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3 A Brief History of Space Flight
The key of space flight is to make the spacecraft with enough flight speed to
overcome and get rid of the constraints of the gravity of the Earth or Sun and fall
back to the Earth’s surface no longer. It is so-called cosmic velocity, a minimum
initial velocity which the spacecrafts launched from the Earth’s surface orbit the
Earth, or escape away from the Earth to fly to interplanetary space, out of the Solar
System or into interstellar space. More generally, the space flights require three
characteristic velocities, the first cosmic velocity, the second cosmic velocity and the
third cosmic velocity, which are respectively 7.9 km/s, 11.2 km/s and 16.7 km/s [2].
At such high speed, what types of engines are used as power sources? The rocket
engines are the ideal power sources for space flights. According to different fuels
and exhaust materials utilized by them, the rocket engines are generally divided into
four types: chemical rocket engines, electric rocket engines, nuclear rocket engines
and photon rocket engines. The chemical rocket engines are the most mature and
widely used engines at present; the electric rocket engines are mainly applied to orbit
keeping for some spacecrafts, the nuclear rocket engines are in the stage of prototype
development; the photon rocket engines are still in the stage of conceptual research.
Although the rockets using the chemical propellants have reached the third cosmic
velocity, the thrusts of the chemical rocket engines are still very small for the explorations of the planets beyond Jupiter in the solar system and the interstellar flight
missions. The planetary gravity assist is a way to change the orbit and velocity
of spacecraft flight by optimizing the launch windows of rockets and the relative
motions and gravitations of the planets or other celestial bodies in the solar system.
When a spacecraft firstly flies near a planet from a long distance, its motion effect
is like that of the spacecraft bounced back by the planet, which is similar to an
elastic collision, but both have not yet come into contact. And thus, the planetary
gravity assist is also called gravitational slingshot effect. Actually, the spacecraft
flies near the planet in hyperbolic orbit, and it can leave the planet in the opposite
direction without starting the orbital engine. Moreover, as long as the spacecraft
escapes from the planetary gravitation, it can achieve a velocity increment equal to
twice the velocity of the planet.
3.2 Ideas and Technologies of Ancient Space Flights
Traveling in space and exploring the universe has always been the oldest, simplest and
most wonderful dream of mankind. For thousands of years, the glorious human civilization has spread many wonderful myths and moving legends about space flights.
However, due to the limitation of scientific and technological development, the exploration of space flight before the twentieth century was only addicted to blind adventure and endless fantasy. The ideologies and technical developments for ancient space
flights can largely be divided into three stages: the first stage is from the thirtieth
century B.C. to the sixth century A.D., the myths and legends about space flight
as well as its idea sprouting; the second stage is from the seventh century to the
sixteenth century, the invention of gunpowder and the development and application
3 A Brief History of Space Flight
The key of space flight is to make the spacecraft with enough flight speed to
overcome and get rid of the constraints of the gravity of the Earth or Sun and fall
back to the Earth’s surface no longer. It is so-called cosmic velocity, a minimum
initial velocity which the spacecrafts launched from the Earth’s surface orbit the
Earth, or escape away from the Earth to fly to interplanetary space, out of the Solar
System or into interstellar space. More generally, the space flights require three
characteristic velocities, the first cosmic velocity, the second cosmic velocity and the
third cosmic velocity, which are respectively 7.9 km/s, 11.2 km/s and 16.7 km/s [2].
At such high speed, what types of engines are used as power sources? The rocket
engines are the ideal power sources for space flights. According to different fuels
and exhaust materials utilized by them, the rocket engines are generally divided into
four types: chemical rocket engines, electric rocket engines, nuclear rocket engines
and photon rocket engines. The chemical rocket engines are the most mature and
widely used engines at present; the electric rocket engines are mainly applied to orbit
keeping for some spacecrafts, the nuclear rocket engines are in the stage of prototype
development; the photon rocket engines are still in the stage of conceptual research.
Although the rockets using the chemical propellants have reached the third cosmic
velocity, the thrusts of the chemical rocket engines are still very small for the explorations of the planets beyond Jupiter in the solar system and the interstellar flight
missions. The planetary gravity assist is a way to change the orbit and velocity
of spacecraft flight by optimizing the launch windows of rockets and the relative
motions and gravitations of the planets or other celestial bodies in the solar system.
When a spacecraft firstly flies near a planet from a long distance, its motion effect
is like that of the spacecraft bounced back by the planet, which is similar to an
elastic collision, but both have not yet come into contact. And thus, the planetary
gravity assist is also called gravitational slingshot effect. Actually, the spacecraft
flies near the planet in hyperbolic orbit, and it can leave the planet in the opposite
direction without starting the orbital engine. Moreover, as long as the spacecraft
escapes from the planetary gravitation, it can achieve a velocity increment equal to
twice the velocity of the planet.
3.2 Ideas and Technologies of Ancient Space Flights
Traveling in space and exploring the universe has always been the oldest, simplest and
most wonderful dream of mankind. For thousands of years, the glorious human civilization has spread many wonderful myths and moving legends about space flights.
However, due to the limitation of scientific and technological development, the exploration of space flight before the twentieth century was only addicted to blind adventure and endless fantasy. The ideologies and technical developments for ancient space
flights can largely be divided into three stages: the first stage is from the thirtieth
century B.C. to the sixth century A.D., the myths and legends about space flight
as well as its idea sprouting; the second stage is from the seventh century to the
sixteenth century, the invention of gunpowder and the development and application