The Insane Engineering of SpaceX’s Merlin Engines Part-1: The Introduction
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.
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| 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.
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| 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.
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| 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.
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| 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.
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| 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.
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| Impellers |
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.
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| 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.
Thanks for Reading!!!
-Ratnadeep Das Choudhury
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