Rocket Propellants

Any manmade objects need some sort of ‘energy rich stuff’ to provide motion. What is this ‘energy rich stuff’?

In cars we use gasoline, a volatile and highly burnable liquid containing hydrocarbons like Octane (C8H18). From chemistry class we may remember that carbon and hydrogen may easily react with oxygen. Left alone separated from air, gasoline cannot burn! But if you introduce air, that means oxygen (about 21% oxygen in air), then lit a match, then you will easily get a powerful chemical reaction. The reaction will create huge amounts of gas products at high temperature. The reaction products, assuming complete reaction, will consist of carbon dioxide (CO2) and water vapor (H2O).

In chemistry we call fuels, reactants. In other words, the substance you want to burn. But as said above, no fuel will burn unless it contains some oxygen or has oxygen present in some form. In chemistry we call substances that contain oxygen, an oxidizer. Pure oxygen (O2) is a strong oxidizer, air is too since it contains 21% oxygen. Potassium nitrate (KNO3) is another example. Potassium nitrate is a solid oxygen rich salt which contains 45.7% oxygen per weight.

Mix or introduce an oxidizing agent with any type of fuel, then you may get a exothermal reaction where gas products are generated at some elevated temperature. Often, we need to kick start the chemical reaction by introducing a spark.

In this article we shall discuss the type of ‘energy rich stuff’ we are using in rockets and how performance of the reaction may be affected. Finally, we will briefly discuss propellants for electric rocket propulsion.

Definition

As we have already discussed, the specific impulse (ISP) is the total impulse per unit mass of propellant consumed. In other words, specific impulse can be seen as a performance parameter. A higher number means better performance. A rocket with higher specific impulse propellant needs to carry less propellant to the same work as a rocket engine with a less effective (lower ISP) propellant. Specific impuls will be important in the discussion of rocket propellants.

The SI-units is Ns/kg, which can be simplified to m/s. Sometimes Specific Impulse is denoted in seconds, and for such cases one need to multiply the value in seconds by the standard sea level gravitational acceleration 9.8066 m/s2 to get to the correct value in SI-units.

Types of Rocket Propellants

In rocket propulsion it is common to differentiate the types of chemical propellants based on their state prior to combustion. We have three types in chemical rocket propulsion:

  1. Solid Propellants
  2. Liquid Propellants
  3. Hybrid Propellants

This article is a part of a pre-course program, used by NAROM in different courses, for example Fly a Rocket!