When he worked at NASA in the 1990s, Tim Conners was the lead propulsion engineer for the agency’s Blackbird program. He discusses getting the SR-71 flying again:
JP-7. People ask about the fuel. I think it’s pretty well known that NASA had a huge amount of JP-7 stockpiled that they received from the Air Force back in the early ’90s. So much that it was in a dedicated tank, like one of the giant jet fuel tanks at Edwards Air Force Base. You know, the tanks up on the ridge.
From what I understand, that supply unfortunately was discarded about 20 years ago, and JP-7 is a unique fuel. Obviously, low volatility. When I poked at what might be the way forward there, it didn’t sound like that was a showstopper. I got the impression that there’s been dialogue underway with refineries for a replacement or a surrogate that would work. So I went from being deflated, hearing that the fuel was gone, to being encouraged that potentially there was a surrogate workaround.
[…]
So that was one of the questions I asked is whether or not that formulation exists still. The answer I got back was that essentially a refinery should be able to recreate a suitable surrogate. If you think about it, JP-7 was unique in that it had a very low volatility characteristic.
What I’ve learned about Jet A from some of the ramjet work that I’ve been doing is that it has a ridiculously low volatility too, of course, by design. So I don’t know that JP-7 is that far apart from the chemical characteristics of a Jet A-class fuel, JP-8-type fuel. JP-8 was a significant change from JP-4, so the Air Force was using JP-4 in their fleet, and then moved to JP-8 to be consistent in the ’90s with the Navy with their formulations. That’s a lower volatility fuel, and at NASA we had to recharacterize the flight performance of all of our jets with JP-8, but it worked fine.
We were concerned about operability – that we were going to have flameouts and engine relight and flight relay problems. Those did not materialize, so I don’t want to oversimplify it, but I am thinking that you might be able to take an existing Jet A and with the right additives knock down the volatility to a level that’s safe for flying in the Blackbird.
[…]
There were dedicated JP-7 tankers for the Blackbird. I don’t know if they can take an existing system and flush it. It would depend on the formulation, right? JP-7 was notorious for its unseemly characteristics. It was toxic.
I had a shirt that got dripped on when I was standing under the wing of the Blackbird once, and I had to throw it out. I could not get the stink out of that shirt from the JP-7. It was a weird fuel. So if you can get one that’s more aligned with the fuels used in the service, then I don’t believe that’s going to be a showstopper as far as tanker support, but you do know tanker support is going to be required for that airplane.
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I just remember it turning my stomach. Like I kept smelling something as the day went on, and I reached over and saw a stain on my shoulder. I don’t recall what happened the rest of the day, but I just remember being repulsed by the odor and then not being able to get it out when I washed the shirt.
JP-7 was developed at Pratt and Whitney by master chemist Clarence Brown (CB) Eichman in 1955 for the Central Intelligence Agency (CIA) for use in the Blackbird;s predecessor, the Lockheed A-12:
JP-7 is a compound mixture composed primarily of hydrocarbons; including alkanes, cycloalkanes, alkylbenzenes, indanes/tetralins, and naphthalenes; with addition of fluorocarbons to increase its lubricant properties, an oxidizing agent to make it burn more efficiently, and a caesium-containing compound known as A-50, which is to aid in disguising the radar and infrared signatures of the exhaust plume. A-50 has been hypothesised to be synthesised by addition of caesium carbonate to dialkyl phosphite. The SR-71 Blackbirds used approximately 36,000–44,000 pounds (16,000–20,000 kg) of fuel per hour of flight.
JP-7 is unusual in that it is not a conventional distillate fuel, but is created from special blending stocks in order to have very low (<3%) concentration of highly volatile components like benzene or toluene, and almost no sulfur, oxygen, and nitrogen impurities. It has a low vapor pressure, and high thermal oxidation stability. The fuel must operate across a wide range of temperatures: from near freezing at high altitude, to the high temperatures of the airframe and engine parts that are being cooled by it at high speed. Its volatility must be low enough to make it flash-resistant at these high temperatures.
The very low volatility, and relative unwillingness of JP-7 to be ignited, required triethylborane (TEB) to be injected into the engine in order to initiate combustion, and allow afterburner operation in flight. The SR-71 had a limited capacity for TEB, and therefore had a limited number of available 'shots' of TEB (usually 16) for restarts, and those had to be managed carefully on long-duration flights with multiple stages of relatively low-altitude air refueling and normal high-altitude cruise flight.

