#51
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In article ,
Paul Hovnanian P.E. wrote: As far as introducing some sort of coolant into the skin, you'd have to determine the amount of heat generated. A good estimate is that all of the shuttle's kinetic energy is converted into heat at the skin interface... No, that's a lousy estimate. There is more than enough kinetic energy in a vehicle reentering from orbit to vaporize the entire vehicle, no matter *what* it's made of. The only reason reentry is practical at all is that with careful design (notably, a very blunt leading surface) only a *tiny* fraction of the heat actually reaches the skin. Then, given the specific heat of various coolants, calculate how many tons of coolant you'd have to haul up at launch and throughout the mission in order to cool the skin. The fast answer will be: too much. The shuttle makes a prolonged reentry with quite high total heat loads, which would require an awful lot of expendable coolant. Expendable coolants work much better with Apollo-style lifting-capsule reentries, which are short and sharp, with higher peak heating rates but much lower total heat loads. That's what an ablative heatshield is: solid expendable coolant. The shuttle is cooled by transferring the heat of friction to the surrounding atmosphere which carries it away. No, the air is in general hotter than the surface. Almost all of the heat is spread into the air and never reaches the surface. What does reach the surface is to some small extent soaked up (heat continues to soak through the tiles for quite a while after reentry -- the cargo-bay temperature actually peaks *after* landing), but mostly radiated. Conveniently, even fairly hot air is essentially transparent to radiated heat. But that does require a very hot surface -- radiated heat flux scales with the fourth power of temperature -- with very good insulation behind it. -- "Think outside the box -- the box isn't our friend." | Henry Spencer -- George Herbert | |
#52
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#53
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In article ,
wrote: It would not have made any great difference to Columbia. Titanium is not *that* much better; the conditions in Columbia's wing were far beyond the working limits of *any* reasonable structural metal. I was wondering about using titanium for just the leading structural member. Comparing Al to Ti the melting points are 669C vs 1660C, quite a bit different. I guess that says nothing of how much strength either loses as they approach the melting point. Exactly. The maximum *working* temperatures -- temperatures at which they still dependably retain most of their strength -- are much, much lower, and if I recall correctly, only a couple of hundred degrees apart. This is significant for aircraft but a very minor advantage for space reentry. If memory serves, the strength loss in aluminum is fast enough that there's little advantage in just making it thicker to reduce the working stresses. With titanium you can go a bit higher by doing that, but there is of course a mass penalty. How hot do they estimate it was inside the leading edge? Don't remember the accident-report numbers, but RCC panels are used only where temperatures exceed about 1250degC, and the stagnation points -- the worst case -- on nose and leading edge are at about 1650degC. -- "Think outside the box -- the box isn't our friend." | Henry Spencer -- George Herbert | |
#54
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Henry Spencer wrote:
Don't remember the accident-report numbers, but RCC panels are used only where temperatures exceed about 1250degC, and the stagnation points -- the worst case -- on nose and leading edge are at about 1650degC. The air entering the hole in the RCC was much hotter. The hottest air at the shock is normally kept some distance from the vehicle, but the hole let that hot air flow directly into the wing. I've seen a figure of 8000 F, enough to erode even the RCC to a thin edge at the boundaries of the enlarged hole. Paul |
#55
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I was wondering about using titanium for just the leading structural
member. Comparing Al to Ti the melting points are 669C vs 1660C, quite a bit different. I guess that says nothing of how much strength either loses as they approach the melting point. Exactly. The maximum *working* temperatures -- temperatures at which they still dependably retain most of their strength -- are much, much lower, and if I recall correctly, only a couple of hundred degrees apart. Additionally, this was a dynamic event. AIUI, Ti has lower thermal conductivity, so in such a case the Al would have the advantage of being able to "cool" the impacted parts of the structure to the rest of it, while the local temperature rise would be faster for Ti, further reducing its advantage. Jan |
#56
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Peter Fairbrother wrote:
Who needs Al or Ti? Carbon fibre is stronger than either weight for weight. Let's make the entire wing structure out of large closed cell CF/phenolic, or RCC foam - or at least fill the spaces with broken aerogel Your idea makes sense, but it would be rather expensive to retrofit the existing shuttle fleet with new wings. NASA improved insulation on the outside of the external cryogenic tank, and they made uncertified repair kit to fix the lost refractory tiles in orbit. I believe that it would be good idea to protect the fragile refractory tiles on the leading edges with disposable plastic foam. A backup water cooling system (like the fire sprinklers) would also help. |
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