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The SR-71 used it to cool its skin and keep it around 300°C during Mach-3 flight. It also used it to cool its engines.
by lultimouomo 8y ago
The SR-71 used it to cool its skin and keep it around 300°C during Mach-3 flight. It also used it to cool its engines.
- benj111 8y agoIsn't it also the SR-71 that has to expand to become fuel tight, and so leaks fuel all over the runway at takeoff.
- enriquto 8y agoI wonder how do this kind of crazy engineering ideas happen. What is gained by not making the thing fuel tight unconditionally? Leaking fuel at takeoff sounds extremely dangerous!
- benj111 8y agoExpansion due to heat, the GP mentions a temperature of 300c. Edit to add: Different materials expand by different amounts due to heat, and different parts of the aircraft are reaching different temperatures. If you know all the bits are going to expand x amount in normal flight, then you work back to what size they are when cold, if the shrinkage/expansion is too great you possibly could go with a different material (with various tradeoffs) or you just accept the shrinkage as they did here.
- sitharus 8y agoIt also helps that the SR-71 fuel was almost impossible to ignite. It couldn’t be spark ignited, instead they used triethylborane, an pyrophoric liquid that burns in contact with air. It’s also used to ignite rocket engines as its reactive enough to burn on contact with liquid oxygen.
- benj111 8y ago"almost impossible to ignite" .... "pyrophoric liquid that burns in contact with air" Those seem mutually exclusive? Do you mean that triethylborane ignites the fuel?
- benj111 8y agoI don't know why I'm getting down votes for this, its a reasonable question. To answer my own question triethylborane is indeed used to ignite the main fuel. https://www.quora.com/Why-was-Triethylborane-used-as-a-starter-fuel-for-the-SR-71-Blackbirds https://www.quora.com/Why-was-Triethylborane-used-as-a-start...
- refurb 8y agoYou’re getting downvoted because you’re calling out an inconsistency that doesn’t exist. The fuel that doesn’t burn is separate from the pyrophoroc stuff used to start it.
- wtallis 8y ago> Leaking fuel at takeoff sounds extremely dangerous! The fuel that was used had to withstand fairly extreme conditions before reaching the engine to be burned, which also made it pretty hard to ignite under ordinary conditions. https://en.wikipedia.org/wiki/JP-7 https://en.wikipedia.org/wiki/JP-7
- deleted 8y ago[deleted]
- frostburg 8y agoIt's lighter and you're inherently accommodating for thermal expansion that would happen anyway and possibly cause issues.
- hyperbovine 8y ago> I wonder how do this kind of crazy engineering ideas happen Three words: the Cold War. See also: Project Sunshine, the Oak Ridge Experiments, SDI, etc. Once you start writing blank checks to the military-industrial complex the crazy ideas start pouring forth.
- refurb 8y agoIf you know everything is going to expand significantly due to heating, I’m sure making it fuel tight at room temperature would involve significant engineering trade offs.
- avmich 8y agoIt's interesting that at the dawn of rocket engines the regenerative cooling was thought to be a bad idea. The reasoning was the hot side will expand too much and disconnect from cool side because of stresses. It's good von Braun actually used one...
- throwaway77384 8y agoIt is!
- twic 8y agoI suppose on a supersonic aircraft, you can't reject heat into the surrounding air, because that's already been heated up too far by your passage through it. Is this technique still in use in supersonic aircraft?