 Welcome back. So, so far we have discussed the what are the rockets, historic perspective of the rocket, the development of rocket science. Now, let us back to the actual course, start discussing rocket propulsion. First of all, the previous class we have talked about what are rockets, the basic definition of rockets. Rockets are the devices which generate thrust by imparting momentum and energy and onboard propellant. This is the catch word onboard propellant. So, the propellant is onboard, rocket is carrying that propellant and it essentially energizes that propellant or adds momentum to it. So, therefore, unlike air breathing engine which draws most of its propellant from air or surrounding environment, a rocket essentially energizes onboard propellant. Now, there are two broad classification of rockets. Rockets can be classified under two broad categories. First is chemical rockets and the second is non-chemical rockets. So, in this course, we will primarily deal with chemical rockets, but towards the end we will have some lectures on non-chemical rockets like the electric propulsion systems. So, first let us look at the chemical rockets. As the name suggests, the chemical rockets generate thrust through the release of internal chemical energy of the propellant. So, thrust is produced by release of internal chemical energy of the propellant and that energy can be released only through chemical reaction. So, the primary source of production of thrust is the chemical reaction in chemical rockets. Now, this is the broader classification of rockets. The chemical rockets also have two primary subcategory. Actually, sometime we can also call it three, but primary two subcategories. First is liquid fuel and second is solid fuel. So, we also have sometime hybrid also, but primary it is liquid fuel or solid fuel where it depicts the phase of the propellant. Either the propellant is in the liquid phase, both oxidizer or and the fuel in the liquid phase or sometime we have a mono propellant where is a single propellant, it can be in the liquid phase or it can be in the solid phase. Again solid phase also we can have separately oxidizer and fuel blended together as a one rod or we can have them separately also. And one subclass of liquid fuel as I have said before also is cryogenic, which essentially means that the propellants which are in their natural state gases, but if they are stored under extreme conditions of pressure and temperature then they are in the liquid form. So, they are stored in the liquid form, but the combustion occurs in the gaseous state by the time they are sent to the combustor actually in cryogenic by the time they reach the combustor one of them have already gasified, other comes the liquid stage, but one is already gasified. So, therefore, but liquid fuel primarily both the fuel and oxidizer are in the liquid state. So, the basic principle of operation as far as the chemistry is concerned is same in both only the rocket design is going to be little different. The sub systems are going to be different for liquid fuel you need to have liquid storage, pumps, gas generator etcetera. So, solid propellant you do not need that this entire pellet is put inside the rocket chamber and it is burned inside that you do not have to have separate systems. However, so coming to the liquid propellant rockets looks at look at the merits and demerits of these two systems. The primary use of liquid propellant rockets are in large boosters. Now, liquid fuels generally have higher enthalpy per unit mass relative to solids. So, if I look at the specific enthalpy or enthalpy per unit mass then typically liquid fuel will have a larger enthalpy compared to solid propellant and there is a reason for it. What is the reason? How do we emit the heat of reaction by bond energy? So, typically the molecules are much closely packed in solids than liquids. So, they can be very easily broken. So, therefore, the energy content will be more in liquid because the very less goes in breaking the bonds. So, energy release will be more whereas, in the solid lot of energy has to go to break the bonds. So, that energy is available. So, therefore, the overall delivery of enthalpy or energy will be less. So, typically the specific enthalpy or the enthalpy per unit mass for liquid fuel is more than that of the solids. Now, this is because of the fact that the intermolecular bonds are weaker in liquids. Therefore, they are easy to break the bonds, easy to vaporize. Now, one thing you have taken course in combustion. I have said that the combustion typically actually occurs only in the gaseous phase. Finally, the combustion to occur it has to be in the gaseous phase. So, therefore, whatever propellant we take first it has to vaporize and only then reaction will occur. Liquid is easy to vaporize, solid is little more difficult to vaporize. Therefore, the combustion process is easier in liquid compared to solid as far as the physics is concerned. Of course, it is easy to light the solid propellant and it will keep on burning, but as far as the energy release is concerned liquid fuel will be much more efficient than solid fuel. Now, one of the prime advantage of using liquid fuel is providing variable thrust. Now, this is very important which is also called throttling. Liquid fuel can provide variable thrust or throttling because we can very easily control the flow rate and flow rate is what essentially dictates how much total energy is produced. So, by controlling the flow rate you can very easily control the total thrust produced by these engines. This is a very important feature as far as the engines are concerned. Because of the fact if you look at mission requirement typically all the missions they have a well defined period for every application. Something has to burn for 5 second at particular thrust, 2 second for particular thrust like that which is very difficult to achieve the solid propellant, but liquid propellant can very easily attain that. Another advantage is allowing throttling is shut off. Liquid propellant can be very easily shut off. So, from safety point of view also it is much safer compared to solid propellant. As I have just said challenger disaster was started with solid propellant booster. Even if you see that something is going wrong cannot do anything about it because once the solid propellants are lighted is impossible to stop it from burning up whereas liquid propellant can be very easily put off. So, shut off is a very important condition. Now shut off provides variable thrust and shut off combination of these two. Essentially allow use to go for multimode missions where at different stage of the mission you will have different requirement, but with solid propellant you just light it once it is gone. So, if it is a very specific well defined motion solid propellant will be better, but if it is a flexible mission we are talking about liquid propellant has to be there because you have that control of controlling the total thrust or the duration of thrust the amount of thrust both are important particularly for very many applications like satellite mission keeping and all. Many times you may not know a priori how much thrust you will be required at what condition for how much time. So, having a liquid propellants is a liquid rocket system can you can provide that amount of thrust required thrust. So, that is the biggest advantage that is why liquid propellants are primarily used in boostation and higher stages also higher stages typically we use liquid propellant rockets. However, they have some disadvantages also one biggest disadvantage is it required the fuel feed system. So, the disadvantage is first fuel feed system feed and storage in order to store this liquid fuel liquid typically we will have much less specific volume than solid gas as even less, but liquid much less specific volume as solid. So, therefore, the weight per unit volume is relatively low as compared to solids. So, in order to store enough mass you need to have larger volume. So, you need storage tanks now this storage tanks do nothing, but the store fuel if you look at the space shuttle it has separate storage tank for storing fuel, but that adds on during lift up light because that has to be carried on board. So, this storage tank they do not take part in production of thrust or any mission requirement, but they have to be there to carry the fuel that adds to the weight. So, this adding storage tank adds to the weight. Now, for multi stage missions they can be discarded, but if you are talking about mission where you do not have to discarded then you are carrying that weight is a dead weight and other is the fuel feed system that is pumps very important. Now, these pumps are special turbo pumps they are again these are also technological marvels. If you look at the total operational time of a liquid fuel rocket let us say you have a liquid rocket in the second stage like PSLV second stage is a liquid fuel right. Now, the first stage is solid propellant it has burned out within fraction of a second you have a second stage right and on a light it you have to light at its full capacity full capacity means almost about 50 60 kg per second of fuel right. So, 50 60 kg of fuel you have to put in per second with one or two second. So, the pump has to pump in that much amount huge amount of flow within such a short duration. So, development of this pump the pump material the pump bearing pump blades everything is very very critical for the operation of a proper operation of a liquid fuel system and that again is dead weight right because it is not participating apart from supplying the fuel. So, once again is a very expensive component is a very difficult component to design and manufacture, but does not directly take part in the first generation. So, it can tell a mission make a mission crippled, but it does not add advantage to the mission. So, therefore, that is one of the biggest problems in liquid fuel systems. So, liquid propellant rockets one of the bottlenecks is the fuel feed system and then you have to maintain the distribution typically there are many many injectors you have to maintain the distribution equal distribution everywhere. So, there are various issues that come into picture then second now because of this they are heavier than solid propellant heavier than solid fuel rockets because these are the weight that you have to carry. So, for the same amount of thrust they will be heavier than solid propellant rockets and then lots of dead weight which you need to carry in a liquid feed system and one of the major problem in liquid propellant rockets is persistence of combustion instabilities. Combustion instabilities a major problem in liquid propellant rockets and what happens this instabilities actually coupled with the feeding system. So, if the instability occur instability is pressure variation in the combustion chamber. Now, the combustion chamber pressure variations are of large amplitude and the fuel feed system the delivery mass flow rate depends on the pressure differential between the feeding system and the combustor. So, if the combustor instabilities of large amplitude there are lot of oscillations in the pressure then the differential also oscillates. So, that feeds back to the fuel feed system and change oscillates the mass flow rate. If you oscillate your thrust will start to oscillate and that actually augments the combustion instability also. So, can lead to disastrous mission failure. So, this is a major problem with the fuel feed system and combustion instability coupling. So, essentially what is done is that in the design stage we design in such a way that this instabilities do not occur because these are system level problems and operating condition level problems. Then the designer specifies that keep away from this operational ranges. So, if you specify the operational range that is the flow rate, temperature, pressure etcetera and you have to operate within that parameter otherwise we may get into combustion instability. So, this is something in the design or development stage has to be addressed and that essentially adds to development cost or time. But without that it becomes a dangerous proposition to operate a liquid fuel engine. On the other hand the solid propellant rockets are simple and reliable as we can see as we have talked about the history. As we have seen in the history the original rockets were developed in 8th or 9th century were all solid propellant without detailed critical predictions or anything they are very simple and reliable. Solid propellant rocket once you light it it is going to fly. It may not fly in the intended manner, but it is going to fly somewhere. So, solid propellants are simple and reliable biggest advantage. This simplicity and reliability gives us robustness or repeatability in operation because of that these are very attractive as weapons. Most of the missiles particularly short range missiles are solid propellant rockets like even the stringer missiles are solid propellant. So, they are essentially preferred for weapons. Advantage is you do not need this storage and everything and huge portability because they are very compact closely packed. So, can be taken very easily carried very easily. So, gives the portability that is why they are preferred as weapons as well as and strap on boosters. Now strap on boosters are the attachment fitted to the rocket vehicle to provide the initial thrust. They have to run for few second provides the huge initial thrust. So, you can have huge solid boosters like in the special program and then let them burn off. Once they burn off you can discard them. So, the cases can come off and then the main rocket can take over, but to provide the initial thrust you need reliability and you need lot of it lot of thrust which can be provided very easily by the solid rocket boosters. So, therefore, these are usually used as strap on boosters. Also they are used as retro rockets and rural rockets for stress separation because those are well defined mission requirement. You fire it for very short duration because when the two stages separate they need to be moved apart. So, one of them will be in the forward motion, other will be in the backward motion. So, that you can move them apart and those are usually achieved by very small solid propellant boosters. Smalls they call it rocket pellets. Small rockets are fired to provides little amount of thrust whether the stages separate. Again the reliability is very critical in that and that is why solid propellants are preferred. Then the propellant density as I have talked about before density is greater than liquid rockets that I have already talked about because they have very high specific volume compared to the liquid propellant. Therefore, the propellant density is greater and the biggest advantage no dead weight apart from the casing. Apart from the casing nothing comes in no rotating part no supply system nothing is required. Apart from the casing it is just a solid rocket which is lighted and it burns. So, you do not need any separate storage you do not need any feed system. Only thing required is the ignition system that is it apart from that nothing is required. So, that is a major advantage no carrying no dead weight. So, which any weight you save adds to your payload what to your range both of them are beneficial. Therefore, this is a major advantage of solid propellant rockets and these are especially attractive for small change in velocity. You do not need very large changes, but for small changes these are very attractive. It provides the initial small changes of again why do you use for the initial booster. We do not require very high velocity there small change in velocity, but you have to lift lot of weight because you are lifting the entire system with all the propellant and everything. So, lot of weight with small velocity change solid propellant is preferred. Once you start to become lighter switch over to liquid propellant and tack it forward because you have already got the initial push as the initials velocity. So, after that the liquid propellant can take over. So, this is very attractive for small change in velocities and as we have just mentioned that the liquid rockets require heavy tanks and pump which they do not and they are usually more stable combustion instabilities are although are there because combustion is a phenomena of the rocket chamber acoustics. So, they are there, but less than this secondly it does not feedback combustion instability if it is there does not feedback to the fuel feed system. So, the combustion system and instability either the combustion or the evaporation and burning process and the instability are kind of decoupled because there is no feedback back to the evaporation rate or burning rate. So, less unstable this is another advantage of this. Disadvantages is liquid propellant had the major advantage of throttling whereas, solid propellant one of the major disadvantage is throttling not possible. So, difficult to throttle major disadvantage difficult to control combustion once the combustion process starts it just have to let it burn because you cannot control it. Nowadays of course, for the various machine requirement the pre programmed and tar pellet with different burning characteristics, but that has to be done a priori it cannot be done on flight in a active manner it has to be passively programmed. So, these are the major disadvantages of solid propellant rocket and there are some instabilities which do exist, but primarily these two difficult to throttle and difficult to shut off. So, once these are fired either they complete their mission or they can have disaster consequences because they work like a missile for space exploration. So, solid propellant if they are fired and that is one of the reasons that most of the launching sites are very close to the areas where there are not much of inhibited not much of people because if once you ride the light the solid booster and mission is a failure you have to abort the mission and then it may take it to a populated area and have disaster consequences. Therefore, all the mission areas are essentially or the launch pads are essentially away from the inhibited areas. So, now in the latest development, but solid propellant because of this energy density and these advantages are still very attractive. Therefore, technology are being developed to somehow provide some control and throttling to the solid propellant rockets and one of the Indian success stories is solid propellant. Indian SLV program is completely solid propellant is a very successful program all the solid propellant rockets are very efficiently used. So, India is one of the world leaders in solid propellant rocket technology. So, far we have been talking about chemical rockets we have discussed the liquid propellant rockets and solid propellant rockets. Now, let us look at the other variant of rockets which are non chemical rockets. So, next we talk about little bit about non chemical rockets. Now, as the name suggests the primary source of energy is not chemical energy in non chemical rockets. So, instead of releasing the chemical energy of the propellant itself the energy is imparted to the propellant from some alternative source. So, energy from some alternative source not directly the chemical energy. Now, this type of scheme of imparting non chemical energy become attractive when large changes in velocity are required without carrying a large amount of propellant. So, we need to change the velocity, but not with large amount of propellant or if we need very small propellant it is to be even if the delta V or the velocity change is not very high. If we can afford to use only small amount of propellant in that case we have no other option, but to use some alternative means of energy production to get that acceleration or velocity increase. So, this extra energy to give the velocity increment delta V is provided by non chemical sources that is why they are called non chemical rockets. There are various examples of non chemical rockets the energy transfer to the propellant can be of different forms. First is thermal energy. So, primary source of energy can be thermal energy. Under this category one of the thing that can be talked about is a nuclear reactor. Nuclear reactor is a very good source a very compact source of energy it can provide very large amount of energy with very little amount of propellant. So, therefore, ideally it will be a very attractive very attractive for rocket proportion and as we have discussed in the previous lecture actually it something like that was proposed by Peltier long time back nuclear proportion. But nuclear reactor although are attractive are I would say not politically correct, because whenever you have a when you have a rocket you have to propel through atmosphere and you do not want to have open nuclear reactor moving through the atmosphere. So, nobody will allow it that is why even though it is a very attractive concept it never became reality. And the issue with nuclear reactors are not purchased energy generation, but the safety issues to contain the radiation or insulation etcetera or the auxiliary systems for power production that becomes heavier. Therefore, although nuclear reactor can be a good source it is not never practically used. Now, one of the ideas of using nuclear reactor or actually nuclear explosion was to have something like a ballistic launch where you can have a nuclear explosion underneath a rocket or a ballistic missile or something and then because of that explosion it will move. But once again it is too risky a business to be practically implemented that is why even though nuclear reactors can be a good source never used so far for space exploration. Now, the other alternative can be laser proportion. Now, lasers are can be used in two ways in proportion. First laser can be used and ignited in a chemical rocket which is essentially a low power laser can be very easily used or laser can actually be used as the source of energy, because laser lights are very intense source of energy and therefore, they can be used to propel up to large distances. So, therefore, laser proportion is one alternative where essentially it is thermal energy where laser light can be used to heat up and that can be used then to propel. Then other alternative is solar collectors. Once again it is a thermal route where solar rays can be collected and they can be used the electricity generated or the power generated by the solar rays can be used to propel. So, that is one of the ways of propelling through thermal route. Solar can also be used in some other way which are called solar cells. This is not C E L L cell, but S A I L cell. In solar cells actually we have large solar arrays which are mounted over the satellite typically used in satellite and they work like the cells that are used in boats. The solar waves have is electromagnetic nature can produce small amount of thrust to this cells. So, these are pretty large devices which can be used to use solar radiation to propel by using this type of cells. Again these are some concept which are also in some time has been used, but they require very large structures. Now, the another route of non chemical rockets can be electrical energy. Electrical energy unlike the thermal energy actually is very widely used in various satellite operations and first of all let us give some examples of electric propulsion systems. One is say arc jet thrusters, then ion thrusters, then plasma dynamics thrusters, etcetera. These devices primarily use electrical energy to produce thrust. So, therefore they are called electrical thrusters or electrical propulsion devices. So, if I look at for I will just take one example later on when we go to electric propulsion systems we will discuss them in detail. Let us just look at an arc jet thruster. Typical schematic of an arc jet thruster is we have a solid cathode and anode. Some propellant is passed between these two in the gap between these two and very high voltage is applied between the two high voltage. Very high voltage is applied across the two because of this high voltage just like a welding machine electric arc is produced in this gap and the propellant which is typically xenon is fed into this gap. This comes in a liquid form. When it crosses this arc across this electric arc is a very high temperature. So, in this arc it evaporates into gas. So, it comes as liquid evaporates into gas and not only gas is actually ionizes. Now, because of that there is a massive decrease in density because liquid has much higher density than the gas. Now, if the density decreases so much in order to maintain the overall mass flow rate it has to accelerate. So, this accelerates gets a very high velocity and this produces the thrust. So, thrust is produced by the high velocity that is produced by this device. So, this is our electric arc and that is responsible for production of the high velocity jet. This is very hot. Temperature can be as high as 20 to 30 thousand Kelvin and this is how an arc jet thruster works. There are other thrusters like ion thrusters very similar principle where we ionize the gas and let the ions pass flow through. Then we have plasma dynamic thruster where we create plasma and the plasma is then allowed to pass through this thing come out of the nozzle and that produces the thrust. So, essentially the basic principle in all these electric propulsion systems is that the electrical energy is responsible for the production of very high velocity exhaust. Now, here we are still using some amount of propellant. Propellant is required we are energizing the propellant only the energization process of propellant is not through a chemical source, but through an electrical source. That is why it is a non considered a non chemical rocket. So, here also the propellant will be used because that is what needs to be accelerated to produce thrust. We need to have an action and that action comes from the propellant momentum. Therefore, this is how the electric propulsion systems work. So, the basic characteristics of electric propulsion devices are first of all they do not produce very large amount of thrust pretty low thrust will be produced by them. They are usually used for low duration missions. Now, since they are producing very low thrust they cannot be typically used within the earth's atmosphere where the forces acting are very large, but if you are in the outer space where there are no frictional forces nothing to slow down the vehicle and the vehicle is moving at a high speed. A small amount of thrust will provide required acceleration to carry out certain maneuver and that is good enough. So, therefore they are very very efficient in outer space. Now, the advantage is since you are not carrying any whether you do not have any chemical propellant you are not you do not need to carry lot of propellant. Therefore, the specific impulse which we will come to later is much higher compared to the chemical rockets. So, this is one of the basic features low thrust and secondly also they have to work for low duration mission they cannot be operate for a long period. Now, one of the examples of application of electric propulsion devices are satellite station keeping where as I have said in the previous lecture that the satellite may deviate from its path. So, that needs to be brought back to the intended orbit that is done by these devices and they can operate over a very long period of time. This is the total lifespan of these devices are much larger compared to chemical rockets. So, that is one of the major applications other is of course, course correction most of the deep space probes like Sassini or Galileo they have some kind of electric propulsion system. Because to them for this type of missions you require to have very long operational life and chemical rockets cannot have that large long operational life. Therefore, most of those missions actually have some electric propulsion system on board. Now, these are the advantages of electric propulsion system. The disadvantages are first of all because we are using electrical power electrical power always produce heating. Now, radiating this excessive heat is a very difficult proposition something that we have to somehow in built in the system. So, radiating this excessive heat then another disadvantages you require some additional power source. Now, for example, here in the arc jet thruster in order to produce the electric arc you need fairly high voltage in the range of kilo volts. Where is the source of energy what you can have is some battery some capacitors if there are batteries they need to be charged. The only source of charging is the solar cells they will take a long time to charge. So, and of course, then that needs to be transformed because solar cells will not produce the kilo kilo volt range of voltage. So, need to be you need to have a transformer to step up. So, all those things essentially act to the weight. So, therefore, this additional power source and the auxiliary components actually act to the weight of the system that is why it is it becomes as a whole bulky. Then because of this additional weight that it needs for the additional power source sometimes the power to weight ratio can be very high that is power produced per unit weight of the entire system is can be as a unmanageable not very high let us say can be unmanageable it may require lot of weight to produce unit power. So, therefore, that makes it not very much economical in the long run. So, these are the disadvantages. Now, so far then what we have done today we have looked at chemical rockets we have looked at solid propellant we have looked at liquid propellant we have looked at non chemical rockets. What we have observed by discussing all those type of various rocket vehicles is that each of them have their own advantage own disadvantage. So, they can be used for specific missions. For example, for the initial boosting solid propellants are the best. So, that is why we use solid boosters, but for the acceleration up to the escape velocity best option is liquid propellant. So, that is why in the later stages we use liquid propellant once we are in the outer space non chemical will be better. So, but if you are talking about small scale applications like satellites keep station keeping etcetera non chemical electrical is the best source, but if you have to produce more powers thermal is the better source. So, if you have to someday think about going far deep into space this may be the options, but solar collector of course will be limited by the range up to be solar laser available, but nuclear and laser can be operated beyond that also. So, essentially the bottom line is we choose the horses for the courses. There are various type of rockets available we choose specific rocket for a particular application. So, the choice of rocket is dictated by the mission requirement. Rockets most suitable for long journey essentially let us say Saturn 5 is not usually very much suitable for launching at earth orbit. So, therefore, you need to have modification and many times even different stages of same vehicle use different type of engine again the biggest example is PSLV where you have a solid liquid solid liquid right GSLV solid liquid liquid cryogenic. So, they have different engine at different stages again that choice depends on the mission requirement. How long a particular stage will be firing what will be the payload fraction for that particular stage based on that we decide what type of rocket to be used, but once we go outside after that depending on the mission typically electric propulsion systems are used. So, with this we complete our discussion on the introductory part of the rocket propulsion. Now, let us go into the more details of rocket propulsion.