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Since you did all that work, perhaps you could answer a question. When Shuttle was flying, I learned they hand grinded the windshield between every flight. That was to remove pitting from micrometeoroid strikes. I tried to convince every Shuttle engineer I talked with to replace the windshield with ALON: AluminoOxyNitride. That was developed in the 1980s under contract for the US army for windows of tanks. It isn't used anymore, now they put a webcam outside the tank, and flat screen display inside. You can't shoot through a window that isn't there. I suspect they put multiple webcams outside in case one gets shot. But the material was developed, and has been used in Afghanistan and Iraq as the windshield on some HMMWV vehicles (Humvee). I wanted to use it for the Shuttle windshield. Just to eliminate any hand-grinding. The engineers I spoke with said their management didn't listen to their ideas. They all said management didn't want to reduce work or reduce cost to process Shuttle, that when they came up with ideas they were told to shut up.
My question: would this have worked? Would ALON have worked as the Shuttle windshield? I notice the Cupola on ISS now uses exactly that same material, so it looks like someone did listen. After all, you can't go outside the station to hand-grind the window. But would this material withstand heat of Shuttle atmospheric entry? What did Shuttle use, was the glass they used any better? Worse? What?
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I don't know much at all about ALON, although the chemical name sounds like a ceramic of some kind. If so, it might make a good windscreen, as long as it has strength and transparency when very hot.
If memory serves, the shuttle windscreen was 3 layers, and at least the outer layer was quartz. That's the same high-temperature windscreen material used on X-15 and SR-71. I don't think any of those birds ever used glass (rather molten at only 900 F) or the structural clear plastics (rather molten just above 300 F). I'd have to run numbers to know, but I remember hearing tales about the -15 and -71 having 2000 F windscreen temperatures. But if you have multiple layers, the inner layers will run cooler if not in direct contact with each other.
There is a corruption effect when you work as the contractor for any government agency long enough. Your management practices and decision-making end up looking just like those of your unusual lone customer. If they don't, you don't get contracts. It's a self-fulfilling prophesy, and it's a self-defeating cultural thing. So I am entirely unsurprised by your tale about the engineers being discouraged from making improvements of any kind.
It's not just space, it's all of aerospace defense, and now far beyond. It's why we spend so much and get so little for the money, these years, when it was so different when I was very young.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Aluminum Oxynitride: chemical forumula Al68O73N5. It's chemical formula is it's name, just with numbers removed.
Manufacturer's website shows a slightly different composition; looks like they added more aluminum nitride.
This is an aluminum ceramic. It's based on synthetic sapphire, which is pure crystalline aluminum oxide. This has aluminum oxide with a little aluminum nitride added, and a trace of yttrium oxide to control grain size. It's made by taking powder, and sintering to fuse the powder into a solid. The trick is to get all air bubbles out. Any air bubbles will make it translucent like frosted glass instead of transparent. Synthetic sapphire actually has slightly greater tensile strength and slightly greater hardness, however sapphire is a crystal so will shatter with any impact. ALON was developed to stop military bullets, so impact damage is its reason for being.
Surmet: ALON Optical Ceramic
Composition: Al23-1/3XO27+XN5-X
Grain Size (typical): 150-250 microns
Structure: Cubic, Spinel
Density: 3.696 - 3.691 g/cc
Form: Polycrystalline
Melting Point: 2150°C
Young's Modulus: 323 GPa
Hardness: 1850 kg/mm2 (Knoop Indent, 200g)
Fracture Toughness: 2.0-2.9MPa-m1/2
Flexure Strength: 380-700 MPa
Thermal Conductivity 13 W/mK
Transmission Limits: 0.22 to 6 microns
This says melting point is 2150°C (3902°F). Documents about making the stuff calls for baking at 1850°C (3362°C) for hours. One says 1800°C (3272°F) for 3 days. As I'm sure you're aware: sintering a ceramic requires melting the surface of the grains so they fuse together. But you don't melt the whole thing to liquid, that wouldn't form ceramic.
Last edited by RobertDyck (2016-02-29 12:51:59)
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That is a very intriguing material. How big and what shapes can you get this stuff? Who makes it? Is that "surmet" a company name?
I've got a completely-different application for stuff that tough that hot, and it doesn't have to be transparent.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Yes, Surmet is the contractor that makes it. Click the link above, or click here. Quoting from that page...
Today Surmet manufactures precision ALON® components with consistently high quality, in batches that are in tonnage quantities, and in increasing sizes and shape complexity. Surmet has capability to produce precisely finished ALON components and is the only supplier of monolithic windows as big as 18x35-inch and windows with engineered seams as large as 24x27-inch to the US Defense market.
Note: difficulty is polishing to optical clarity. They polish with diamond dust because the stuff is so hard. Shapes tend to be flat or hemispherical because of the polishing process.
Be warned: I also saw a TV interview with a US Air Force general who gushed about this stuff. He wanted to put it on all his aircraft. Surmet can't make complex shapes of a fighter aircraft canopy, but they could use it for flat windows of cargo aircraft. The general said in that same interview that the reason they don't is cost. This stuff is too expensive for the Air Force.
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