Cadet Competition Written Practice Exam

What gas is typically expelled from rockets to produce thrust?

1. Helium

2. Carbon dioxide

3. Hydrogen

4. Water vapor

The correct answer is: Hydrogen

The gas that is typically expelled from rockets to produce thrust is hydrogen. In many rocket propulsion systems, particularly those utilizing liquid rocket fuels, hydrogen plays a critical role. When liquid hydrogen is burned in the presence of an oxidizer, such as liquid oxygen, it undergoes a chemical reaction that produces water vapor and a significant amount of thermal energy. This rapid expansion of gases is what generates thrust, propelling the rocket upward. Moreover, liquid hydrogen is favored in many advanced rocket engines due to its high specific impulse, meaning it provides more thrust per unit of fuel compared to other options. This efficiency is essential for reaching and maintaining the velocity required to escape Earth’s gravitational pull. While other gases may be present in some propulsion systems, hydrogen is the key component in many of the most powerful and efficient rocket engines.