More accurately, they never found a glue that was both impermeable to fuel that would also stick to titanium. It wasn't the intent for the OXCART/Sr-71 to leak all over the runway; it was an unfortunate consequence of materials science not catching up with the design parameters of the CIA.
Same reason it took two ganged Buick 455s at redline to start the engines, even though it used triethylborane as lighter fluid (something so perniciously flammable that exposure to air causes it to explode). The sorts of lubricants that are happy in a Mach 3 turbojet have a viscosity somewhere south of tree sap at room temperature. Same reason its tires are more aluminum than rubber and filled with 200PSI nitrogen - the heat they experienced at cruise would cause normal tires to explode. This led to runways that not only had to be longer than LAX, they had to be flat to within an eighth of an inch and swept mercilessly clean before and after every launch. Debris taller than a quarter would blow tires. Etc. Etc. Etc.
I know more about the OXCART project than most people I know, which is saying something. I could go on for hours. My cousin owns the crazy-expensive $2500 version of Brian Shul's book and it's something to behold; I borrowed it for six months for research. So rather than just dork out incessantly for half an hour, lemme share one of my proudest possessions:
This is one of the most esoteric books I own and I own a few esoteric books. Long story short, the CIA codename for the development of the U-2 was RAINBOW. The CIA codename for the development of the A-12 (single-seat, classified-until-the-90's version of the SR-71 that flew before anyone ever heard of the SR-71) was GUSTO. The Lockheed Skunkworks codename for the U-2 was ARCHANGEL. The U-2 was technically ARCHANGEL-1. The A-12 was ARCHANGEL-12. In between there were 11 other iterations. And they are freaky.
Yeah. Lockheed came up with that in 1957. It was concurrent with this:
Anyway. I digress. Crazy book about crazy planes that crazy-smart people designed AND DISCARDED along the way towards coming up with what became the SR-71. It's not a cheap book. But I bought it used, off Amazon, "some marks", for $9 out of a cute little shop in Burbank.
Yeah. Kelly Johnson's last wife (he outlived 2 others) was named Nancy, and she passed away recently.
I have the book that Paul Suhler sent to Kelly Johnson's widow, signed.
Anyway. Thanks for letting me share. The opportunity kind of makes up for the fact that craft service put a bowl full of barbecue sauce (NOT rum sauce, as you might expect) next to the bread pudding last night.