Abstract
Aerospace engineering is so dependent on fluid mechanics because it is about spacecraft or aircraft in a flow of gas.
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Aerospace propulsion is using the fundamentals of fluid mechanics plus thermodynamics to study aircraft and spacecraft engines, and the common gas turbine propulsion systems and applicability determination of each.
Aerospace propulsion engines are split into 2 main part which are breathing engines and non breathing engines and these 2 branches themselves split into many examples. This article talks about each branch of propulsion systems and explain the way it works and where is it used.
Some of these engines and engines we saw in our lifetime at least once.Introduction
Have you ever been into a plane? If yes then you must have seen the huge engine under the wing, have you ever wondered how it works ?
Actually the principles are simple but it needs a lot of precision and deep check while engineering these parts.
There are different applications of aerospace propulsion systems such as turbomachinery, rockets and ramjets, piston engines, spacecraft systems, engine systems, fuels and combustion.
Here the turbojet, turbofan, turboshaft, turboprop, after burner, ramjet, scramjet and the rocket engine will be discussed with motions of other types.
Aerospace industries have experienced a huge growth over the past years and it is expected that this growth will continue in the future. There is continuous development of existing aerospace propulsion systems.
What does propulsion mean? The word is derived from two Latin words: pro meaning before or forwards and pellere meaning to drive. Propulsion itself means to push forward.
Propulsion is a system that produces thrust or exhaust to push an object.
The principle of propulsion is Newton’s 3rd law which states for every action there is a reaction that is equal in force and opposite in direction, and that is the working principle of a propulsion engine, in and airplane the engine sucks the air from the front and throws it from the back that pushes the engine forward and because it is attached to the plane it moves the plane forward. The air is accelerated by the engine, and the reaction to this acceleration produces a force on the engine that moves the plane.
Different propulsion systems produce thrust in different ways but they all follow the same principles.
Aerospace propulsion systems divide into categories as the following:
Air breathings are engines that use the air around to produce the ignition in the combustion chamber and sucking it in, without air they cannot work. Such example for this is the airplane engine or the helicopter engine. And the other type which is the non air breathers are engines that do not need air around to work such example for this is the rocket engine which uses its own oxidizer that is carried with it in the rocket at the bottom along with the fuel.
An airplane propulsion engine must serve 2 proposes. 1st the thrust must balance the drag produced on the plane from the air while it is cruising and 2nd the thrust must exceed the drag for the plane to accelerate and move forward because that way the equal force will be pointing forward, actually the bigger the difference between the thrust and the drag, (called the excess thrust) the faster the airplane will accelerate.
For some aircraft, like airliners and cargo planes they spend most of their time in cruising and that means that they don’t necessarily need excess thrust but what they most need is high efficiency of the engine and also low fuel consumption but since thrust depends on the amount of gas and the velocity we can generate high thrust by acceleration of a large mass of gas by small amount or small mass of gas by a large amount but it is more efficient to accelerate a large mass by a small amount because of propellers and fans aerodynamic efficiency
It is the opposite for fighter jets because they need high speeds which mean they need very high thrust more than engine efficiency. New fighter jet aircrafts have afterburners on a low bypass turbofan core. Future hypersonic aircraft will have types or ram jets.
So the different types of aerospace propulsion have two classes, a breathing and non air breathing essentially. One uses air from the atmosphere. The other one doesn't.
As you might expect, air breathing is an atmospheric based propulsion system. Not everything generally use in space. Well, you can use non breathing in the atmosphere. So you can see that we've got on the air breathing high speed engines, piston engines, experimental and future systems, electrical and electromagnetic and the chemical rods there are more than this.
Turbojet Engine
So the turbo jet engine is composed of 3 main chambers. First the compressor chamber in it the air is compressed and its temperature and pressure rise. Second chamber is the combustion chamber, in it the compressed and heated air is mixed with fuel that is sprayed into the chamber and combusted and a directed explosion happens that is forwarded to the back with the flow of air. The air at this point is further increased in temperature, pressure and speed. The thrust force of the engine depends on the fuel sprayed in the combustion chamber, when the pilot wants to increase speed he increases the fuel consumption of the engine by increasing the fuel injection into the combustion chamber. Third and final chamber is the turbine, in it the air loses some of its temperature and pressure by moving the turbine and the movement of the turbine itself generates power to move the compressor. After that the air is still in a high temperature, pressure and speed state and is expelled from the back of the engine with its high speed causing thrust and because of the 3rd law of Newton this will move the plane forward.
Turbojet engines are mostly now used in a military aircraft. Not so much in modern aircraft. It's the basis of all modern air breathing propulsion. It is, however, quite noisy and less efficient.
Turbofan Engine
The turbo fan engine is based on the turbo jet engine. These are used in all modern airliners because they're a lot more efficient and a bit less noisy. Basically, we have a core turbo jet and a giant fan at the front. This fan produces some thrust, but a lot of the thrust is produced by the bypass section. The fan sucks in the air and the air is split into 2 parts. One that goes into the turbojet and one go around the turbojet and is expelled at the back with around the air that is expelled by the turbojet, this makes the engine more efficient and less noisy. There can be 2 compressors and 2 turbine one is low pressure and one is high pressure.
Turboshaft Engine
Turboshaft engines are the engines used in helicopters and these are also called a bypass engines. Turboshaft engine is based on the turbo jet again. But instead of using high speed gas, the thrust moves a shaft, spins the shaft at a high speed. So a propeller is attached and generates thrust. So this works the same as a turbojet but at the end when the turbine is moved by the air the movment is sent to a shaft at the beginning in front of the compressor and is connected with gears to turn the movement 90 degrees and turn the propeller.
They're also used for power generation.
Piston Engine
We've got the piston engine, the original propulsion system used for about the first 40, 50 years of propulsion in aerospace.
This is a V8 in a star configuration.
This tool used in a lot of smaller general aviation planes it is quite heavy and inefficient.
Turboprop Engine
This engine is like a turbofan engine. The only deference is that with the turboprop engine the huge fan in the front is actually outside the engine chamber.
Afterburner
The afterburner is a technique to further increase the thrust and to help get supersonic.
It is basically an added chamber which comes after the turbine and injects fuel to the air going outside another time to further increase its energy and speed and produce much more thrust. Afterburners can be turned on and off by the pilot.
They are basically used in fighter jets and were used in the Concord which is the only civil aircraft that can go supersonic and it reached Mach 2.
Chemical Engine (Rocket Engine)
So this is used it rockets, missiles and space shuttles and almost every spacecraft. This engine is a non breathing engine. It carries with it the oxidizer and the fuel to produce a high speed exhaust and therefore thrust. The chemical engine is a very wide branch and there are many types of it that use deferent pumps, oxidizers, fuels, structures and much more variables.
In general there are solid fuels and liquid fuels. Liquid fuels are used in the space shuttle and they are used more because they are more efficient and also they can be controlled unlike the solid fuel that is used in the smaller rockets helping the space shuttle lift off they can’t be controlled because once they are ignited they can’t be turned off they will keep producing thrust until the fuel is fully consumed.
Basically the fuel and the oxidizer and pushed by a pump from their tanks to the combustion chamber and are ignited and that explosion is directed to the conversing diverging nozzle so the combustion move in the conversing section it speeds up with its high temperature, pressure and speed until it reaches the throat and a normal shock wave happens and the combustion becomes supersonic and goes through the diverging part (which can be seen at the end of a rocket) where it gains more speed and is exhausted with a very high speed and because of the Newton’s 3rd law that gives the spacecraft a huge thrust.
In their website NASA writes that to simulate the elements of the Space Launch System, they use the Loci/CHEM software, a C++ library and declarative programming framework being utilized at NASA's Marshall Space Flight Center. Full-scale geometry and liquid hydrogen as the fluid simulation cases were run. they found that there was a strictly non-uniform flow field generated by the liquid hydrogen duct geometry and components, but the largest contributor to flow non-uniformity near the feed line/engine interface is the low pressure fuel pump.
Ramjet
So obviously there is no longer this compressor combusted turbine exit stage. Now there's just this spike at the front. This is a high speed engine. It doesn’t work unless the spacecraft is supersonic. The disadvantages they have to be going faster than the speed of sound to start so they are not the main engines of the spacecraft they only help the engine by producing more thrust. They have very few moving parts because their working principle is that the air come in the the cross section area of the ramjet gets smaller and smaller and this compresses the air and it becomes subsonic then fuel is injected and the air just burns. An exhausted combustion takes place and speeds the air and makes it supersonic again and moves towards the end of the ramjet while the cross section area is increasing giving the air more speed and the air exists faster than it entered giving the spacecraft thrust.
Scramjet
The scramjet is based on the ramjet, but it's used at hypersonic speeds, which is faster than Mach 5 or 5 times the speed of sound. They're highly experimental. NASA's been testing one for the last couple of years. It is the same as the ramjet engine but the air entering slows down from hypersonic to supersonic at the combustion where fuel is injected and the air exits in a hypersonic speed faster than it entered. They all have different structures, but. But in this case, combustion of air occurs at a supersonic speed while in a ramjet that happens in a subsonic speed.
Electric and Electromagnetic Engines
The electric and electromagnetic propulsion systems it does look a little bit like something from science fiction, but they have been used on spacecraft for quite a while. They're actually quite an old system. They're still experimental. They're still being improved. But they are very efficient. They produce a very low thrust, though. They are usually only used in space.
Experimental Engine Types
So the non-hybrid is the experimental types solar sails, nuclear fusion and gravity tethers, magnetic beam power. So we've got a solar sail, magnetic being powered. They’ve been used in various experimental crafts for the last few years but still there is a lot of ongoing research into them.
Conclusion
So these are the types of engines used in aerospace propulsion.
They all follow the same rule of motion which is the 3rd law of Newton that states for every force applied there is an equal and opposite force applied too.
As it has been seen here that aerospace engineering is a very wide branch and aerospace propulsion alone will need an entire book to be explained fully.
In the future we might see aircrafts or spacecrafts engines working in different way and the experimental types we have now could become practical in the near future. Because fuel will be over one day so experiments on engine that don’t require fuel are ongoing and will not stop until propulsion engines are fully practical.
Many new engines are being experimented and made nowadays in many aircraft or spacecraft companies but at the end they all follow the same principles of motion and combustion and when one disregard the laws of nature the unforeseen consequences can be catastrophic.
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