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I'm curious as to why planes like the U2 Dragon Lady and the SR71 Blackbird couldn't fly higher. What physical constraint set their operational ceiling?

Pilots wore spacesuits, so that wasn't the limiting factor. Was the air too thin to give enough lift? Was there not enough oxygen for the engines? Some other reason?

EDIT: My question is specifically about planes designed for high-altitude flight, not for general aircraft. To narrow the scope of my question, consider the SR-71 as the prototypical example. What set the operational ceiling of the SR-71?

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    $\begingroup$ Possible duplicate of What determines the maximum altitude a plane can reach? $\endgroup$ – fooot Jun 23 '17 at 18:20
  • $\begingroup$ Not duplicate. Edited question to narrow scope. $\endgroup$ – Nelson Jun 23 '17 at 18:35
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    $\begingroup$ Even with your narrowing, I don't see how the other question would not apply here, tbh. Physics is the same for all. $\endgroup$ – Federico Jun 23 '17 at 18:52
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    $\begingroup$ Maybe a dupe in the case of the U-2 (limited by low IAS / high Mach #), but almost certainly NOT a duplicate with regard to the SR-71. As a first guess, I suspect that aerodynamic heating may have been part of what limited the SR-71 from flying higher/faster, but I'd be interested in an answer more informed than my guess is. Good question, IMHO. $\endgroup$ – Ralph J Jun 23 '17 at 20:01
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    $\begingroup$ @Ralph J, the max cruise limitations on the SR-71 are predominantly powerplant and heat, but the altitude limitations as far as I know remain classified. Inlet issues are probably primary limits at 3.4, but the A-12 had a flight with higher speeds (and was lighter). $\endgroup$ – mongo Jun 23 '17 at 21:24
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The limiting factor for subsonic aircraft, including the U-2, is well explained here.

For supersonic aircraft this answer simply says the limit is "a combination of wing loading and maximum speed". If you look at the flight envelope of the SR-71 below, it becomes clear that more altitude can be best bought with more speed.

SR-71 flight envelope

SR-71 flight envelope (picture source). Tower buzzing at Mach 3 is clearly impossible.

Supersonic speed limits

  1. Intake design: If the kinetic energy of the flow cannot be efficiently converted into pressure in the intake, thrust will suffer and will drop when the flight Mach number is increased beyond the limits of the intake.
  2. Airframe efficiency: If the leading edge sweep of flight surfaces is not high enough to keep those leading edges within the Mach cone, drag will rise and limit the top speed of a design. The desire to reach Mach 2+ speeds was the driver for the many swing wing designs of the 1960s.
  3. Compressed gas temperature: Once the compression heating in the intake brings the gas temperature close to its dissociation temperature, the chemical energy in the fuel cannot be fully converted into heat. This reduces engine efficiency and is the reason for supersonic combustion in designs for speeds in excess of Mach 4 or 5.
  4. Aerodynamic heating: Metals and composites show decreasing strength with increased temperature. Fly fast enough for some time, and the structure cannot tolerate the flight loads, even if dynamic pressure is kept constant.

The order in which I listed those limits ranks them with increasing speed. Once you move beyond Mach 1.6, every consecutive tenth of the top flight Mach number must be bought with increasing expenses and compromises. Going beyond Mach 5 with current technology will only be possible with rockets, so those designs quickly become low-orbit satellites. In the end, it is simply not worth it to push the limit yet further.


EDIT: It seems the answer is not explicit enough. If we try a thought experiment and modify the SR-71 to reach higher altitudes, the possible options are:

  1. Just pull on the stick: This helps in the short term, but flying stationary at lower density would require a higher lift coefficient and a higher angle of attack. This would lower the overall L/D of the plane and slow it down because the engines could not develop sufficient thrust.
  2. Increase engine thrust: This could be tried in flight by advancing the throttles at top speed, and the aircraft would accelerate. This would quickly exceed the limit of the compressor inlet temperature, though, leading to a shorter life time or even damage of the engine's hot section. Next, range would suffer due to the higher fuel consumption. If the engine is improved by using better materials, a moderate increase in cruise Mach and, consequently, flight altitude is possible.
  3. Lower wing loading: A lighter aircraft can cruise at a lower density, all other parameters being equal. At the end of a trip the SR-71 could reach the highest altitude, just like any other plane. Structural changes to lighten the structure beyond removal of all reconnaissance equipment, however, would have only limited potential: The SR-71 was already designed efficiently, so there is very little potential for weight savings without compromising structural strength. And removing the cameras and side-looking radars would strip the plane of its operational value.
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    $\begingroup$ Excellent, well-sourced answer. However, it only half-answers the question. You established that the SR71 couldn't go higher because it couldn't go faster. Why did it have to go faster in order to go higher? Was there not enough lift at lower speeds? Not enough air for the engines? $\endgroup$ – Nelson Jun 23 '17 at 23:09
  • $\begingroup$ Interesting answer on speed, but OP asked about operational ceiling. $\endgroup$ – mongo Jun 24 '17 at 0:58
  • $\begingroup$ @Nelson: We here frown on copying other answers into a new one, and prefer linking. Read the linked answers. If this still does not answer your question, you are free to post a new one. Make sure to explain why the other answers could not help. $\endgroup$ – Peter Kämpf Jun 24 '17 at 8:46
  • $\begingroup$ @PeterKämpf I'm not sure what you mean by "copying other answers into a new one". What is it I'm not supposed to do? I don't mean to cause any trouble around here :) $\endgroup$ – Nelson Jun 26 '17 at 21:05
  • $\begingroup$ To reduce wing loading (def: weight divided by wing area), you could decrease the weight, or you could increase the wing area. Could hypothetical solution #4 to the thought experiment above be "make the wings larger"? I suppose that could add some other issues (trade-off with increased drag, weight, etc), so it may be a bit too advanced of a discussion for a simple SE question... $\endgroup$ – Nelson Jun 26 '17 at 21:09
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Above 100,000 feet, there is almost no air at all, so no oxygen for air breathing engines to burn, and no air to produce lift or for control surfaces to react against.

In the 1960's, the USAF had two research aircraft that could exceed 100,000 feet. Might have had more, but these are the two that I remember:

The NF-104 could go slightly over 100,000 feet using a rocket engine above 70,000 feet for propulsion, and small reaction rockets to provide attitude control. It was built as a low cost X-15 trainer as its high flights simulated the X-15's operating characteristics.

The X-15 could go considerably over 100,000 feet, using a rocket engine at all times (after being launched from a B52), and small reaction rockets to provide attitude control.

Point being - by the time those aircraft exceeded 100,000 feet, they weren't flying. They were following a ballistic arc, propelled by a rocket, and kept aloft entirely by inertia, not lift.

So - why didn't the U2 and SR71 fly higher? One reason is - they would need rocket engines with oxydiser on board for propulsion, which is fairly short ranged, and very tempermental.

The requirement for rocket propulsion would negate the primary advantage of both aircraft: the ability to stay at high altitude for long periods of time.

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    $\begingroup$ A 1 m^2 opening at 100,000 feet crosses 29 kg per second of atmosphere at mach 5. A 1m^2 opening at sea level at mach 0.5 crosses 208 kg/s. That is "only" a factor of 10. Getting an aircraft with a huge cone to get enough air and have it survive those speeds may be very tricky, but doesn't look physically impossible at 100000 feet. $\endgroup$ – Yakk Jun 24 '17 at 14:56
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In the case of the U-2 and the SR-71, the altitudes they operate at provide threat protection and more importantly, area coverage for ISR (intelligence, surveillance and reconnaissance) sensors. Going to 100,000 feet does not provide a significant intelligence benefit, nor does it provide greater threat protection.

Service ceilings can be overcome by providing different powerplants. ECS (environmental control) can be redesigned (if needed) to handle higher altitudes.

Edit #2: In the specific case of the SR-71, the thrust capabilities and the wing surface area limited the max sustained flight. Practically, that was just below about 85,000 feet. However in certain situations, altitudes above that were flown.

Edit #1: Above about 100,000 feet the atmospheric density drops off, which provides a practical limitation to airfoil and air breathing powerplant operation.

As I understand it, from working most of my career with SR-71, U-2 and satellite assets, the real issue is that there has not been established the need for aircraft to go higher, and therefore, there is no business case to develop aircraft to do so.

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    $\begingroup$ No greater threat protection at 100k vs 80k? Really??? An SA-2 / SA-3 / SA-whatever missile with enough energy to reach a Mach 3 target at 80k might have the energy to reach the same target at 100k, but its engagement envelope would be far smaller, and it would be easier to defeat with a small turn. There were reasons (satellites) the SR-71 wasn't kept around, but that isn't what this question is asking. $\endgroup$ – Ralph J Jun 23 '17 at 20:12
  • $\begingroup$ The question asked what the physical constraint is, and the reality is that there are not substantial physical constraints. Rather they are need based constraints, and aircraft which would do ISR (the example class he mentioned) at 120,000 feet did not offer substantial benefit to justify the cost. In other words, higher altitude flight is readily solvable, it just isn't economically justifiable. $\endgroup$ – mongo Jun 23 '17 at 20:57
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    $\begingroup$ Worth noting that the SR-71 routinely flew above 80,000, although as far as I know the max operational altitude remains classified. A friend who flew the NASA SR-71 aircraft acknowledged that the max altitude was still classified when assigned to that aircraft. $\endgroup$ – mongo Jun 23 '17 at 21:18
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    $\begingroup$ BTW, it is notable that the A-12 had a published service ceiling of 95,000 feet. It weighed a little less than the SR-71, as the payload pods were configured differently. NICE GIRL was a flyoff between both aircraft, and differentiated some of their capabilities. Weight is everything, and the YF-12 had a published ceiling of 90,000 feet. The SR-71 was heavier than both, but accepted more useful payloads. $\endgroup$ – mongo Jun 24 '17 at 1:22
  • $\begingroup$ It might look like density drops of at 100k ft, but only when you plot it with linear scales. Here I have used the NRLMSISE00 model for a logarithmic plot, and as can be expected there is no change in the slope at or around 100k ft. $\endgroup$ – Peter Kämpf Jun 24 '17 at 21:07

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