Question

Which of the following is not used as a rocket fuel?
A) Synthetic rubber
B) Liquid Hydrogen
C) Paraffin
D) Liquid Nitrogen

Answer 1

Hint
Rocket fuels are those which burn rapidly and are highly combustible. We see the properties of all the given materials and eliminate the one which is not suitable i.e., has limitations to be used as a rocket fuel.

Complete step by step answer
Rockets fuels are those which burn when combined with oxidizer for propulsion. To be considered as a rocket fuel, it should be highly combustible, and should burn rapidly. At high temperatures, pressure, it should emit a large volume of gases. Left outs after combustion should contain nothing. Synthetic rubber is one of the most popular rocket fuels. It's high energy and being safe makes it one of the best. Liquid hydrogen has the lowest molecular weight but burns rapidly, it has the highest specific impulse when comes in contact with liquid oxygen. Paraffin wax has high regression rates and being a solid fuel, it has less limitation and is used as a rocket fuel. Liquid nitrogen on the other side, it is difficult to store nitrogen as liquid. When nitrogen boils, excess pressure will be created which can explode the nitrogen tank. These limitations will stop liquid nitrogen from being a rocket fuel.
Hence option (D) is correct.

Note
Generally, we tend to put synthetic rubber as the answer but it is not. Synthetic rubber is one of the most popular rocket fuels. Few solid fuels other than paraffin are ammonium nitrate, ammonium dinitramide, ammonium perchlorate or potassium nitrate. Nitrogen is in liquid state at very low temperature (-417).