 Hello and welcome to lecture number 39 of this lecture series on introduction to well jet propulsion. So, in this lecture series on jet aircraft propulsion, we have had quite some interesting discussion on different components of jet aircraft engine. Before that of course, we also discussed about the cycle analysis both the ideal cycle as well as the real cycle analysis of jet engines. Subsequently, we also got an opportunity to talk in depth about different components which constitute a jet engine starting from the intake, the fan compressor, combustion chamber, the turbines and of course, the nozzles. We have seen different types of these components like different types of intakes, different types of compressors, this axial and the centrifugal compressor, the combustors, the turbines and also the different classifications of nozzles. So, after having discussed these different components, these individual components in detail, we also started some discussion on matching of the components. Subsequently, we have had some discussion in the last few lectures on ramjets and pulse jets. So, I think I have mentioned several times that ramjets are very simple types of engines and if in fact pulse jets are also very simple engines in the sense that both these engines do not really have the rotating components which are characteristic of the traditional or conventional jet engines like turbojets or turbo fans and so on. So, the absence of these rotating machinery makes the whole engine at least conceptually very simple, but so you may wonder if they are very simple, why is it that they are not used and why do we have to really use these complicated machinery in actual practice. Well, there are practical limitations of using these engine concepts and for example, ramjets cannot generate any static thrust. So, if you were to use ramjets on an aircraft, the aircraft cannot take off because ramjets cannot generate any static thrust and ramjets are more efficient only in supersonic Mach numbers. So, unless of course, we come up with a concept of using both these engines together that is let us say conventional turbojet engine which will take off and accelerate to the supersonic Mach numbers. Subsequently, the ramjets can take over and so on. There are some of these concepts which I think we will discuss in the next class which will be our last class on last lecture on this lecture series. So, some of these advanced concepts which involve integration of the traditional jet engines with some of the more advanced jet engines like the ramjets will be discussed in little bit detail in the next lecture. In today's lecture, we are going to discuss about the ramjets, the components of ramjets and how do we take a look at in detail what are the different constituents of a ramjet and how can we analyze ramjets and so on. So, in today's lecture, let us take a look at what are the topics that we are going to discuss. So, we are going to talk about the components of ramjets and pulse jets. We will also be talking about ramjet combustors in little bit detail which we will see very soon that they are very similar to the afterburners that you might have discussed during the turbojet engines. So, ramjet combustors will be taken up in some detail. We will also see how we can analyze ramjet combustors and determine total pressure loss across combustors and so on. We will subsequently take up the different types of pulse jets engines, the valve and the valve less pulse jet engines. We will also be talking about in very brief about a conceptual cycle based on pulse jets which are known as the pulse detonation engines. So, these are some of the topics that we are going to take up for discussion in today's lecture. So, we are going to have primarily an overview of the different components and we are not going to analyze some of them in detail because we have already done. So, for example, an air intake that we have discussed in very much detail in some of our earlier lectures. Similarly, nozzles we have discussed in detail. So, we are not going to revisit these concepts and these topics all over again. We are going to assume that you have already understood the working of air intakes and nozzles because it is the same concept which is to be used here as well. But combustors we will see in little bit detail on how we can analyze combustors, how we can calculate pressure loss and temperature rise in combustors etcetera. So, let us begin with ramjets and as I have mentioned ramjets are probably one of the most simplest forms of jet engines which have which have been evolved which have worked. In fact, the Germans had used ramjets very extensively during the second world war and some of their advanced military aircraft had the ramjet based propulsion system and they were very successful and had created lot of problems for the allied forces to resist the power of some of these advanced types of engines which were unknown to the rest of the world at that time. And so, ramjets have been used for a long time, but they have not really been used for civil aviation and in fact, the majority of application of ramjets have so far been limited to military application. They are still used primarily for missile propulsion. So, many of the missiles which have to have longer range will benefit by using an engine like a ramjet because ramjets are air breathing propulsion system which means that a missile which uses ramjet engines does not have to carry an oxidizer. It just has to carry a fuel and since ramjets are everything that will improve or increase the range of such missiles and some of the long range missiles actually use ramjet engines. Of course, the initial part of the missile will still have a booster phase which will be a rocket propellant or rocket engine which could be either solid propellant or a liquid propellant which will take the missile to a certain Mach number from which the ramjets can begin to operate. And so, they have been used and they are still used in many of the modern day missiles, but they have not really been used in aircraft application or at least now, but some of the earlier generation aircraft which had supersonic cruise used ramjet engines, but they are not used anymore and they have definitely not been used in civil aviation. But ramjets being very simple in the term of at least conceptually very simple to design and so, besides the fact that ramjets do not have rotating machinery makes the design of ramjets even more simpler. And that is the reason why ramjets still have lot of potential in terms of their application to civil or military aviation. So, let us take a look at a few salient features of ramjets before we go on to the components of a ramjet. So, in a ramjet there are basically three components as we know ramjets consist of intake, they consist of combustion chamber and the nozzle. So, how does a ramjet work? We have seen that little bit detail in the last class. In a ramjet the compression process is taken care of purely by the intake, because ramjets as we know as even the name suggests depend basically upon the ram compression of air. That is as the engine is moving at very high speed the incoming air is decelerated from very high Mach number to very low Mach number subsonic Mach numbers and in that process there is a substantial increase in pressure. So, that is the basic principle behind compression of air incoming air just within the intake. So, intake of a ramjet not only serves the purpose of an intake as such they also take care of the compression as such. So, the entire compression process is handled by the intake. Downstream of the intake we have a combustion chamber, combustion chamber as we will see very soon is very similar to that of an after burner and these combustion chambers have certain flame holding devices to ensure that the flame is held stable within the combustion chamber and does not really move out. And downstream of the combustion chamber we have the nozzle which accelerates the combustion products through the through them resulting in thrust which is generated and that is what propels the ramjet forward. So, with the use of just three simple components ramjets are able to generate thrust and ramjets have over the years been demonstrated to be very successful, but there are lot of disadvantages which we have been discussing the main disadvantage being the fact that ramjets cannot generate static thrust. So, in a ramjet engine the intakes in fact all the three components are important, but the intakes will play a little more importance in a ramjet because they have to take care of not just the capturing air from the free stream intakes also need to carry out the compression process. So, intakes form a very important component of ramjets and after the intake as we have discussed the compressed air from the intake goes into the combustor and combustion products are expanded through the nozzle to generate thrust. So, let us take a look at these components one by one ram intakes we will not discuss in too much detail because these intakes are very similar to the supersonic intakes that we have discussed in detail long back. So, supersonic intakes as we have seen will basically consist of a conversion diversion section where in the flow is first taken through a section where the area is decreasing and conversion and the area becomes minimum at what is known as the throat and downstream of the throat we have an increasing area. And we have seen that the intake operation is very sensitive to the inlet and exit conditions which means that if we have a fixed intake then it operates efficiently only for its design condition. And under off design conditions the intake performance can be grossly suboptimal and therefore, these intakes have to necessary that the these intakes have certain variable geometry features incorporated in them. So, that when the aircraft is operating under off design conditions the intakes can still deliver thrust which is thrust and other performance parameters which are not very far from the optimal value. And typical ramjets ramjet intakes will have a spike or a center body which can be adjusted to adjust the location of the shocks. So, a ramjet as we have seen is more efficient when it operates at supersonic Mach numbers which means that the deceleration in the intake occurs through shock waves. And we have seen different types of intakes we have seen external compression, mixed compression, internal compression intakes and fixed and variable geometry and so on. So, most of the ramjets that have flown so far have axisymmetric geometry they do not really have a 2D geometry because the whole engine is very simple. And so if it is 2D then the 2D geometry has to be really converted to circular in the combustion chamber and then again the nozzle. So, normally the ramjets that have been demonstrated over the years have an axisymmetric geometry. And so the intake center body is also an axisymmetric spike which can be adjusted so as to locate the shocks according to our desired positions. And usually the intakes ramp will have or will generate 2 or 3 oblique shocks eventually ending in a normal shock because in a ramjet the combustion can take place while combustion takes place in subsonic speeds. So, a supersonic Mach number flow has to be decelerated using these shocks and then it becomes subsonic before it enters the combustion chamber. So, through these shocks of course, there is there are stagnation pressure losses which are incurred and we have seen that, but this can be controlled or kept minimal by ensuring that there are enough number of oblique shocks before a normal shock. So, we may not want to really use a single normal shock to decelerate a supersonic Mach number to a subsonic Mach number because that incurs a lot of stagnation pressure loss which will eventually result in thrust loss. So, use of few oblique shocks 2 to 3 are the ones which are commonly used 2 or 3 oblique shocks followed by a normal shock that would be a kind of an optimum configuration for a typical ramjet intake. So, after the normal shock of a ramjet intake then the intake then progresses and becomes a purely subsonic diffuser because after the normal shock the flow is subsonic. So, then downstream of the normal shock we have a diverging area diverging area in a subsonic flow leads to deceleration. So, the flow is further decelerated from high subsonic to low subsonic before the flow enters the combustion chamber because in the combustion chamber we would like to have relatively lower velocities. So, that there is enough time for the fuel to burn within the length of the combustion chamber because if we have a flow which is coming in at very high speed there are 2 issues firstly it may lead to stability issues the flame may not be stable in the combustion chamber and the other thing is that that would also necessitate a larger length of the combustion chamber. So, that the fuel has enough time to ignite and burn within the combustion chamber. So, these are to be avoided and so they we would like to decelerate the flow to relatively low velocity before the flow enters the combustion chamber. So, these are the different functions of the intake we have already discussed intake in lot more detail earlier on. So, I will not go into the details of that all over again, but let us look at the next component which is the combustor we have already seen and discussed about combustors the main combustors of jet engines and there we have seen the different types of combustors the can type the can annular and canular type and so on. So, in a ramjet combustor the main combustor is different ramjet combustor is very different from that of the main combustor of a jet engine, but the ramjet combustor can be very similar to an after burner. We also seen some after burner geometries earlier on. So, ramjet combustor has some similarities with an after burning or an after burner of let us say a turbo jet engine and so we will discuss about some of the components or details of ramjet a typical ramjet combustor. So, unlike in turbo jets where there are rotating components ramjets do not have any rotating components. So, that is a big advantage for a ramjet because we can now afford to have temperatures which are much higher than what were used in conventional jet engines. So, there are a maximum temperatures as high as 3000 Kelvin or have been commonly used in ramjets which are unthinkable for a conventional jet engines because there are limitations by the turbine blade which limits the temperature to about 1500 or 1600 Kelvin or not beyond that. Whereas, in ramjets we can use temperatures as high as 3000 Kelvin which is a substantially higher temperature and ramjet combustors as I mentioned are very similar to the after burners which are used in turbo jet engines. Ramjet combustors are characterized by flame holders. So, flame holders are basically devices or certain bluff bodies which have been designed aerodynamically to ensure that the flame remains stable within the combustion chamber because flame stability is a very important issue. Unlike in traditional or conventional combustion chambers where there are liners and other things which can take care of stability plus there are swallows and lot of other physical features which have been provided to ensure stability ramjet combustors have flame stabilizers or flame holders which will ensure that the flame remains stable and that the combustion can be completed within the length of the combustion chamber itself and that is a very important component which will constitute a typical ramjet combustor. But of course, the presence of these flame holders may also lead to stagnation pressure loss and so the designers or designers of combustors would need to keep in mind although we would like to have a larger surface area of these flame holders for better flame stability increase in surface area can obviously lead to increase in stagnation pressure loss. So, there is an optimization problem there one where one would like to keep the flame stability limits as high as possible at the same time we would also like to ensure that the stagnation pressure losses are kept minimal because stagnation pressure loss in the combustion chamber will lead to thrust loss eventually it will all affect the thrust. So, designers would need to carry out an optimization to ensure that the flame stabilizers or flame holders not only give a fairly reasonable stability margin it also ensures the pressure losses stagnation pressure losses are kept minimal. So, that is one of the things that the designers would like to keep in mind. So, this is a typical schematic of a ramjet combustor as you might have seen it is very similar to that of an after burner. Now, in this schematic let me explain what this is all about. So, here we have the intake or intake exit. So, the air which is coming in from the intake. So, exits at this point. So, this is probably the exit of the intake and somewhere on the center body one might want to keep the fuel injectors. So, fuel is injected through the center body here and then this entire length from here all the way up to the nozzle entry constitutes the combustion chamber. And somewhere here we might have the flame holders which I mentioned were are typically bluff bodies which are aerodynamically designed and these flame holders ensure that the flame is restricted to the combustion zone. So, combustion region is somewhere here we will see the operation of a flame holder in little more detail later on. So, the flame is held stable or tried to help been tried to be stable within this combustion zone. After the combustion region of course, we have the nozzle where this is the nozzle entry we have the convergent section then ending in a throat followed by a divergent section. So, this is a typical ramjet combustor and as you can see here the intake exit also has a divergent section because we are continuously decelerating the flow in a subsonic Mach number a diverging area leads to deceleration. So, that is how we accomplish deceleration by increasing the area fuel is injected here and it gets ignited because of the and the flame is held stable by the use of flame holders which ensure that the flame is held stable within the combustion region itself. So, I mentioned about the flame stabilizers let us now look at what is it that flame stabilizers basically do. So, to understand the working of flame stabilizer let us consider two cases let us consider a case where there is a spark plug which is used to ignite an air and fuel mixture and on one hand we have an intermittent spark and let us say we also have a continuous spark. So, if we have an intermittent spark so every time the spark is initiated there is a combustion which gets started. So, the combustion products which are indicated or the flame front indicated by these wavy red circles will propagate downstream because there is a certain speed associated velocity associated with the incoming mixture air and fuel mixture and as the spark is ignite or the spark is started it ignites the air fuel mixture and the flame front as we can see here propagates radially outward and as it moves downstream the flame front gets larger and larger. So, this is how a flame front would propagate if we have an intermittent spark on the other hand if we have a continuous spark then there are infinite number of these flame fronts which are generated and. So, we what we see will be flame front which looks like this we see a continuous flame front because there is a continuous spark which ignites and causes the flame front to propagate radially outward and. So, since there are infinite number of these we do not really see individual flame fronts as we see here which is on account of the intermittent nature of the spark here we have a continuous spark which ensures that the flame front also is continuous. In a ramjet one would like to have flame front which is very similar to that of a continuous spark we do not really want an intermittent spark we would like to initiate combustion using a spark plug let us say, but once ignited the flame should be self sustaining. So, in a ramjet combustor what we do is that we provide a bluff body. So, you can see here the bluff body which is what which is very similar to the bluff body I had shown for the ramjet combustor. So, let us go back there. So, these are the flame holders as we can see. So, downstream of this what happens is what they are taking a look at. So, this is the flame holder a difficult flame holder a bluff body and those of you who have undergone courses on aerodynamics would immediately realize that since this is a bluff body downstream of the bluff body should be a wake. Wake is a region where there is a vortex where the numerous vortices the high wake is characterized by high levels of turbulence and recirculating flow. So, downstream of the vortex downstream of the bluff body we have a vortex dominated recirculating flow and. So, here we have the flame well the air and fuel mixture coming in and let us say once the flame is ignited using some means by a spark plug say once the flame is ignited the flame front will propagate as shown here. Because of the fact that once you ignite downstream of the bluff body we have a lower velocity much lower velocity flow and there is recirculation hot recirculation of the gas. As a result of this the bluff bodies themselves or the recirculated region behind the bluff body will itself act like a continuous spark. So, in a continuous spark we have seen that the flame front will assume a shape like what you see here because there are infinite number of flame fronts which are present. So, the presence of the bluff body and a consequent as a consequence of that the hot recirculated gas downstream of the bluff body causes generation of these flame fronts which will ensure that the flame is held stable and within the limits of the combustion chamber. So, this is how a typical flame holder is going to work that is downstream of the bluff body or the flame holder. We have hot recirculated gas which will ensure that the flame fronts are held or kept in a stationary or stable mode to ensure that the flame is held stable within the limits of the combustion chamber. So, a flame front is so the presence of a flame holder is essential for ensuring that we have a stable combustion within a ramjet combustor. So, in a typical after burner also you would have a flame stabilizer or a flame holder very similar to what we have discussed. So, the presence of flame holder ensures that the flame is held stable within the combustion chamber. So, what we are going to do next is to analyze typical combustion chamber in a very simple manner. We will carry out a one dimensional analysis of flow through a combustion chamber carry out momentum balance and see how we can calculate or estimate the pressure loss stagnation pressure loss in the combustion chamber as well as this changes stagnation temperature across the combustion chamber. So, we will use a very simple 1 D model to calculate the pressure loss as well as the stagnation temperature rise in a combustion chamber. So, what we will see is that even if there are no frictional losses there will still be a certain amount of pressure loss because of the fact that we are adding heat in a constant area duct. So, those of you who would have studied Rayleigh flow which is basically flow through constant area duct with heat transfer will understand the fact that there will be a pressure loss associated with heat addition in a constant area duct. And this is not because of the fact that there is friction even if we assume no frictional losses there will still be a certain amount of total pressure loss which comes into picture. So, we are going to analyze flow where we will calculate the total pressure loss not just because of friction, friction also we will assume there is something present. But even otherwise we will see that even if the frictional losses are 0 there will still be a certain amount of pressure loss. So, to analyze that we will consider a 1 D flow in after burner or a combustor we will obviously assume that the flow entering and leaving the combustor are uniform. And that the flame holders will exert a certain drag which we will denote by D on the flow. So, this drag is towards the left hand side as we will see little later. So, this is the simplistic combustor model that we are talking about U 2 refers station 2 refers to the inlet of the combustor station 4 refers to the exit of the combustor. So, let us say U 2 is the flow axial velocity entering combustor and U 4 is the axial velocity leaving the combustor. And within the combustor zone which is indicated by these dotted lines we have these flame holders as well. So, this is a very simplified model of a combustion chamber flow. And we are going to analyze the flow that is that is basically applicable for this kind of a very simple 1 D combustion flow. So, flow through this we will as I had said the flame holders exert a certain drag which is towards the left hand side. And therefore, we denoted by the parameter D which is basically drag difference in static pressure at the inlet and exit is possible. And therefore, there is a corresponding force associated with that and that is denoted by P 2 minus P 4 multiplied by the cross sectional area. So, this is the net change in momentum of the force which is basically the difference in the momentum of the flow between the exit and the inlet. So, exit momentum is basically mass flow rate at the outlet multiplied by the corresponding velocity. Similarly, the mass flow rate at the inlet multiplied by the corresponding velocity. So, of course, mass flow rate at the inlet and outlet are more or less same except for the fact that m dot 4 also includes certain amount of fuel which is added in the combustion chamber. So, this we can simplify now when we simplify that we get P 2 minus P 4 is rho 4 u 4 square minus rho 2 u 2 square basically because mass flow rate is rho into a into u. So, a is common factor here. So, which gets cancelled out and therefore, mass flow rate becomes rho times u 4 multiplied by u 4. So, rho u square minus rho 2 u 2 square plus k times half rho rho 2 u 2 square here k is assumed to be the pressure drop due to friction. So, that also accounts for the drag which has been accounted for in this parameter k. Now, we shall now express this in terms of Mach number. So, Mach number we know is m square which is basically the ratio of velocity square to speed of sound which is square root of gamma r t which is gamma P by rho. So, we express this equation that we have written P 2 by P 4 as 1 plus we have expressed all these which have all these velocities ratio of density and velocity square in the form of Mach number. So, we have P 2 by P 4 is 1 plus gamma M 4 square minus gamma M 2 square into P 2 by P 4 plus k times gamma M 2 square by 2 into P 2 by P 4. So, this is simplifying this equation that we have written here we have simplified that into in the form of P 2 by P 4. So, basically we divide throughout by P 4 we get P 2 by P 4 minus 1 is equal to this and that is how we get 1 plus gamma M 4 square and so on. So, this again can be simplified we have simplified that in the form of what is seen here because we have P 2 by P 4 on both the sides. So, if we express that in the form of the static pressure ratio we have P 2 by P 4 is 1 plus gamma M 4 square divided by 1 plus gamma M 2 square into 1 minus k by 2. Here k is the parameter which is associated with the pressure drop. So, which means that even if k is 0 which means if there is no frictional losses being assumed there will be always a certain pressure difference because of the fact that the Mach number is different as you add heat in a constant area duct. So, we will now proceed to express the static pressure ratio in the form of stagnation pressure ratio. So, P 2 by P 4 in terms of stagnation pressure ratio from the isentropic relations. So, the above equation which we can simplify we get 1 plus gamma M 2 square into 1 minus k by 2 divided by 1 plus gamma M 4 square this whole thing multiplied by 1 plus gamma minus 1 by 2 into gamma M 4 square divided by 1 plus gamma minus 1 by 2 into gamma M 2 square whole raise to gamma by gamma minus 1. So, here we have expressed the total pressure ratio the previous equation was for static pressure ratio we have now expressed that in terms of the total pressure ratio. So, if we assume that the fuel added is much less as compared to the mass flow rate of air itself we have M dot 4 is equal to M dot 2 and from state equation we have P is equal to rho R T. Then we can also express the static pressure ratio in the form of stagnation temperature ratios. So, we have P 2 by P 4 is U 4 by U 2 into T 2 by T 4 which is M 4 by M 2 square root of gamma square root of T 2 by T 4 which again we have expressed in terms of the isentropic relation square root of T 0 2 by T 0 4 1 plus gamma minus 1 by 2 M 4 square by 1 plus gamma minus 1 by 2 M 2 square. So, stagnation temperature ratio can therefore, be expressed because we have stagnation temperature ratio here we have simplified this equation we can express the stagnation temperature ratio T 0 4 by T 0 2 as 1 M 4 square by M 2 square into 1 plus gamma minus 1 by 2 M 4 square by 1 plus gamma minus 1 by 2 M 2 square this multiplied by 1 plus gamma M 2 square into 1 minus k by 2 this whole square divided by 1 plus gamma M 4 square the whole square. So, we have now two equations we have which we have derived from fundamental one dimensional analysis of flow through combustors one of them is pertaining to the stagnation pressure drop across the combustion chamber and the other one pertaining to the stagnation temperature rise across the combustion chamber. And both these equation we have seen there is a parameter k which is denoting the pressure loss in the pressure loss as a result of or frictional losses associated with the presence of the effects of viscous forces or viscous effects. Even if we assume k to be 0 we will still see that P 0 4 P 0 2 by P 0 4 and T 0 4 by T 0 2 still have a certain number associated with them which means that there will still be a certain stagnation pressure loss occurring not just because of friction, but because of the fact that we are adding energy or heat into a constant area duct and in the absence of friction also it leads to certain pressure loss. So, this is one way in which one can analyze flow through combustors in a very simple manner based on certain parameters which can be calculated like Mach number at inlet and exit and the static pressures and so on. So, based on these parameters one can estimate and calculate the stagnation pressure loss which combustor is likely to incur as a result of frictional losses as well as as a result of heat addition in a constant area duct. So, we have analyzed combustion chamber in little bit detail so far and this combustion chamber analysis as we have seen it is primarily with one dimensional assumption which is fairly reasonable because in a ramjet combustor we have seen it is a much more simpler combustion chamber as compared to let us say the main combustor of a jet engine a gas turbine engine where the combustion chambers can be little more complicated. And so, 1 d analysis for such combustors may not really be true whereas, ramjet combustors 1 d analysis can still be assumed to be reasonably good estimate of what is happening within the combustion chamber. So, we will now take a look at the next component that constitutes a ramjet which is the nozzle. We have already discussed nozzle in great detail in some of the earlier lectures we have seen the different types of nozzles a convergent nozzle is used in subsonic flows a convergent divergent nozzle is what one would use in a supersonic flow. As I said ramjets are primarily operational in supersonic flow ramjets would need to have a c d nozzle that is a convergent divergent nozzle. The reason being that one would like to accelerate the flow to supersonic speeds. Most of the ramjets as I have said have an axisymmetric geometry and so, the nozzles also have an axisymmetric geometry with a provision to with or without provision to vary the area. And so, if a ramjet has to operate over a variety of range of operation in terms of mach number and altitude then the use of variable area nozzle becomes significant. Because, if ramjet is design a nozzle is design for a particular operating condition it may not be very optimal or efficient under off design conditions. So, to ensure that the ramjet and the nozzle operation is still fairly optimal or efficient at other of conditions other off design conditions. One would like to incorporate a variable nozzle geometry. So, some of the ramjets also have a variable nozzle geometry incorporated in them. And so, most of the nozzles as I said are axisymmetric and variable geometry is basically required for optimal operation under various operating conditions. So, we will not discuss more details of nozzles because we have already discussed one full chapter on nozzles different types of nozzles. Now, there is another aspect of ramjet that ramjets can be of different types. So far we have seen ramjets the ones which we have discussed at least the schematic that I have shown in the last class was true for one type of ramjet. And there are several other variants or types of ramjets which are which have been proposed some of them have been used. And some of them continue to be on the conceptual level, but seem to show promise. So, let us see what are the different types of ramjets which are possible again we will not going to too much details of the geometries etcetera because that is not really in the scope of the syllabus of this course. So, there are variants of ramjets and there are different types of configurations which can be thought of. So, the conventional ramjets which is what we were discussing so far are sometimes referred to as can type ramjets because their combustors are similar to the can type of combustion chamber of gas turbine in it. So, these are basically called can type ramjets sometimes denoted by C R J. Depending upon the fuel that is used in ramjets one could have either solid fuel ramjets which are S F R J or liquid fuel ramjets or gaseous fuel ramjets. So, either of all these three possibilities are there the liquid fuel ramjets being little more popular than the others. Now, ramjets can be integrated with a rocket because as I said ramjets cannot take off on their own which means they need some assistance in taking off or reaching their starting speed and most often rockets are used for carrying the ramjets to the desired speed. So, such ramjets are called integral rocket ramjets or IRR. Now, again depending upon the kind of fuel which is used you may have a solid fuel integral rocket ramjets or liquid fuel integral ramjets or even a gaseous fuel integral ramjets. And it is also possible that one can combine a ramjet with one of the conventional engines ramjets combined with let us say a turbojet engine. So, turbojet will take off the aircraft take it to a certain desired speed before the ramjets can start operation. So, such engines which are combination of ramjets and conventional engines are called the combined cycle mode and these are called the air turbo ramjet. I think in the next class in the that is the last lecture we will see some more details of air turbo ramjets and what is meant by air turbo ramjets and how they operate and so on. We will leave those details for the next class. There is also another class of ramjet engines which are known as ejector type ramjet wherein ejector stream is used for propelling the ramjet forward and so on. So, as you can see there are different types of ramjets which are possible. We have only discussed one of them in detail that is the conventional ramjet or the can type ramjets and of course, there could be variants of these ramjets depending upon the cycle they are operating in their function that these ramjets are required to operate upon. So, this winds up our discussion on ramjets in some detail. Let us move on to pulse jets. We have also already discussed pulse jets and their cycle analysis in the last class. Today we are going to discuss about different types of pulse jets and we will see what are the different features of these different types of pulse jets. Now pulse jets as we have discussed in the last class are yet another class of simple or so called simple jet engines and I call them simple because they do not require the use of very complicated turbo machinery like compressors and turbines because presence of these rotating machinery makes the design extremely complicated and something that is very difficult to design and optimize and develop. Whereas ramjets and pulse jets wherein at least pulse jets for that matter are so simple that one can actually try and develop them in a laboratory scale and many of them have been actually demonstrated on a very amateurish lab scale unlike jet engines which cannot really be demonstrated on such a scale which is why pulse jet engines are much more simple and even in fact simpler than ramjet engine for that matter. So, there are two distinct types or classes of pulse jet engines. One of them are known one set of pulse jets are known as valve type pulse jets and the other set of engines are known as valve less pulse jets. So, we will discuss little bit about both of these types of pulse jets. In fact little more of the valve less pulse jets because you already seen valve pulse jets in some detail in the last two lectures. So, in a valve pulse jet engine we use a series of valves which open and close depending upon the operating condition that is as combustion is initiated due to the pressure develop within the combustion chamber the valves upstream of the combustion chamber close and the combustion products are forced to move through the tail pipe and this results in a thrust. So, as the engine generates thrust the valves again are forced to open because it will also draw in air from the ambient and then again we initiate or ignite or inject fuel in the combustion chamber and that again leads to combustion and the valves close and combustion products are exhausted through the tail pipe and this continues. So, you can immediately see that the thrust generated in a valve in fact in pulse jets are of pulsating or oscillating nature they do not generate a continuous thrust. So, that is one disadvantage of pulse jet in the fact that they cannot generate continuous thrust. So, the in a valve pulse jet one would use mechanical valves sometimes they are referred to as reed valves and so on. These are valves which are one way valves basically which will prevent the flow from the combustion chamber to move upstream it will ensure that the flow always moves only downstream and depending upon the cycle operation if it is combustion phase the valves are closed resulting in combustion products being exhausted through the tail pipe. After exhaust process is over the valves open because there is a partial vacuum created in the combustion chamber which forces the valve open and suction air from the ambient and then this cycle is repeated. So, in a valve combustion chamber or in a valve type pulse jet use of mechanical valves is or the presence of mechanical valves is significant and that is what ensures the operation of the pulse jet as such. So, in the case of valve engines in fact that is true for valve is as well the combustion process is basically self sustaining like any other jet engine one would not have to ignite the fuel all over again every time. So, once you ignite it and the engine starts then the combustion is basically self sustaining and the valves basically operate when the combustion initiates in the combustion chamber the valves close and the combustion products are expelled or expelled through the tail pipe to create thrust. So, this is a valve type of combustion of pulse jet, but there is a big disadvantage for valve type of combustion valve type of pulse jet. The major disadvantage being the fact that presence of these mechanical valves means that there are a lot of possibilities of wear and tear in which indeed happens that valve type of combustion pulse jet do not really have a large life in the sense that they can operate for about 30 minutes or so beyond which the valves of the mechanical valves will wear out and the operation becomes extremely inefficient. So, that is one major disadvantage of a valve type of pulse jet the presence of mechanical valves. So, this can be overcome by avoiding the use of mechanical valves all together, but then how do you create a pulsing effect and generate thrust. So, this can be accomplished by using what are known as a valve less pulse jet engines which use aerodynamic valves they do not really have mechanical valves, but they have aerodynamic valves. So, in a valve less pulse jet engine the major disadvantage being the presence of mechanical valves there are of course, wear and tear reliability issues and of course, noise problem which can be partly overcome by using valve less pulse jet. So, valve less pulse jet do not have mechanical valves, but they have aerodynamic valves. Now, the problem of noise is still continuing even in a valve less pulse jet one would still have a lot of noise to deal with, but the issue of wear and tear and reliability is much better in valve less pulse jet as compared to the valve type of pulse jets which is why valve less pulse jets also have been explored some of them have actually been used and flown at least to demonstrate the concept. So, one of the most popular forms of valve less pulse jet engines which have been demonstrated and in fact, one of the aircraft actually flew at least the test aircraft flew with a valve less pulse jet to demonstrate the concept. So, the one which has been most popular is known as the Lockweed Healer design because they were the inventors of the valve less this particular design of valve less pulse jet. This basically has a U bend that is the exhaust has after the combustion chamber there is a long U bend in fact, the intake and the exhaust face each other that is they face the same direction and since we are using aerodynamic valve here these valves will leak there will be a certain amount of leakage associated with the valve, but that can be minimized to generate a net thrust forward. So, both the intake and exhaust pipe will say for face the same direction and that is the whole concept of using a U bend which will cause certain blockage to mimic that of a valve a mechanical valve. So, using these the bend that we are going to see in the next slide. So, in this particular design that you can see this is a schematic of a typical valve less pulse jet engine. So, fuel is initiated or ignited or injected here this is the intake of the pulse jet engine. So, the air is drawn into the pulse jet fuel is injected here and ignited and once ignited the combustion products will be expelled through the exhaust. You can see that there is the length of these two are differential and so combustion products are expelled or exhausted through this some amount of these combustion products also get exhausted through this, but that is very minimal. So, majority of the exhaust goes through this exhaust which is shown here and so this results in a net forward thrust. So, this is one of the designs of a valve less pulse jet engine and there are many other designs if you search in the net you will find numerous designs for valve less pulse jet engine this is the one which have been most popular and successful. So, in this combustion product process it generates two shock waves and by appropriately tuning the system adjusting the length of the system one can achieve a stable resonating combustion which leads to considerable thrust generation. So, one can tune the length of these two different sections the length of this exhaust pipe and the inlet to ensure that one can achieve a stable resonating combustion which leads to lot of a considerable thrust. Now, here the combustion products are combustion process is deflagrating and as a result of these valve less pulse jet engines have a clean combustion that is more or less complete combustion occurs in the valve less pulse jets. They have not been used in actual aircraft they have been demonstrated, but they have been demonstrated successfully for a range of aircraft from very small size to very large sizes. So, this is one example one picture of an aircraft a French aircraft which has actually got a Lockweed Healer type of a valve less pulse jet you can see the engine here this is the valve less pulse jet engine where you can see the U bend and the long exhaust pipe. So, valve less pulse jet engines are characterized by a rather long exhaust pipes you can see the long exhaust pipe here. So, this is one example of an aircraft which had a valve less pulse jet engine. Now, there is another concept which is also gaining lot of significance that is known as the pulse detonation engine and they are expected to have efficiencies much higher than conventional gas turbine engines. These engines also do not have any moving parts like a ramjet or a pulse jet and pulse jet detonation engines actually have a detonation rather than deflagration. Detonation involves supersonic combustion of the fuel unlike deflagration which is subsonic combustion pulse detonation engine as the name itself suggest has detonation which means fuel ignites at supersonic speeds. So, they are basically been envisaged to be used in supersonic flights unlike the valve less pulse jet engines which are meant for subsonic speeds. Now, in this pulse detonation engine one would use intermittent detonation waves to generate thrust and the main advantage of the PDE is that they are not governed by acoustics of the system unlike valve less pulse jet engines or the valve pulse jet engines which means then since they are not governed by acoustics there is a better control of the engine unlike the conventional pulse jet engines. And PDE is generate much higher thrust which are for a comparable ramjet even at subsonic speeds unlike ramjets which cannot really generate too much thrust at subsonic speeds pulse detonation engines can generate substantial thrust for a comparable ramjet even at subsonic speeds. So, that is a very significant advantage of pulse detonation engine. And so there are different modes of operation of pulse detonation engine like it can be used as either as standalone engine where is wherein we have just a series of these tubes which constitute the PDE and then all of them generate operate simultaneously to generate thrust or you can integrate with other cycles. You can have a combined cycle or you can have a hybrid cycle. In combined cycle one would add like to add a PDE pulse detonation engine to flow path of a ramjet or a scramjet. And therefore, it enables operation even at subsonic speeds and also at hypersonic speeds. So, a combined cycle PDE can actually enable operation of an engine from all the way from subsonic to hypersonic speeds. And the third type of PDE are known as hybrid engines. Hybrid engines would involve use of PDE along with a conventional jet engine like a turbofan or a turbojet typically a turbofan where one would like to place the PDE in the bypass duct. And in addition to the core duct which is like the normal engine, we can place the pulse detonation engines in the bypass duct a few of them annular in the annular location and they also generate additional thrust. So, of course, these are all concept at the at the moment they are all conceptual. And maybe in the future one might see some of these engines being used in operation. And since they are at least theoretically supposed to have much higher efficiencies than conventional engines, it makes a lot of sense to try and explore the possibility of using pulse detonation engine for aerospace propulsion. So, that was very quick overview of different types of pulse jets valve type valve less types and the pulse detonation engine. Let me recap what we discussed in today's lecture. We started off our lecture with discussion on ramjets. We had quite some discussion on ramjets basically focusing on the combustion chamber of a ramjet because we already discussed intakes and nozzles in lot more detail early on. Ramjet combustors were discussed in rather great detail. Subsequently, we discussed about the pulse jets different types of pulse jets the valve type the valve less type of pulse jet. And towards then we also discussed about the pulse detonation engine. So, with this I would like to wind up this lecture where we had been discussing about ramjet engines types of ramjets and the combustion chamber of a ramjet in detail and types of pulse jet and the pulse detonation engine. So, we would be winding up our lecture series in the next lecture where we will we will also be talking about some of the future concepts of aircraft propulsion and other emerging concepts which we may see in operation a few years from now. So, that brings us to the end of this particular lecture which was lecture number 39.