A rocket works because of two of the laws that you will probably
already have studied:
(a) action and reaction are equal and opposite;
(b) momentum is
conserved.
The simplest way of seeing this is to blow up a balloon and then let it go.
The balloon rushes all over the
place as the air shoots out of the open end. It is behaving like a rocket - the ACTION of
pushing out the air produces a REACTION that pushes the balloon forwards. (See Figure
1)
The momentum of the air is equal and opposite to the momentum of the
balloon.
In an actual rocket it is the exhaust gases that rush out of the back. The
velocity of these exhaust gases depends on the fuel used. (Figure 2)
For example, if hydrogen is used as the fuel and
pure oxygen as the oxidant, the velocity of the gases is 6000 m/s.
The
rocket does not have to push against the launching pad, the reaction on the combustion
chamber pushes it forwards. It is partly for this reason that rockets are able to operate in
space. Rockets work better where there is no air to get in the way and this makes it unlike
the jet engine. The rocket carries its own oxygen with it and so is an ideal power unit for
space travel.
There are some
strange ideas for new types of rocket. One shoots out a stream of high speed ions while one
of the strangest uses small nuclear bombs going off behind it to push the rocket
forwards!
Most rockets in use today are liquid fuel ones and we will look at this type
more closely.
Figure 3 shows the basic design of this type of rocket. The fuel and
oxidant (a chemical rich in oxygen) are pumped into the combustion chamber and lit. This
produces a mass of hot gases which shoot out of the nozzle of the rocket so pushing the
rocket forwards. The oxidant is carried as a very cold liquid and can only be put into the
rocket just before take-off.
The largest rockets ever built
were the Saturn V series used in the USA Apollo Moon flights. Each rocket stood over 100 m
tall and had a take-off mass of a staggering 3 000 000 kg. (See Figure 4)
Each flight and its development cost around £100
million and the first stage engines used as much fuel in a SECOND as a normal car would
use in THREE YEARS!
Two simple rocket experiments can be done in the lab. One
uses a rocket powered by compressed air that shoots out a jet of water behind the rocket,
and the other uses a carbon dioxide capsule of the kind used in a soda siphon (Figure
5).
