Where does the momentum exchange happen in a rocket?

Question

Where does the momentum exchange happen in a rocket. Is it in the combustion chamber alone?

Answer 1

Consider the rocket and its fuel as single system .Now ,initially this system has 0 momentum When the fuel is burnt ,the rocket's nozzle is setup in this way so that the pressure of gas formed due to burning fuel is released in downward direction.But as we know initially it has 0 momentum ,so according to the law of conservation of momentum if gasses produced are moving downward the rocket should get the same momentum but in upward direction.That's why rocket moves in upward direction in space also.From here you can conclude that if gasses are pushed downward than rocket should be pushed upward so the net force on system remain 0 or the third law is followed(action-gasses pushed downward reaction-rocked pushed upwards.)

Answer 2

How does a rocket push on its exhaust and how does the exhaust push back on the rocket?

Actually, it is not the rocket that starts to push. It is the fuel. Simply outlined:

When the fuel in the fuel chamber ignites and burns, it expands due to the creation of reaction product gasses that fill more volume than the initial fuel. This expansion will push on everything around it - the pressure increases enormously. It pushes on the walls. If there is an opening - an exhaust valve or nozzle - the gas will escape through there by pushing on the opposite wall. Thereby the rocket gets a forward push. A thrust.

Also, why is the upward motion of a rocket linked to Newtons third law?

The force applied on the rocket is also felt by the fuel. This is Newton's 3rd. Fuel moves one way (out through the exhaust nozzle as combustion gasses) and the rocket moves the opposite way, since they both feel the same both opposite force.

Answer 3

When combustion of the fuel within the fuel tank occurs, exhaust gas is produced, and the particles of the exhaust gas move very quickly and collide frequently with the walls, leading to high pressure. Thus, when they exit the open end of the engine, they are at high velocities. Hence, due to the high pressure, an immense amount of force is exerted by the rocket, backwards.

Newton's third law states that for every action force, there will be a resultant reaction force acting in the opposite direction. Hence, the rocket would be propelled upwards in the opposite direction as the force exerted by the rocket.

Conservation of momentum (COM) can also be used to explain this phenomenon, but ultimately it boils down to Newton's third law because theory of COM is derived from Newton's third law.