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Interview with
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HearEversfield via RealAudio: 14.4 | 28.8 | ISDN Get RealAudio software |
EVERSFIELD: Right. There's a universal gas law, which says that
pressure times forty and times the temperature—the absolute temperature—which we measure in kelvin, degrees kelvin, that is actually a constant for any
particular gas. And that really is the principal on which the whole balloon is
flying. So this envelope which is about twelve and a half tons in weight with
the payload, has to lift that mass. So at launch, we need roughly one kilogram
of lift, is one meter cubed, or one cubic meter to lift one kilogram. So it's
a lift of twelve and a half tons. We need twelve thousand, five hundred meters
cubed of helium at sea level at about fifteen degrees centigrade. If we just had an envelope of that volume, the moment it starts to lift, the pressure inside that envelope would increase and we'd end up with a super pressure balloon. And that would induce high stresses into the fabric and potentially burst. So this balloon is a zero pressure balloon. So we actually made the pressure larger than the twelve and a half thousand meters cubed we need; in fact, we have a volume of thirty-one thousand, one hundred, fifty meters cubed. And that allows the gas at sea level—we have a roughly forty percent fill at launch, and as the balloon rises, the air around the balloon as we go up in altitude, gets thinner and extends so the gas inside the balloon expands. As that expands, it gradually fills the envelope. The temperature is, obviously decreasing as we decrease with altitude. And our target altitude is about forty thousand feet, where it's about minus forty degrees centigrade. So when we do the calculations, you can see that with the pressure, volume and temperature that the balloon would actually float at equilibrium at thirty thousand feet. With twelve and a half tons at thirty one thousand, one hundred, fifty meters cubed. Now, at launch, in order to get that movement upwards, we need this free lift. And on average we have about ten percent free lift at launch. So that will be about a ton and a half of extra force, if you like, trying to lift us up into the air. With that free lift, we will gradually start accelerating upwards and we will have ascended to our thirty thousand feet. The gas will have expanded then, fully, to occupy the complete volume of the sphere. Now, once it reaches that point, we have excess lift, if you like, we still have an excess lift and it has to get out of the balloon. Otherwise, it starts to over-pressurize. So we have three appendices, which are, effectively, overflow pipes, if you like, in terms of some sort of analogy. But the helium then goes through these pipes and overspills. And that gets rid of the excess helium pressure inside the envelope. Now, if that cannot escape quickly enough, the pressure will, in fact, keep rising. It's like having a hole in a pipe—the smaller the hole, the harder it is to blow through the volumetric flow-rate that you would require to get that excess volume out. So the three appendices we have have been calculated that that will diminish the pressure sufficiently quickly enough, once it reaches its float altitude. NOVA: So the pilots will be constantly concerned about what their altitude is and whether they're rising or descending, right? Of course at launch they want zero wind conditions, but at altitude they are at the whims of the jet stream. |
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Hear Eversfield via RealAudio: 14.4 | 28.8 | ISDN |
EVERSFIELD: At launch, when we take off, we want zero wind conditions.
The problem is, the chances of getting zero wind conditions for launch is
fairly, fairly rare, so we need to take the opportunity when it arises.
Consequently, the jet streams may not be ideal overhead at the time. So this
may mean we wish to loiter, let's say, ten thousand feet, or fifteen thousand
feet for one or two days, which will at least give the pilots a chance to
acclimatize and get used to the new environment. So we can ballast the balloon off at that stage to hold them at a pre-determined float altitude below the ideal thirty thousand that they wish to achieve. And, let's say, after a couple of days there, there's a strong jet stream ahead at normally thirty thousand feet, they can dump some ballast and gradually, the envelope will rise and take them to that altitude. They will float to that altitude to whichever they've ballasted out to, and during the day, that should remain fairly constant. As the sun comes out it will heat the gas and the pressure temperature law comes into its own and we will gradually rise a bit. And as the night time approaches, the temperature will drop and then gas will contract and the balloon will lose altitude. Now, sometimes the jet stream can be as thin as two to three thousand feet thick. So to remain inside that jet stream, we need to heat the gas in the evening so that we can maintain that float altitude—that ideal float altitude right in the heart of that jet stream so that we don't fall out of it. Because, obviously, the jet streams are carrying us on the course that we wish. Now, also the jet stream may be taking us off on the wrong course. So we may need to change course and the only way we can steer this balloon is by vertically ascending or descending. That's the only real means of direction or control of the balloon, unlike an aircraft where you can apply weather to change direction. So in order to do that, we would need to valve helium to be able to descend out of one jet stream into another. The danger, obviously, in valving gas when you're at that sort of altitude, it's very, very cold, somewhere down around the minus forty, minus fifty degrees centigrade, there's always the small chance of ice forming. And if the valve was to be iced open, for example, that could be potentially a dangerous situation to be in because the valve wouldn't close and we could be losing helium, even at a slow rate, quite constantly. NOVA: How do you stop the valve from icing? EVERSFIELD: We don't. We have a single valve in this balloon. There was a big debate whether we should have two or three valves but if you've got three valves, you've got three more chances of a valve going wrong. So this valve has been designed and built and tested right the way down to minus sixty degrees centigrade in a chamber that was sprayed with water to simulate more extreme conditions than we'll ever actually see. And in fact, John Ackroyd actually took it down to the test chambers to actually perform that series of tests, and that was found to be extremely successful. And it's actually powered by a BAC-111 Trim Tab Control motor which has unknown failures—It doesn't have any known failures, and so it's an extremely reliable system. NOVA: How do you solve your leakage problem with helium? EVERSFIELD: The leakage problem is—first of all, you need to know what your leakage rate is through your base fabric. So there's a lot of design time and effort put into making sure that the base material is helium-tight. The biggest leak path tends to come from any joints. Because obviously, the fabric is not big enough to make in one big sphere. So it has to be split into orange peel-type segments called gores. And we can either stitch and tape these together—the stitching giving it the strength, the tape giving it the sealing characteristics, we could glue it, or we could weld it. There are several ways of welding it. We can use RF welding, radiofrequency welding. We can use impulse welding, which is rather like a hot iron. And we can hot air weld, which is the technique we actually finally adopted after a tremendous test program that we went through. Now, like any process, it's not just a matter of jumping onto a machine and operating it. All our operators in the welding process went through a test whereby they welded up a small test balloon and this was pressure tested and the pressure at which it burst was recorded. And not until they'd reached the level we'd required for structure integrity, were they then allowed to go on to actually manufacturing the Global Balloon. Because, obviously, we need the strength in the balloon at the welds, and we also need the correct helium porosity. Now, in order to ascertain what that helium porosity was on the seams, we had a machine specifically designed for us using mass spectrometry and, essentially, it is a machine which tests one-meter lengths at a time, you can put a seam in of the material. It has an upper chamber and a lower chamber. The upper chamber passes air over the top of it. It clamps it together and seals the top and bottom sides. Air passes over the top and it checks to make sure there isn't what we call a gross leak. For example, if you put a pin hole in it, the air would rush through that pin hole quite quickly and you'd know you'd have a gross leak. If it passed that test, helium, being such a small molecule, being able to get though much smaller holes, would then be shot across the top of this chamber and sucked, if you like, to the underside of the chamber. And any helium migrating from the top chamber to the bottom chamber would then be recorded by the mass spectrometer. That was calibrated against known leaks. In other words, if we put a pin hole in, we knew what helium leak that would give us. The budget for the whole balloon was three hundred kilograms loss of lift for the three weeks. We can afford to lose three hundred kilograms in the balloon over the three weeks. Now, the base material, without any welds in it, assuming everything was perfect, would lose one hundred kilograms in the three weeks. So we had to ensure that the seams, of which there's approximately two and three—three miles, minimum of seams to be welded, that those seams were, in fact, helium tight. That allowed us two hundred kilograms of loss of helium through those seams. So from that figure, by calculating the length of the seams that were welded, the two hundred kilograms loss of lift in the three weeks, we could work out what our helium leakage was for every seam. So as the envelope was welded, we would then put it through our helium leak detector and record every single meter of the seam that was welded to find out what its leak rate was. That would be added up and, if necessary, remedial action could be taken, i.e. repairs or whatever, to ensure that the helium leak was maintained at the level which we could accept. NOVA: What are the fabrics that will be used? EVERSFIELD: We have three types of fabric on this particular balloon. The upper cap is made from a polyester fabric with polyurethane coating on either side and it is extremely tear-resistant. If I can just give you an analogy here, you can have a very, very strong material in tension, but it can be very weak in tear. A standard metal Coca Cola can, for example, a metal Coca Cola can, has equal strength in tension as our fabric. So that both have exactly the same strength. However, if you just put a small tear in the Coke can, you can easily tear it. So although it's very strong, it tears very easily. Our cap material, you cannot tear by hand. So we have a far more tear resistant structure. And that's very important for something like this. So that's the top cap, which is effectively the top hemisphere of the balloon. That sees the highest pressure. If you imagine filling a football with water and holding it at the top, the bottom of the ball would have the highest pressure. And the top of the ball, although the bottom of the ball, although there's water in it, there's very little pressure, acting due to gravity. So the top cap carries the highest stress. We then have a lower hemisphere, which we've referred to as the bladder, which is the part that has the lowest stress in it. This is actually made from a rip stop type of hot air balloon fabric, which also has a very thin polyurethane coating on it. And that has actually been stitched and welded with a tape—welded across the top of it to seal it, as opposed to the upper cap material, which has just been purely welded. And what we had to do with the top cap was to seal it with a small bead of contact adhesive to actually stop any helium leaks progressing through the edges of the fabric. And then, the lower cone section which leads roughly from below the equator of the balloon down to the flying wires on top of the capsule, that's actually a standard hot air fabric, which contains the heat within the envelope. So those are three main materials used for the balloon. And that is—the hot air cone is also just purely stitched, exactly as you would do with a standard hot air balloon. NOVA: What do you mean by welding fabric?  EVERSFIELD: The fabric, which has—can either be a nylon-based fabric
or a polyester-based fabric, is essentially just like your shirt or blouse
material. And then on both sides of that, we coat it with a thin coat of
polyurethane, which is a thermoplastic material which, when you heat it,
liquifies, and when you cool it, it resets again. So, because the whole fabric is coated with that, that acts not only as the helium barrier, but also allows us—when we put the two components—one piece of fabric over the top of another one, we actually heat in between it with hot air to a specified temperature, which melts the polyurethane and, just immediately behind that jet of hot air running in to weld it, there's a roller which just supplies a little bit of pressure to squeeze the two materials together and it also cools it on its way out. So over the space of about six inches, the whole welding process is performed. NOVA: So, in essence, you think that the welded seams could be stronger than the fabric itself? EVERSFIELD: The welded seams have to be stronger than the fabric itself. That's the usual design process. However, we have an extremely strong fabric here so, whereas the ideal is to design it so that the failure would never be in the joint, whether it's a glued joint or whether it's a welded joint. That's because we have such a very strong material, we would need quite a large overlap of weld in order to get the full strength in. But at room temperature, yes, the weld is comparable to the strength of the material. We've taken our temperature testing regime to all the extremes we could possibly encounter. That's down to minus sixty, we've tested. We've tested at room temperature. We've tested up to plus one hundred degrees centigrade, which is the temperature of boiling water. And although it's very unlikely, for example, the top cap will never see that temperature, it gives us a good quality control process by which, if there is a problem, it highlights it much earlier on and much easier to detect that you can detect at room temperature. So those are some of the factors of safety we've built into this particular balloon design. NOVA: What amount of propane will you need for life support, and how do you calculate that without actually doing it? EVERSFIELD: Well, with the Pacific and Atlantic crossings where they've done before—they've flown on continuous engine running all the time. At Oswestry in Shropshire, we had the engine running for virtually five hundred hours simulated continuous run. So that's already been done. The five hundred hours is equivalent to three weeks. So we do have a fuel burn, but of course, that's been obviously at sea level, and it's going to be very much different at altitude. But from the Atlantic and the Pacific crossings, they've got a good idea of what their fuel burns are likely to be at altitude. But it's the hot air part of it, if you like, on the Pacific and Atlantic, being hot air balloons, was significantly more than we're ever going to burn on the Global helium balloon. NOVA: With a third person in there, would that change some of the variables as well? EVERSFIELD: Not really, because we have a bigger balloon—I mean, the helium is providing the lift. With the hot air balloon, the hot air provides the lift and that hot air is always having to be fed into that balloon continuously unless the balloon has been designed such that the solar radiation during the day can heat the balloon sufficiently so you don't need to burn. That is, in fact, what is done on quite a few balloons. But here with a helium balloon, it will just maintain a reasonably constant altitude. There's no energy input, as such. The balloon will just float. It's a lot easier to fly and, in fact, a lot less input is required by the pilot. NOVA: Do you have any fun experiments that kids or interested students could try at home to convey some of the concepts we've discussed, using this balloon adventure as a model? |
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Hear Eversfield via RealAudio: 14.4 | 28.8 | ISDN |
EVERSFIELD: There is a nice little test that they can do, which is to
get a cork with a little bit of weight hanging on the cork and put it inside a
bottle with—like a champagne bottle, or if you're not as rich as we are, a
wine bottle—fill the bottle with water, put your little cork with the weight
on it inside the bottle, and it will probably just float. And then, when you
press the cork to close the bottle, as the pressure gets higher from the
compression of the air on the top of the bottle, so the cork with a little
weight will gradually sink and you can actually stop it, depending on how hard
you press on the cork on the top. And that will explain what buoyancy is all about. Because really, we're talking about buoyancy. It's like a submarine in the water. It's exactly the same principle but we're doing it in air. So if you can look at how a submarine works by filling a tank with water and it will sink and reach a certain point ,then you can let some of that water out and it will rise, this experiment shows that sort of principle of buoyancy, which is certainly how the balloon works. It's simply a buoyancy experiment. NOVA: Great, thank you. EVERSFIELD: Thank you. Interviews: Ackroyd | Branson | Erickson | Eversfield | Kendrick Photos: Aaron Strong Global Contenders '97/'98 | Expedition '96/'97 | Fossett | Virtual Flight Science of Ballooning | Teacher's Guide | Resources | Transcript | Balloon Home Editor's Picks | Previous Sites | Join Us/E-mail | TV/Web Schedule About NOVA | Teachers | Site Map | Shop | Jobs | Search | To print PBS Online | NOVA Online | WGBH © | Updated October 2000 |
NOVA: Leon, can you tell us exactly what you do on this project?