 So, now the next time when we meet which will be on I think 14th of January, we will go back in history and we will try to look at how this technology first came into being and then subsequently how it went ahead further. I also want to inform you about the first assignment that we will be looking at in this course. I will announce it next time because it is to do with what we cover in the next lecture. At this point, I think we can take some questions or doubts that you may have regarding the modern aerostat or airships based on what you saw or if anything else you have in mind. We can take these questions. Yes, please mention your name as you before you speak. So, my name is 3 side. So, my question is what is the basic conception difference between a manned airship and an airship? There is no difference except a manned airship normally is designed with higher factor of safety for every structural component and in manned airships use of hydrogen as a lifting gas is not permitted by law. So, the best of my information that is the only difference between a manned airship and an unmanned system. So, can I say that a manned airship with some tethering can be used as an airship? Yes, there have been examples of some hybrid systems in which people have used a manned airship with a tether. So, now whether you will call it as a tethered airship or a man carrying aerostat that is up to you. But yes, but what kind of application did you have in mind for a manned aerostat? No, I just thought about very high service and very long time duration. We say the endurance for a manned airship is around 48 hours. I was just thinking what is the conception of the personal application. Okay. Let me correct you the 48 hours endurance is for a manned airship for an aerostat the endurance could be even 6 months, just a function of how much gas tight the envelope can be made. And over how much period of time the gas will leak out sufficiently to that the buoyancy is completely lost and she stops coming down or loses height so much that it becomes ineffective. Okay. So, you are talking of an application where you have a tether and then you have an airship. So, if we move around or will it remain stationary? So if it remains stationary then why do you want to put a man on board? What will be the benefit? The differences are only this that the usage of the LTA gas there are some constraints as well as because of human being on board and the safety issues that are high factor of safety in the design. Anybody else has any question? Yes. Name? Does everybody know you also? Ramya. I know her of course, but others should also know. Yes Ramya. Oh yes. We will have a special lecture on materials used for LTA systems in which we will discuss what is done to ensure that the gas leakage is minimized. So, I will just tell you basically what we do is there are two approaches. One approach is to use a double chamber envelope. So, you can have like a football internal bladder will be only for controlling the gas leakage or to hold the gas and the external envelope will be to take care of all the loads and the scrubbing and other weather issues or with the modern technology you can have a single fabric with either laminates or coatings. So, you can think of a multiple laminate. Let us say two envelopes. One example is Tedler and Mylar these are dew point patented materials. So, many people use Tedler, Mylar, laminate. Similarly, what most people do is they go for coatings. So, they take a base fabric for strength, they coat inside for gas retention, they coat outside for atmospheric UV protection or ability to withstand the scrubbing and other loads. So, this is what is normally done. Any other question anybody has? Yes. But you have not mentioned your name. Gauri. Gauri, okay. How do airships come down? It is a very good question because I have learned that whatever goes up must come down. But in the case of airships, how do they come down? So first question is how do they go up? The mechanism which is used to make them go up, I would say a kind of reverse of that is used to bring them down. For example, you saw in the airship video that the power plant was tilted to give vector thrust. So, with vector thrust, we were overcoming gravity to go up. Of course, gravity is overcome by buoyancy, but you want to physically go up, create an imbalance force, upward force, you can do it by tilting the engine. So, you can do the reverse to bring it down. That is one way of doing it. That is a very technologically expensive way of doing things. Of course, airships do have thrust vectoring mechanism. So, can someone help in this? It is a very interesting question and it needs a little bit of thought on how do you bring it down? So, what would you do to bring an airship down? Yeah. Main. Vineet. Vineet. From which department are you? Okay. They can bring you down. They pull you down. When a boat reaches near the shore, they throw a rope, there is a big pole that I rounded and then some people pull it. Similarly, you can bring someone or some airship down by physically pulling it down. This is one way of doing it. Okay. But then you have to, either you have to fly very near the ground so that there is always a rope hanging below. Suppose you are above that, let us say you are at 1000 feet. Now you can, you can throw 1000 feet rope and ask people to pull you down. It is difficult. Anything else? Yes. Okay. This is one way of doing it. You have a small airbag inside the gas bag. So, when you have the right amount of air along with the lifting gas inside such that lift is more than or equal to weight, it will go up. But when you want to make it heavy, you can collect air from atmosphere in that airbag. Okay. So, you can have a system which just takes in ambient air. Now this air is heavier than the gas, the gas inside. So the net weight increases. So she will slowly start sinking down. This is one way of doing it. Yes. Yes. Controlled venting of helium is a way to bring people down, airship down, but very expensive way of doing it. Because then you are losing, it is like consuming fuel or consuming your gas. Okay. So, it is done but only in emergencies when other things fail and you desperately want to come down. Then venting of helium gases perhaps last is sort of available to you to reduce the lift. Any other way of bringing an airship down? Yeah. It was first or third vectoring. Correct. So, you create a downward force and you bring it down. Anything else you can do? Yes. Decrease the size of it when you try to create it. So that is what, that is what Pratik said that you release the gas. Reduce the volume of the balloon. How do you do that? It has some mechanism which is called rotating air. Okay. So what you are saying is that you will do something so that the envelope volume is reduced. Yes. It can be done. It can be done. This is a very nice innovative way of doing it. Compartment. Okay. Think about it. Which we have to think about. When we do aerostatics, at that point we will revisit whether you can have a multi-compartment gas bag and then you can push gas from say 3 bags into 1 bag and then collapse 3 bags. So that the volume of the envelope reduces but the mass of the gas remains the same. Okay. We have to revisit this when we come to the aerostatics. There was something that Omkar was trying to say. Yes. Liquefy the gas. Liquefy the gas inside. Okay. Very, very complicated mechanism because the system that you have to carry on board to liquefy the gas will be difficult. I will tell you simpler methods of doing it. Yes. Yes. Condense the atmospheric. This is what I was going to say. This is what many people do. Collect water vapor from the atmosphere, condense it, you get water which is very heavy and that can bring. But then you have to wait for a situation when there is water vapor. If it is above Sahara Desert, no water in the atmosphere, you cannot come down. We have to wait for rain. You cannot do that. So it is done. Collection of water vapor. In fact, what also we do is the exhaust of the engine. Okay. Even there there is some water vapor sometimes. So there are. So we will see that. All this will be part of the course. Any other way of bringing it down. So why not do the following? Why not make it heavier than air in the first place? So that when you want to bring it down, do nothing, it will come down. So what you do is, remember that when you are flying at some speed because of the shape, it will generate some dynamic lift. If you are a clever aerospace designer, you will be able to give a shape that gives you very good lift or more lift than drag. You know that is what we do. We want to have higher L by D, lift over drag. So if I can carefully shape the envelope in such a way that when it starts moving, it starts generating lift. Let us say 10%, 15% of the total lift comes from dynamic lift. So you are heavy on the ground. Your total buoyancy is let us say 1000 and the vehicle weight is 1200. So 200 kg force you create by maybe tilting the engine or 230 kg force. So you start slowly moving up. You acquire some speed. Now the dynamic lift starts coming and then you can relieve the engine. And when you fly at some particular speed, you might be able to manage comfortably with the dynamic lift plus static lift equal to weight supplemented with the thrust vectoring. And when you want to come down, just stop flying. Now this is what we do when we fly our airships. Our airships are normally heavier than air for this only reason that we want them to come down. If things go wrong, what do I want? I want the airship to come down. So although I call them LTA vehicles, in reality I cheat and fly an HTA vehicle. So you can call it as a buoyant heavier than air vehicle to be very precise. So this is one solution. Yes, Tim. In what way will the endurance come down? Because you are consuming energy for all the time and in case you have an actual LTA vehicle, then you can save the energy. I mean save the energy for turning the engines on and off or maneuvering. True. Very true. So you see one has to do a trade-off between what is the total consumption of fuel, either you will have to always fly lighter, you fly lighter than air and then have some system which will bring it down forcefully. So which of them will consume more power or more fuel, we do not know right now. So from safety point of view, normally airships are flown statically heavy, a static heaviness of a typical airship with 15 passengers around 500 kg, which means there is a 500 kg force acting down always. So in other words, you are offsetting gravity with a 500 kg lag so that if something goes wrong, you can slowly come down. Anything else? Yes. Yes. So this is called a static heaviness. So you can make it around 6 to 10 percent statically heavy. So the extent of buoyant lift will be only 90 or 95 percent. 5 percent you will like to create by aerodynamic forces. You can what we do is in our flight, we mount the engine at a slight angle. So as she takes off, it is not horizontal flight, it is always giving. So one component of the engine is always giving me that minus, that gravitational or downward force. So when the engine stops, comes down. Okay. Anything else Pratik? You have something? You have a compressor right on board because you are using a turbine system. You have a compressor, like you have two engines, so you have two compressors right next to you. So what if you can route the wheel in the balloon to your, it is still running, it is still compressing the helium and you need it there. So which means you are going to introduce helium from the balloon into the air stream of the engine. No. Instead of that, I would say why don't you carry small separate systems, which do only this. Because if you mix air and helium, how do you recover only helium from that? You have only helium, but you are bleeding it right. So it was said, I think I was told, but you said it will be very heavy. No, what someone said is, what someone said is simply release the helium. Liquifaction, what has been attempted is a small liquefaction system inside the envelope, which when commanded, liquefies gas and pushes in a cylinder. What I am saying is liquefaction requires a lot of compression, what I am saying is not no compressive to that extent, but to a certain extent. But you see now the same engine is having an air stream. In that you would introduce a helium stream and that and then the fan will compress it and then you have to push it back inside the envelope or where will it go? It can have intermittent things like that. It is very complicated. I know it is understandable. It is doable. See whether you have an integrated system or a dedicated system, we decide based on our assessment of the cost, complexity and weight. It can be done. I mean things can be done to attempt it. So now last time I mentioned something on the Moodle page for the Moodle page and one person has responded to that also. We have got some information about helium now on the Moodle page. I would urge other students to also now proceed further. So the question for you to attempt at the end of this lecture is find out photographs, videos, description of very innovative uses of airships and aerostats which will add value to our understanding. So if you locate something about an application which I have not shown or which you think will really add value or even excitement to our study, please give links of that on the Moodle page. And the second question is think of other innovative ways people have used for bringing an airship down or the buoyancy control. Okay, on that note we will stop for the day.