Comment by tetris11 | Hacker News Reader

Comment by tetris11 | Hacker News Reader

https://theaviationgeekclub.com/u-2-vs-sr-71-former-usaf-avi...

Why the SR-71 was retired over the U-2 which is still in service today:

> For a mission over a target in the Middle East with the planes based in England, [the SR-71] needed to have multiple airliner size aircraft in the air to support it. > It was not capable of taking off with a full load of fuel (or rather, not advisable for many reasons), so in order to take off, there had to be a tanker waiting in the air to top it off once it got airborne.

> It would then make a high speed run to the target zone where it would meet up with another tanker before doing its Mach 3 run over the target. It would then need to tank up again once the recon run was done for the trip home, and then there would still be a tanker in the air on standby in case it needed gas before landing or in case it needed to divert for weather or something.

> So that’s a minimum of 4 airliner sized support aircraft, including all of their parts, fuel use, and crew, flying out of a minimum of 2 different bases just to make a single SR-71 operational mission happen.

> And that’s on top of all the crew, fuel, parts and mechanics just to operate the SR-71.

This sounds like dev hell, and yet a viable dependable product. I wonder how many big corps operate the same behind the scenes.

There's a term "hangar queen" for aircraft that fly a little and need a lot of maintenance.

Sometimes you want extreme performance at any cost. But a lot of the time you want a good enough performance at low cost.

The amount of supporting resources and length of maintenance breaks etc affect cost, or the amount of aircraft you need to get in order to be able to do the mission.

If you have 20 aircraft but 15 are being maintained, you only have 5 flyable ones, you still had to pay for acquisition of all the 20, and have to pay for the ongoing maintenance for the 15.

If you have 10 but only 5 are maintained at any given time, you have the same flying capacity of five aircraft but your total costs are likely going to be much less as you only had to acquire ten aircraft and only have to have maintenance people enough for 5 at a time.

This is really hard to explain to a lot of people in the software industry by the way. They say a feature is complete, my job is done. But if it's extremely hard to configure, deploy, monitor, maintain etc then it might turn out to be a net negative.

It's the whole iceberg that counts, not the tip.

As the saying goes NKAWTG - Nobody Kicks Ass Without Tanker Gas

> So that’s a minimum of 4 airliner sized support aircraft, including all of their parts, fuel use, and crew, flying out of a minimum of 2 different bases just to make a single SR-71 operational mission happen.

Why can't it be 2 support aircraft? Is the SR-71 mission too long for the tanker to stay in the air? The tankers can also service other aircraft so they aren't necessarily put in the air exclusively for the SR-71.

It's still an expensive plane though.

The SR-71 won't be anywhere near the first tanker when it needs fuel again. The fuel itself isn't common to most aircraft, which means the tanker has to carry fuel specifically for it so it.

Correct. The SR-71 uses JP-7 fuel, and is the only aircraft in the fleet known to do so. So, the USAF has to have custom KC-135Q tankers just for SR-71s. They can't be used for any other fuel.

The thing that made it obvious to me that there was a replacement for the SR-71 was that when they retired the aircraft, they didn't retire any of the tankers. That meant there was now some other unknown aircraft flying that also used JP-7, and it must therefore be the replacement for the SR-71.

Note that JP-7 is so hard to light on fire that if you throw a match into a bucket filled with JP-7, the match will get snuffed out. You've got to be a lot hotter than a match to light that stuff.

Your anecdote about JP7 being hard to light in fire is also true for pump gas. The liquid gas is not flammable, only the aerosolized gas is.

But if you did the same experiment with 87-93 octane gasoline, you would set the fumes on fire as the match went into the bucket. And those fumes would continue to burn.

The same is not true of JP-7.

The SR-71 used a different high flashpoint fuel (JP-7) to most aircraft, which require separate tankers (KC-135Q) with segregated storage.

No. The Q's ran on JP-7. They had the turbojet engines, while most other tankers had turbofans.

The Q like most early KC-135 used used Turbojets(J57) later upgraded to Turbofans (CFM56) as the T

J57: https://en.wikipedia.org/wiki/Pratt_%26_Whitney_J57

CFM56: https://en.wikipedia.org/wiki/CFM_International_CFM56

As far as I know only the A-12 family and the X-51 used JP-7 given it needed Triethylborane (TEB) to ignite.

https://en.wikipedia.org/wiki/JP-7

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

More on the KC-135Q/T

https://www.blackbirds.net/tankers/kc135q.html

http://www.habu.org/kc-135q/kc135q.html

When they were still flying, I worked on them at Beale AFB. You always knew when an SR was going to take off. About an hour prior, 2 KC-135Q's would take off like a bat out of hell. They would always use water injection and you could see their black smoke trails.

An hour later, you would hear a giant rumbling coming from the flight line. It would get louder and louder until it was all you could hear. As the SR shot off into the distance, you could slowly make out the din of all the car alarms on base.

They would sometimes need 4 tankers and they would shoot off in pairs. I was told they always had two tankers per fill up. They would grab some from one and then some more from the other.

Back in the 90's it was estimated that each SR flight had a cost of at least a million dollars with fuel costs, wages, etc.

Most of them. It is Excel workbooks all the way down.

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