A jet engine that needs forward motion to compress air is called a "ram jet" which is only used in a few applications even though it is so simple. Airplanes no

A jet engine that needs forward motion to compress air is called a "ram jet" which is only used in a few applications even though it is so simple. Airplanes normally use either "turbo jets" or "turbo fans". These use the exhaust to spin a turbine at the back of the engine which in turn spins a compressor at the front (and also a larger "fan" in the case of a turbo fan). So the airplane can be completely stopped at the end of the runway and rev up the engine to full power to start the takeoff. A normal plane only needs its engines to have enough thrust that is a fraction of the craft's weight to overcome drag enough to keep it flying at cruise speed. So pointing it down would not lift the plane off the ground. But there have been designs with powerful enough jet engines: https://en.wikipedia.org/wiki/Ryan_X-13_Vertijet https://en.wikipedia.org/wiki/Ryan_X-13_Vertijet An alternative is to rotate just the engine instead of the whole plane, and even more practical is to simply divert the exhaust of a horizontal engine downwards when needed: https://en.wikipedia.org/wiki/Hawker_Siddeley_Harrier https://en.wikipedia.org/wiki/Hawker_Siddeley_Harrier