Hannah Forgi-Stephens, of Port Chalmers School, asks :-
Why do you go into space in a rocket and not in a plane?
Marlyn Jakub, a physicist at Otago University, responded.
To develop an upward "lifting" force, one must push something else downward. On airplanes, this upward force is usually provided by the wing shape that turns the surrounding airflow into a slightly downward direction. Without air there is no lifting force from airplane wings!
Similarly, to develop a forward push through air or space, one must push something else backwards. This forward push is usually called "thrust" and comes from the engines. Engines on propeller-driven airplanes use oxygen in the air to burn fuel, and other air is also pushed backwards by the engine-driven propellers. With turbojet engines, outside air is combined with fuel to cause hot combustion gases to be ejected backwards from the airplane.
Unfortunately, outer space does not have air, so airplane wings produce no lift and airplane engines produce thrust only when operating within the atmosphere of planet Earth, where outside air provides oxygen to burn the fuel in the engines.
Rocket engines differ from jet engines. Rockets carry both fuel and oxygen (or "oxidizer") in tanks attached to the space vehicle. Once the fuel and oxidizer are mixed, the ignition and burning can generate hot gases to be emitted backwards from the rocket engine, thus giving forward thrust for the space vehicle. Outside air is no longer the oxidizing agent for burning the fuel, so the vehicle can travel outside the Earth's atmosphere.