 We discussed in the previous class about the turbojet turbo prop and the turbo fan now let us look at the next engine that got developed that is the ramjet before I go there I just want to ask you something suppose I do not add heat I do not have the combustion chamber in a turbojet engine what happens then will it produce thrust why it is an external power to run that without in starting if you run the compressor compressor is run compressor is run then you have the turbine yes true correct so you are saying that if I do not have the heat addition then it is impossible to produce any meaningful thrust okay now the concept of a ramjet it is evolved from a turbojet the idea here is let us say I am flying at a very high Mach number of around 2.5 okay fine so let us do that calculation if if I have a flight Mach number of around 2.25 okay then what is the stagnation pressure that you get P0 by P this ratio is yeah goes as 1 plus ? – 1 by 2 M this you will if you do this calculation you will find is around 11.2 right if you remember when we discussed about the turbojet and when we are talking about efficiencies we talked about diffuser efficiency right and we said diffuser efficiencies somewhere around 60 to 90% and it depends on the flight Mach number so even if you assume something like 60 to 70% efficiency of diffuser so if you assume efficiency of the diffuser to be around 60 to 70% this number works out to be around the actual pressure recovery from the stream works out to be something like around 7 now you have a fluid that is already compressed okay to something like 7 times the ambient pressure so I do not need a compressor so if I do not need a compressor I do not need a turbine and so you do not have any moving parts and that is the essential idea of a ramjet so if you see this figure here this is a typical schematic diagram of a ramjet engine what you have here is a supersonic diffuser and then you have at this point the pressure of the incoming flow is already higher compared to the ambient pressure okay and then you have fuel injection here and the fuel flame holder here and the combustion takes place and then you have the flow expanding through the nozzle okay so you do not have any turbine you do not have any compressor so a ramjet has no moving parts and that sense it is very simple to design it operates very efficiently between a Mach number range of 2 to 4 now what do you think if you compare a ramjet with a turbojet what do you think would be the thrust to weight ratio of this compared to a turbojet you think would be higher lower it will be higher so the thrust to weight is higher higher thrust to weight would be something like 160 to 170 per kg for a ramjet and for a turbojet this is for a ramjet okay now if you have a turbojet the thrust to weight ratio would be something like 40 to 70 Newton per kg and for a turbo fan it would be lower than the turbojet so okay so the thrust to weight of a ramjet engine is the highest because it has no compressor there is no turbine there are very few parts so it can be very very light okay what about SFC what do you think would be the SFC of a ramjet compared to a turbojet would it be higher lower or same as turbojet I think it is going to be the same actually if you look at a ramjet engine that you have here this ramjet engine is very similar to what you have an afterburner in a turbojet engine right what happens to the SFC in the afterburner is it higher or lower than the main engine lower higher right in a turbojet engine the SFC of with the afterburner on is higher than the SFC with the without the afterburner on so a ramjet engine is very similar to the afterburner of a turbojet engine okay and therefore you will find the SFCs are much higher so if you compare SFCs for ramjet you will find ramjet SFCs to be 60 to 90 milligrams per second this was around 31 to 36 milligrams per second and 16 to 94 milligrams per newton second so you see that in the ramjet the SFCs are much higher the main reason for this is you are adding heat in a ramjet at a much lower pressure than what you do in a turbojet meaning with the main a with only the main engine that is without the turbo fan turbo afterburner on if you are adding heat in the main engine then you are adding heat at a very high pressure so therefore the availability is more so you can extract more work out of it remember we are always going to expand only for pressure we can expand only for pressure okay so if you add heat at a very high pressure you can extract more work out of it so the availability is more so in a turbojet engine you are adding heat at a much higher pressure compared to a ramjet engine therefore you will find the SFC of a turbojet engine being lower than that of a ramjet engine okay and one thing about the ramjet engine is that you know somehow you should go to a high Mach number right so if you do not go to a high Mach number or if you do not go beyond the speed of sound beyond Mach 1 then the compression will not be large so you need it cannot be a self-starting system so you need something to take it to from 0 to Mach number or something like that from where it can carry on otherwise it is not a self-starting system that is one of the drawbacks and usually rocket motors are used to take it from 0 to the design Mach number and then these things operate at that particular Mach number now what are the applications what are the applications of ramjet these are typically used in missiles okay so and in the Indian scenario you have Akash and Brahmos that are ramjets okay there is also an aircraft very famous aircraft that used a ramjet SR 71 yes SR 71 was a unique design it is a turbojet and a ramjet put together that is the only aircraft that did that and it had a very very high yes so it had a very high Mach number because it used that ramjet it could bypass and operate in different Mach number regimes differently at high Mach numbers it was operating as a ramjet at lower Mach numbers differently okay so SR 71 was the only aircraft that is used this otherwise it is primarily used in only missiles and if you look at this I have put together some data here wherein you see for different missiles okay the diameter and the weight and the thrust and the maximum speed it can go to okay some of them are ramjets plane ramjets and the others are integral ramrockets we will talk about what integral ramrockets are a little later in the course the first and the last ones are ramjets okay and the others are other two are integral ramrockets okay fine now in a sense from the piston engine plus propeller which was flying at very low speeds we have come from piston engine plus propeller to a ramjet which can take us to something like Mach 4 right why not beyond it what is the trouble beyond it why cannot we go beyond Mach 4 with a ramjet sorry drag okay any other okay yeah scramjet stands for supersonic ramjet now if you take a look at a ramjet wherein the flow Mach number as you rightly said is brought down from ambient condition to low subsonic inside the combustion chamber if you take a look at that thank you okay now if we were to do the calculation of what is the temperature at the end of this compression process okay for different Mach numbers for the entry Mach numbers I will put that as M infinity and if we were to calculate t stagnation we will find that at a Mach number of 4 this stagnation temperature is somewhere around 980 Kelvin and around 6 goes to 1788 Kelvin and 7 it goes to 3300 Kelvin now the trouble is if you are using kerosene as the fuel okay if we remember our previous discussions the adiabatic flame temperature that you can get with with kerosene air combination is somewhere around 2300 Kelvin now if you look at this table you have t stagnation itself going to something like 2260 at a Mach number of 7 so what it tells you is beyond this point it will be very difficult to add any sensible heat to the flow by burning a fuel right which is why I asked you the earlier question suppose I do not add heat what will happen right you would not get any thrust even here so you need to add heat and to be able to add heat you need to ensure that the temperatures do not go to such high values okay and what you do in a ramjet combustion is you bring down the velocities to nearly subsonic conditions at the entry of the combustion chamber now the idea is if you do not do that if you let the flow velocities be supersonic even at the entry of the combustion chamber then the temperatures will not be so large and therefore you will be able to add heat okay and that is the idea of a supersonic ramjet so let us say you have a flight Mach number of around 8 then in the combustion chamber if you can allow a Mach number of around 2.7 okay then the temperature of flow at the entry point at the entry of the combustion chamber will be something like 1200 Kelvin now surely this is the first temperature but it is still not very large so there is scope for adding some more sensible heat by burning a fuel and then you can expand the flow through a nozzle there is also another problem that I did not talk about here that is once you go to such high temperatures the oxygen and the nitrogen in air they start to react okay and the oxygen content in the air slowly begins to deplete you would not have the same amount of oxygen content if you bring back bring it from bring the flow from somewhere around Mach number 7 to around stagnation conditions so if you take a look at those numbers of the oxygen in the air it ranges from something like 0.21 so the oxygen content you see is decreasing so because of these two problems we have looked at what is known as a scrum jet now scrum jet it is easier said let us say we can bring down the Mach number from 8 to 2.7 and then do the combustion at 2.7 Mach number it is very easy to say this but very very difficult to do this the idea of a scrum jet has been there for a very long time more than 30 years now and yet no country has ever flown anything with a scrum jet the Americans have recently tested something the Australians have recently tested something the Japanese have a program Indians have a program but Indians have only tested it without fuel addition is rose tested it very recently without any fuel addition only aerodynamic testing the Americans have tested it with the fuel addition and they claim to have produced positive thrust we will see why is it that it has taken us this long from the concept to realization stage what are all the difficulties involved with scrum jet why is it taking us this long from concept to realization is the first problem is if you look at this Mach number here you are not bringing down to stagnation conditions so therefore the static pressure in the combustion chamber will be low right because of this now reaction rate between fuel and oxidizer depends on this static pressure it goes as reaction rates goes as square of the square of the pressure typically now if the static pressure is lower then the reaction rates won't be faster the reaction between fuel and oxidizer is not happening at a much rapid rate that you wanted to be so reaction rates are very low now there is an additional problem you are saying let us somehow bring this Mach number to something like 2.7 typically in gas turbine engines as well as in ramjet engines this Mach number would be 0.3 right from 0.3 to nearly 10 times that is a Mach number that you are looking at so the flow speeds is very very large through the combustion chamber okay flow speeds are large means what if you have a particular length of the combustion chamber then the residence time of the reactants is very small so this is like a double run firstly your reaction rates lower and on top of it you are saying residence time is going to be small so it is like a no when situation that we are getting into right so that is a part of the reason why it has been so difficult for realizing this the other part is if you look at the typical gas turbine engines that we use for aircrafts Boeing makes the airframe right Boeing or Airbus makes the airframe and Pratt and Whitney or G makes the engines so expertise of engine making and airframe making are not in the same place they can be distributed you can make the aircraft and then match the power and fit this engine and go about it with regards to any other aircraft we are making the airframe for the LCA using a GE engine right now to power it right so airframe development can take place separately and the engine development can take place separately this is because there is no great aerodynamic coupling that is there between the two of them the aerodynamic coupling between the structure and the engine is not too large in most systems but in this case there is a very strong coupling between the airframe as well as the engine so you cannot develop them separately you have to do them together okay and that is one of the major drawbacks interaction between engine and airframe is very high in the case of scramjets so you cannot have expertise in different places you have to have them together which is not always easy okay and even testing you cannot if you are looking at any of the turbojet or turbo prop engines the engines are developed separately they tested somewhere else and then they are fitted here all the testing and other things will have to be done on the airframe and the engine being integrated together if you are looking at a scramjet and that is not always easy okay the other major problem here is drag if you look at the drag both the pressure drag as well as the viscous drag is high here because you are going at very high Mach numbers the drag is very high now you want inside the engine Mach number of 2.7 right if you are wanting that and if you want to ensure a reasonable residence time then the length of the vehicle will have to be long which means that the drag will also be long I mean will also be large okay so drag in this case is very large so in most engine test flight test unless people come up with realistic numbers as in they put out these numbers it is very difficult to believe whether the system produced any positive thrust or not okay please also remember that even on the inside there is a viscous drag because the flow Mach numbers inside the engine is also very large and typically the engine length is comparable to the entire vehicle length in this case so the drag is a serious problem here very large drag and lastly testing of this is very difficult because you need to test for the engine as well as the structure right so and if you are to build a wind tunnel for testing this it is very very expensive because as you go to higher and higher Mach numbers the power required to keep it running for a small time is very large okay so testing is a problem you can test only small scale right but the other problem is how good is your scalability that is if you test something at small scale can you go ahead and use it at a larger scale and be confident that whatever you have done at a smaller scale is going to be valid it is not going to be easy if you have a circular intake so which is why people go in for a rectangular intake because you can add them up if you have one rectangular intake and do the testing on that and do everything then you can make a large engine by simply adding up all these rectangular intakes so that is one way in which people have looked to address this problem okay so you can have a rectangular cross section so that you have modularity of design why then are we interested in this we simply seem to be putting too much of problems here we have only stating problems in the way it has been stated makes one feel that there is no hope it is also true that people say it is not very easy people in the field were doing this not very easy probably to get an accelerating system in a ramjet a scramjet probably possible if you take it to that kind of Mach number somehow it can probably sustain itself that is it can just overcome the drag okay why do you think people are looking at this what is the motivation for just flying faster is not sufficient no the kind of money that is being invested in this kind of activity is very very large there must be some other reason why people are doing this no okay if you look there are two applications or in this case three applications if you look at a launch vehicle application because in India both ISRO and DRDO are pursuing a separate program okay and there must be a reason why they are pursuing it if you look at a launch vehicle reason as in if you are using this to launch something into orbit typically currently launch vehicles use rockets to do this now we will come to what rockets are a little later in the course but just for this argument rockets have to carry their own fuel and oxidizer on board which means that their weight of the system goes up right as compared to that if you look at any of these air breathing engines they will use the atmospheric oxygen which is usually a few times larger than the fuel that is required right so they end up using atmospheric oxygen and use carry fuel on board now with a ramjet we have a limitation of around Mach 4 right that is the limit at which we can go up to right now but if you go for this you can go to a larger Mach number inside the atmosphere and therefore if you can develop this successfully the cost of launching any satellite would be much smaller if you have a large portion of it being air breathing okay that is the idea why ISRO is pursuing it fine now if you look at a military application why do you think a military would be interested in this if you have a missile that is built around a scramjet and if it can go at Mach 8 it is very very difficult to track it and shoot it down okay you might argue that you already have ballistic missiles why the hell do we want all these different kind of things they also go at very high Mach numbers but there is a difference if you look at what a ballistic missile does it is powered only for a part of the flight the remaining part it is going as a stone it can maneuver itself depending on the aerodynamics but it is not powered during the entire flight period okay it is only powered during a portion of the flight period but if you have this it is powered through the entire period it is flying and therefore your maneuverability is very very high not only are your speeds high your maneuverability is also high therefore it makes it very difficult for the enemy to shoot it down as a reason why military is going ahead for this and if you look at any civilian application typically current day technology that we have the San Francisco to Tokyo flight takes something like 14 hours okay people say that if you have a scramjet engine then you can do this within 2 hours would it be nice if you can fly between any two places that were within just 2 hours so that is the motivation behind looking at this from a civilian perspective although the civilian perspective is very far away right now people are looking at only the military and the launch vehicle application most innovations that we have will always go through this especially related to aircrafts and airspace industry will always go through this cycle firstly it will be the military that will put it to use or develop the technology for its use then the civilian application comes in right it has been like this for all the engines if you look at turbojet engines turbojet engines were invented for a military purpose right and slowly it it got changed and mocked and then you have this civilian application that is coming in right so any kind of engine development or any kind of such activity firstly it will be the military application and then you will have the civilian application okay I think I will stop here and continue in the next class