Men have always wanted to fly like birds. Birds can fly easily because they are light,but men's bodies are heavier. Men first went up into the air in balloons. These were big bags,and they were filled with gas. Hydrogen   is a useful gas for balloons. It is lighter than air. _ is also lighter than air,but it costs a lot of money. So balloons are usually filled with hydrogen. Balloons have to fly with the wind as they have no engines   to drive them. Later,men made airships. They are balloons with engines. They are also filled with hydrogen. Aircrafts   with wings now take people across the world. Powerful engines drive these machines across the sky. Some of the engines are like the engines of cars,but they are more powerful. There is another kind of engine,we call it the jet   engine. An English engineer invented the jet engine. At the same time,Germans were also building a jet engine. In May 1941,the English engineer's new engine was fixed in an aircraft,and the aircraft flew quite well. Jet engines are very powerful. Usually two to four are enough for an airplane,but some big aircraft need six. Anyone in a moving jet plane can feel the power of the engines. Jet planes can travel faster than sound. As a flying jet plane leaves its noise behind,we do not hear it until it has gone. We don't hear the flying jet plane's noise until it has gone because  _  .
jet planes are very powerful jet planes have enough jet engines anyone can feel the power of the engines jet planes travel faster than sound