In today’s aerospace world, SpaceX needs no introduction. Being the largest private space technological company, SpaceX proved everyone wrong. But, to make this happen in the first place, they need a solid and reliable engine for their rocket’s and here they introduced their Merlin Engines.

Known for their sheer power and reliability along with the reusable nature of the rockets, the SpaceX’s brought the launching cost dramatically lower than previous generations of rockets.

Elon Musk - Founder, CEO and chief engineer of SpaceX

To introduce you to the insane engineering they used for developing the Merlin engines, I decided to make a series of articles through which I will explain to you the mechanism behind it in a very simple way.

Merlin Engine

To understand the mechanism, you need to know some technical terms and the working of the really important parts. So, let’s start from that.

Important Terminologies

Before starting, I want to introduce some of the important terminologies which are very important in order to understand the rocket science behind these magnificent machines. Remember, that these components are used in various applications in mechanical engineering, but here, I am only going to explain to you about their usage in aerospace engineering. So, let’s start.

1) Cryogenics

Cryogenics is the branch to study the production and behaviour of materials at a very low temperature. So, a cryogenic engine means an engine that uses cryogenic tech for its functionality. Now, here I oversimplified the word cryogenic tech. So, let’s go a little bit deeper.

Here, cryogenic tech includes the cryogenic fuel and materials & engineering to handle them. To propel rockets we need efficient fuels. The most preferred ones are oxygen and hydrogen. Why???.... We will discuss it later. For now, just remember these fuels.

Liquid Hydrogen

The problem with Oxygen and Hydrogen is that they are in the gaseous state at room temperature. Since they are in a gaseous state, so we cannot store enough quantity of oxygen and hydrogen in a compact fuel tank for the whole journey of the rocket. To solve this problem, we use the knowledge of cryogenics. We can store them in liquid form and to do so we have to cool them down and store them in a pressured tank. Along with this, the materials used to manufacture these tanks have to specialise for this purpose.

Liquid Oxygen storage facility

2) Propellent

In simple terms, the propellant is a substance that propels a vehicle in a particular direction. As we have already discussed in the previous point about liquid hydrogen (LH2) and liquid oxygen (LOX), they are the fuel or in technical terms the propellant of the rocket. In LH2 and LOX combination, the LH2 is called the rocket fuel component and LOX is known as the oxidizer.

One must note that Merlin engines don’t use liquid hydrogen and liquid oxygen combination for the fuel rather they use the combination of liquid oxygen (as oxidizer) and RP-1(as rocket fuel) as propellants. RP-1 stands for Rocket Propellent-1 or Refined Petroleum-1. This is a highly refined version of kerosene which is primarily used as rocket fuel. Its biggest advantage is that it's cheaper and stable at room temperature as compared to liquid hydrogen. But at the same time, it has also a lower specific impulse (see point 6 for details) than liquid hydrogen. These all characteristics made RP-1 a preferable fuel for merlin engines and Falcon rockets as their main aim is to reduce the cost of each launch even while maintaining high efficiency.

Liquid Hydrogen and RP-1 storage facility

3) Impeller

The impeller is a very essential component in any rocket, especially in turbopumps which I will cover in my next article. For now, I will try to explain it in a very simple way. In mechanical engineering, an impeller is a component that rotates to increase the pressure and thus control the flow of a fluid. Here, in our case the fluid is propellent. Impellers used in cryogenic engines like merlin have a series of curved vanes fitted inside shroud plates. The whole structure is rounded. The middle portion of the impeller where the blades are connected together is called the eye.


When the impeller rotates, the liquid propellent surrounding the rocket also rotates. Due to curved vanes and rotation of the impeller, the propellent moves radially out due to centrifugal force and thus increasing the pressure & kinetic energy of the propellant as the rotational mechanical energy is transferred from the impeller to the propellent. At the eye of the impeller, since the propellent get displaced, so a negative pressure is created and thus helping the impeller to suck in more propellent from the tank.

4) Pintle Injector

In a cryogenic rocket engine, if you only have propellants, effective combustion cannot take place for effective propulsion as it will be very difficult to mix those propellents. Here, the pintle injector comes into the picture. It helps to ensure appropriate intermixing of the propellants. It consists of two concentric tubes and one opening. Once the propellants get mixed it goes through atomization, then it goes to the main combustion chamber.

5) Pyrophoric Ignition

To understand this, you first have to understand the meaning of pyrophoricity. A pyrophoric substance is a substance that ignites instantly once it gets in contact with oxygen. So, pyrophoric ignition means the ignition in the combustion chamber by using pyrophoric material. SpaceX merlin uses TEA-TEB pyrophoric ignition method. TEA stands for Triethylaluminium and TEB stands for Triethylboron. Both are colourless pyrophoric liquids.

6) Specific Impulse

In simple terms, specific impulse means the thrust which has produced by a propellent per unit rate of the same. Its usually represented by ISP.

7) Calorific Value

Calorific value is the amount of energy actually stored in a certain kind of fuel or propellant. It can be measured by measuring the total amount of heat energy is produced after its complete combustion. Its unit is Joules per Kilogram.

8) Thrust Vector Control (TVC)

Controlling a rocket is a very difficult task, especially when you know that those rockets are made for carrying out operations in space. The problem is that in space, the density of air is either thin or null. So, the control methods we are familiar with using in aeroplanes fail to execute and do their job in space. Here, the technology of thrust vector control comes into the picture. In this technique, the direction, altitude, or angular velocity of rockets can be controlled by the engine's thrust. Its primarily achieved by controlling the direction of the rocket’s nozzle. There are many other complex methods to achieve this capability, which I am not going to discuss here as it is out of this article's scope.

9) Converging-Diverging (CD) Nozzle

As the name revers, this type of nozzles has a converging and diverging shape. It was first invented by a Swedish engineer named Gustaf De Laval and that’s why this type of nozzle is also known as the de Laval nozzle. This type of nozzle was first used in rockets by Robert Goddard. CD nozzle’s primary job is to accelerate the gas passing through it to supersonic speed by converting the heat energy to kinetic energy and pressurising the gas, all due to its shape.

Nine CD nozzles configuration at the base of Falcon-9 rocket 

Although there are a hell lot of components which is needed to be discussed before moving forward, for now, this much knowledge is enough to understand the basics of turbopump, which is the topic for our next article.

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