4 ms·

To me, the real recognition of their proficiency was learning that SR-71s leak fuel like crazy when they are on the ground... because their body expands from he

by dekhn 3y ago

To me, the real recognition of their proficiency was learning that SR-71s leak fuel like crazy when they are on the ground... because their body expands from heat during flight. At first I thought that was incredibly wasteful, then I realized it was actually brilliant, given the circumstances in which the plane was used.

bostonsre 3y ago
Yea, that and the external engine used to start the engines were pretty great ideas to simplify the project and still meet their requirements.
- bombcar 3y ago
  If it’s crazy and it works, it’s not crazy.
- adolph 3y ago
  Each of the two Buick V-8s had eight straight exhaust pipes that exited out of the bottom of the AG330. Each engine had a Hydromatic transmission with electric shift, and the two transmissions were coupled together by a 12 inch wide, toothed Gilmer belt. The pilot told the Crew Chief via intercom "Engage Buicks". The Buick operator pressed "Jet Start", and movement of the Buick throttle (one throttle lever controls both engines) downward engaged the transmissions. . . . The pilot would watch his onboard gauges to confirm minimum oil pressure, fuel pressure, and rising RPM, and then set the jet throttle to idle. At that point 30cc of TEB was injected into the burner cans, a characteristic green flame was emitted, the J58 lit off, accelerated and started to run on its own. The accelerating jet started to unload the Buicks, and when 3,200 rpm was reached, the pilot called "Buicks out", the Crew Chief signaled "cut", the Buick operator hit "Cart Shutdown", the probe fell free passively, and the cart throttle automatically returned to idle. https://www.thesr71blackbird.com/Aircraft/Engines/starting-the-sr-71-blackbirds-j58-engines-ag330-start-cart https://www.thesr71blackbird.com/Aircraft/Engines/starting-t...
- lloeki 3y ago
  Well, every car IC engine is started by another "engine", albeit an electric one, and these days embedded. But then again these IC engines used to be started by an external crank. So the gist of the idea is not that far fetched. (Now I'm wondering how the huge Diesel engines on massive boats are being started, I wouldn't be surprised if they were started by a smaller IC engine)
  - flopsamjetsam 3y ago
    And still used by some racing cars e.g. F1 has an external (electric) starter, that the mechanic inserts into a hole in the back of the car that goes into the back of the diff -> gearbox -> ICE.
  - bradfox2 3y ago
    Compressed air usually.
injb 3y ago
Why is leaking fuel brilliant?
- 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.

by dekhn 3y ago

bostonsre 3y ago

Yea, that and the external engine used to start the engines were pretty great ideas to simplify the project and still meet their requirements.

bombcar 3y ago
If it’s crazy and it works, it’s not crazy.
adolph 3y ago
Each of the two Buick V-8s had eight straight exhaust pipes that exited out of the bottom of the AG330. Each engine had a Hydromatic transmission with electric shift, and the two transmissions were coupled together by a 12 inch wide, toothed Gilmer belt. The pilot told the Crew Chief via intercom "Engage Buicks". The Buick operator pressed "Jet Start", and movement of the Buick throttle (one throttle lever controls both engines) downward engaged the transmissions. . . . The pilot would watch his onboard gauges to confirm minimum oil pressure, fuel pressure, and rising RPM, and then set the jet throttle to idle. At that point 30cc of TEB was injected into the burner cans, a characteristic green flame was emitted, the J58 lit off, accelerated and started to run on its own. The accelerating jet started to unload the Buicks, and when 3,200 rpm was reached, the pilot called "Buicks out", the Crew Chief signaled "cut", the Buick operator hit "Cart Shutdown", the probe fell free passively, and the cart throttle automatically returned to idle. https://www.thesr71blackbird.com/Aircraft/Engines/starting-the-sr-71-blackbirds-j58-engines-ag330-start-cart https://www.thesr71blackbird.com/Aircraft/Engines/starting-t...
lloeki 3y ago
Well, every car IC engine is started by another "engine", albeit an electric one, and these days embedded. But then again these IC engines used to be started by an external crank. So the gist of the idea is not that far fetched. (Now I'm wondering how the huge Diesel engines on massive boats are being started, I wouldn't be surprised if they were started by a smaller IC engine)
- flopsamjetsam 3y ago
  And still used by some racing cars e.g. F1 has an external (electric) starter, that the mechanic inserts into a hole in the back of the car that goes into the back of the diff -> gearbox -> ICE.
- bradfox2 3y ago
  Compressed air usually.

injb 3y ago

Why is leaking fuel brilliant?

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.