§1. Sapphire boules and the Chinese solar cell market [02:38]
The company that grows this aluminum oxide, or builds the equipment for it, is up here in New Hampshire. The equipment is about a quarter the size of this room and about as tall as this room. The melting temperature of aluminum oxide is about 2,000 centigrade, so it gets pretty hot.
They make what they call a boule — b-o-u-l-e — of sapphire, of aluminum oxide, about this big. That boule, like a great big rugby ball without points on the ends, is worth about a hundred thousand dollars. But it takes about a month to six weeks in the furnace, and it's a one or two million dollar furnace. And this company in New Hampshire, who do they sell it to? The Chinese.
I said, well, who needs all this single-crystal aluminum oxide? It turns out every LED has got a substrate of aluminum oxide. This stuff gets cut up into little squares or rectangles, and then they put the gallium arsenide laser or whatever on top of that to make the LED. Probably because of coefficient of thermal expansion matching.
That chunk as it is, is worth a few dollars. And then we had to get a diamond tool to cut off the sharp edges, because they just put it in a great big press — aluminum oxide has better fracture toughness than most ceramics but it's still pretty lousy, so they just smash it to break it into little chunks because it's so hard to cut. The only thing harder on the natural scale than aluminum oxide is diamond. We use aluminum oxide, which we call corundum, as a grinding grit. You go to a hardware store and ask for aluminum oxide or corundum — it's basically the same thing, but not in big grains. They like to have big grains.
This company had previously sold several thousand furnaces — similar, didn't go to quite as high a temperature, a little different design — to grow silicon for solar cells. They sold thousands of these furnaces to the Chinese. The Chinese government decided about eight or ten years ago they were going to take over the solar cell market for the world, and they were going to invest as a nation. Now they're not supposed to do this technically — this is one of these externalities I talked about in the first day. They're not supposed to as a nation go out to compete against the rest of the world; it's supposed to be individual companies. But in China it's sort of fuzzy about whether the company is owned by the government or not.
The government had a national policy that they were going to have a certain amount of solar cell production for nice environmental reasons. They didn't have the technology, so they bought it from this New Hampshire company. And they basically did the same thing: put silicon in a furnace, heat it up — it only takes about 1600 degrees centigrade rather than 2,000, which is a lot easier — heat it up and let it cool down, and you get these large crystals of silicon. They're boules, except these are actually grown in graphite, so they don't end up as a football shape; they actually have some flat surfaces. Great big crystals of silicon.
Now, when people started making solar cells forty years ago, they were taking single-crystal silicon, saw-cutting it with diamonds, and making flat sheets. When Intel makes a single crystal silicon, they buy that from the Taiwanese. A company called Taiwan Semiconductor Manufacturing went into the silicon foundry business — they grow great big single crystals of silicon and they beat out everybody else in the world. So Bell Labs and Intel and all these other people just said, oh well, we'll buy our silicon wafers from Taiwan.
But those are single crystals. You don't need single crystals for solar cells. I'm going to pass this around — you're welcome to pick them up, be careful, they're very sharp. [Tom passes a silicon solar cell sample around the class.] This is like a razor blade, but you can pick it up. This was actually grown by a company out west of Boston. It was started by Professor Sachs in mechanical engineering. They use edge-defined film growth, where they have a little carbon fiber on either side and they draw these fibers out of the molten bath in a temperature gradient and grow plate sheets of this stuff.
They're sort of fragile — in fact they're ultra fragile. That was just a chunk of silicon, grown very slowly, cooled very slowly. Takes about a month — well, it only takes a few days to heat the great big thing up and melt it, and then you cool it down a degree per hour or something to solidify it, and get great big grains because it's very pure. Actually it's got some boron or phosphorus in there. You can see the full width sheet, and here's a piece on top.
There's a silicon solar cell market, and people were predicting that because we were getting these cheaper ways to grow silicon — whether it's that way with the big chunk, or this, which came from the same place. The silicon things come out huge. This is a piece that's been diamond saw cut; you'll see the chips on the edges, and that's just from passing it around. It's pretty brittle stuff. It's not exactly a structural material from a brittleness point of view, but it is a functional material for solar cells. Lots of wasted material in just the width of the saw cut to cut this stuff, and I've got a little thing to talk about that a little bit later.
§2. Solyndra, edge-defined film growth, and the cost of new materials [09:54]
This company up in New Hampshire has sold hundreds, if not thousands, of these furnaces to the Chinese, who are building a thirty billion dollar investment to try to create a trillion dollar market in solar cells. Meanwhile, remember during the election, they were hitting up Obama for the Department of Energy having loaned three or four hundred million dollars to Solyndra in southern California, which was supposed to be doing solar cells in the United States. They tanked. So the federal government defaulted on the federal guaranteed loans for four or five hundred million dollars.
I went up to a place in Canada about ten years ago that was a solar cell manufacturing facility. The Canadian government had plowed a quarter billion dollars into this place — huge manufacturing facility for solar cells — and they tanked. If you really look at the economics of solar cells, you can't quite get there by the old way: make a huge ingot — doesn't have to be a single crystal but even a great large grain polycrystal — and then cut it with diamond saws. It's just too much saw cutting. That's why Professor Sachs started this company with edge-defined film growth, so you'd grow it as a thin sheet and you don't have to saw cut it.
One of the problems with this edge-defined film growth — I was out there at that lab, and they have nice machines and it works, but it's really slow. When you're pulling something that thin and that light, you don't make very many pounds per hour. This is where you get to Sprague's law: whenever you first hear about a new material, that's the best properties you'll ever have. And Williams's corollary: whenever you first hear about the cost of a new material, that's the lowest cost it'll ever have.
A lot of times people don't even run the numbers to say how fast can I grow it, how fast can I make it. If you take my casting lecture, I prove why nanotechnology will never be a structural material. Nanotechnology has to be formed from the vapor phase, and the kinetic theory of gases says the fastest you can go is about a millimeter an hour. How many Golden Gate bridges or Oakland Bay bridges are you going to build at a millimeter-an-hour growth rate with million-dollar equipment?
This New Hampshire company — very nice facilities, they're making a lot of money — they're going to make it for a very short period of time. They're going to sell the technology to make these large grain sapphire, large grain silicon, to the Chinese, who are then going to put their low-cost labor into it with their government subsidies and everything else, and they're going to wipe out all the other people who are trying to get into the business in other countries. These are the externalities and the political realities of trying to make advanced materials in an area that politically is good because it's environmentally clean.
Frankly, you have to have government subsidies right now to be able to justify putting solar cells on your house. You know that we passed the law in Massachusetts that ten percent of our electricity will come from renewable energy. It's a wonderful thing, it encourages things. They're building this huge wind farm off Nantucket, and people in Nantucket or Martha's Vineyard or wherever are very unhappy because wherever they look they're going to see propellers going around very slowly. But it will be renewable energy. And we all get to pay about 25 cents per kilowatt hour for the privilege.
Now, you don't know what you're paying because you're not homeowners, most of you. I'm paying 15 cents a kilowatt hour. If only ten percent of my cost comes in at 25 cents a kilowatt hour, I only see about another penny. It's a six or seven percent increase on my electric bill. But a six or seven percent increase on everyone's electric bill in New England is billions of dollars. So they pass a law that basically says we will have renewable energy at least ten percent, and we will — but you're going to end up paying for it. It's just a hidden tax. But it's a hidden tax that's good for the public good; we don't make CO2 and things like that.
§3. Reading *Advanced Materials and Processes*: the 34-story wooden building [15:22]
For about thirty-five years I've been getting this magazine, which is the monthly magazine of the American Society for Metals. They changed their name to ASM International back in the 90s, when everyone thought metals were dying and they didn't want to be part of a dying industry, so they're Advanced Materials and Processes. I was looking at this the other day and I thought, there's a lot of things in here that you've learned enough about material selection to explain.
This is the type of media reporting that you get from supposedly professional societies. In the beginning of the thing — I put some yellow highlights on — they'll have little quick news articles about some new development. A company may have some person, or some professor may decide he wants to promote his research, so he'll write up a little spiel for them and send them a picture, and they're just looking for copy to fill up their magazine. They'll report it without any critique. This has all the scientific rigor of the internet, but it's been around longer than the internet.
Here's a skyscraper that was designed in Sweden — they're going back to a 34-story wooden residential building. I thought, wait a second. The 34 stories caught my eye because the reason New York City went from 10-story buildings to skyscrapers was because Bethlehem Steel learned how to roll I-beams around 1900. Before that, without Otis elevator and without steel for the core structure, brick buildings and wooden buildings couldn't be more than 10 stories tall, and no one wanted to walk more than 10 stories up and down to get to their office.
So 34 stories out of wood — you're really pushing the limits of whether the wood will collapse and split under its own weight, because wood has very anisotropic properties. But you read a little further: it will be built over a wooden construction with a concrete core. Ah, now I know where they're getting the strength. Concrete has tremendous strength in compression, not so great in tension, but it's got very good strength in compression. So they're going to build a wooden and concrete structure. Now how can you even build a concrete structure 34 stories tall, because ten percent of it will be steel reinforcement inside the concrete. So don't tell anybody about that.
But read on: wood production has no waste products. To a certain extent that's true — we generate sawdust and the wood chips that come out of the bark, but you can sprinkle those on your lawn as mulch for your garden, and you can burn some of that in wood pellets. So yeah, we don't have a lot of waste wood products, I'll go along with that. Binds CO2 — wood binds CO2 because trees breathe carbon dioxide. We breathe oxygen and exhale carbon monoxide [dioxide]; trees breathe carbon dioxide and exhale oxygen. Sort of a symbiotic relationship between mammals and plants.
But binding CO2 — it only does that when it's living. Once you cut it down it's no longer binding that CO2. Nonetheless, you've got to have forest to grow the wood. But let's think not just about the wood in this building, let's think about the other things, like the concrete. Where does concrete come from? Limestone. Limestone has a chemical formula of calcium carbonate. You heat that up, and you make burned lime, calcium oxide, plus CO2. So is this a CO2-free building? Not exactly, because there's probably more weight of concrete in this than there is of wood, given that the concrete's three and a half times as dense.
Now I'm cheating there, because I use the word weight rather than volume. If you want to do it on a volume basis, maybe it's equal, I don't know. But there's just simple little words like, am I comparing on weight or am I comparing on volume? If the two things have very different density, you have to ask, should you be comparing on a volume basis for a structural material or a weight basis? Might make more sense to be a volume basis. We usually talk about weight, and it turns out wood is very strong compared to its density. This is the strength-to-weight ratio, the specific strength: yield strength divided by density.
§4. Specific properties and the conductive polymer hype [21:02]
People can use those phrases all the time, and they do that for the unaware. You should not be the unaware — you're MIT grads or soon will be. I was sitting in the old Shipman room, probably 20 or 25 years ago. A young professor, Gary Wenick, was in polymer science. This was at a time when people had just discovered things like polyacetylene, which were electrically conductive polymers. He was giving a talk to the visiting committee or somebody about his research. He said, these conductive polymers have a specific electrical conductivity — that means the electrical conductivity, which is the inverse of resistance, divided by the density — that is better than copper.
I thought, wow, that's pretty good, because copper is the best we got. Copper is a huge industry. At first I thought, is this going to wipe out the copper industry? And he said, because of this specific conductivity, we will be using — this was like 1985 or maybe 1990 — we will be using electrically conductive polymers in rotating machinery: electric motors, big generators, whatever.
That all sort of made sense — it's spinning, you want lightweight, and if it's got better specific electrical conductivity. The next morning while I was shaving, I almost cut myself, I realized: wait a second, that is not the criterion for selection of the material that you're winding the motors out of. At that time, 1987 or whenever this was, we were winding motors out of aluminum. Aluminum has a better specific electrical conductivity than copper. It's 40 percent lower electrical conductivity, but it's got one-third the density. So aluminum has twice the specific electrical conductivity of copper, but we were making motors out of aluminum.
But we were not making great big generators out of aluminum. The bigger the thing, the more you want lightweight, right? We're still making them out of copper. I thought, well, if specific electrical conductivity is the parameter of interest in the design of this material, why am I still using copper in these million-dollar machines? Aluminum's cheaper than copper, I could use it. You have to be careful — people will sell you on a specific material property, not specific in terms of divided by density all the time, but they will sell you on a material property that is not the property of interest. It's the one they want to sell.
We do have electrically conductive polymers now, and we use them for touch screens and things like that, but they haven't replaced copper in big generators and a lot of motors. I figured that out the next morning after he gave his talk, back in the 1980s. But I was still reading all through the 1990s how electrically conductive polymers were going to take over the copper industry in the world. Don't think so, folks. I realized out of common sense — remember, I told you, you actually already know the answer, you may not have thought about it. It was when I was shaving the next morning that I realized aluminum was already a better specific electrical conductivity than copper, and we hadn't switched over to aluminum. It would have been an easy thing to switch over to aluminum, but we hadn't. So obviously there must have been something more in the design that I didn't know about. It turns out we are still not using these conductive polymers to anywhere near the extent that all the researchers were touting them.
§5. Hype cycles: structural ceramics and high-temperature superconductors [25:35]
Now I've been through about a dozen of these tremendous revolutions. One of the first was structural ceramics, in the early 1980s. Ceramics had excellent high-temperature strength, they were supposedly corrosion resistant — they're not, but they were supposedly. Certainly they're corrosion resistant to a lot of things that metals corrode in, but ceramics will corrode in other things. The world was spending billions of dollars in research for what they called fine ceramics.
So now 30 years later, what do we make out of fine ceramics? They were going to make automobile engines, they were going to make turbine engines for aircraft. What do we make? I've got some knives at home — excellent at slicing onions, because of the sharpness, they don't crush the cells and release all those things that make you cry. They actually cut through the cells and don't release as much of the juice. In the mid-80s I used to give this talk and say, we're not going to have structural ceramics because they don't have fracture toughness. Look at that piece of silicon, the little flat piece with the chips on it. Look at my knives in my kitchen — they don't have tips on them, they've got chips on them. They retain their sharpness as long as they don't chip off and turn into something blunt.
And they only cost 100 bucks apiece for a knife, where you can buy one for five or ten bucks out of steel. But I like them. My daughter-in-law really liked them — she used to chip them all the time. In any case, it never became the huge market. Then there was conductive polymers. Then there was high-temperature superconductivity. I told some of you offline the story: IAP in the department, mid-90s, I had to give a talk over at little Kresge to a bunch of people from industry.
Someone asked me a question about the new high-temperature superconductors that had just been discovered five or six years before. I said, they're fantastic functional materials, we're going to use them for detecting magnetic fields, optical sensors, things like that. But all this stuff you hear about high-field magnets — we won't see those in our lifetimes or our grandchildren's lifetimes. I got back to my office about five or six in the evening, got a phone call from a faculty member in the department. He had already heard what I'd said half an hour before. He just lit into me: you don't know anything about superconductivity. I said, Mike, I did my doctoral thesis in superconductivity, I think I know something about it.
He just blessed me out. I finally said, quit acting like an eight-year-old, I have a right to my opinion. Well, he took me to the dean. The three of us sat in the dean's office, and the dean said, Mike says you called him an eight-year-old. I said, well, I'll quit calling him an eight-year-old when he quits talking like an eight-year-old. The dean thought he was going to smooth this over, but I'm not — don't try to smooth it over with me. Anyway, this faculty member, because he had research in high-temperature superconductors, he told me we would be riding on levitated trains within five years.
It's now about 17 years later. If I ever go over to the cancer center and see this guy again, I'm going to ask him where the levitated trains are, because I haven't been able to get on one yet. There are other problems. You actually have to know some details about something to understand it. You can't get your knowledge off the front page of the Wall Street Journal. I go over and teach a class over at Sloan, and these guys and gals — I got in trouble once for saying that, men and women — they're reading the Wall Street Journal before class in the morning. I used to tell them, by the time you read it in the Wall Street Journal, it's not news. You need to get there a little earlier. And just because you read it there doesn't mean it's true.
One of the things right now: they've run some articles on additive manufacturing in the Wall Street Journal. So every CEO in America thinks, why are we doing machining and cutting things apart? We should be adding metal, we should be doing 3D printing. You read all this stuff about 3D printing. Well, let's talk about rates of manufacturing. It's great — Tiffany's, when they're making the Super Bowl rings, they make plastic molds in a 3D printer, shooting wax, making wax molds, and then they do lost wax casting to make the thing because they're only going to make 150 rings. You say, why do you need 150 rings for a team of 53? Well, there's all the coaches, and there's the lawyers that work for the owner, and there's the owner, and the owner gives rings to his friends, so they actually probably do make 100 to 150.
They're very valuable rings, and you can get even more for them if you can find — unless OJ wants to take it back. So when you read these things about wooden buildings, kind of read between the lines. To me, this is sort of like — I don't need to read the comics, I can read the A and MP, latest and greatest science.
§6. The Siemens cold-rolling sale and the second-hand mill problem [31:44]
Siemens Metal Technologies is going to sell a 140 million dollar plant to some Chinese to build a complete cold rolling complex. So the Chinese don't have the latest and greatest process controls for rolling steel — what do they buy? They buy it from us. In the 1980s, I showed you the big productivity increase in steel, like a twofold increase in productivity from 1980 to 1990. There was the change to continuous casting, which in the 1970s went from 65 percent yield to 97 percent yield. All of a sudden the world had thirty percent more steel-making capacity than they needed, and the price of steel dropped.
A lot of those companies that first went out of business would sell their old equipment to try to make a little money — to Brazil or Argentina or Saudi Arabia. After about five years they realized their other mills couldn't compete, because they were now competing with their own equipment that they had just shipped and sold for 10 cents on the dollar to some other country. By the late 90s, if they went out of business, they would sell their mill for scrap rather than sell it to someone else who's going to re-enter the market using their equipment against them.
This has happened many times. I ran into this in shoe manufacturing back when I was an assistant professor. New England used to be the center of shoe manufacturing. The Gordon McKay School of Engineering at Harvard, of Applied Sciences — Gordon McKay was a wealthy shoe merchant in Boston who gave 10 million dollars to Harvard in the early 1900s, and now the Gordon McKay name is all over their School of Applied Sciences, professorships and everything, out of shoe manufacturing. I had to go up to the North Shore — there was a guy going to these old mills, taking the old shoe machines from the 1930s, rebuilding them, and selling them to third world countries. They would make footwear which would come back to the United States as low-cost shoes.
So whether it's the New Hampshire company selling the technology to make silicon solar cells or sapphire substrates, or steel companies selling old or new equipment technology — the more technologically advanced countries are going to sell their equipment to the less technologically advanced, and then they're going to see those people come in and beat their socks off in the marketplace, and then they complain about it. Well, okay, you better keep developing technology and keep ahead of the game.
§7. Polyamide oil sumps and the bolt-spacing tell [34:45]
So I read this, and it talks about plastics and oil sumps driving down vehicle weight. They're using polyamide 6. Anybody know what polyamide 6 is? We have a more common name for it. It's nylon. Rather than calling it nylon in this article, they say polyamide 6 as if it's a new material. This was new in 1935. Glass reinforced — we've been doing that for about 50 years. Heat stabilized — I don't know exactly what they mean by that, maybe they remove some of the stresses.
Sixty percent lighter compared to the old aluminum or steel oil pan. They had to do a curb impact test — I don't know how you hit your oil pan against the curb unless you're jumping the curb, but never mind. Stone impact tests — you could get stones kicking up. And an engine drop test — I don't know what those mean. But does anybody notice what struck me in looking at the picture of the screws? Look how closely spaced those screws are around that oil pan. Anyone ever looked underneath the car on a steel or aluminum oil pan? Does it have screws that close? No. Why?
You're a material scientist now. This is not as stiff. The steel is stiffer, and the modulus, the stiffness of the material, determines how closely you have to put the bolts in order to get the seal. If there's a rubber gasket in between, the thing's just going to bow and you're going to lose your seal. So I just doubled the cost of my bolts — I don't know if they included that. And I also have to install and tighten all those bolts. If anyone ever has to take the oil pan off, they've got to loosen all those bolts.
So there are trade-offs. I'm not saying this isn't an interesting development. You can see they made a honeycomb structure so they could get the strength they think they need in impact resistance. It may be a great application. But the nylon is going to go for four dollars a pound, six dollars a pound. The aluminum will go for two dollars a pound, the steel will go for a buck a pound. This is lighter and you don't need as many pounds. And if we're talking volume, this is a lot thicker. In any case, there are some interesting material selection issues here.
§8. Aluminum-lithium and composite fan cases [37:38]
They also announced in this magazine that Alcoa in the United Kingdom is putting in its third-generation aluminum-lithium alloys. We haven't gotten to specific aluminum alloys, but they can put up to 10 percent lithium in aluminum. It's very interesting because ordinarily alloying doesn't change the modulus of the material very much. But 10 percent lithium — that's by weight, so it's actually a very large atomic percentage when you're dealing with something as light as lithium — increases the modulus from about 10 million psi to 11 or 12 million, depending on how much lithium is in there.
A 10 or 20 percent increase in modulus is great, but it's also so light that the density is less. So the specific modulus is like 30 percent greater. When you're designing an aircraft, specific modulus is the metric you want to design lightweight structures for. In the mid 70s they first came up with this and it was a huge research project; it took them about 20 years to resolve some of the corrosion resistance problems. The article says "better stiffness" — it is better stiffness, but it's even better specific stiffness.
Damage tolerance — I've used a different term for damage tolerance. Fracture toughness. Same thing, just different words. One of the things you need to learn is what all this vocabulary is. They say stiffness — to a material scientist, stiffness is modulus. What's damage tolerance to a mechanical engineer or materials engineer? Fracture toughness. What's corrosion resistance? Corrosion resistance. People use different terms to impress people.
Down here, contract to produce composite aft fan cases. This is for the Rolls-Royce engine that is going to power the A350. The fan case they're talking about is, I think, this one right back here, the aft. This whole thing is the fan part of the engine, and the aft is in the back. They're talking about this being a composite material. I'm going to spend some time talking about composites and their advantages and disadvantages. In fact composites are inherently going to be more expensive. But if I'm in the aerospace industry at 200 a pound, or even more if it's off the engine weight, which saves weight on the rest of the structure, I can afford to use something that might cost me a thousand dollars a pound.
This is potentially a good application for composites. Rolls-Royce just gave the contract to ATK Alliant — Alliant Technology, I can't remember what the K stands for. They're in Magna, Utah. What were they famous for? Anybody know what Alliant built that blew up? Space shuttle Challenger — the seals. That was all Alliant technology. They designed the seals for the Challenger that caused the Challenger to blow up. An engineer, Boss Juliet [Boisjoly] at Alliant, said don't, it's too cold, don't fly it. But they were going to fly the Challenger. Anyone remember the reason why they flew the Challenger?
Well, what they said wasn't really the real reason. The real externality was that Christa McAuliffe, the school teacher from New Hampshire, was going to talk to President Reagan during his State of the Union address from space that night. None of the top managers wanted to cancel this for some stupid technical reason, because they had a big political windfall — the first school teacher in space was going to talk to the President of the United States by satellite phone during his State of the Union address that night.
If she wasn't up in space, it was "here I am from the Holiday Inn in Cape Kennedy," rather than "here I am from 120 miles above." So there was a political reality there. The engineer says, it's too cold, the O-rings are too stiff because they're so cold, it's not going to work. They had previous history of O-ring damage — they had double O-rings for redundancy, and they had almost blown one of these things before when the outside temperature was like 40 degrees, and now it was 28 degrees. They said, oh well, we've flown cold missions before — yeah, but never this cold. That's just someone trying to gloss it over.
Student: I had a question back about the aircraft engine. Are they able to make composite fail a little bit more — I mean because I don't know, like C4 tests? Like, I guess if you have an aluminum covering, it's a little bit more like dumbbell failure or plastic.
Frankly, if you get something that hits it hard enough, that aluminum is going to dent. Once it dents, it's not going to operate properly anyway — the fans, the blades are going to go flying off, because once you get oval. So the composite is not as damage tolerant. Have they designed as much damage tolerance as they need into the composite? You bet they have, they're concerned about that. It's a five million dollar engine at that point, and it's a 10 million dollar nacelle, and it's a 200 million dollar aircraft, and it's a billion dollar lawsuit for the people who die. There are financial consequences of all this.
Believe me, the people in the aerospace industry are sensitive to this, they're good engineers, they know what they're doing. So I read an article about the aft fan case — this is not done for political reasons. In my experience, I've never seen anything in the aviation industry where people were doing it for purely political reasons. They're doing it for economic or safety reasons. Or just you couldn't do it otherwise.
§9. The V-22 Osprey and the helicopter speed limit [44:28]
I actually have a picture in here of the V-22 Osprey. The tilt rotor. The two rotors on either end will tilt down, and they used one of these to get Osama bin Laden. You have to understand — does anyone not know why a regular helicopter is limited to about a 200 mile-an-hour forward speed?
It's the blade speed, the tip blade speed, and specifically the advancing tip blade speed. When that goes supersonic, you run into all kinds of stability problems. At Langley, Virginia, at the materials lab right outside, they have a helicopter that they once put some rocket motors behind, and they actually flew it with the rocket motors pushing it forward to like 240 miles an hour. The blade tips had gone supersonic on the advancing blade tip. So maybe it's only 200 miles an hour going forward for the fuselage, but the advancing blade tip is going a lot faster than that.
Once it gets to 600 miles an hour, or 650, whatever Mach 1 is, you get all types of instabilities. That can cause the blades to break up, and that's not a good day for the pilot or any of the passengers. But they've done it because NASA wanted to do it. I'm sure the guy had a parachute to eject and everything, because they wanted to study the instability to see if they could get around it. But basically the instabilities are not something you get around.
So Bell and Boeing developed the tilt rotor, so you can operate in helicopter mode with the rotors up, or you can tilt the rotors forward to have a propeller going around like a regular aircraft. These can go about 300, 350 miles an hour. They also don't make as much noise as a helicopter blade running this way, as opposed to a propeller running this way. So they can't hear you coming, and you're coming at 350 miles an hour, not 200 miles an hour. The Marines consider this a significant advantage. They did use one in getting Osama bin Laden, although most of them were Sikorsky Blackhawks. I'll tell you some more about Blackhawks later.
§10. The turbofan revolution: momentum versus kinetic energy [47:15]
Does anyone know why the aircraft engines look like this, with the big — instead of the old engines back when I was your age. You'd look at the side of the jet and it had this hot-dog-shaped thing, it didn't have this great big fan up front. Anyone know why in the 1990s we switched from pure jet engines, where you just take the gas in, combust it and shoot it out the back, to the turbofan, which is different than a turboprop? A turbofan is a jet engine with a great big propeller on the front.
Does anyone know the basic physics of why they went from the old hot-dog sausage shape to this great big thing? In fact, they would like to get rid of this shroud. General Electric had a design that was just a propeller up front with no shroud around it. Yes?
Student: [inaudible question about oxygen/gas flow]
It has to do with the gas flow, not specifically the oxygen. They already are compressing in the compressors, they've gotten up to like 30 atmospheres. At 30 atmospheres at those temperatures you can't compress a lot more — they have gone higher than that, but you've got about all the compression you can get efficiently. Yes?
Student: Like you need to have a higher pressure region or lower right here, since you're running the same — to compress it off the same shaft you can't —
You're getting there. They actually do have to have two shafts, because there's one shaft that's going to spin this fan at a different speed than the compressor inside. So it is a double-shafted engine, it's a lot more complex. Price of engines went from two million to five million. But it gets back to: there's something called kinetic energy, one-half mv squared, and there's something called momentum, mass times velocity, if we're talking about the gas.
There's a certain mass of gas coming out the back of the engine, and it goes through the compressor and everything and generates all this extra energy from combustion. That's the one-half mv squared thrust. But you can get a lot more thrust if you can actually pass gas, not at near sonic velocities out the back, but at less than sonic velocities, over a much bigger volume. That's the fan up front. In simplistic terms, the fan is the momentum, pushing air to give you thrust. The kinetic energy is what's coming out the tailpipe, giving you the kinetic energy thrust. These are hot gases, these are cold gases. But you get more thrust out of these cold gases now than you ever got out of those other gases.
Now, this is a divergence between military engines and commercial engines. Through the 1970s and early 80s, military engines would push the technology, and then the civilians would come along and say, oh great, we'll just take that technology improvement and efficiency and put it on our civilian aircraft. Starting in the 80s, when General Electric and Rolls-Royce and everybody else started looking at this basic concept of, you really want to move the greatest mass of gas — you want this m to be huge, and that's what goes through the fan. The m that goes through the engine itself is a lot smaller. Before, the two m's were the same.
When they decoupled and had a fan, the engine and the fan are two different masses, and you have a much bigger mass, lower velocity, but a huge mass — gives me more thrust than small mass and huge velocity. The aeronautical engineers worked all this out. All of a sudden, the military engine development, which wants to go supersonic — you can't go supersonic with a fan, just like with the helicopters, those fan blades start wobbling and they break off.
But the military wants to go supersonic for strategic and tactical reasons. So they don't have big fan engines except on their transports; on their fighter jets it's a completely different engine now. Until the 1980s they were basically the same — sort of dual-use technology. Now it's not dual-use, it's a problem economically.
So I've done enough of that. I'll go back tomorrow, I'll do some steel processing, and show you some other things that happened in the last 25 years that have taken steel about a tenfold improvement in productivity. And then we'll do aluminum. Then we're going to get into each specific metal and go that way.