QUESTIONS CORNER
Rocket propulsion
How do rocket engines propel rockets through the vacuum in space?
P.K. VASUDEVAN
Kochi, Kerala
Rocket propulsion takes place due to a fundamental principle of physics which states that to every action of force there exists an equal and opposite reaction force. The reaction forces are helpful in almost all modes of movement. When a fish swings its fins backwards in water, the reaction force pushes the fish forward; when we push down our feet backwards on the ground, we are able to move forward while walking or running.
When a rocket is fired, a suitable combination of fuel is made to burn in order to produce gaseous combustion products at extremely high temperature and pressure in its “tail chamber” fitted with an exhaust jet. For example, in the modern rockets a mixture of liquefied hydrogen and oxygen is used as a fuel, the burning of which produces water in gaseous form at very high temperature and pressure. This gas escapes through the jet with enormous force which causes the reaction force to act in the opposite direction on the rocket body. Thus the rocket gets propelled up.
The atmospheric air is not at all required for the propulsion process; in fact, it adds to the damping of the motion due to frictional drag while the rocket is in the atmosphere. The drag force, in general, retards the motion of a moving body; more the speed of the body, more is the drag due to the surrounding atmosphere.
The jet planes are also propelled by the hot exhaust gases pushed out through the jets causing a large reaction thrust on the body of the plane. Of course, the jet planes move in the air filled atmosphere, while the rockets cross past the atmosphere and enter the space where it moves in an environment of vacuum.
The gas produced by the rocket immediately diffuses away in the extremely low pressure zone, restoring the condition of “vacuum”. Anyway the rocket moves into undisturbed regions of space. In vacuum, there exists no air to drag the rocket back and so the entire reaction force is used for propelling the rocket alone.
H. K. SAHU
Scientific Officer
Indira Gandhi Centre for Atomic Research
Kalpakkam, Tamil Nadu
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