08. How do rockets work? (A K2S Question)
From The Space Library
A vertically launched rocket must have a thrust greater than its weight or it doesn't get off the ground. Increasing the momentum of the exhaust can increase the rocket thrust. To make the best use of the limited amount of propellant on the vehicle, increasing its temperature and pressure as much as possible increases the exhaust velocity and thus the momentum. The maximum achievable temperature and pressure is determined by the specific energy of the propellant and by the melting point of the engine materials. High-exhaust velocities are achieved in the rocket engine by flowing the hot gas through a throat and then expanding the gas in a nozzle. This is called a converging–diverging nozzle. The temperature determines the velocity of the individual molecules of gas inside the combustion chamber, but the velocities are in random directions. As the gas passes the throat of the converging-diverging nozzle the velocities become almost parallel. Then the expanding gas inside the nozzle pushes the expanding gas in front of it to even faster velocities, increasing the exhaust momentum. The reactive force of the expanding gas pressing against the nozzle surface provides the vehicle thrust. A properly designed nozzle can convert more than 95 percent of the thermal energy of the gas in the combustion chamber into kinetic energy (energy of motion) of the same gas in the exhaust.
Answer provided by John W. Cole
Question and Answer extracted from the book Kids to Space - by Lonnie Schorer
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