 The next set of motors that we are looking at or next set of engines that we look at is liquid rocket engines okay, now just like we had certain classifications of solid rocket motors depending on whether it is a double base propellant or a homogenous propellant we also can categorize the liquid rocket engines into mono propellant a mono propellant system is one wherein the propellant decomposes okay in an exothermic fashion and then the energy is released whereas in a bi propellant system you have fuel and oxidizer which makes and burn and then you get the high pressure high temperature right we will discuss this a little more in detail but firstly the bi propellant systems are themselves further classified as hypergolic non-hypergolic earth storable cryogenic and pump fed and now what do we mean by hypergolic and non-hypergolic hypergolic systems are those wherein if you introduce the liquids as soon as they come in contact with each other the reactions take place they do not need any source of ignition okay whereas a non-hypergolic system you need some ignition energy to initiate the reactions this does not need any ignition and earth storable and cryogenic the earth storable systems are those wherein the propellants can be stored under ambient conditions ambient conditions of pressure and temperature whereas if you look at cryogenic engines they use typically hydrogen and oxygen or kerosene and oxygen kerosene and oxygen is a semi cryogenic system because kerosene is in some sense earth storable and oxygen liquid oxygen if you use that is a cryogenic cryogenic essentially means that these are gases at ambient conditions of pressure and temperature and you need to cool them in order to ensure that they become liquids why do we need to have liquids primarily because the density of liquids is much more than the density of gases and if you store them as liquids then you can store them in a smaller volume gases you can argue this way that you can pressurize them and increase their density right that is one of the other roots but when you pressurize them we will see that the thickness of the pressure vessel keeps on increasing depending on the pressure so and therefore it does not become useful after some time to store them as gases at high pressure okay so therefore it is seen that if you go to liquids then it is a lot better so cryogenic liquids typically are liquid oxygen and liquid hydrogen systems okay and last is pump fed or pressure fed you know liquids when you store them in certain tanks okay they need to be pumped in or fed in to the engine now how do you do it is what determines this classification if you have a turbo pump fed system okay then it is called as a pump fed system and if you just pressurize the tank and let the liquid come out at high pressure that is called as a pressure fed system usually pressure fed systems are low thrust devices whereas turbo pump fed systems are very very high thrust devices we will discuss them a little more in detail firstly starting with the mono propellant thruster now if you look at this figure this is the typical mono propellant thrust system you have an inert gas bottle that pressurizes the liquid here why do we need to use inert gas it is because you do not want reactions to take place inside the tanks itself right you want the reactions to take place where you have the thrust chamber so you pressurize it with an inert gas either helium or nitrogen helium is if you use it it is very obvious that helium has a lower molecular weight so if you even if you have a very large tank then the molecular weight being low for helium the weight of the gas in this tank will be lower but unfortunately if you want to store them for a longer time right because this is very low molecular weight it is usually seen that it tends to leak over time and you cannot store it for very long times and therefore people also use nitrogen as a pressure okay nitrogen you can store it for a much longer time then you have a pressure regulator which regulates the downstream pressure the pressure in the inert gas bottle is typically of the order of 350 bar okay so 350 bar you come down to something like 25 or 35 bar and then you have this liquid mono propellant tank you have a diaphragm here right that is basically to ensure that you know most of this mono propellant systems are operated in space or under very low gravity conditions now under normal gravity conditions you will always find that the liquid will be at the bottom so therefore if you have gas that is pressurizing it will expel it out but there is no guarantee of that happening in very low gravity conditions so therefore you need a diaphragm so as to make sure that the pressure end is on this side okay otherwise the situation could be inverted to and you would end up pushing in inert gases only then you have the liquid mono propellant here there are two liquid mono propellants that are typically used one is hydrazine into H4 the other is hydrogen peroxide or H2O2 okay both of these are unstable compounds and if you see here this hydrazine breaks up into ammonia and hydrogen and nitrogen and this breaks up into water oxygen and hydrogen so this is the mono propellant tank and after this there is a there are thrusters small thrusters a cluster of them these are typically used in satellite systems for station keeping applications okay there in you need very very low thrust now there are solenoid valves attached here solenoid valves are typically off and if you give them a particular voltage and current they turn on so they can be remotely operated okay and it then feeds into what is seen here as a thruster if you compare this with a solid rocket motor a solid rocket motor the propellants and the thrust chamber were some identical I mean there was no other thing right they were both stored in the same place and which is why we said that the specific impulse of a solid rocket motor would be lower because you need to have compatibility between fuel and oxidizer so you are not free to choose the best oxidizer and the best fuel whereas in a liquid rocket motor you have this freedom of choosing the best oxidizer and the best fuel so therefore you need to have storage and thrust chamber separate this portion above the thruster is known as the feed system or the storage tanks and this is the thruster here now in this thruster there is something known as a catalytic bed a typically used catalyst is aluminum pellets aluminum pellets coated with iridium and because of this catalyst the decomposition of either hydrazine or hydrogen peroxide takes place okay and if you look at hydrogen pero hydrazine decomposition that is N2H4 it can decompose into alpha D NH3 1-alpha D by 2 N2 here if you decompose it completely to nitrogen and hydrogen okay you will not get a very high temperature typically the T adiabatic if it completely decomposes is something like 867 Kelvin okay but you won't want that kind of low temperatures so what is done is there is an optimal value that is if this alpha D is somewhere around 4 by 3 right then the temperatures are T adiabatic corresponding to this would be 1650 Kelvin and ISPs would be something like okay as you can see here this has a lower ISP compared to solids but these are used as I said in satellite applications primarily because you do not want a continuous operation you want a pulsed operation right and liquids are very convenient for such an application and therefore although it is of a lower ISP you tend to use this the typical thrust that these thrusters will have to deliver is thrust varies from 0.1 Newton to something like 20 Newton it is very small primarily because if you look at the application where it is supposed to be used it is in space so there is no or the sense of pressure there is very very low right there is no sensible atmosphere and correspondingly there is very very little drag so if you give it a very large thrust you typically wanted to be corrected by small magnitudes if you give it a very large thrust it could move very far away from the stated position so therefore you tend to have very small corrections that are made so as to make sure that it goes to the position that it has to be in and these thrusters will have to be operated typically around undergo a few million cycles that is they are switched on and off and the burn time is typically very very small so there if you look at the thrust time curve for them it will be like okay they are switched on and switched off after a certain time okay they are not in continuous operation and if you look at the life of the satellite right the one that determines how long the satellite will stay is how much propellant you can carry okay and in some sense if you can have a higher specific impulse for this then you will find that the satellite can be kept in position for a much longer time okay which is why people are now looking at plasma thrusters and other electrical thrusters the good thing about these electrical thrusters is they have very low thrust which is what is needed for this operation but their ISPs are typically two orders of magnitude more than what is seen here so therefore your operation time will increase if you have a higher ISP so that is the direction in which people are moving now let us move on from the mono propellant thrusters to bi propellant systems let us first look at this pump fed and pressure fed systems this here figure here shows a bi propellant pressure fed system you will have inert gas bottles and then the pressure regulator as I said you have both nitrogen and helium and then you have a separate oxidizer and fuel tank okay and if you look at the pressure downstream of this gas bottle in the tanks it is somewhere around 30 to 40 and in the combustion chamber it comes down to something like 20 to 30 so these are basically fed by because of the high pressure here okay and then you have the combustion chamber and then the nozzle as opposed to this if you remember if you add heat at the highest pressure right then you can extract more work out of that system so therefore if you also if you want larger thrust if you remember the thrust equation is nothing but CFPC 80 so if you have a very low pressure then the thrust will also be lower and if you have a very high pressure your thrust can be much more larger so that is the idea behind pump fed system and the schematic of that is shown here what you see is again pressure bottles but these pressure bottles are much smaller in size and what happens here is you do not pressurize it to something like 30 or 40 bar you pressurize it to something like 3 to 6 bar which is higher than the vapor pressure why that is so we will discuss a little later in the course okay and therefore the tanks are essentially maintained at low pressures they are not at high pressures they are not at 20 bar or 30 bar right they are at very low pressures so therefore the thickness of the tank material can be smaller and then you have turbo pumps here okay you have a pump separate pump for fuel and separate pump for oxidizer and then you have a if you have to run these pumps you need a turbine right so this turbine uses a part of this fuel and oxidizer system okay part of this feed is given to the gas generator indicated by G here the pumps are indicated by P and the turbine is indicated by T now if you remember what you just studied earlier in an earlier course that is on gas turbine engines you will see that the turbine inlet temperature needs to be smaller okay they cannot be very high although here in the combustion chamber of the rocket motor you can go up to 3300 there it is restricted to something like 1600 to 1800 Kelvin so here in the gas generator you need to be careful so as to ensure that the temperatures do not exceed that and then you have a turbine which essentially powers the two pumps okay and if you have to start it initially right initially if you have to start the system you need a small propellant charge typically a solid propellant is used to provide this and that will first run the turbine right if the pumps are not run then you do not have anything coming into the gas generator so initially to run the turbine a small solid propellant system is used and then the turbines are turned on and then the pumps become operational and they start feeding in the propellant and the gas generator can then be used to run the turbine and then you have in the combustion chamber here you can go to something like 50 to 200 bar okay the combustion chamber pressures are very very high here in space shuttle to the combustion chamber pressure in the space shuttle main engine liquid engine is somewhere around 200 bar so somewhere in the pipeline you will have pressures exceeding 300 bar very very high pressure systems if you have a high pressure system the other advantage is that your size of the thrust chamber right the size of this rocket motor can be very very small and as I said if you want a very large thrust thrust is dependent on PC right so if you have a very high chamber pressure then you can get a very high thrust so this also in some sense needs a lot more cooling because of the high heat that is generated here and typically a fuel part of the fuel is pumped through a regenerative jacket okay there is a jacket around the nozzle as well as the thrust chamber okay and typically liquids are pumped through this and these liquids take in the heat and then this is used up again the combustion chamber so in a sense what we had discussed when we would discuss nozzles right we said there is a loss because there is a loss of heat to the ambient that is minimized if you have regenerative cooling because you are essentially using what is been lost to the walls again is recirculated back and this as I said is going to give you very high thrust it is important to note this that if you want to decide and what system to use for a particular application remember we said we the satellites because it is a low thrust system we wanted to use mono propellant systems when do we use a bi propellant system when do we use a pressure fed when do we use turbo pump fed now what is shown here in this plot is you have on the x axis total impulse total impulse is nothing but impulse is nothing but f into operational time okay or T burn so either if you have a very large thrust or if you want to have the operation for a very long time right then your impulse becomes larger now if you see this is x axis total impulse and y axis is system weight okay if you see this if you can use cold gas for certain applications that is when the impulse requirement is very small then this becomes a very good system right if you have impulse requirement of the order of 10 power – 1 there is no point in using a bi propellant system bi propellant system will in fact have a very heavy weight because you need to have pumps you need to have all kinds of devices so it is better to have cold gas if you are you using it for a very small impulse and then further this curve the mono propellant system comes in that is if your impulse is up to 10 power 1 it is better to use a mono propellant device and then pressure fed by propellant device and then lastly turbo pump fed by propellant system now the difference between the by propellant the pressure fed system and turbo pump fed system is essentially because of the tank weight if you notice in the earlier figures in the pressure fed system the gas bottle is the one that is pressurizing so it needs to have a very large volume in order to fill the entire tank volume so if your thrust is large and as well as burn time is large then you are going to have a lot of liquid in the tank so the tank needs to be filled at something like 30 to 40 bar so the inert gas bottle weight goes up as well as the tank weight goes okay because if you remember from your structures it is Pd by 2t right so the thickness goes up as the pressure goes up so therefore the system weight is going to increase and that is why you will find that beyond a certain fraction of the impulse if you use a pressure fed system then it will not be very beneficial its system weight is much higher than compared to something like a pump fed system if you have something like 10 power 5 you can see that the system weight will be something like 10,000 kgs if you use turbo pump fed system but whereas if you use a pressure fed system it will probably be something like 10 power 7 or 10 power 8 okay so in a sense this graph gives you an indication of what is to be used if as per your requirement so the essential thing to notice you need to know what is your impulse if you know the thrust time curve you can calculate the impulse and therefore you can choose the appropriate system we will stop here and in the next class we will look at little more in detail on the liquid propellant systems okay thank you.