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The Lockheed U-2 was first introduced in the mid-1950s, close to the dawn of the jet age. It seems reasonable to think that the technology required to design and build it then is rudimentary by modern standards, and, as a result, has become far more accessible. And, yet, the U-2 stands in a class on its own, with few, if any, equivalents in performance.

Let's say that I wanted to make my own U-2. Are there design challenges that would still be hard to handle even 70 years on? Or is it more that the requirements of human flight to 80K' are so expensive to meet, that, when compared to the price of a satellite, it's not worth the time?

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    $\begingroup$ I think physics and human body haven't changed in the past few decades 😅 So the same limitations still apply 😉 $\endgroup$
    – sophit
    Feb 11 at 15:35
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    $\begingroup$ While operating slighter lower we have the privately built globalflyer en.wikipedia.org/wiki/Virgin_Atlantic_GlobalFlyer and there are various new build drones that routinely operate in the U2 environment en.wikipedia.org/wiki/Northrop_Grumman_RQ-4_Global_Hawk. $\endgroup$
    – GremlinWranger
    Feb 11 at 23:19
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    $\begingroup$ @sophit physics might not have changed, but materials science and manufacturing abilities definitely have improved, so we might see a different outcome because we can do more with less today. So no, the same limitations dont apply 🙂 $\endgroup$
    – Moo
    Feb 12 at 1:49
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    $\begingroup$ @user3528438 I can't agree with that. The SR-71 and MIG-25 are planes which were designed to push the limits on how fast a plane could fly. The U2's job was just to go high and loiter. It's much harder to go Mach 3 than it is to hold the plane steady at 95kts. $\endgroup$
    – Kenn Sebesta
    Feb 12 at 16:11
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    $\begingroup$ @Moo : yet even with better materials and science, it would still be extremely expensive and difficult. Not as difficult as back then, but still difficult, so if the same missions can be performed by other means (and the existing U2 can perform them still), it's not worth the investment. $\endgroup$
    – vsz
    Feb 13 at 12:45

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The really big challenge on the U2, which is actually based on a F-104 Starfighter fuselage as a foundation, was creating an air breathing engine that would run at 70000 ft. P&W produced a modified J-57 engine (J-75 on later airplanes) with near zero compressor blade tip clearance to get it to run at that altitude (that is, the blade tips would initially brush the outer wall of the compressor case and there would be a kind of "break-in" to wear off just enough to make a tiny clearance, way less than the regular engine).

The other issues were mainly stability and control challenges related to the extremely tight operating envelope, than can easily be dealt with via software today.

It has an L/D of 23:1, making it kind of a heavy, oversize powered version of a Schweitzer 1-26. It'd need a mighty big and strong thermal to be able to climb unpowered though.

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    $\begingroup$ I was exaggerating a little bit for dramatic effect, but it did start out with an F-104 fuselage as the foundation you might say. If you look at an A-model, cover up the fuselage aft of the intakes, you'd think it was a 104. $\endgroup$
    – John K
    Feb 11 at 19:40
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    $\begingroup$ Ah... TIP clearance. Another edit to make. Near zero tip clearance is common now, but in the mid 50s it was a radical thing P&W proposed to Lockheed to get the J-57 to go that high without flaming out. $\endgroup$
    – John K
    Feb 11 at 22:45
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    $\begingroup$ Is it right to assume that tighter compressor blade tip clearances would make for fewer losses due to shear flow between compressed and uncompressed air, as well as slightly more usable air overall? Or is there more to it? $\endgroup$
    – Jules
    Feb 12 at 6:22
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    $\begingroup$ @IanKemp IIRC the blades would just make brushing contact and would wear off enough material until there was no contact. This was radical at the time, but modern engines are made with extremely small tip clearances. The CF 34 had a core lock problem due to this, where the core could jam from blade contact following a high power flameout due to shrinkage in the case. It became a normal production test procedure to stop the core in flight, and if it couldn't be got windmilling again by a certain diving airspeed, there was a "break-in" procedure to follow to wear the blade tips down. $\endgroup$
    – John K
    Feb 12 at 15:11
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    $\begingroup$ Fascinating answer, thanks. So I just have to find a donor 1-26 on barnstormers, hook up a few dozen close-tolerance model airplane ducted fans, and wheeeeee! $\endgroup$
    – Kenn Sebesta
    Feb 12 at 16:16
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What U2 design challenges are still challenging?

I think that most of the technological limitations of that era would be today quite a bit less restrictive, basically in all fields of competencies.

Off the top of my head, modern technologies would bring the following improvements if available back then.

  • propulsion $\rightarrow$ modern engine would supply a better specific fuel consumption i.e. more thrust for less fuel; this would give more weight and space for the payload and/or more range and height;

  • structure $\rightarrow$ using carbon fiber reinforced plastic wherever possible instead of aluminium would give some 15% lighter structure, also improving payload and/or range;

  • aerodynamic $\rightarrow$ airfoils could be better optimised; for example using some supercritical airfoil, less drag at transonic speed could be expected, enlarging the coffin corner toward higher speeds making the flight envelope a bit less sensitive to speed changes;

  • avionics $\rightarrow$ well, who needs a pilot anymore? 🙂

Are there design challenges which would still be hard to handle

If we really need a pilot in the loop, I think that this would still be the only limiting factor today as it was back in the 1950s, because its presence would entail the need for a cockpit, a seat, a pressurisation system, windshield, suicide pill, ... all things using up space and weight.

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    $\begingroup$ Given the excessive regulations and safety precautions of today, much of those gains would be eaten up. +1. $\endgroup$
    – Peter Kämpf
    Feb 13 at 20:52
  • $\begingroup$ Well, as said the human factor is the limiting force still in action 😅 Thanks for upvoting $\endgroup$
    – sophit
    Feb 13 at 21:20
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lets say I wanted to make my own U-2

The advantage these days is real-time transmission of data rather than bringing film back and developing it.

Back in the 1950s it was essential to get the results into the hands of decision-makers as soon as possible. ICBMs and submarine launched BMs made this situation even worse.

So, modern, low budget "U-2" would be ... a balloon, possibly solar powered with propellers mounted in a duct to avoid detection. If this was made in a "stealthy" form, it could be virtually invisible. No need to fly near Mach speed leaving a hot jet trail behind.

The U-2's advantage was that its height originally made it impervious to interception even though it was radar detectable. Once missiles were able to accurately reach that height it was time to move on (to the SR-71 and satellites).

Modern defenders need to find ways to watch out for these balloons. A clever military tactic would allow a few "easy" ones to be caught, while many others remain undetected.

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    $\begingroup$ I think NORAD is already "watch[ing] out for these balloons"? $\endgroup$
    – Someone
    Feb 12 at 17:04
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    $\begingroup$ I have a hard time believing that the recent spy balloon/s really could maneuver other than possibly some limited ability to release ballast and helium/hydrogen (whichever it was) to navigate vertically. If they were meant to move horizontally at a speed other than the wind speed, they would be shaped like blimps! $\endgroup$
    – quiet flyer
    Feb 12 at 17:21
  • $\begingroup$ Good answer WRT the relevance of the U2 from a mission standpoint. Not sure that’s what the OP was thinking though… $\endgroup$
    – Michael Hall
    Feb 13 at 15:41
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    $\begingroup$ @MichaelHall Total pipe dream is to build my own U-2 so I can get to 80k' and see what the world looks like from there. But that got me to thinking about why no one else has built what is otherwise a very simple machine: a self-launching glider with a massive jet engine. $\endgroup$
    – Kenn Sebesta
    Feb 14 at 2:22
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    $\begingroup$ @KennSebesta, that is a pretty lofty dream! (Pun intended). Complexity will come from other than power and aerodynamics though: Pressurization, oxygen, IFR certification, etc. $\endgroup$
    – Michael Hall
    Feb 14 at 2:52
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The major issue with the U2 / TR1 is that at altitude the stall speed and sound barrier are nearly the same. With the thin air there is less lift, requiring the large sailplane like wings. At the same time the speed of sound decreases with the lower air pressure, but it is still dense enough to create a shockwave if the speed of sound is exceeded.

If the wing stalls, you normally drop the nose to gain more airspeed, but this can cause you to break the sound barrier and the resulting shockwave will damage the wings, possibly severely. Flying the U2 / TR1 was always a balancing act to keep above stall speed but below Mach 1.

Another fun part was trying to land. At low altitudes the wings generated a lot of lift even at low speeds. You basically had to fly it on to the runway and hold it down until it stopped moving. You did this while it balanced on one wheel. The wheels out on the wings fall off during takeoff as the wings don't have room to retract them. Early designs had two trucks with mechanics riding in the back trying to reattach the wheel as the plane was flying down the runway.

Please note I am not a pilot; I was in aircraft maintenance, but I worked B52 and KC135 aircraft. I knew people who had worked on the U2 / TR1, but I never did so myself.

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    $\begingroup$ There may have been attempts to catch the wingtip before the plane stopped moving after the main wheel was on the ground, but I can't believe they were actually trying to re-attach the tip wheels at any point when the plane was in motion, to say nothing of when the plane was still in the air! $\endgroup$
    – quiet flyer
    Feb 13 at 16:48
  • $\begingroup$ Do you know how wide the U-2's "coffin corner" is at altitude? $\endgroup$
    – Rodrigo de Azevedo
    Feb 13 at 16:49
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    $\begingroup$ The range between stall speed and Mach 1, the coffin corner, eventually becomes 0 at some altitude above 75,000 feet. $\endgroup$
    – Jim
    Feb 13 at 16:54
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    $\begingroup$ There was reason they stopped trying to reattach the wheels, the wings were modified to add a skid plate. once the plane slowed down enough one of the wings tips would simply start dragging on the ground, The plane would then shutdown engines and get towed off of the runway. $\endgroup$
    – Jim
    Feb 13 at 16:56
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    $\begingroup$ For an ideal gas, pressure does not affect the speed of sound, but temperature does. For the earth's atmosphere, the temperature will decrease with altitude up to the stratosphere. Above the stratosphere the temperature will actually start to increase, but at those altitudes we tend to not use wings, but rely on pure thrust instead. With aircraft instead of relying on temperature to indicate altitude, we use air pressure instead. $\endgroup$
    – Jim
    Feb 13 at 20:03
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Using today's technology, you could [re]design the U-2 using more "stealth" technology as well as using a more traditional tricycle undercarriage. Everything else on the aircraft is fine the way it is! Of course the use of more carbon fiber components for further weight reduction would be a good idea as well. For every pound of weight removed is a pound of weight of more sensors, or higher in altitude you could climb. Physiologically speaking the pilot would still need a Full-Pressure-Suit, in the event of a cabin depressurization. Do away with the pilot you say? Ask Northrop-Grumman how their RQ-4 Drone (that was supposed to replace the U-2) worked out?

There is a limit to the altitude you can climb while using a traditional "Air-Breathing" turbofan engine. The higher you climb the less air(oxygen) molecules there are to support combustion, unless you go super/hyper-sonic and use a RAM/SCRAM-jet engine, but that uses A LOT of fuel. Traditional Jet engines can only consume subsonic air (No shockwaves allowed in the inlet). To fly higher you must either add more wing [area] (for subsonic flight) or fly faster!

Any aircraft that is optimized for high-altitude(thin-air) flight will always be a "pig" to land in the thick air. The U-2 isn't so difficult to fly, it is VERY difficult to land! More Wing Area for higher altitude flight would make it a bigger "pig" to operate/land in the thicker air. The U-2 was designed to "Loiter" for many hours on end and flying supersonic uses up all the fuel too quickly (Ask a SR-71 driver).

So, your DIY U-2 needs to be designed to be more stealthy, using lighter/more-exotic materials, with a slightly larger wing [area] (for higher flight) and tricycle gear. The orignal prototype took 18 months to deliver to the CIA for a cost of \$ 1-million USD (\$11,163,059.70 USD today). So, your biggest challenge today is designing and building one for \$11,163,059.70 in today's money! Not gonna happen, so just keep using the ones that are already paid for with 1986 money (those are the most current U-2S').

PS...U-2s rarely ever carry a "wet-film" framing camera anymore. They are up to "modern" standards in avionics/navigation and sensors.

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