocket with stages.
You can see the entire rocket in Figure 1 on the right. The main rocket engine is colored red. You light it with a fuse (14),
which burns up and ignites disks of fuel (12). Once all the fuel is burned up and the rocket has reached a fairly high altitude,
the second rocket (blue) mounted on top ignites, separates, and fires off even higher. Because the second rocket weighs much less than the first one, a certain amount of fuel will make it rise very much higher into the sky than if that fuel had to lift both rockets together.
The rocket keeps its stability by spinning round at high speed as it flies along, just like a bullet fired from a gun. Figure 3 shows how this happens. It's a cross-section through the rocket at the point marked 3—3 in Figure 1 (where the blue and red rockets meet). Inserts of fuel (16) burn and send jets of hot gas outward at tangents, making the rocket body rotate. Unlike the main rocket engine, the spinning jets are ignited by an electrical circuit shown as 18, 19, and 20, which enables them to fire simultaneously. In practice, you'd fire up these tangential rockets to make the rocket spin around on its stand (Figure 5), on ball bearings (22) and, once it's spinning, light the main fuse (14) to blast it into the sky.
The business part of the rocket—the part that does our useful work—is the payload section on the top. This is shown in Figure 2 on the left. Goddard's rocket was designed for taking photographs from high altitude, so we have a camera (orange, 36) and a gyroscope and induction motor (purple, top) which keeps it pointing in the same direction while the rocket spins.
So it's nothing like as complicated as it looks!
Artwork: Early design for a high-altitude rocket camera from
US Patent: 1,102,653: Rocket Apparatus by Robert Hutchings Goddard, July 7, 1914, courtesy of US Patent and Trademark Office (with some details removed and colors added for ease of explanation).
A typical space rocket: the Atlas Centaur
Photo: An early Atlas rocket photographed in 1963.
Picture courtesy of NASA on the Commons.
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
- 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.
The Space Shuttle: the rocket that kept coming back!
The development of NASA's reusable
space-plane, the Space Shuttle, launched a whole new age of space exploration. Previous spacecraft
had lasted only for one mission, but the Shuttle, which took off like a
rocket and flew back like a plane, could be reused up to 100 times.
Between its maiden flight in 1981, and its final voyage in 2011,
the Shuttle made 135 missions, successfully launching
and repairing numerous satellites and the
Hubble Space Telescope, and playing a major role in assembling the International
Space Station. Now it's retired, we bid it a very fond farewell. So long Space Shuttle!
Photo: Relaunch of Space Shuttle (mission
STS-26) in 1988. Picture courtesy of NASA on the Commons.
How did the Shuttle work?
The main component of the Shuttle was a spacecraft about two thirds
the size of a 747 airplane ("Jumbo Jet") called the orbiter. This was
launched by two solid rocket boosters (SRBs) fixed to its underside, which burned solid
fuel inside them. In between the SRBs, a giant external tank (ET) fed
around 528,000 gallons (2 million liters) of liquid fuel to the
orbiter's three engines. This vast amount of fuel was needed to
accelerate the Shuttle to a speed of roughly 17,500 mph (28,000 km/h)
to reach an orbit of 190–330 miles (304–528 km) above Earth.
During liftoff, the Shuttle's main engines burned fuel so quickly that
they could drain a family-sized swimming pool in just 25 seconds!
Each orbiter cost roughly $2 billion to build and each Shuttle mission
cost roughly $450 million.
Photo: Cutaway of the Space Shuttle orbiter.
Picture courtesy of NASA Marshall Space Flight Center (NASA-MSFC).
Browse the hi-resolution version of this image.
Life in the Shuttle
Each Space Shuttle mission lasted up to two weeks, so the orbiter needed a
comfortable-but-compact, two-story living area. On the top level were
the pilot's seats and cockpit controls. A ladder led down to a large
sleeping area, galley kitchen, storage lockers, gym—and a vacuum toilet
that worked even in space. The astronauts ate their food with metal
knives and forks held to metal trays with magnets; this stopped them
floating away in the Shuttle's near-zero gravity.
Space Shuttle missions often made headline news around the world.
The maiden voyage on April 12, 1981, confirmed the Shuttle could
successfully return from space. Another notable flight in April 1984
involved astronauts repairing a crippled satellite in the Shuttle's
cargo bay before returning it successfully to space. Two years later,
the orbiter Challenger exploded shortly after takeoff killing
all seven crew members. Flights resumed in late 1988 and the Hubble
Space Telescope was launched in 1990. Disaster struck again in February 2003,
when the Space Shuttle Columbia was destroyed as it returned to Earth.
The final Shuttle voyage launched on July 8, 2011, after which the
four remaining orbiters were retired to science and aviation museums
spread across the United States.
From takeoff to landing
During a typical one-week voyage, the Shuttle launched from its base
at Kennedy Space Center (KSC), Florida, and carried out its mission
several hundred miles above Earth's surface. When the mission was
complete, it returned to Earth's atmosphere and made an unpowered
landing, like a glider, either at KSC or at Edwards Air Force Base in
California. At 15,000 ft (4572 m) long, the Edwards landing strip is roughly
twice the length of a typical airport runway.
The Shuttle's unique feature was its ability to venture into space
and return to Earth intact. The main problem with this, however, was that friction would heat up the orbiter
to nearly 1927°C (3500°F) as it passed into Earth's atmosphere,
so it was coated with about 20,000 heat-resistant
ceramic "tiles" (refractory bricks)
to stop it burning up on reentry. A tough material called reinforced carbon-carbon was used in the tiles
on the wing edges, the nose, and other areas where temperatures could exceed
1260°C (2300°F). Black high-purity silica tiles 1–5 inches
(2.5–12.7 cm) thick were individually cemented to the underside of the
orbiter to protect it from temperatures of 649–1260°C
(1200–2300°F). Less heat reached the upper surface of the craft, so
it was protected either by white tiles or by a blanket made from a
There and back—in seven easy steps!
- At liftoff, three engines on the orbiter burned liquid fuel from
the large brown tank underneath. Two SRBs either side burned solid fuel
and made over 70 per cent of the liftoff thrust. Together, these five
engines produced some 7.3 million pounds of thrust—150 times as much as
a single Jumbo Jet engine! During this part of the flight, the temperature inside the Shuttle's
engine was two thirds as hot as the surface of the Sun.
- Two minutes later and 32 miles (48 km) into the sky, the empty SRBs were detached.
- When the Shuttle reached its orbit, the ET was detached and burned up on its way back to Earth.
- A typical mission lasted 4–7 days.
- Once the mission was over, the orbiter flew backward for the first part of its descent.
- As the orbiter turned, its nose was raised so its heat-resistant undercarriage met Earth's atmosphere first.
- During descent, the orbiter slowed from 28,000 km/h (17,500 mph)
to 350 km/h (220 mph) and a parachute brought it completely to rest
as it touched down to land.
Photo: Above right: Returning to Earth and landing safely was always the hardest part of a Space Shuttle mission. This is Space Shuttle Endeavour, landing in 2002. Photo courtesy of NASA Dryden Flight Research Center.
Inside the Shuttle cargo bay
At 60 ft (18.3 m) long and 15 ft (4.6 m) wide, the orbiter's cargo
bay was big enough to hold a a satellite or a couple of trucks parked
side by side. It contained a variety of sensors and
scientific instruments and a 50 ft (15 m) grabber arm, used for
launching and retrieving satellites. The reflective inside doors of the
cargo bay doubled up as heat shields to protect the cargo from solar
The Shuttle could not launch a satellite directly into geostationary
orbit (a fixed orbit over a certain place on Earth). Instead, it spun
the satellite slowly out of the cargo bay. When the satellite was clear,
its own rocket motors would fire and power it into position.
Photo: Astronaut Bruce McCandless walking above the
cargo bay of the Space Shuttle (mission STS-41B) in 1984.
Picture courtesy of
NASA on the Commons.
On this website
On other websites
- Cosmos Close-up by Giles Sparrow. Quercus, 2011. A visual tour of the universe.
- Space by Carole Stott et al. DK, 2010. What's out there, beyond the sky, in the darkness where we can only imagine? A colorful introduction from Dorling Kindersley.
- Man on the Moon by Andrew Chaikin. Penguin, 2010. An engaging read that charts the historic Apollo missions and the men who made them possible.
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