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Comment by cyberferret | Hacker News Reader

It's a common bug bear of all aircraft with swept wings designed for supersonic flight. Terribly inneficient at take off/landing speeds.

Which is why most military aircraft have to hook up to a tanker after take off to fill the tanks so they can continue on their mission. Concorde (and other passenger aircraft) don't have this luxury due to the risks involved, so I am thinking that most of the high cost of fuel vs passenger load that is being discussed in this thread is because of the voracious thirst that the plane has in the early stages.

Most supersonic aircraft are quite efficient in the cruise stage. The Lockheed SR-71 for instance, derives a fair percentage of thrust just from the shape of the airframe, so the pilots actually throttle back a little once they reach Mach 2+!

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It is interesting to hear the SR-71 was so efficient once it got going. Certainly when it was stationary it was possibly the least fuel efficient aircraft ever - its body was designed to leak fuel when on the tarmac. The hot airframe would, once in flight, expand to seal the gaps!

The SR-71 was an interesting beast. I've read that due to the constant leaking while on the tarmac they would have to immediately rendezvous with a tanker to refuel after takeoff.

I highly recommend you guys try and grab a copy of "Skunk Works" by Ben Rich which is a fantastic story about Kelly Johnson and the Lockheed facility that built the SR-71, U2 etc.

Ben worked alongside Kelly, and said that the guy had an unerring knack for designing aircraft. Ben (an aerodynamics engineer) would spend days designing an intake, and Kelly would come along and take one look at it and say "That is 20% too big", and sure enough, when Ben went back over his calculations, he would find it to be about 18% oversized.

Another great book is "Sled Driver" by Brian Shul - probably one of the best aviation books I've read (and I am a former pilot).

Sadly, I believe both books are out of print now, but you may be able to find a copy somewhere second hand. I've seen "Sled Driver" on eBay for about $3000 from time to time!!

I have a copy of "Skunk Works" and it is indeed a great book. Amazon still has it for under $12[1]. And seeing as it's quite popular, most large libraries should have a copy as well.

It's great that you brought up "Sled Driver", I'm actually currently saving up to buy a copy. Brian Shul still has new copies available on his website for $250[2]. I think the copies that go for very high prices on eBay are the first editions or some of the special commemorative versions.

May I ask your opinion on the print quality of "Sled Driver"? I know Shul is a photographer, in addition to being a former SR-71 pilot, so I assume the photographic print quality is quite high. Have your read his companion book "The Untouchables"?

[1]https://www.amazon.com/Skunk-Works-Personal-Memoir-Lockheed/...

[2]http://galleryoneimages.com/Hangar/index.php?cPath=22

Yeah, I think mine is a first edition - a friend gave it to me about 25+ years ago. It is a treasured book in my house.

The photos in there are nice, but I would say it is on par with most coffee table style books that I've seen. Wonderful photos, but the print quality is about the same as most books of that era. To be honest, I got so caught up in the stories he tells in there that I can't recall a lot about the photography - I will have to revisit it and check it out.

Great that he has started republishing the books again. I remember about 10 years ago when it was out of print, copies were going for about $6000+ on eBay !! o_O I thought I was sitting on a goldmine. :D

I haven't read "The Untouchables", but will put a copy on my wishlist for this year. Can't get enough of books about great planes and great pilots.

Thanks for the response. I didn't realize Sled Driver was first published that long ago. I'm certainly glad he is offering newer editions, even if they are still quite pricey.

Also recommended: Omega Tau's hour-long, detailed interview with an SR-71 pilot:

http://omegataupodcast.net/91-flying-the-sr-71/

> I've read that due to the constant leaking while on the tarmac they would have to immediately rendezvous with a tanker to refuel after takeoff.

To reduce structural loads the aircraft took off with a very light fuel load and refuelled in flight.

Rather interestingly, the modern fascination with SR-71 stories and facts has turned a rather mundane "weeping" of fuel into the much more exciting image of fuel almost gushing out of cracks in the fuselage at such a rate it had to be refuelled immediately after takeoff!

It was a long time before aircraft designers figured out how to stop the hydraulics from leaking. My dad (a P51 driver) said you could always tell where a P51 had parked due to the slick it left behind.

The trouble was they just didn't have good materials for the seals. Things have gotten a lot better in that department.

The "constant leaking" on the ground gets overblown a bit. The primary reason for the immediate refueling is that they would take off with a minimal fuel load, because the plane performed very poorly one engine. Fully loaded with fuel they would have basically no chance of recovering from an engine failure on takeoff.

I think military aircraft mostly like to tank after takeoff so that they can carry more ordnance. The maximum weight they can have in flight tends to be higher than the maximum weight they can take off with (especially when operating from short runways), so if you take off with lots of weapons and partial fuel, then refuel in flight, you can carry more than if you took off with full fuel.

I love thinking (noun) like this. It's perhaps clichéd to call it out-of-the-box, particularly as it's likely common knowledge within the community, but breaking down the situation, looking at the characteristics of the plane on the ground versus in the air, and how the characteristics and options are different in each encourage me to similarly challenge assumptions when I approach other challenges.

Military planes are always overloaded. WW2 aircraft needed every inch of runway, and any hiccup in any engine in the first few minutes of flight was certain doom for the aircrews (given that it was barely flying, and overloaded with bombs and fuel).

> The Lockheed SR-71 for instance, derives a fair percentage of thrust just from the shape of the airframe, so the pilots actually throttle back a little once they reach Mach 2+!

That sounds...unlikely. Do you have a source for this?

I have a suspicion you might be conflating the "turboramjet" aspect of the engine design, with the air-frame actually generating thrust.

I believe he's referring to the the high pressure acting against the aft-facing region of the inlet compression centerbody. Kelly Johnson famously claimed that at cruise, the inlet contributed more to thrust than the engines did. That was a bit of a deliberately provocative statement, made with an element of jest, and based on the accounting conventions one uses for for thrust. The pressure distribution necessary to achieve these results would not be possible without the engine serving as a pump for the inlet-captured mass flow, and providing the high-pressure boundary condition at the outlet of the inlet.

Ah, so I guess it comes down to whether you consider the inlet and exhaust nozzles to be a part of the engine, or just a part of the air-frame.

Personally I'm of the mindset the inlet and exhaust nozzles are an integral part of the installed engine, particularly in this case given the rather unique intake/engine/exhaust design.

Yes, agreed. If we were discussing hypersonic airbreathing configurations, for example, no one would be making those kinds of statements. Once you get into the realm of high speeds, the integration between the propulsion components becomes necessarily deeper and tighter, and trying to parcel out that kind of credit is really more of a basis for engineers to rib each other over during happy hour, rather than a rigorously defensible position.

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