JP-7 | Hacker News Reader

JP-7

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59 pointswin6634 comments

It's crazy to me that once they had this high-performance jet fuel, they also used it as a coolant and as hydraulic fluid.

I guess the main requirement for the latter two purposes is that it doesn't break down at really high temperatures. The jet fuel needs to be stable at high temperatures too (additionally, you need to be able to burn it to make the engine go) so once you put all this research into finding the right jet fuel you have a liquid that will do OK on the other jobs.

Edit: and also it was used for lubrication! Good fuels definitely do not necessarily make good lubricants...it must just be really hard to find anything with the right thermal stability.

The F-1 rocket engine on the first stage of the Saturn V used a similar arrangement, with the fuel also doing double duty as hydraulic fluid, coolant and lubricant.

That's pretty standard on most liquid propellant rocket engines.

> they also used it as a coolant and as hydraulic fluid.

Potentially a much smaller headache than pumping one or two more fluids around inside a jet engine and keeping them from breaking down. The jet fuel only needs to not break down for a single pass through the engine. A closed lube or cooling system would need to not break down for multiple passes (i.e. much longer duration at high temperature).

>it must just be really hard to find anything with the right thermal stability.

A total loss lubrication system doesn't need to be actively cooled (assuming your lubricant input is of suitable temperature) which saves a ton of weight and complexity which is why rocket and jet engines often use the fuel as a lubricant.

FWIW JP-7 was not the only hydraulic fluid, it was only used as hydraulic fluid in a few places where that was convenient, or possibly where they'd have needed a different hydraulic fluid due to the temperature.

I wonder how do you actually make it catch fire when you need it to combust

Compress the fuel in a small area and then apply some high voltage. It’s probably not much different than a spark plug in a car.

They actually injected triethylborane to act as a match and initiate combustion (TEB auto-ignites at freezing temperatures and burns ridiculously hot).

The plane had a limited number of TEB "shots" and those were needed after each afterburner use, or when restarting the engine in mid-air (as crew needed to do under some conditions), so they had to be carefully managed.

Many tactical aircraft to this day use their fuel for the same purposes. I know the F-35 uses its fuel as a heat sink. The F-35B variant specifically uses fuel as the hydraulic working fluid (aka "fueldraulics")for much of the work of the 3 bearing swivel nozzle.

Most two-stroke internal combustion engines are also lubricated by the fuel.

Well, sort of. Every 2 stroke engine I've seen needed oil in the fuel, so if you just ran straight petrol you'd have a problem.

According to the article, the lubricants in JP-7 are also additives.

I was an inflight-refueling technician in the USAF and there was a variant of the tanker (KC-135T) that had isolated tanks to specifically carry this stuff, for refueling an SR-71. Sadly that was before my time. A neat little factoid tho, I guess.

Wikipedia indicates it was originally KC-135Q with the unique fuel system (which separates the exotic SR-71 fuel payload from the typical fuel to fly the refueling plane). KC-135T was the re-engined variant using modernized engines like the rest of the fleet.

"Manufacturing several hundred thousand gallons of the new fuel required the petroleum byproducts Shell normally used to make its Flit insecticide, causing a nationwide shortage of that product that year."

Can anyone verify this claim? It sounds too good to be true.

According to Wikipedia, FLIT was a product and large consumer-facing brand of Standard Oil, not Shell.

"According to one source, some raw material (possibly the solvent) used for the production of FLIT was similar to that used for LF-1A fuel for the Lockheed U-2 high altitude reconnaissance aircraft, causing a nationwide shortage of bug spray in 1955. Fuel LF-1A was produced by Shell.[6]"

https://en.wikipedia.org/wiki/FLIT

The citation is a book source and it's a book I don't own I can't vouch for the legitimacy.

It's discussed in Ben Rich's excellent memoir, Skunk Works.

https://www.google.com/books/edition/Skunk_Works/oL4bHPIt7XI...

I can’t verify that exact claim, but as a chemist, it wouldn’t be surprising. Pesticides are mostly organic (carbon containing) compounds and oil is often used as feedstock.

So I could see some key starting material for the pesticide being diverted to make a fuel component.

> However, very high skin temperatures are generated at this speed due to friction with the air.

I remember reading somewhere that this is a common misconception and that most of the heat is generated not by friction but by air compression.

But that may have been about the Space Shuttle re-entry, which has considerably higher speeds.

They used two massive Buick V8s to get the turbines started. They eventually switched to Chevy V8s. Another person commented about fuel leaking, it must have been a sight to watch one start up.

https://youtu.be/JjdyQpEUYzI

About 2:42 in this video you can hear the side by side V8's revving up and then the J58's whirl.

https://www.youtube.com/watch?v=nFO--wzFfUs

I've read many years ago that in low temperatures, the SR-71 was prone to leaking fuel while in its hangar. But that was not dangerous because JP-7 won't catch flame without TEB.

EDIT: oh yes https://theaviationgeekclub.com/the-story-of-the-sr-71-black...

An SR-71 video that gives more context https://youtu.be/dG6gnJc6XeU

The SR-71 fuel tanks didn't fully seal until the plane was at cruising speed. The heat on the airframe tightened up the tanks.

https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird

If ever in Washington D.C. I highly recommend the Udvar-Hazy Center at Dulles airport. There is an SR-71 on display there (along with just about every other important plane in US aviation history) that you can walk right up to. It's surprisingly small IMO.

Not if you are a child and never meet your heroes as a grown-up!

Funny, I saw the SR71 at Udvar-Hazy, and thought it was HUGE!

Haven't seen the one in DC, but I've been within arms reach of one of the old NASA SR-71's that's on display (not accessible to the public unfortunately) several times. "Small" is just about the last word I would have ever used to describe it.

Oblig. link to the story about the fastest guys in the desert.

https://oppositelock.kinja.com/favorite-sr-71-story-10791270...

Recording of him reciting it in person:

https://www.youtube.com/watch?v=8AyHH9G9et0

Yeah but how does it compare to Skinny Fuel?

So this state of the art technology is no longer used for newer jets?

State of the art for the late 50s. Most jets then and now don't operate in regimes where a fuel like this would be necessary.

The fuel was formulated to meet the engineering requirements of a very particular product that was rendered outmoded by developments in satellite and UAV technology.

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