Rockets need to achieve speeds greater than the escape velocity of 11.81 km/s (or about 7.35 miles per second) in order to overcome the gravitational pull of Earth and escape its gravitational field. Here's why:
Overcoming gravitational pull: Gravity is a force that attracts objects towards the center of mass of a celestial body, such as the Earth. In order to break free from Earth's gravitational pull, a rocket needs to achieve a speed that can counteract this force. The escape velocity represents the minimum speed required to escape from the gravitational field without any further propulsion.
Counteracting atmospheric drag: As a rocket ascends through Earth's atmosphere, it encounters air resistance or drag. The faster the rocket travels, the more drag it experiences. By achieving speeds greater than escape velocity, the rocket can overcome the drag and continue to accelerate upwards.
Achieving desired trajectory: Rockets typically have specific trajectories they need to follow to reach their intended destinations, such as Earth orbit or interplanetary missions. By attaining speeds greater than escape velocity, rockets can attain the necessary velocity to enter and maintain their desired orbits or trajectories.
It's important to note that not all rockets or spacecraft need to reach escape velocity. Some missions, like those in low Earth orbit or to other nearby celestial bodies, require lower velocities that can be achieved with less powerful rockets or by using gravitational assists from other bodies. Escape velocity is specifically relevant for leaving Earth's gravitational field entirely.