ME05548Notes-8 (1).pdf

Jet Propulsion

INTRODUCTION The principle of jet propulsion is based on the Newton's second and third laws of motion. Momentum is imparted to a fluid in such a way that the reaction of the imparted momentum provides a propulsive force. This is done by expanding a high pressure, high temperature gas through a nozzle due to which a jet of gases with high velocity comes out into atmosphere and its reaction in the opposite direction gives propulsive force. (Force necessary to move a vehicle forward). Jet propulsion has got its name because the driving force to move a vehicle comes from the reaction of a high velocity jet of gases. For jet propulsion systems, and open cycle gas turbine is a most suitable choice.

The working fluid is partially expanded in a gas turbine to develop necessary power to drive the compressor and accessories and the rest of expansion takes place in a nozzle which is placed just after the turbine. The gases from turbine while passing through the nozzle will be accelerated and come out in the form of a jet with very high velocity. The reaction of this jet propels the vehicle forward. (In the opposite direction of jet).

CLASSIFICATION OF PROPULSIVE DEVICES Basically, propulsive devices are of two types : 1. Devices which make use of atmospheric air as the main propulsive fluid. These are called

as Atmospheric jet engines or air breathing engines. 2. Devices which carry their own propulsive fluid. These are known as - Rockets or Non air

breathing engines.

The fig. 8.1 shows the complete classification of propulsive devices.

FIGURE 8.1: TYPES OF PROPULSIVE DEVICES

Air breathing engines; take oxygen from atmospheric air for the combustion of fuel. So, their performance depends upon the speed of the engine and the pressure and temperature of atmospheric air.

In a rocket engine, the fuel and the oxidiser (substance which supplies oxygen for the combustion of fuel) are contained in the device itself. So, they don't depend upon the atmospheric air for their operation.

DETAILS OF AIR BREATHING ENGINES Details of various air breathing engines are given here briefly.

PROPELLER It is an indirect reaction device. It handles relatively large mass of air and accelerates it backward at comparatively low speed. The reaction to the rate of change of momentum of mass of air known as - Thrust makes the device or vehicle to move forward. The function of engine is only to revolve the propeller at the desired speed. The formation of shock waves over the section of blades of propeller at high speed and the blade slip at high altitudes are the main draw backs of the propeller which limit its efficiency. As the thrust is not provided by expansion of high pressure gases; this is called an indirect reaction devices. Now a days, this is used in small air crafts only.

TURBOJET This is the most important direct reaction device - Device that makes use of expansion of high pressure gases to provide necessary thrust to propel the device. It makes use of a gas turbine plant. Infact, a turbo jet is a modified form of simple open cycle gas turbine. After partial expansion in the turbine, (to get enough power to drive the compressor accessories). The exhaust from turbine which is still at a higher pressure than atmospheric expands in a nozzle which is placed immediately after turbine and a high velocity jet of gases comes out. Compared to propeller, relatively small amount of air flows through the turbo jet but it has high rearward velocity. They are very efficient at high speed and high altitude but inefficient at low speed and low altitude. These are widely used in air crafts.

TURBO PROP It is also known as - propjet.

It is a combination of direct and indirect reaction devices. (Propeller and turbojet). The thrust is provided by both devices. It also uses a gas turbine plant. Along with compressor, the turbine drives the propeller also. It is also used in air crafts. It has the thermal advantage of turbojet combined with the advantages of propeller for efficient take off of air craft especially for heavily loaded aircraft.

ATHODYD (RAM JET AND PULSE JET) These are direct reaction devices. Athyodyds has got its name from - Aero thermodynamic ducts. These are straight duct type of jet engines without compressor and turbine. In these devices, the entire compression is obtained by ram compression and hence there is no need of turbine and compressor. These are used in pilot less air crafts, helicopters, missiles etc.

TURBOJET ENGINE The fig. 8.2 shows a typical turbojet engine.

FIGURE 8.2: TURBOJET PLANT

The basic components of a turbojet engine are diffuser, compressor, combustion chamber, turbine and jet exit nozzle.

The variation of pressure, temperature and velocity in different components of the engine are shown in fig. 8.3.

Figure 8.3: Variation of Pressure, Temperature And Velocity In A Turbojet.

The basic thermodynamic cycle for then turbojet engine is the joule or Brayton cycle shown in fig. 8.4.

FIGURE 8.4: T-S DIAGRAM OF TURBOJET The diffuser which is in the front portion slows down the entering atmospheric air (reduces the velocity of equal to that of the plane speed) and in doing so, transforms part of the kinetic energy of air stream into pressure energy (With increase in pressure, the air gets compressed). This type of compression is called - ram compression. The process of compression (1-2) may be assumed to be isentropic.

Now, the air is further compressed in a rotary compressor (usually of axial flow type) and the pressure of air is raised further. The compression in the compressor is shown by the process 2 ' - 3. Major part of pressure rise is accomplished in the compressor which is driven by the turbine.

The compressed air then goes to the combustion chamber where fuel is injected by a fuel pump which is also driven by the turbine. The mixture is generally ignited by a spark and combustion takes place. The heat addition takes place at constant pressure (3-4) (p3 = p4). The hot gases then enter the gas turbine and partial expansion takes place in the turbine to generate just enough power to drive compressor and other auxiliary equipment. 4-5 shows the expansion in the turbine.

The exhaust gases from the turbine, which are at a higher pressure than atmospheric are expanded in the nozzle, which is located after the turbine. The nozzle converts the thermal energy of exhaust gases into kinetic energy. As a result of this expansion in the nozzle, a jet of gases with high, velocity comes out and the increased velocity of air produces a thrust. The process of expansion in the nozzle is shown by 5' - 6.

Merits and Demerits of Turbo Jet Engines : Advantages:

1. Simplicity - The construction in simpler compared to reciprocating engines and there are no unbalanced forces of reciprocating parts.

2. Low specific weight - compared to reciprocating engines, the specific weight is much less. 3. High power output - much higher power than a reciprocating engine can be developed. It

can operate on a larger range of mixture strength. 4. Much higher speeds can be obtained. 5. No lubrication is needed.

6. Small frontal area. 7. Compared to reciprocating engines, weight to power ratio is higher. 8. Because of no reciprocating parts, it is vibration free and greater reliability is thus achieved. 9. At high speeds and at high altitudes, the efficiency is much higher than a propeller. 10. Combustion and power output are continuous and there are no fluctuations of pressure.

Disadvantages : A turbojet engine suffers from the following disadvantages : 1. At low altitudes and low speeds, the fuel consumption is more than that of a reciprocating

engine. 2. Life of unit is relatively shorter. 3. Materials are costlier and noise is more. 4. During take off, the thrust is low;

TURBOJET PERFORMANCE PARAMETERS 1. Thrust T: The jet engines are compared on the basis of engine thrust. Thrust is the force produced due to change of momentum of working fluid. Let Va = Velocity of air craft through the air. It is also called as velocity of

approach of air m/sec Vj - Velocity of jet relative to air craft m/sec. ma = Mass flow of air kgs mf = Mass of fuel kgs.

Mass of fuel is very less and hence can be neglected. Assuming expansion upto atmospheric pressure (There is no pressure thrust and the vehicle is moving in still air), thrust of turbojet:

2. Thrust Power : T.P: It is defined as the rate at which work must be developed by the engine if the vehicle is to be kept moving at a constant velocity Va against friction force or drag.

Thrust power T.P = Thrust x velocity of vehicle.

Neglecting mass of fuel, T.P = ma. (Vj-Va) . Va -watts

= (VJ Va) . Va - watts/kg of air

3. Propulsive Power : P.P : It represents the energy required to change the momentum of mass flow of air and is

expressed as the difference between rate of kinetic energies of entering air a exhaust gases.

Neglecting mass of fuel,

Also, P.P = Thrust power + kinetic energy loss.

4. Propulsive Efficiency : p: It is defined as the ratio of thrust power to propulsive power. Neglecting mass of fuel,

The propulsive efficiency increases with an increase in aircraft velocity but the thrust decreases. The propulsive efficiency becomes 100% when Va approaches Vj. Then the thrust reduces to zero. So, a lower value of propulsive efficiency is used to obtain reasonable thrust.

The kinetic energy loss is the kinetic energy of jet dissipated as the stream of jet comes to rest relative to the surroundings. There must be a loss wherever the effective jet velocity is not equal in magnitude to the speed of the flight through the surroundings.

5. Thermal Efficiency : th: It is defined as the ratio of propulsive power to heat energy supplied.

Neglecting mass of fuel,

6. Overall Efficiency : ov : ov = th x p = Thermal efficiency x propulsive efficiency.

Also, ov = Propulsive efficiency x Turbine efficiency x nozzle efficiency.

7. Jet Efficiency : j : It is defined as

ANALYSIS OF BASIC CYCLE Or i O'RBO JET

Refer the fig. 10.4 Consider 1 kg of working fluid through the system.

Diffuser : During process 1 -2, the air entering from atmosphere is diffused isentropically from velocity V1 = Va to zero. (V2 = 0), Process 1 - 2' is the actual process.

Between states 1 and 2; energy equation is

In and ideal diffuser, V2 = 0; Q1-2 = 0 and w1-2 = 0 h = Enthalpy at a state points Q = Heat supplied during the process. W = Work done during the process.

2

2V= Kinetic energy of the fluid at that state point / unit mass of working fluid.

= 1 . C p . T

Compressor: Process 2' - 3 represents isentropic compression of air. 2' - 3' shows actual compression process.

Between states 2 and 3, energy equaion is

Assuming that changes in potential and kinetic energies are negligible.

Ideal work consumed by the compressor = h3' - h2

= Cp(T3 -T2) Actual work consumed by the compressor = h3' - h2

c= Efficiency of compressor.

Combustion Chamber: Process 3-4 represents ideal addition of heat at constant pressure in the combustion chamber. Process 3' - 4 shows the actual addition of heat.

Ideal heat supplied Q = h4- h3 = Cp(T- T3)

Cpg and Cpa - Specific heats of gases and air at constant pressure.

Turbine : Process 4-5 represents isentropic expansionof gases in the turbine. Process 4-5' shows actual expansion.

Between states 4 and 5; energy equation is

If Q4-5 = 0; then turbine work

If change in kinetic energy is neglected, then Wt=(h4-h5) = Cp(T4-T5)

Actual turbine work = (h4 - h5') = Cp(T4-T5')

= ( )'54. TTC pt t = Efficiency of turbine.

Neglecting the work consumed by the auxiliary equipment; Turbine work supplied = compressor work consumed.

Nozzle : Process 5' - 6 represents isentropic expansion of gas in the nozzle. Process 5' - 6' shows actual expansion.

Between states 5 and; energy equation

If C5' 2 is very less compated to V6 2, then

n = Efficiency of nozzle. Thermal efficiency of the cycle

THRUST AUGMENTATION One of the drawbacks of a turbojet engine is that relatively small power will be available at the time of take off and climb compared to reciprocating engine. So, temporary thrust augmentation is necessary for take off and climb. It is also necessary for combat and emergency power requirements of military air crafts.

The thrust may be increased by increasing the mass flow rate of air or by increasing the jet exit velocity or by increasing both. Two principal methods of thrust augmentation are : 1. After burning (reheating). 2. Water - methanol injection.

TURBOJET WITH AFTER BURNER After burning is the most effective means of thrust augmentation but uneconomical and so it is used for short durations only whenever necessary rather than continuously.

The fig. 8.5 shows a turbojet engine equipped for after burning or tail pipe burning.

FIGURE 8.5: TURBOJET WITH AFTER BURNER

After burning consists of introducing and burning the fuel between the turbine and nozzle exit and has the same effect as a reheater. Its function is to increase the exit temperature, which

results in an increased jet velocity thus increasing the thrust.

A turbojet operates with 4-5 times as much air as is required for the combustion of fuel to limit the gas temperature at entrance to turbine. So, the turbine exhaust has enough air to support further combustion. For a turbojet engine with after burner, the inlet velocities must be sufficiently low to support stable combustion and to avoid excessive pressure loss; a diffuser is provided between turbine outlet and tail pipe burner inlet.

The burning of fuel increases volume of gas and hence causes the stream to be ejected from the jet pipe nozzle at an increased velocity so increasing the thrust produced. The amount of fuel injection is limited by the temperature the tail pipe or reheat pipe can withstand.

In jet engines, there is an optimum nozzle area for particular conditions. With after burner, a larger exit area is required for burning gases. This is incorporated by providing a variable area nozzle.

The after burner may be considered as a ram jet engine attached to the end of a turbojet engine. The turbojet engine "rams" the air into the after burner. So, the combination of turbojet engine and after burner is called - "Turbo ramjet". Besides a thrust augmentation device, the turbo-ramjet engine may be considered as special type of engine for flight at super sonic speed.

WATER - METHANOL INJECTION It consists of spraying a mixture of water and methanol into the inlet of compressor. This increases the thrust by causing and increase in mass flow due to the inlet air temperature being reduced plus the addition of mass of water - methanol itself.

The combustion of methanol produces high enough heat to offset the cooling effect of water otherwise the flow of fuel would have to be increased abnormally. This method may increase the thrust by 25%. The disadvantage of this method is that besides fuel, another liquid is required to be carried.

BY PASS TURBOJET ENGINE This increases the thrust without adversely affecting the propulsive efficiency and fuel economy.

The fig. 8.6 shows a by pass turbojet engine.

FIGURE 8.6: TURBOJET WITH BYPASS ARRANGEMENT

A fan is located in the front portion and it is driven by the main shaft. Part of the air drawn by

the fan is sent over the combustion chamber via suitable ducting to the exhaust unit thus by passing the engine and hence the name - "By pass".

A portion of air is sent to the engine compressor with an added advantage of creating a super charging effect. The by pass ratio is selected according to the requirement.

The advantage of by pass arrangement is that the lower velocity of jet efflux gives a better propulsive efficiency and better fuel economy than a straight jet for a given thrust as less kinetic energy is wasted to atmosphere. This applies particularly at air craft speed below sonic and for long range air craft.

TURBO PROP ENGINE When a gas turbine is used to drive a propeller, the engine is known as - Turbo propeller engine.

The fig. 8.7 shows a turbo prop consisting of two independent turbines.

FIGURE 8.7: TURBO PROPENGINE

The fig. 8.8 shows T-S diagram of a turbo prop engine.

FIGURE 8.8: TURBO PROP CYCLE - ON T-S DIAGRAM

The turboprop consists of a geared propeller attached to the turbojet engine. The power developed by the turbine is consumed in driving the compressor and the propeller. To add more power to the propeller shaft the turbine is modified by adding additional stages.

About 80-90% of the power developed is used to drive the shaft and the remaining is used to obtain thrust from the jet. The control of the engine is governed by changing the pitch of the propeller and the quantity of fuel burned.

In the turboprop engine shown; one turbine drives the compressor while the other drives the propeller through a reduction gear.

The cycle of turboprop is same as that of turbojet engine cycle except the turbine expansion process is greater. Regenerators, inter coolers and reheaters may be incorporated to improve its power and efficiency.

The propeller and jet produced by the nozzle together give forward motion to the aircraft. For a propeller driven aircraft, the change of propulsive efficiency is greater initially but it falls at higher speeds. The jet propulsion efficiency goes on increasing with sped of air craft. A simple turbojet engine is inefficient at low speeds because large jet velocities are necessary to achieve appreciable thrust.

A turboprop combines the advantages of a turbojet with the advantages of a propeller. The advantages of turbojet are low specific weight, small frontal area, and simplicity in design, less vibrations. The advantages of a propeller are - higher power for take off and climb, high propulsive efficiency at low speeds (below 800 km/hr), high thrust per unit frontal area and fuel economy.

ATHODYDS (RAM JETS AND PULSE JETS) At high aircraft speed, the turbojet is designed to take the advantage of ram compression. At Mach No. 2, the ideal ram compression ratio is about 8. At such high ram compression, there is no need of a compressor. In a turbojet, the turbine is only to drive the compressor. So, when compressor is eliminated, turbine also gets eliminated. Ramjet and pulse jet engine work on this principle. The work 'Aythodyd' is derived from -Aero-Thermodynamic duct.

RAM JET It is also called as - Lorin tube or flying stovepipe. Ram jet engines have the capability to fly at super sonic speeds.

The ramjet consists of an inlet diffuser, combustion chamber and exit nozzle or a tail pipe. It is a steady combustion or continuous flow engine and has simplest construction of any propulsion engine.

The fig. 8.9 shows the ramjet and its corresponding ruling cycle on T-S diagram,

FIGURE 8.9: RAM JET ENGINE

The air enters the plant with super sonic speed and it is slowed down to sonic velocity in the super sonic diffuser. The air is compressed due to compression shock waves as the velocity changes. Further compression and hence pressure rise takes place in the subsonic diffuser. The velocity of air passing through the diffuser decreases and hence the pressure increases. This is called - Ram compression. A 'pressure barrier' (To say simply, some mass of air with high pressure which acts as a wall) is created after the end of diffuser.

The fuel is injected through nozzles into the combustion chamber where it is ignited by a spark plug. The expansion of gases towards the entrance of diffuser is restricted by the pressure barrier and so the gases are constrained to expand through the tail pipe and exit nozzle with super sonic velocity. The high velocity gases provide the thrust to the unit.

As the ram jet has no turbine, the temperature of gases of combustion is not limited to a low value as in turbojet engine. Air-fuel ratios of about 15:1 are used.

Advantages: 1. It is simple in construction. 2. It has no moving parts and so free from unbalancing and vibrations. 3. Except rocket, thrust developed per unit engine weight is more than any other propulsion unit. 4. Wide range of fuels can be used.

Limitations: 1. At low and moderate speeds, the fuel consumption is high.

2. It will not operate statically (when not moving); since it needs velocity of air incoming for compression. So, it needs a turbojet or rocket to boost it upto a speed of 400 km/hr or more before it produces any thrust and propels itself.

3. To obtain steady combustion and flame stabilization, devices like flame holders are required.

The ramjet is useful for Mach number 0.7 to Mach number 3.0 (850 km/hr to 3600 km/hr). The performance is superior at speeds of Mach 2.0 and above. It has been used in missile applications where boost is provided by a rocket and subsonic ramjets have been used for driving propeller of a helicopter.

PULSE JET ENGINE Like ram jet engine, this also develops power or thrust by a high velocity jet of exhaust gases without the need of a compressor or turbine.

A pulsejet engine is an intermittent combustion engine and operates on a cycle similar to a reciprocating engine whereas turbojet and ramjet are continuous in operation and are based on Brayton cycle.

The fig. 8.10 shows a pulse jet engine.

FIGURE 8.10: PULSE JET ENGINE

The compression of incoming air is done by ram effect in the diffuser. The grid passages are opened and closed by V-shaped non return valves.

The fuel is injected into the combustion chamber by fuel injectors and the combustion is initiated by a spark plug. Once the combustion starts, spark is turned off and the residual flame in the combustion chamber is used for subsequent combustion.

As a result of combustion, the pressure and temperature of products of combustions increase. The pressure of the products of combustion is higher than ram pressure and hence the non-return valves get closed and as a result, the hot gases flow through the tail pipe with a high velocity and give a forward thrust to the unit.

After the passage of hot gases to atmosphere, the pressure in the chamber falls and the high-pressure air in the diffuser forces the vales to open and fresh air enters the combustion chamber and the cycle repeats.

As the airflow in not continuous; this is an intermittent combustion engine.

Advantages : 1. This is very inexpensive compared to a turbojet engine. 2. It is capable of producing static thrust and produces thrust in excess of drag at much

lower speeds than a ram jet.

Limitations : 1. Noise level is high. 2. High rate of fuel consumption. 3. Thermodynamic efficiency is low. 4. Serious limitation to mechanical valve arrangement. 5. Vibrations are severe. 6. The operating altitude is limited by air density consideration.

ROCKETS A rocket doesn't depend upon atmospheric air for its operation and so it can operate at any place - even in vacuum.

The fig. 8.11 shows the schematic diagram of a rocket engine.

FIGURE 8.11: ROCKET ENGINE

It consists of an injection system, combustion chamber and an exit nozzle. The fuel and oxidiser. (A substance which supplies air for combustion) burn in the combustion chamber and high pressure is developed. The developed pressure depends upon mass flow rate of propellants, cross sectional area of nozzle throat and the chemical characteristics of the propellants.

The products of combustion are ejected to the atmosphere at supersonic speed through the nozzle. In the nozzles, pressure energy is converted into kinetic energy. The reaction of this high velocity jet emerging from the nozzle produces the thrust to move the vehicle.

The fuel and oxidiser together is termed as propellant. Solid propellants and liquids

propellants are the generally used propellants in rockets.

There two types of solid propellant rockets: Restricted burning and unrestricted burning. In restricted burning rocket, the propellant is constrained to burn from one surface only. It is similar to burning of a candle or cigarette.

In unrestricted burning, the propellant is free to burn from all surfaces at the same time. This type of rocket is used when small thrust for a relatively long period is required.

The rockets are discussed in detail in the next chapter.

F0RMULAE 1. For a turbojet engine : Let Va - Velocity of air craft m/sec.

Vj = Velocity of jet relative to air craft m/sec ma = Mass flow of air kgs mf = Mass flow of fuel kgs

Then = Mass of the products leaving the nozzle for 1 kg of air/ Neglecting the mass of fuel;

(a) Thrust of turbojet = ma . (Vj - Va) N/sec (b) Thrust power = thrust x velocity of vehicle.

T.P = ma. (Vj-Va).Va- watts.

ROCKETS

INTRODUCTION In recent years, the principle of rocket has been adopted for propulsion of air crafts, missiles and space crafts. Unlike jet engines, the rocket engine caries its own oxygen and so not dependent on atmosphere for its working. Similar to jet propulsion, the thrust required for rocket propulsion is produced by high velocity jet of gases passing through the nozzle. The main difference between a jet engine and a rocket engine is that in case of jet engine, the oxygen required for combustion is taken from atmosphere and the fuel is stored whereas in a

rocket engine, both fuel and oxygen are contained. The jet engine can function satisfactorily upto a certain attitude only being dependent on air in atmosphere. The rocket engine is not dependent on atmosphere for its working and so it is the only suitable power plant for use at very high attitudes or in outer space (in vacuum). So, one of the important applications of rockets is to launch satellites.

CLASSIFICATION OF ROCKETS The rockets may be mainly classified as : 1. According to type of propellants :

(a) Solid propellant rocket. (b) Liquid propellant rocket.

2. According to number of motors ; (a) Single stage rocket (one motor only) (b) Multi stage rocket (more than one motor)

A hybrid rocket combines solid fuel with an oxidiser (substance that contains oxygen necessary for combustion). Fuel along with oxidiser is known as propellant.

APPLICATIONS OF ROCKETS The following are the important applications of rockets : 1. Missiles. 2. Satellites launching. 3. Signalling and fire work display. 4. Space ships. 5. Research. 6. Long range artillery. 7. Air crafts.

REQUIREMENTS OF AN IDEAL ROCKET PROPELLANT A good rocket propellant should have the following characteristics : 1. High calorific value. 2. High density so that it occupies less space. 3. Stability and ease of handling. 4. Reliable smooth ignition. 5. Low toxicity and corrosiveness.

SOLID PROPELLANT ROCKET Fig. 8.12 shows the schematic diagram of a solid propellant rocket.

Fig:8.12 Schematic Diagram of a solid propellant Rocket It consists of a seamless tube usually made of steel closed at one end. The tube is filled with solid propellant which contains both fuel and oxidiser. The propellant is ignited electrically and burns continuously. There is no method to stop the burning of propellant. The open end contains convergent-divergent nozzle through which gases are ejected out at supersonic speed. The reaction to ejection of high velocity gases produces the thrust of the rocket.

The configuration of the solid propellant varies according to the required thrust-time programme for the engine. The rate of burning is controlled by the configuration of the propellant.

There are 2 important types of solid propellants : 1. Composite or heterogeneous propellant. 2. Homogeneous double base propellant.

The composite solid propellant consists of an inorganic oxidiser such as potassium perchlorate or sodium nitrate dispersed in a fuel matrix. The double base propellant consists of colloid of Nitroglycerin -Nitrocellulose.

The main advantage of solid propellant rocket is its simplicity as it has no moving parts; because of absence of any fuel system. But, it has to be large enough to store the entire propellant and also strong enough to withstand high pressures and high temperatures. There's no provision to cool the combustion chamber. These rockets are suitable for producing thrust for shorter durations.

They are used to power projectiles, guided missiles and as boosters for aircrafts and space crafts.

LIQUID PROPELLANT ROCKETS The liquid propellants are of 2 types : 1. Mono propellant. 2. Bi propellant. A monopropellant is a liquid that requires no oxidiser for releasing the energy. Ni-tromethane, propyl nitrate and hydrogen peroxide are the general monopropellants. With mono propellants, storage tank for oxidiser is eliminated and control system is simpler. But the operation will continue for less time.

In liquid propellant rockets, separate tanks are provided for fuel and oxidiser. Both these mix

in a combustion chamber where combustion takes place. In case of solid propellant rockets, the steel tube in which propellant is filled forms the combustion chamber.

Some propellants are ignited with an electrical igniter. Some propellants ignite upon contact with each other. These self igniting propellants are called-Hypergolic propellants.

Liquid propellant rockets operate on 2 systems : 1. Pressure feed system. 2. Pump feed system. The fig. 8.13 shows rockets that operate on these systems.

Fig:8.13 Liquid propellant rockets In pressure feed systems, an inert gas is used to force fuel and oxidiser into combustion chamber with pressure. In pump feed system, separate pumps are used to force fuel and oxidiser which are driven by a gas turbine. The pumps and the turbine are mounted on a common shaft. The gas turbine may use main rocket fuel and oxidiser may be taken from combustion products bled off from the main rocket motor (engine). The products of combustion are discharged through the nozzle.

The pressure feed system is suitable for shorter duration and pump feed system is suitable for rockets of high power and longer duration.

The use of additional thrust by rocket motor at take off is termed as - Jet assistance take off (JATO) or Rocket assistance take off (RATO). Pressure feed system has been used for RATO.

The liquid propellant rockets posses certain advantages over solid propellant rockets. They are :

1. In liquid propellant rockets, the propellant may be used to cool the rocket motor by circulation of fuel around the walls of combustion chamber and nozzle. The maximum duration of uncooled rocket motor is about 30 seconds whereas cooled rocket motor can operate as long as the fuel lasts. So, it is suitable for long space flights.

2. For a given thrust, size of combustion chamber may be reduced compared to a solid propellant rocket.

3. Better control over the rate of combustion. In a solid propellant rocket, there are no methods to stop burning once ignition of propellant takes place. In liquid propellant rockets, combustion can be cut off and restart at any time.

THEORY OF ROCKET ENGINES The thrust is equal to rate of change of momentum of the working medium. The maximum velocity from nozzle is obtained when gases are expanded upto ambient pressure. In this situation, the thrust is only due to velocity and the thrust due to pressure is zero. When operating at high attitudes, this is not practicable because complete expansion would require a nozzle having very large exit to throat area ratio. This implies a long nozzle with considerable friction loss. Though theoretically, the thrust is maximum with complete expansion. (At incomplete expansion, pressure thrust is not able to compensate fully for loss of momentum thrust), due to friction effect, a smaller expansion ratio doesn't mean much loss of thrust.

The thrust with incomplete expansion : T = Momentum thrust + Pressure thrust = mp-Vj + AJ(PJ-pa) where

Where mp = Mass rate of propellant consumption -Kgs/sec Vj = Jet exit velocity relative to nozzle - m/sec pj = Exit static pressure - bar pa = Ambient pressure - bar Aj = Exit area - m2

Lower the ambient pressure, higher is the thrust. Maximum thrust is obtained when pressure is zero i.e. in vacuum. Unlike other jet engines, the thrust increases with attitude.

EFFECTIVE JET EXIT VELOCITY Vje This is a term used in rocket motor testing. Thrust T = m . Vje

= mp- Vj + Aj . (pj-pa)

THRUST POWER Tp It is the thrust multiplied by the flight velocity.

TP = T. Va = mp . Vje . Va Va = Velocity of air relative to tne engine.

This expression is different from thrust for a turbojet or propeller because the initial velocity of propellants is zero in case of rocket as air is assumed to be at rest.

PROPULSIVE EFFICIENCY OF A ROCKET P Propulsive efficiency

As in case of Jet, propulsive efficiency of a rocket approaches 100% as Vj/Va approaches unity and is lower on either side. For the propeller and turbojet, the flight velocity can't exceed the Jet velocity if the thrust is to be positive and for the rocket, the jet velocity is independent of forward motion, the effective jet exit velocity VJe can be less than, equal to or greater than Va.

The variation of propulsive efficiency with aircraft speed is shown in fig. 8.14.

Figure 8.14: Variation of Propulsive Efficiency

The propulsive efficiency of a rocket is lower than both propeller and turbojet and so can't compete with them in economy. But, rocket is the only power plant that can be used for speeds above 4000 Kms/hr and for altitudes above 30,000 metres.

SPECIFIC IMPULSE It is another term used in the performance of a rocket. It is defined as the ratio of thrust force to rate of propellant expenditure.

Specific impulse gives a direct comparison of effectiveness of propellants. Propellants with greatest possible specific impulse should be used as it allows a greater load to be carried for given overall rocket weight.

Specific impulse is an important parameter in the performance of a rocket. High combustion temperature is necessary to get high specific impulse. In general, fuels having hydrogen content give high specific impulse without giving excessively high combustion temperatures.

IMPORTANT POINTS 1. A rocket is a device which works on the principle of jet propulsion, carries its own

oxygen for combustion of fuel and so it can work at high altitudes and in vacuum also. 2. Rockets can be mainly classified based on the type of propellant and number of stages. 3. Fuel along with oxidiser (substance that contains oxygen - usually liquified oxygsn is

used) is known as propellant. 4. The unit of specific impulse is - seconds. 5. For given nozzle and reactant, thrust of rocket at any altitude is a function of combustion

pressure. 6. The specific thrust increases with increasing value of combustion temperature and low

molecular weight of gases. 7. With liquid propellants, rate of combustions can be controlled better. 8. Propulsive efficiency of a rocket is less than propeller and turbo jet. 9. Cryogenics is the science of maintaining extremely low temperatures. The oxidisers can

generally be liquified at very low temperatures. A crygoneic engine is one in which the oxidiser is maintained in liquified state at very low temperatures.