Principles of jet propulsion
Non-reciprocating aircraft engines, all of which operate on the principle of jet propulsion, include the turbojet, the turboprop, the ramjet, and the rocket engine. The turboprop, turbojet, and turbofan engines, which are modifications of the turbojet engine, are gas turbine engines, in which the air that enters the intake of the engine is first compressed in a compressor. Fuel is then added to bum the oxygen in the air, increasing the gas temperature and its volume. The high pressure gases are then partially expanded through a turbine which drives the compressor (and the propeller in a turboprop engine). The residual gas that is now at intermediate pressure is accelerated by expansion through a rear-facing nozzle, to produce a high leaving velocity and, with it, the desired thrust. Turboprop engines are efficient for medium-sized planes at speeds up to about 480 to 640 km/hr (about 300 to 400 mph). At higher speeds, turbojet or turbofan engines perform better. The performance of a propeller reaches such a low level of efficiency that jet engines are used exclusively on aircraft that operate above 800 km/hr (about 500 mph).The ramjet engine is an internal-combustion engine, in which the air compression needed for combustion is obtained from the speed of forward motion alone. As in the turbojet, its total power output is delivered as the jet thrust of its expelled gases. Although the ramjet can be applied to piloted aircraft, its present rate of fuel consumption is so prohibitively high that it is used only in guided-missle applications.
Like the ramjet, the rocket engine has its chief application in guided Missiles. A solid propellant rocket, rocket-assisted takeoff (RATO), is used for supplementary initial power in the takeoff of heavily loaded aircraft.
The principle of rocket propulsion depends on the following two laws: -
(i) Newton 's third law of motion
(ii) Law of conservation of momentum
We have already read about these laws, and now we will see how they can be applied for propelling the rocket.
The motion of a rocket is an interesting application of Newton 's third law of motion & momentum principle. The rocket expels a jet of hot gases from its tail. This is say, an action force. The jet of hot gases exerts a force on the rocket, propelling it forward; this is the reaction force.
From the momentum point of view, the hot gases acquire momentum in the backward direction & the rocket acquires an equal amount of momentum in the forward direction.
The simplest example to understand the propulsion of rockets is that of a balloon.
A balloon shooting forward (when the mouth of the balloon filled with air is released) and a rocket hurtling into space are propelled by similar forces. The air in a closed balloon exerts a uniform outward force. But when air rushes out of its neck (similar to exhaust gases leaving rockets) disturbs this equilibrium. Thus an equal and opposite force is exerted on the surface opposite to the neck. This drives the balloon forward.
As we have seen in the previous section propellants are used to provide thrust to the rockets. These propellants on burning produces large amount of gas, which are allowed to pass through nozzle. On passing through the nozzle, high pressure is generated i.e. gas comes out with high pressure.
Now to increase the thrust, one basic property is used while designing the nozzle. The neck of the nozzle is kept very small as compared to the body of the rocket. So the pressure of the gas increases and so does the velocity. Thus high thrust is achieved.
Let us now see the different types of propulsion techniques classified according to present-day and future techniques.
Rocket engines operate in a very different mannerthan that of reciprocal engines and jet engines, in that its main component ofthrust is rocket fuel, and it operates with different characteristics. Inshort, rocket engines operate better at higher speeds,burn more fuel, and are more dangerous.
Rocket engines, unlike other types, do not requirean external supply of oxygen like jet engines, since they burn fuel internally.Rocket engines burn some combination of fuels, either fluid or solid. Withengines which burn solid fuel, there is one type of fuel, but withfluid-propellant engines, two fuels are often used, such as liquid hydrogen(the fuel) and liquid oxygen (the oxidizer).
What are Jet Engines?
Although lighter than reciprocating engines, thesejet engines provide much more thrust. The only downside to these engines isthat they are not as efficient at lower speeds. There are four types of jetengines, turbojet, turboprop, turbofan, and ramjet. However, the ramjet isprimarily used for unmanned craft, such as guided missiles.
Jet engines operate on the basis of Newton’sThird Law of Motion. The gases are expelled from the nozzle at incredible speeds, so as areactive force, the engine is pushed forward with the same force - thrust.However, because jet engines accelerate a relatively small amount of air tovery high speeds, there is much energy in the form of heat that is unused forthrust.
Any solid propellant consists oftwo parts:
• an oxidizer
• a fuel (or a reducer)
In solid propellants, the fueland oxidizer components are prepared separately and are then mixed together.This is because the oxidizer is in powder form and the fuel is a fluid ofvarying consistency. They are then blended together and poured into the rocketcase under carefully controlled conditions
In addition to fuel and oxidizer,some other compounds are added to increase the efficiency of the propellants.To understand this, let us see an example of solid propellants used in shuttles.
• The oxidizer is ammonium per chlorate (NH4ClO4) (69.93 %).
• The fuel is a form of powdered aluminium (16 %).
• The catalyst(increases rate of combustion) is iron oxidizer powder (0.07 %).
• The binder (holds mixture together) is polybutadiene acrylic acid acrylonitrile (12.04 %).
• An epoxy-curingagent (1.96 %).
The binder and epoxy also burn as the fuel burns, thus contributing to the thrust produced.
By changing the shape and size of the perforation we can control the rate and duration of
burning and thus control the thrust. More the thrust required, larger will be the perforation but the fuel will burn for smaller time. Lesser the thrust required, smaller will be the perforation but the fuel will burn for a very long time.The burning period and the thrust depends upon the type of perforation in the fuel.
There are two types of solid propellants:
• Homogeneous solid propellants
• Composite solid propellants
Homogeneous solid propellants are of two types:
• Simple base homogeneous solid propellants
• Double base homogeneous solid propellants
Simple base propellants are those propellants which consist of only one compound having both oxidation and reduction properties. This compound is usually nitrocellulose.
Double base propellants consists of two compounds, usually nitrocellulose and nitroglycerine, along with aplasticizer (added to impart flexibility). The advantage of this type ofpropellant is that it does not produce smoke. Thus increases energy and burn ingrate.
Composite propellants are heterogeneous mixtures. These use crystallized or finely ground mineral salt as oxidizer. This oxidizer forms the bulk of the propellant. The fuel used is aluminum. A polymeric binder holds the propellant together. Sometimes catalyst is added to improve burning.
• They are stable and easily storable.
• They do not require turbo pumps or complex propellant feeding devices.
• The solid propellant motor cannot be shut down. The fuel once ignited burns till the end.
• The propellant is temperature sensitive.
Liquid propellants are nothingbut rocket propulsion fuels in liquid state.
They are made up of 2 parts
• An oxidizer
• A fuel
Both the oxidizer and fuel are inliquid form.
Liquid propellants are moredifficult to handle than solid propellants and they require separate oxidizerand fuel tanks. Lightweight pumps and injectors are used to spray thepropellants into the combustion chamber.
The combustion of liquidpropellants can be controlled easily by controlling the rate at which the pumpsspray the liquid into the combustion chamber. Shutting off the pumps completelycan easily stop the combustion. Thus controlling, stopping and starting thecombustion is very easy when liquid propellants are used.
In order to start the combustionprocess spark plugs, igniters, explosives are used.
Liquid propellants used in launchvehicles can be classified into:
• Cryogenic propellants
• Hypergolic propellants
Refining crude oil produces petroleum fuels. Crude oil simply means unprocessed oil. Crude oil is a fossil fuel as it has been formed in the earth's crust by decaying of the fossils of plants and animals. These fossils are subjected to high temperature and pressure for millions of years to form crude oil.
The oil is made up of mixture of complex hydrocarbons (compounds containing mainly carbon and hydrogen).
Crude oil consists of large number of components mixed together. Therefore to obtain kerosene which is used as a propellant various components of crude oil will have to be separated. To separate the components of crude oil 2 processes have been employed
They can also be employed to separate the components of crude oil. These include breaking of large hydrocarbons into smaller hydrocarbons, combining small hydrocarbons to make larger one and rearranging to obtain desired hydrocarbons.
After obtaining kerosene from one of the above processes it is used in combination with liquid oxygen as the oxidizer.Kerosene obtained is referred to as RP-1 (highly refined petroleum). RP-1 delivers a specific impulse considerably less than cryogenic fuels. (See cryogenic fuels).
Kerosene was used to power the first-stages of the Saturn 1B and Saturn V rockets.
In a cryogenic propellant thefuel and the oxidizer are in the form of very cold, liquefied gases. Theseliquefied gases are referred to as super cooled as they stay in liquid formeven though they are at a temperature lower than the freezing point. Thus wecan say that super cooled gases used as liquid fuels are called cryogenicfuels.
These propellants are gases at normal atmospheric conditions. But to store these propellants aboard a rocket is a very difficult task as they have very low densities. Hence extremely huge tanks will be required to store the propellants. Thus by cooling and compressing them into liquids, we can vastly increase their density and make it possible to store them in large quantities in smaller tanks. Normally the propellant combination used is that of liquid oxygen and liquid hydrogen, Liquid oxygen being the oxidizer and liquid hydrogen being the fuel. Liquid oxygen boils at 297oF and liquid hydrogen boils at 423oF.
A hypergolic propellant is composed of a fuel andoxidizer that ignite when they come into contact with each other. There is noneed of an ignition mechanism in order to bring about combustion.
In hypergolic propellants, the fuel part normallyincludes:
The oxidizer is generally:
- Mono Methyl Hydrazine- MMH
- Unsymmetrical Di-Methyl Hydrazine- UMDH
The easy start and restart capability of hypergolic propellants make them ideal for spacecraft maneuvering systems. They are also used for orbital insertion as their combustion can be easily controlled and thus allows the precise adjustments required for insertion into orbit. Hypergolic propellants are also employed for altitude control.
- Nitrogen tetroxide N2O4 or
- Nitric acid HNO3
As we now know the properties of cryogenic and hypergolic propellants let us compare them.
• Hypergolic propellants remain in liquid state at normal temperatures. They do not need the temperature-controlled storage as in case of cryogenic propellants.
• As compared to cryogenic propellants,hypergolic propellants are less energetic. That is they produce less energy per unit mass. For example: in a moon bound shuttle, 75% of the on board mass would be fuel, in case of cryogenic propellants. But in case of hypergolic propellants, the number raises to 90%.
• In comparison to cryogenic propellants,hypergolic propellants are very poisonous. They react with living tissues aswell cause injuries. So it is mandatory for technicians to wear full-body Self-Contained Atmospheric Protection Ensemble (SCAPE) suits.
• They are corrosive therefore storage requires special containers and safety facilities. It is necessary that they be stored safely, with no possible contacts between the fuel parts.
One example of terrible outcome of improper handling is the fate of the unmanned Mars Observer in August 1993. Just three days before it would have entered orbit around Mars, an accidental mixing of mono methyl hydrazine and nitrogen tetroxide is believed to have caused uncontrollable spinning and subsequent loss of that spacecraft.