The initial action in a rocket launch is the combustion of fuel within the rocket engine. Here's how it works:

1. Fuel and oxidizer: Inside the rocket engine, a fuel (like liquid hydrogen or kerosene) and an oxidizer (like liquid oxygen) are mixed.

2. Ignition: The fuel and oxidizer are ignited, usually with a spark or a pyrotechnic igniter.

3. Combustion: This ignition starts a rapid chemical reaction, burning the fuel and oxidizer.

4. Hot gas expansion: The combustion creates extremely hot gases that expand rapidly.

5. Thrust: These expanding gases are directed out of the rocket engine through a nozzle, creating a force called thrust. This thrust pushes the rocket upwards, overcoming the force of gravity.

Essentially, the rocket engine transforms chemical energy (from the fuel and oxidizer) into kinetic energy (the motion of the rocket).