Which one of the following is not used as fuel in rocket propellants? (a) Liquid oxygen (b) Liquid hydrogen (c) Liquid hydrazine (d) Kerosene oil

Rocket fuels, also known as propellants, are crucial for the thrust that propels rockets into space. These consist of a fuel and an oxidizer which together undergo a chemical reaction to produce large amounts of energy. Examples include liquid hydrogen and kerosene oil.

Liquid hydrogen is a popular choice for its high specific impulse—a measure of the efficiency of rocket propellants. Kerosene, on the other hand, is valued for its storability and energy density. Each fuel type has specific characteristics that influence the rocket's performance and suitability for different missions.

For students grappling with these concepts, it's essential to understand that fuels are substances that can release energy through chemical or nuclear reactions. In the context of space exploration, chemical energy is paramount, and selecting the appropriate fuel significantly affects the spacecraft's capability to reach its target destination effectively.

In the context of rocketry, oxidizers are substances that provide the chemical oxygen needed for the fuel to burn in the combustion chamber. Since space lacks an atmosphere, rockets must carry their own oxidizers. Liquid oxygen (LOX) is one of the most commonly used oxidizers because it is highly reactive with many fuels like hydrogen and kerosene.

The role of an oxidizer is as important as the fuel because the energy released during a rocket's flight is the result of the combustion between the fuel and the oxidizer. Insightfully, students should note that while there are substances like LOX and nitrogen tetroxide that are adopted purely as oxidizers, all fuels require an oxidizing agent to release energy efficiently.

Understanding the purpose of oxidizers transcends beyond just knowing their names; it includes being aware of how they contribute to the overall balance and efficiency of the propulsion system.

The heart of rocket propulsion lies in the chemical reactions that take place within the engine. During a launch, the fuel and oxidizer react exothermically, meaning they produce heat. This results in the expansion of gases, which then exit the rocket nozzle at high speeds, propelling the rocket forward according to Newton's third law of motion.

For instance, when liquid hydrogen reacts with liquid oxygen, water vapor and heat are produced. This reaction can be represented by the chemical equation: \[ 2H_2 + O_2 \rightarrow 2H_2O + \text{energy} \]. This simplicity belies the complexity of engineering efficient combustion and the nozzle design which must maximize the exhaust velocity for optimum thrust.

Recognizing the intricacies of these reactions allows students to appreciate the precision needed in rocketry. It's not just about the substances used, but also about the exact proportions and the conditions under which they combine that ultimately determine the success of the rocket launch.