25-08-2017, 09:32 PM
Rocket Science
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Introduction
Non est ad astra mollis e terris via
(There is no easy way from the Earth to the stars)
Seneca, circa AD 50
On October 4, 1957, Sputnik 1 became the first artificial satellite.
It was launched into orbit by the former Soviet Union. The media
coverage following the Soviet’s success meant that the general public
quickly became aware that rocket science was a scientific endeavour
and no longer in the realms of science fiction. Rocket science
has always been perceived as very challenging and the difficulties
the Americans faced with their early launch failures reinforced this
idea. Wernher von Braun, a major contributor to the development of
rocket technology, both in Germany and later in the USA, said:
It takes sixty-five thousand errors before you are qualified to make
a rocket.
After the success of Sputnik 1, the launch and operation of satellites
became very politically sensitive and so the brightest scientists
and engineers were often employed as rocket scientists. It therefore
became thought of as a subject only for the most intelligent.
There are other fields of study that are arguably more challenging
than rocket science, but, other than brain surgery, none have
entered the mainstream vocabulary as a difficult thing to do.
Gravity
Gravity causes all objects to attract each other. On the Earth the
effect of gravity can be seen when an item, such as an apple, falls
to the ground. The Earth and the apple are attracted to each other
with the same-sized force. However, as the apple is much lighter
than the Earth it accelerates much faster, until they crash into each
other. The apple usually comes off worse.
Gravity acts along a line between the two centres of the items
involved, in the above example this is between the centre of
the apple and the centre of the Earth. Therefore, gravity will ensure
that, on the Earth, objects are always attracted towards the centre
of the Earth or, as seen from the surface, downwards. The Earth
will always be attracted towards the centre of the object, but this
is not noticeable to us.
Propulsion
On the Earth, forward motion is usually achieved by pushing on
some medium, such as the ground for a car and the sea for a motorboat.
We walk forwards by pushing back against the floor with our
feet. This is why it is difficult to walk on ice. Although most
propulsion
systems do push on something, the act of throwing something
out in the opposite direction can also produce forward motion.
This can be seen if a child carries a heavy ball while standing on
a skateboard. If the child throws the ball away, both the child and
the skateboard will move in opposite directions. A jet aeroplane
works in a similar way, it takes in air, squeezes it in a compressor,
mixes it with fuel, and the gases from the resulting explosion are
thrown out behind at a faster speed. These methods of propulsion
use an action that causes a reaction, which was described in 1687
by Sir Isaac Newton in his third law of motion “For every action
there is an equal and opposite reaction”. Although Newton’s third
law of motion about action and reaction sounds simple, it is often
misunderstood and many people assume that for something to be
propelled, it must push against something.
General Information
Rocket science has developed very slowly. The Chinese probably
invented rockets about 1,400 years ago. In comparison, Sir Frank
Whittle’s thesis on jet propulsion was written about 80 years ago,
and similarly the study of atomic energy dates from Henri Becquerel’s
work just over 100 years ago. Rocket science has a large scope
that includes many technical and scientific subjects such as engineering,
mathematics, astronomy, physics, chemistry, biology, geography,
meteorology, medicine and also law. Politics is also involved,
as the funds used on such expensive projects need to be justified.
Each subject can be further subdivided, and the amount of in-depth
knowledge required to design, build and launch even a simple Earthorbiting
satellite is probably beyond any one person’s capabilities or
budget. The early pioneers, such as the head Soviet rocket engineer
and designer Sergei P. Korolev and the German, and later US citizen,
Wernher von Braun, had teams of experts helping them.
Attitude Control and Movement
The attitude, or direction the spacecraft is pointing, must be controlled
and kept stable once in space. This is so that the communications
antenna can be accurately pointed towards the Earth and
any onboard instruments can obtain data from known directions.
Also, propulsive manoeuvres used to change the orbit must be
conducted when the satellite is orientated in the correct direction,
otherwise the new orbit will be incorrect. Some instruments or
equipment, such as solar panels and optical equipment, need to
be able to move independently in relation to the main body of the
satellite and so accurate attitude determination is essential. The
method of calculating the attitude is explained in detail in Chapter
6 – “Navigation in Three Dimensions”.
While a rocket is still within the Earth’s atmosphere, fins
can be used to control the direction of flight. However, once outside
the atmosphere, fins will no longer work and to influence the
flight path or the attitude of the spacecraft a change in the direction
of the thrust is required. This change is achieved by angling
the exhaust nozzles. Once in freefall, the rocket’s attitude has no
influence on the path.