Aerospace Ed – Takeoff

Aerospace Ed here with an answer to the question, “Why don’t airplanes take off straight up like a rocket?”

An airplane does not have engines that are powerful enough to take off straight up. The fuel stores potential energy that is converted to kinetic energy by the engines. The airplane accumulates this kinetic energy as it accelerates on the runway. When the wings are going through the air fast enough to fly, the airplane takes off. If the engines are not strong enough to accelerate to flying speed, the plane will not lift off the runway. If you think this is impossible, check with the pilot who did not realize he had his feet on the brakes!

A float plane takes off by riding on its floats, accelerating to its maximum floating speed. The floats have a lot of drag in the water. Then the pilot momentarily pulls back on the elevator control to get the floats up on the “step”, so they are skiing on the water and able to accelerate to flying speed.

A helicopter can take off straight up because it accelerates its large rotor instead of accelerating on a runway. When the rotor is going fast enough, the helicopter can lift off.

An airplane accelerates as long as its thrust is greater than its drag. An airplane climbs as long as its lift is greater than its weight. Changing the attitude of the nose of the airplane trades off speed and lift. Because a spacecraft does not have drag in the vacuum of space, it can accelerate as long has it has fuel to burn.

A rocket has engines that produce thrust that is greater than its weight. What if the engines were not producing enough thrust? It could sit on the launch pad not accelerating like an airplane on a runway. It would be wasting energy. There is an exception to this scenario. What if as the fuel is used the rocket becomes lighter than the rocket thrust? It could eventually lift off.

There is an old joke similar to this. The B-36 was a huge airplane, with six pusher props and Convair added four jet engines to give it enough power. This aircraft was on the boundary between propeller planes and the jet age. The joke was that the B-36 never really climbed, it just took advantage of the curvature of the Earth.