 Let us look at some common type of loadable fabrics. You can very easily you will see single ply fabrics. One can have a single ply, single ply means a single material, a uniform material. It can have coating on both sides or it can have coating on one side and an adhesive film on the other side and this adhesive film is applied and then heat sealed. So there is a company in Nasik called as Entremonde Poly E Quoters. So we have visited that company once. They showed us all these processes. So at some point of time if we get an opportunity we might have a class trip to this company where if they open the factory to us we will like to go and visit and see how they do the coating of various fabrics and how they touch them. So they are the ones who supply the fabric to the airship and aerostat operators globally as well as to the DRDO Laboratories in India they are using their services for coating of fabric. So let us see if we can arrange a trip where we can see these fabrics. That is another company in Mumbai which develops the basic fabric. They do not have any coating facility but they develop basic fabric. So maybe we can invite them once to give a talk on this special topic. I do not know where their factory is. I have not visited their factory but it will be interesting to go and see how these things are done. I will welcome all of you to visit LTA lab and you can see the fabrics that we have already worked with. We have already done experimentation with around 6 different types of fabrics. I will be showcasing to you very soon some of our fabrics. So you could have single ply fabrics with coating on one or both sides or you could have coating only on one side or you may have coating on one side and film on the other side. Then you could also have laminates of coated fabrics. So there are 2 fabrics which are joined together. Then you can have laminates of films and strength bearing fabrics. So 3 types. So central member, one film on top, one film on bottom. Now what is the difference between a film and a lamina or a film and a fabric? Essentially film is a very very thin fabric. It is the same thing but when the thickness is very less we call it as a film rather than a fabric. You could have 2 ply fabric woven and non-woven with coating on both sides. Here is an example of, this slide is only meant to give you some kind of a mental picture of the weights. How much does it weigh? So a typical load bearing member would be around 100 grams per square meter, 100 GSM. You coat it on outside and inside, there will be a difference in the coating required. The coating that is required for providing the environmental protection weighs more than the coating that is required for gas retention. So around 92 GSM or almost double the weight is created by the coating from outside and nearly half the weight is added by coating from inside. So the typical weight will come to around 234 grams per square meter with a plus minus 20 percent variation and you can notice that the load bearing member is nearly half 43 percent, outside is 40 percent, inside is 20 percent. This is a typical distribution of a double coated single ply fabric. These numbers have come from actual data. I am not able to reveal the details of the material but I can tell you that this is a standard data for a fabric that is available. Now here is a picture which explains to you what is meant by the warp direction and what is meant by the weft direction. In a typical woven fabric, you use long longitudinal fabrics and then you weave them with lateral fabrics and the lateral fabrics normally are continuous which go along the width. So they are called as the weft which are used by the weavers and the warp is along the length. So as you can see the strength along the warp direction is generally slightly higher than the strength along the weft direction. So if you use different types of coatings, you can see the look at the first fabric which has got the PU1 coating. The GSM is very high, it is 120 plus minus 20 but the strength is 155. So 155 kilogram will be required by a 5 centimeter width strip to create strain such that the fabric starts elongating. It may not break but it may just keep elongating endlessly and then there will be a neck formation and then it will tear. So a lighter fabric PU2 you know it has got 280 and the strength is almost half in both the directions. Then you could have PU3, it is heavier fabric and hence the strength is also larger. If you look at neoprene which is basically a light weight compound, it weighs half but the strength is also much lower. So generally there is a tradeoff between the strength that you can get and the weight. So you have to design it very carefully when you select a fabric you have to look at both the dimensions. So what we do normally is we first calculate how much strength is needed that means what are the loads coming on the fabric, then you put a factor of safety and then we look at the load carrying capacity of the fabric. And we assume that this capacity will be reduced slightly further because of imperfections or some other issues. So either you can inflate the factor of safety or you can put another extra mile it is the same thing. So the factor of safety is normally 5, typically we use factor of safety of 5 in fabrics because the degradation with atmospheric changes, degradation because of the fabrication imperfections is quite large. So in an aircraft aluminum structure because the behavior is predictable to some extent we use lower factor of safety. Whenever you have larger predictability, less predictability you have higher factor of safety. It is between 4 to 5 for the fabrics of an LTS system. If you can look at the load bearing layer, these are 4 candidates for the load bearing layer. Polyvester fabric is very commonly used including for shirting and suiting. The same fabric can also be used for base load carrying member of the fabric. So you can see that it has a tensile strength of 1 Giga Pascal. And if you use nylon, nylon has got a lower specific gravity that means it is lesser in weight. It will weigh less but the strength also will reduce. Kevlar is a very interesting fabric although it is a bit heavier from 1.39 to 1.45 but tensile strength is more than 2.5 to 3 times. So that is why tensile modulus is very high. So therefore Kevlar is one very commonly available fabric. However, Kevlar is very expensive because it is a proprietary fabric. So if you want to make an airship with Kevlar then you have to have deep pockets. It is going to be expensive. So in a Kevlar institute we cannot afford it unless some donor gives us Kevlar fabric. We cannot afford it. I am coming to that. There are many exotic fabrics, Decron, Vectron, Dyneema, these are all trade names of fabrics which have been created by companies like DuPont especially for LTA applications. So they can be used, they can be used but how much coating is to be put on top and bottom depends on the application or depends on the situation where you are going to use it. For example, if you look at cotton and rubber it was used in the early 20s for the old airships. The strength is only 80 pounds per inch. The weight, now the weight of the fabric I have kept this intentionally in the non-standard units because when you look at the data in open literature you will find all kinds of units. Grams per square meter is used by us mostly but if you go to the West in US for example you will find ounce per square yard as a very, very commonly used parameter or unit for measuring the weight of the fabric. One can easily convert one to the other by getting the conversion factors. So we see in this particular figure that Dyneema which is a special purpose laminated fabric aimed for stratospheric airships. Now stratospheric airships are still being designed. None of them has been deployed for a long time for actual application. The only long term deployment of a test balloon was by a company in Texas but that was also not like a functional system. It was just to test that and airship can be taken to a height of around 12,000 feet and kept there for 8 to 10 to 12 hours. So these are very large sized. The typical lengths are 150 meters, 200 meters in length. So one of my students is doing research on design of high altitude airships. So he will be coming and making a presentation on the technology of high altitude airships. We will discuss it little bit later. So here are some applications. So GZ or GZ is one airship which was used by the Second World War. It was very popular in Second World War. You can see that it has polyester and neoprene as a combination. And then ZPG-3W are the airships which came almost towards the mid 60s to 70s. They used slightly better fabric but it is very heavy. Skyship 600 is the airship which revived LTA technology in the modern times and that is the question in the quiz also. That particular airship you can see the fabric is only 210 GSM. So it can be easily concluded that the coming back of airships has been mainly possible because of the developments in material technology allowing us low weight better quality fabrics compared to the past. So from 320 it has come down to 210 and you can see that that can greatly reduce the self-weight and hence it will give you. Even though the Skyship 600 has an envelope of nearly 8000 m3, it can lift around 67 passengers. And then Vectron. Now Vectron is a very, very strong fabric, very, very strong fabric, 740 pounds per square inch. So this is recommended for low altitude applications and you can see that even though the strength is so large, the self-weight is only 7.6 ounce per square yard. And the stratospheric airships require very strong fabrics and the weight is further. And then the Japanese have come up with a new material called PBO and some very long name. That has got much better properties than even Dyneema and that is being recommended. So I will leave it to the student to come and talk about that in more detail.