Which aircraft endure the highest Max Q's (aerodynamic pressure)?

Question

What are the highest aerodynamic pressures encountered in any aircraft? And does this limit the maximum speed of any aircraft at certain altitudes? Broadly, 'how fast can you go [at a given altitude]?'

Where dynamic pressure $q$ is: $$q = \frac{1}{2} \rho v^2$$

$\rho$ is local air density, and $v$ is true airspeed.

I'm mostly concerned with high performance fighters/bombers. Here's some data I've culled (Mk-12A and 787 added for reference):

#   | Airframe      | Mach       | Speed     | Altitude    | q (kgf/m^2)
----|---------------|------------|-----------|-------------|------------
1.  | Mk-12A        | Mach  6.15 | 2,100 m/s |       0 ft  | 273,000
2.  | Mk-12A        | Mach 22.6  | 6,900 m/s | 100,000 ft  |  39,300
3.  | X-51A         | Mach  5.1  | 1,500 m/s |  64,000 ft  |  11,100
4.  | HTV-2         | Mach 20.   | 5,812 m/s | 125,000 ft? |  10,700
5.  | F-111         | Mach  1.2  |   410 m/s |       0 ft  |  10,400
6.  | B-1A          | Mach  1.2  |   410 m/s |       0 ft  |  10,400
7.  | B-1B          | Mach  0.9  |   310 m/s |       0 ft  |   6,100
8.  | X-43A         | Mach  9.6  | 3,000 m/s | 109,000 ft  |   4,800
9.  | Skylon        | Mach  5.5  | 1,630 m/s |  85,300 ft  |   4,600
10. | SR-72         | Mach  6    | 1,800 m/s |  90,000 ft? |   4,400
11. | Space Shuttle | Mach  1.3? |   400 m/s |  35,000 ft  |   3,200?
12. | XB-70         | Mach  3.0  |   880 m/s |  73,000 ft  |   2,500
13. | SR-71         | Mach  3.2  |   930 m/s |  78,740 ft  |   2,300
14. | Concorde      | Mach  2.0  |   600 m/s |  60,000 ft  |   2,100
15. | Boeing 787    | Mach   .85 |   250 m/s |  35,000 ft  |   1,200

Airframes #3, #4, and #5/6 are quite different from each other, so I'm surprised they share roughly the same max $Q$. Is 11,000 kgf/m^2 a common limit?

Note: All figures are for level flight except for the Mk-12A (reentry), Space Shuttle (ascent), and HTV-2 (glide).

• [Mk-12A]: Current reentry vehicle for the Minuteman III ICBM, houses the W78 warhead (350 kT). Figures are for reentry.
• [HTV-2]: Rocket glider.
• [B-1B]: "[top speed] is limited by the need to avoid damage to its structure and air intakes," according to Wikipedia. The B-1B used less structural Titanium and a simpler fixed geometry inlet when the speed requirement was lowered from the B-1A.
• [Skylon]: Obviously doesn't exist yet. Reaction Engines' "user manual" for the Skylon discusses the flight profile.
• [SR-72]: Obviously doesn't exist yet. Lockheed Martin discussed Mach 6 cruise. 90,000 ft cruising altitude is just my placeholder value.
• [Space Shuttle]: Figures for ascent.

---

Some background and purpose: For fun, I'm trying to design a hypersonic long-range heavy bomber. I was curious what would limit the maximum low/mid-altitude speed: drag (available engine thrust), max $Q$ (structural strength), or skin temperature (thermal protection system)... but I'll get to that in another question.

I've read through nearly all the aerodynamic, military, aircraft design, aircraft performance, and jet engine tags, looking for answers. (Really, this is a marvelous community.) Finally realized I could ask my own question.

I've tried to follow the etiquette as best as I can, but I'm pretty new to Stack Exchange, so let me know if I should change anything :) Thanks!

Answer 1

First, your example list doesn't distinguish between stationary and instationary trajectories. A reentry vehicle will see the highest dynamic pressure only momentarily at the end of its trip, and aerodynamic heating can be tolerated either by ablative cooling or a decently sized heat sink. All it has to sustain is the dynamic pressure at the tip.

The first samples on your list which need to continuously sustain the given dynamic pressure are the low-altitude attack aircraft B-1B and F-111. Here the dynamic pressure is given by wing loading, aerodynamics and raw thrust to keep the aircraft moving at its maximum speed. Aerodynamic heating comes on top at high altitude, because the stagnation point heat is independent of air density.

Reasons to limit the maximum dynamic pressure are:

• gust loads at low altitude. Hitting the same updraft at higher speed will proportionally increase the load factor.
• Aeroelastic deformation: Higher dynamic pressure means also higher torsional and bending loads, and the deformed structure might become unstable when the deformation increases the loads which lead to this deformation. Flying at higher dynamic pressure requires a stiffer and heavier structure.

If you look at the flight envelope of the SR-71 (below), it should be obvious that at each altitude only a narrow band of speed was available due to the high wing loading (= high minimum dynamic pressure) and the maximum dynamic pressure (at low altitude) rsp. the aerodynamic heating (at high altitude).

SR-71 flight envelope (picture source)

Answer 2

Used to be a fighter pilot in the F-111 and the F-105. They have the highest Q limit I have encountered or read about. (To us high speed on the deck) That limit is 810 knots/Mach 1.2 down low. Both aircraft could sustain that airspeed, I have personally done it in the F105 below sea level in Death Valley. Attached are the Technical order airspeed limits:

I do not believe the B-1B is capable of these speeds, there were all sorts of additional limits placed on it. Modern fighters are no longer designed for this regime.