Space rockets
Last updated: January 16, 2010.
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 early Atlas rocket photographed in 1963.
Picture courtesy of Great
Images in NASA.
History of rockets
Photo: The father of modern rocketry, Robert Hutchings Goddard.
Picture courtesy of Great
Images in NASA.
The first rockets were firework missiles used by the Chinese in 1232
C.E. 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.
How rockets work
Like jet airplanes, space rockets work on a principle called "action
and reaction." 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 mean it has at least enough kinetic energy to do that.
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.)
Blast off
One of the most successful space rockets ever developed is the Atlas
Centaur produced by the Lockheed Martin company. Atlas rockets have
launched around 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, will be used from late 2001 as a launch vehicle for government
and commercial satellites.
Atlas rocket
In an Atlas Centaur rocket, a 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.
Launching 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.
Rocket engines
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.
Unlike airplane jet engines, which take in air as they fly through the sky,
space rockets have to carry their own oxygen supplies with them because there is no air in space.
Liquid-fuel engines pump liquid hydrogen fuel and liquid oxygen gas
into a combustion chamber, burn the mixture, 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.
Photo: Test firing the Space Shuttle's main engine. Picture courtesy of Great
Images in NASA.
