Re: "I'll put my money on a Skylon."
if you have toughened graphite, which would probably be a classified compound on a shuttle, then aerogel, and then carbon, you will have a light weight super strong hull which would survive
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Though I've worked with a range of graphite composites (carbon-carbon to carbon-reinforced bismaleimide) in my career, I'm not quite sure what you're getting at. "Toughened graphite" isn't a standard material product, and any "toughening" of graphite is relative - graphite is always a soft, brittle material, even when its tensile strength heads for 1 million psi / 6,900 megapascals. I used to chop and prepare ultra-high strength, woven graphite sheets with common steel scissors. I never had to sharpen the scissors once, there was no wear of the steel from such brittle, soft materials. (I did learn to wear a filter mask after I began coughing and sneezing black snot while chopping the graphite, but that's a different matter.)
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space which the US air force x-37b hull is probably made of.
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The X-37B's outer skin uses conventional, shuttle-type heat shielding. The nose cap is a carbon-carbon composite with silicon carbide oxidation barriers, like the shuttle's leading edge. The belly is lightweight, fiber-reinforced silicaceous material, a modest improvement on the shuttle's tiles. The underlying frame is mostly polymer composite (where it's not aluminum) like high temperature bismaleimide-carbon composites. (I'm not sure of the exact choice of resin, but BMI functions up to 250C, better than conventional aerospace aluminum alloys, and is a good example for this discussion until I can confirm it.) So, the X-37B's framework is in the family of composites with the 787 and F-35 aircraft, not "toughened graphite and aerogel."
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Given the X-37B's relatively restricted budget, you're not going to see a lot of exotic material choices - truly new material systems really take upward of 10 to 20 years to go from the lab to operational hardware.
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then if you spray with a silicon spray, your hull will then be tough like a diamond,
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1) Diamonds are not very tough; their KIC (K-one-cee, I need to learn HTML subscript codes) fracture toughness is low compared to metals and polymers. They are hard and can be quite strong, but "toughness" is not something usually associated with diamonds.
2) Spraying silicon onto carbon usually results in either carbon with silicon on it or - if you pick an exotic process like plasma-assisted chemical vapor deposition - you can get a thin layer of silicon carbide below the excess silicon. It's usually better to deposit your final material (you seem to like silicon carbide) directly on the substrate rather than hoping for less-controlled reactions between raw elements.
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The chemical vapor deposition industry has some precursor gases that will decompose and reliably produce silicon carbide on graphite. I used to do that when making carbon-carbon composites to track the speed of graphite deposition. To figure out graphite deposition speeds, we'd interrupt hydrocarbon flow through the reactor and flow...it was a pyrophoric methyl silane compound with some chlorine, but I don't remember the exact name, just the fireballs when it leaked into air...anyway, a silane compound that would break down and deposit silicon carbide. You'd have visually distinct SiC layers between graphite can could figure out the millimeters-per-hour of graphite growth. Or you could make graphite-reinforced silicon carbide and skip the graphite deposition, though that wasn't my employer's goal.
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Speaking of reinforced silicon carbide, you might want to ask if carbon and aerogels are the way you really want to go. There are some fantastic high temperature materials out there, like the tantalum and hafnium carbides, and fiber-reinforced silicon carbides have been in limited production for decades.
Biting the hand that feeds IT
