Rocket Propellants


Every time we see a rocket propelling in space, there comes a question in mind that from where it gets the energy to fly? The answer is because of their efficient propellants. They are basically nothing but just a mixture of chemicals that ignites with the help of an oxidizer (a substance which promotes the proper combustion of the fuel). There are many space exploration companies like SpaceX, NASA, etc. that use the best rocket propellants in their projects available to date.

There are various scientific terms related to propellants as their chemistry is so much complex. The mixture ratio is the ratio in which the oxidizer is present in the propellant. We will discuss here on what basis these companies select the propellant and use it in their propulsion systems.

Efficient Rocket Propellants Selection

Rockets fly on the principle of thrust or propulsion by pushing them from the back. In reaction, the rocket propels forward. The speed at which the rocket travels depends on the thrust developed by your propellant. In other words, The faster propellant gets out of the rocket from the back, the faster your rocket will move. Specific impulse is the change in momentum per unit mass. It gives us a rough measure of the speed of the propellant discharged from the back.

Another important property of the rocket propellant is its density. A high-density propellant is easy to be contained in a smaller tank and lighter one rather than a bulky propellant which is hard to contain in a relatively smaller vessel. For example, hydrogen is a very good energy-yielding fuel source but it requires huge space to store.

Corrosiveness is a major problem in almost every propellant. They are often toxic and inflammable too. They require safe and proper handling. Some propellants are more corrosive than some of the war gases and may even react with the air or any metal to get ignited. Therefore only a few substances can handle them properly.

Availability is another consideration to be made when choosing the best propellant for rocket propulsion. Some propellants require a chemical manufacture plant for their products for which different raw materials must be available.

Classification of Rocket Propellants

In this section, we will look at various rocket propellants and their characteristics. They are classified into many types according to different groups. There are three types of propellants according to their state:

  1. Solid Chemical Propellants
  2. Liquid Chemical Propellants
  3. Hybrid Chemical Propellant (Mixture of Solid and Liquid)

Solid Chemical Propellants

There are two types of solid propellants available for use in the industry nowadays. One of them is a double-base propellant. It composes of nitrocellulose and nitroglycerine along with there are some additives present. No separate oxidizer is used as the molecules are unstable. Due to which they easily break apart when ignited a rearrange themselves extracting a huge amount of heat.

Composite is another type of propellant. A mixture of fuel, oxidized chemicals, and solid grain are its basic components. The oxidizer is usually ammonium nitrate, potassium chlorate, or ammonium chlorate, and often comprises as much as four-fifths or more of the whole propellant mix. Space exploration companies use hydrocarbons as usual as fuels which include asphaltic-type compounds or plastics. The fuel must be efficient as well as it should also supply the rigidity to the grain as oxidizer has minimal structural strength. There is still a lot of research ongoing on this type of propellant which mainly focuses on improving the physical properties of the fuel.

The main advantage of solid rocket propellants is that they require minimum maintenance and instant readiness. But also at the same time, they require carefully controlled storage conditions which is a serious concern when handling large sizes containers. Mechanical shocks or sudden temperature changes may crack the gain so it is handled with care.

Liquid Chemical Propellants

Usually, liquid chemical rockets use two separate propellants: a fuel and an oxidizer. Typical fuels include kerosene, alcohol, hydrazine and its derivatives, and liquid hydrogen, etc. Oxidizers include nitric acid, nitrogen tetroxide, liquid oxygen, and liquid fluorine. Some of the best oxidizers are liquefied gases, such as oxygen and fluorine, which exist as liquids only at very low temperatures; due to which it is difficult to use them in rockets. Almost every fuel is liquid except hydrogen when talking about ordinary temperatures.

Certain rocket propellant combinations are hypergolic; that is, they do not need any igniter to them the burning reaction. The reason for this is that they react vigorously upon contact of the fuel and oxidizer. While other propellants cannot ignite without a good ignition source.

Liquid propellants are better than solid propellants in such a way that they generate more specific impulses than the solid propellants. On the other hand, they require more complex engine systems.

Hybrid Chemical Propellants

Hybrid propellants as the name implies, are the mixture of solid and liquid propellants. One of the substances such as fuel is solid while the oxidizer is fluid. The liquid injects into the solid, whose fuel reservoir also serves as the combustion chamber. This concept cannot be applied for very large thrusts therefore, hybrid propellant engines are very uncommon.

Hence, we can conclude that each one of the rocket propellants has its own uses and benefits.


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