 So, what we have done is, we have developed a methodology which is now being used all over the world by many people to design aerostats which looks at these important parameters which either the user will give you based on the operating scenario or you have to assume based on your experience or based on what you think will be needed. Let us look one by one. Deployment altitude above mean sea level, this is the altitude at the ground of the location at which you will deploy. Are you deploying it in Leh or in Mumbai or Manali? That decides the altitude at the ground because the temperature, the pressures will depend on that and the delta H. The next one is called as the floating altitude or the altitude at which you want the aerostat to float. This is above the ground level. Then the next important thing is called as the diurnal temperature variation. The word diurnal means day and night. At any location you know that there is some maximum daytime temperature and some minimum night time temperature. This difference plus 15 in the day and minus 5 in the night that means 20 degrees is the temperature difference which this envelope will encounter. And it should not lose buoyancy so much with this much temperature change or it should not gain so much super pressure because of this heating that it starts creating a tension. So this delta T also decides the size of the ballonet so it is important to know that. Obviously in places where we have very high delta T you will need a larger ballonet system and hence a bigger envelope for the same payload and places where temperature remains almost constant. Let us say night temperature is maximum plus 5 sorry minimum plus 5 daytime is maximum plus 10 then you have only 5 degree delta T. So the ballonet will be small. The ballonet required will be small at that location okay. Maximum deployment duration helps you to determine how much extra volume to keep to take care of leakages. Ambient wind speed will decide the two things that tether profile will be affected very badly with that very heavily with that and also the load coming on the aerostat because under heavy wind conditions you will have heavy loads and your tether should be able to withstand that load. So while selecting the tether, while selecting the tether you will be able to estimate the tension coming and hence you might say I cannot use this I have to use that that will be heavier than this one so this will play a role there. To load weight obviously this is what you have to carry permissible blow by so what is meant by blow by blow by is a technical term in aerostat technology which is basically the horizontal displacement of the aerostat because of winds. So for example if I can just sketch a small aerostat and show you let us say this is our aerostat system. So let us say this is our aerostat system during some wind condition let us say V is equal to X meters per second ambient wind. So from the vertical take any reference point let us say we take the CG for example then this distance is called as the blow by okay. Now as the wind speed increases and suppose the tether length is kept fixed what will happen the same balloon will actually come maybe like this correct. The balloon because the length is kept being kept fixed so therefore there will be a loss in the altitude so the blow by will increase. Now suppose I do not want to allow any loss in the altitude what could be the reason I do not want to allow any loss in altitude because if I lose altitude my camera will not point at the place where I want it to point or my payload may not work let us say I want to collect air sample at height of 1 kilometer near a chimney for measuring the pollution level you cannot say no no there is blow by you can take sample at 500 meters no I want at 1 kilometer so what will you do so you will release tether when you fly a kite you do that right deal that right what do you do you give it because you want it to go to a longer distance or stay at a high altitude but the wind is pulling it so you release tether same thing will happen here the difference is in a kite if you release more rope or thread it is you who is flying it so you are facing the tension in your hand is it true that when the wind is very heavy you have to hold it more tightly sometimes it just pulls you also sometimes your hands get cut because it is so harsh it is pulling you so in the case of tether what will happen is that the weight of the tether is going to be carried by the aerostat envelope and it is the weight of the tether will be subtracted from the payload carrying capacity isn't it because the balloon has some net buoyant lift so if I want to carry 300 kilograms of payload and if the tether itself is 100 grams I am actually going to carry 400 grams of payload if I release another 50 meters of tether actually I will not be able to reach that altitude so in a scenario when the tether length is fixed and that is blow by or wind increase blow by causes lower altitude if the altitude needs to be maintained you have to release more tether this results in reduction in tow carrying capacity because the weight of the additional tether is also going to eat away the lifting force so somebody might say that look I am operating this aerostat for aerial photography of the IPL match therefore the blow by air allow is only 20 meters because after that it will start looking at the hockey match not at the cricket match there is point somewhere else so the user might say permissible blow by is only so many meters so then you have to do the calculations to find out how do I ensure then the next one is permissible loss in altitude that means as I said you want to deploy the aerostat at some place for some work how much loss in altitude because of temperature variation or because of any other reason because of loss of gas are you permitting so in 14 days it should not lose height more than 10 meters this is what somebody might say and in these 14 days the maximum delta T between daytime and iron temperature can be 30 degrees so with that much delta T it should not come down below some distance or some altitude that could be a requirement type of the LTA gas somebody might say hydrogen somebody might say no we cannot use hydrogen it is unsafe we will use helium somebody might say no I want to make a low-cost aerostat so can we use steam or some other LTA gas that will affect the size of the envelope and everything else and type of payload here what I mean is this payload is it self-contained from all considerations that means does it have onboard battery which will power it or you want to send the electrical current for it from the ground you could send it by an additional cable along with the tether or you could send it inside the tether if you have a special conducting tether so conducting tether are available in which you have central high strength membrane which is interround over the optical signal carrying fibers or LAN cables or electrical cables if you use a conducting tether then you can do all communication etc from ground but this is heavy and this is costly if you on the lens and no I am going to use a small camera with onboard battery then I can use a simple nylon rope or any other high strength or strong enough rope that will be cheaper it will be lighter so type of payload will determine what kind of tether is permitted okay then there are some design options these are options which are chosen by the designer based on availability of material these are not normally given by the user user does not know these things for example are you going to use a single envelope or a double envelope that means are you going to have envelope inside the envelope or are you going to have a single envelope and gas inside so what could be the positives and negatives of both of these choices so why would you use double chamber envelope lesser leakage rate why so you are saying instead of one you use two membranes so the leakage will be through one and then through the other one okay that is not the reason but there is a there is a related reason yeah you are right what you say will happen but we do not put double chamber because of that yes the atmospheric conditions like so how will double chamber help so it is a okay you are talking about outer covering as a protective envelope over the aerostat okay this could be one reason basically I will tell you the basic reason the issue is this the requirements from the envelope material are very very stringent and very very contradictory it has to be lightweight as well as strong as well as less permeable to gas as well as able to withstand infrared radiation as well as able to withstand scrubbing and rubbing etc. we are handling because you will put hooks and other things on that so people will pull it so some people said can we not have two envelopes one envelope will take care of only a lightweight gas barrier bag a bladder like football bladder the other one will be like a parachute cloth which can take scrubbing you can put hooks on that it will be it can be coated to have infrared property etc so some people have gone this path saying building the requirement into two one for gas retention and shape generation like a football can come with without tires are there tubeless tires and tube tires similarly you can have a tubeless aerostat and tube aerostat if you have the envelope technology such that one material can take care of all the requirements then a single chamber is always lighter may not be cheaper it will be lighter and easy to handle one less headache but if you do not have it it is better to go for two chambers so that you can de-link the requirements of the two so the conflicting requirements can be handled by putting two so many students when they build their first aerostat they may go for a single chamber because only one envelope to build less work but their envelope should be able to take care so both of them are possible this decision is taken by the designer so in some applications we can say okay let us go for double chamber for example the people with whom I work in Brazil they have gone for a double chamber aerostat in fact they have not they have not made it they are buying it so they use a double chamber aerostat but all the stuff that we have built so far are all single chamber because we were able to get a single material meeting most of our requirements okay envelope shape what should be the envelope shape now if you look purely at the requirements of buoyancy and weight then the best shape is spherical because for a given volume a spherical shape has the least surface area therefore least self-weight but spherical shape has got very high drag it has a very high drag coefficient plus if you look at the vortex structure behind a spherical shape it is very disturbing as compared to slender long shapes so depending on the application now when you go on the highway and you see these small balloons which are used to popularize some trade show or some they have a spherical balloon which just keeps shaking so they are not doing a great scientific work they are just cheap low cost system they make it spherical but when you use but you will be surprised even very serious scientific work is done sometimes by spherical balloons depending on the application so in outer space for example you would say let us use spherical there is no drag because there is no there is no ambient wind so depending on the application you can choose other than that many shapes have been suggested by people there is one shape called as GNVR about which we will see today very briefly given by professor GNV Rao of IISC Bangalore this shape is used by all aerostats designed by ADRD in Agra have I spoken to you about the shape so you have some idea this is one standard shape it is a good shape then we have a shape called Lynx this has come to us from a company in Russia called as Ross Aero systems they they have an aerostat called Lynx so I call this as a Lynx shape they have not given any name to it I just call it as a Lynx shape then there is a teardrop shape which is the name given the shapes made by a company called T comp in USA interestingly as a SAC is a shape given by space application center in Ahmedabad they also made one small aerostat and whatever shape they used is called as a SAC shape so interestingly many companies which make aerostats they do not share their shape data with you that is a proprietary item just like aircraft manufacturers do not give you the airfoil data they will say use NACA 0012 or NACA 24013 but if you ask them what is your airfoil many of them do not reveal they will say modified so and so airfoil similarly whenever I meet aerostat manufacturers and I talk to them and I say look we work in aerostats what is your shape they smile and they say oh this is proprietary we have acquired this over so many years etc so getting data about standard shapes is easy getting data about actual shapes is not easy so what is the layout let us say you have an aerostat from T comp so the only idea is take pictures and read and read the coordinates that is what we do there is no other option the balloon type could be a various types one could have an integral balloon or one could have an elastic cordage this is what we have used in all small aerostats so on the bottom of the aerostat you put elastic like a shoelace and tighten it little bit so when there is super pressure the envelope wants to expand the elastic takes the load and allows expansion when temperature falls and the aerostat wants to contract elastic allows it to contract so without disturbing the shape too much it actually allows it to control the buoyancy to some extent one very interesting chordage I saw in the aerostat developed by TIFR the sac shape they put elastic cordage from the nose to the tail inside so their idea is that as the aerostat wants to shrink let us shrink the tail by pulling the tail from the back and as it wants to expand the elastic expands and the aerostat becomes fuller along the length I have already mentioned you about type of tether conducting one conducting winch could be manual or electrical a manual winch is used for low altitude aerostats 50 feet 100 feet why should you put electrical but you might say no I want to bring it down quickly within half a minute so then you cannot and then sometimes the load required you know we are designing one small aerostat for a college and we came to know that the tension in the tether is 250 kilogram under 25 meter per second wind so it will require Herculean power to pull that by hand and wind it so obviously we have to use some kind of a pneumatic or electrical winch but in aerostats which we have made for some small small aerostats for colleges able-bodied human beings are enough to winch it down okay.