3 ms·

Why is leaking fuel brilliant?

by injb 3y ago

tverbeure 3y ago
Probably because it’s a thinking out of the box solution: fixing the leaks seems the logical and conventional thing to do, but chances are that it would have required trade-offs that made conditions at high speed worse (e.g. more stress on the body.) I’m totally speculating here.
jazzyjackson 3y ago
"...given the circumstances in which the plane was used" I wasn't there but I have the impression that kerosene was not the limiting factor back then. So they solved an engineering problem (body expanding in multiple directions) by declaring, not a problem!
FirmwareBurner 3y ago
Because there are no rubber gaskets that can operate at those temperatures, they'd all just melt, so there's literally no way to proper seal them.
adgjlsfhk1 3y ago
cause leaking fuel costs a few hundred dollars per flight and they don't get a lot of flights. solving the problem "correctly" would probably have taken at least a year or two and probably a couple hundred thousand dollars per plane.
genezeta 3y ago
Usually a fuel leak is a problem. It's something that occurs. You have your design, you build it and somewhere in the whole process a problem occurs and there's a fuel leak, usually at high/operating temperature. Maybe the design didn't take into account some temperature expansion, or the materials used were not appropriate, whatever. But then again, maybe while designing you observe that there's going to be a problem. You do realize that, say, the very different temperatures will inevitably produce too big an expansion for the connecting pipes to be leak-proof while flying, or maybe the expansion will be so big that the material will break when the engine reaches some temperature. So you try to design a solution to that problem. Maybe it would require a new material, which means almost inevitably years of research and a lot of money. Maybe you could design some very specialised connection that can operate at different temperatures and modify its geometry so that it doesn't leak at any temperature; which again may take a lot of time and money in research. So, here comes the brilliance of the solution: You turn the design around. You define the connections' "normal state" to be the one at high temperature. You design for that, make sure that while in operation, there's no leak, the connections don't overpressure, the fit is right. But obviously then what will happen is at low temperature the contraction of the material will produce leaks. But you think about it and realise that this only happens at landing/take-off, for the "brief" time that the plane is moving on the ground, which is both a much safer situation and a shorter amount of time. In those circumstances you can afford to have some leaks without it being so much of a problem. So, it's not so much that the solution is "leaking fuel is brilliant", but that the design acknowledges the inevitability of a leak, and then moves the leak to the moment where its presence is less problematic. So you sacrifice some non-dangerous fuel loss on each mission, while allowing for a better, more secure solution on the moments the plane operates nominally. And all without needing those years/money spent on complex research.
- bradknowles 3y ago
  Secondary factors here are that: 1. There are no separate fuel tanks on the A-12/SR-71. The body of the aircraft is also the fuel tank. If there were internal fuel tanks separate from the body of the aircraft, then there wouldn't be any leaks. But then that would add weight and bulk that they couldn't afford. 2. The fuel they're burning is JP-7, which is so hard to ignite that it will put out a match, or if dumped on a fire, it will put out the fire. You have to get it to much higher temperatures for it to ignite. So, leaks that do happen at low temperatures (on the ground, early in the flight envelope, etc...) just aren't that big of a deal. There's not many things that would be able it ignite the fuel when it leaks at those times.

Why is leaking fuel brilliant?

by injb 3y ago

Why is leaking fuel brilliant?

Probably because it’s a thinking out of the box solution: fixing the leaks seems the logical and conventional thing to do, but chances are that it would have required trade-offs that made conditions at high speed worse (e.g. more stress on the body.) I’m totally speculating here.

jazzyjackson 3y ago

"...given the circumstances in which the plane was used" I wasn't there but I have the impression that kerosene was not the limiting factor back then. So they solved an engineering problem (body expanding in multiple directions) by declaring, not a problem!

FirmwareBurner 3y ago

Because there are no rubber gaskets that can operate at those temperatures, they'd all just melt, so there's literally no way to proper seal them.

adgjlsfhk1 3y ago

cause leaking fuel costs a few hundred dollars per flight and they don't get a lot of flights. solving the problem "correctly" would probably have taken at least a year or two and probably a couple hundred thousand dollars per plane.

genezeta 3y ago

Usually a fuel leak is a problem. It's something that occurs. You have your design, you build it and somewhere in the whole process a problem occurs and there's a fuel leak, usually at high/operating temperature. Maybe the design didn't take into account some temperature expansion, or the materials used were not appropriate, whatever. But then again, maybe while designing you observe that there's going to be a problem. You do realize that, say, the very different temperatures will inevitably produce too big an expansion for the connecting pipes to be leak-proof while flying, or maybe the expansion will be so big that the material will break when the engine reaches some temperature. So you try to design a solution to that problem. Maybe it would require a new material, which means almost inevitably years of research and a lot of money. Maybe you could design some very specialised connection that can operate at different temperatures and modify its geometry so that it doesn't leak at any temperature; which again may take a lot of time and money in research. So, here comes the brilliance of the solution: You turn the design around. You define the connections' "normal state" to be the one at high temperature. You design for that, make sure that while in operation, there's no leak, the connections don't overpressure, the fit is right. But obviously then what will happen is at low temperature the contraction of the material will produce leaks. But you think about it and realise that this only happens at landing/take-off, for the "brief" time that the plane is moving on the ground, which is both a much safer situation and a shorter amount of time. In those circumstances you can afford to have some leaks without it being so much of a problem. So, it's not so much that the solution is "leaking fuel is brilliant", but that the design acknowledges the inevitability of a leak, and then moves the leak to the moment where its presence is less problematic. So you sacrifice some non-dangerous fuel loss on each mission, while allowing for a better, more secure solution on the moments the plane operates nominally. And all without needing those years/money spent on complex research.

bradknowles 3y ago
Secondary factors here are that: 1. There are no separate fuel tanks on the A-12/SR-71. The body of the aircraft is also the fuel tank. If there were internal fuel tanks separate from the body of the aircraft, then there wouldn't be any leaks. But then that would add weight and bulk that they couldn't afford. 2. The fuel they're burning is JP-7, which is so hard to ignite that it will put out a match, or if dumped on a fire, it will put out the fire. You have to get it to much higher temperatures for it to ignite. So, leaks that do happen at low temperatures (on the ground, early in the flight envelope, etc...) just aren't that big of a deal. There's not many things that would be able it ignite the fuel when it leaks at those times.