In this article we will discuss about:- 1. Introduction to Rockets 2. Classification of Rockets 3. Solar Rocket Propulsion 4. Staging 5. Applications.
Contents:
- Introduction to Rockets
- Classification of Rockets
- Solar Rocket Propulsion
- Staging of Rockets
- Applications of Rocket Engines
1. Introduction to Rockets:
Rockets are becoming important as an aircraft power plant. The primary use of rockets is in military applications, but they offer the promise for long range, high speed transport aircraft and also as a power plant for space travel. Efforts of scientists made Neil Armstrong to land on Moon in 1969.
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The major difference between rocket engine power plant and other jet propulsion systems is that it carries the entire propellant (fuel and oxidizer) with it, while the other jet propulsion systems depend on atmospheric air. i.e., Air breathing jet propulsion systems utilize oxygen from the atmosphere (surroundings), whereas rocket engine utilizes oxidizer which is carried in its tanks. This fundamental distinction between the two power plants produces different performance capabilities, which are summarised in the Table 35.1.
2. Classification of Rockets
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The classification of rockets is based on the form of energy and momentum utilised for producing the thrust.
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Thus rockets are classified into three basic types:
The oldest types of rockets are chemical rockets, in which two or more chemicals mix together, producing a chemical reaction. The reaction produces hot gases that are forced out of a nozzle, pushing the rocket in the other direction.
There are two basic types of chemical rocket engines, based on the fuel used:
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(a) Liquid propellant’
(b) Solid propellant’
(a) Liquid Propellant Rocket Engines:
These engines may be divided into various categories. These include cryogenic, monopropellant, storable bi-propellant system and hybrid system. Hybrid system is a combination of liquid and solid propellant systems. A liquid bi-propellant system has greater potential.
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It consists of an oxidizer tank, fuel tank, control valve lines, and the rocket motor, which is the heart of the rocket engine. The basic rocket motor consists of an injection plate I, combustion chamber C, and discharge nozzle N. The injection plate receives liquid oxidizer and fuel, which after mixing produce a chemical reaction in the combustion chamber.
The very high pressure and temperature gases produced in the combustion chamber expand in the nozzle and produce a high supersonic exit velocity (1500 – 3000 m/s). The net thrust produced is the product of exit velocity and mass flow rate of gases.
An ignition system uses hypergolic combinations for igniting the mixture. Hypergolic fuels commonly used are hydrazine, and monomethyl hydrazine. They ignite spontaneously on contact. A cooling system is also necessary to keep the walls of the motor from melting because the temperature during reaction exceeds 2700°C.
The pressure at which the propellants have to be supplied from the storage tank should be higher than the pressure in the combustion chamber. There are two methods which are used for transporting liquid propellants from their storage tanks to the rocket motor.
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They are:
(i) Gas pressurisation system and
(ii) Pump pressurisation system.
(i) Gas Pressurisation System:
In this system, an inert gas (nitrogen) is used. This gas is stored at high pressure and is supplied through pressure regulator valves to force the liquid propellants through the lines, control valves, injector plate and into the combustion chamber.
The mixture ratio supplied is 3 to 5 (oxidizer to fuel flow). The system is first energized by opening the system energizing valve and then the rocket motor is turned on by opening of bi-propellant valve. The motor can be stopped and restarted at will by closing or opening of the bi-propellant valve.
This system is simple. Its disadvantage is heavy weight of the storage tanks because of the high pressures at which they are to be stored. This limitation makes gas pressurisation system applicable only to short duration operations because lesser quantity of fuel can be stored due to weight considerations.
(ii) Pump Pressurisation System:
In this system liquid oxidizer and fuel are stored at low pressure so that tanks are light in weight and forced into rocket motor at high pressure by fuel and oxidizer pumps. The power driving the pumps is supplied by gas turbine which is supplied with steam and oxygen obtained by decomposing hydrogen peroxide by a catalyst.
Because of the use of third liquid (H2O2), gas turbine, pumps and additional lines that are necessary, pump pressurisation system is more complex than gas pressurisation system. The design of pumps is the biggest problem because any leakage of liquids will lead to explosion. Liquid oxidizers are generally acids, liquid oxygen, concentrated hydrogen peroxide (H2O2), nitrogen tetra-oxide (N2O4) etc. and hence special pump materials are required for the oxidizer pump and impeller.
Rocket Propellants:
The desirable properties of rocket propellants are:
1. It should produce a high combustion chamber temperature. This means the calorific value of the propellant should be high.
2. The molecular weight of the products of combustion should be low, which will cause high jet velocity and specific thrust.
3. Easy to store and handle.
4. It should be readily ignitable.
5. It should not react with motor system, tanks, piping, valves and ignition nozzles.
6. It should have high density to reduce the overall size and weight of the system. Requirements (1) and (2) are contradictory and hence a balance should be reached.
An ideal propellant is not yet available. For example- if liquid hydrogen is used as a fuel, then the jet velocities are very high but the size of the fuel tank is large due to the low specific weight of hydrogen. Also, very low temperatures are required for the storage of liquid hydrogen. The liquid propellants used are given in the Table 35.2.
(b) Solid Propellant:
Solid propellant motors are the simplest of all rocket designs. They consist of a casing, usually steel, filled with a mixture of solid compounds (fuel and oxidizer) which burn at a rapid rate, expelling hot gases through nozzle to produce thrust. When ignited, a solid propellant burns from the centre towards the sides of the casing.
The shape of the centre channel determines the rate and pattern of burning, thus providing a means to control thrust. Unlike liquid propellant engines, solid propellant motors cannot be shut down. Once they are ignited, they will burn till all the propellant is exhausted.
Solid propellants are of two types:
Homogeneous and composite. Both are dense, stable at ordinary temperatures and easily storable.
Working of Solid Propellants:
By changing the shape and size of perforation, we can control the rate and duration of burning and thus control thrust.
If more thrust is required, the perforation should be larger, but the fuel will burn for a short time and vice versa. The burning period and the thrust depend on the type of perforation in the fuel.
Advantages and disadvantages of solid propellants are as following:
Advantages:
1. They are stable and easily storable.
2. They do not require turbopumps and complex propellant feeding devices.
Disadvantages:
1. The solid propellant motor cannot be shut down. Once ignited, fuel burns till the end.
2. The propellant is temperature sensitive.
(ii) Nuclear Rocket Engine:
Work on nuclear rocket engine began in 1956 and further development was done by NASA (National Aeronautics and Space Administration, USA) for space flight applications, for manned and unmanned space exploration.
Nuclear thermal rocket is very simple concept. It consists of a propellant (H2) tank, a pump (P), valve (V), nuclear fission reactor (R) and discharge nozzle (N).
Hydrogen runs through the reactor (gets heated) and into the nozzle. Hydrogen gives the best exhaust velocity. But the exhaust velocity is limited or fixed by the melting point of the reactor. The reactor elements have to be durable, since erosion will contaminate the exhaust with fissionable materials. The temperature in the reactor may go upto 3000°C and hence cooling of nozzle is required to prevent it from melting.
(iii) Electric Propulsion Engines:
The basic requirement for space propulsion is the generation of high exhaust velocities so that the propellant consumption is low. Chemical rocket engines produce low exhaust velocities meaning requiring more fuel. Hence electric propulsion engines are used which produce 4 to 100 times more exhaust velocities.
There are three main types of electric rocket propulsion engines:
(a) Arc plasma rocket
(b) Ion rocket
(c) Magneto plasma rocket.
The energy supplied to these is from a separate propulsion device:
(a) Arc Plasma Rocket Engine:
It is one of the simplest types of electrical propulsion systems.
The major components of this system are:
(i) Propellant tank
(ii) Thrust chamber
(iii) Electric supply system
(iv) Cooling system, and
(v) Pumps.
The thrust chamber contains two electrodes. The electrode within the chamber is the anode and the nozzle wall serves as cathode. The propellant gets heated to electrically neutral plasma in passing through the arc formed between the two electrodes. Thrust results from the expansion of heated plasma through the nozzle. A velocity of 4000 – 15000 m/s is produced. The propellant can be used to cool the chamber re-generatively before passing into the chamber.
The arc plasma or electro-thermal rocket is similar to the chemical rocket, the main difference being that electrical rather than chemical energy is used for heating the propellant.
(b) Ion Rocket Engine:
The major components of ion or electrostatic rocket engine are:
(i) Propellant tank and feed mechanism
(ii) Thrust chamber
(iii) Electric power supply.
The thrust chamber incorporates a vaporising chamber, an ionisation chamber, an accelerating grid and an electron emitter. In the vaporization chamber, the propellant is heated and vaporised from where it goes to the ionisation chamber where electrons are stripped to ionize it.
The ions are then accelerated electrostatically by electrodes to produce thrust. For examples, potassium seeded argon is ionized and ions are accelerated. Though it has admirably high exhaust velocity, there are theoretical limits that ensure all ion drives have low thrust.
(c) Magneto Plasma Rocket Engine:
It is a plasma rocket. It creates plasma under extremely hot conditions and then expels plasma to produce thrust.
There are three basic cells in this engine:
(i) Forward Cell:
Propellant gas, typically hydrogen, is injected into this cell and ionized to create plasma.
(ii) Central Cell:
This cell acts as an amplifier to further heat the plasma with electromagnetic energy. Radio- waves are used to add energy to the plasma, similar to how a microwave oven works.
(iii) After Cells:
A magnetic nozzle converts the energy of the plasma into velocity of the jet exhaust. The magnetic field, that is used to expel the plasma, also protects the spacecraft because it keeps the plasma from touching the shell of the space-craft.
Plasma would likely destroy any material that it comes in contact. The temperature of plasma exiting from the nozzle is as hot as 100 million degree Celsius which is 25000 times hotter than the gases expelled from the space shuttle.
3. Solar Rocket Propulsion
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It is a form of space-craft propulsion that makes use of solar power to directly heat reaction mass and hence does not require electrical generator. A solar thermal rocket only has to carry a means of capturing solar energy, such as concentrators and mirrors. The heated propellant is fed through a conventional rocket nozzle to produce thrust. The engine thrust is directly related to the surface area of the solar collector and to the local intensity of solar radiation. But still it is in the developmental stage.
Most proposed designs for solar thermal rockets use hydrogen as propellant due to its low molecular weight which gives excellent specific impulse of 900 seconds (9 kNs/kg).
In the shorter term, solar thermal propulsion has been proposed as good candidates for use in reusable inter- orbital tugs, as it is a high efficiency, low thrust system that can be refueled with relative ease.
4. Staging of Rockets
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Staging of ‘rockets is done to improve the payload capability of vehicles with a high change in velocity (ΔV) requirement, such as launch vehicles or interplanetary spacecraft. In a multi-stage rocket, the propellant is stored in smaller, separate tanks rather than in a larger single tank as in a single stage rocket.
As each tank is discarded when empty, energy expended to accelerate the empty tanks, so a higher total ΔV is obtained. For convenience, separate tanks are usually bundled with their own engines, with each discardable unit called a “stage”.
The first stage is at the bottom and is usually the largest the second stage is above it and is usually the next largest, etc. In the typical case of linear staging, when the first stage’s motor fire, the entire rocket is propelled upwards.
When the first stage’s motor runs out of fuel, the first stage are detached from the rest of the rocket (usually with some kind of explosive charge) and falls away. This leaves a smaller rocket, with the second stage on the bottom, which then fires. This process is repeated until the final stage’s motor burns out.
Advantages of Multi-Stage Rockets:
1. The space and structure of each stage are useless and only add weight to the vehicle, which slows down its future acceleration. By dropping the stages which are no longer useful, the rocket lightens itself.
2. The weight of the future stages is able to provide more acceleration, than if the earlier stages were still attached.
3. Less total fuel is required to reach given velocity or altitude.
Disadvantages of Multi-Stage Rockets:
1. Multi-staging requires the vehicle to lift motors which not required until later hence rocket is more complex and difficult to build.
2. Cost of launching is very high.
5. Applications of Rocket Engines
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Rocket engines are used for launching of space-craft and satellites into space. The rockets that have been used are Atlas-Centaur (1962), Titan-II (1964), Saturn-V (1967), Space shuttle (1981), Delta-II (1989), etc.
To launch satellites, the velocity of rocket should be greater than the escape velocity (11 m/s) for which multistage rockets should be used.
Some other applications are:
1. Jet assisted take-off.
2. Lethal weapons.
3. Long range artillery.
4. Research.