4
$\begingroup$

The Blackbird SR71 could travel faster than Mach 3 but its speed was limited to prevent its skin from melting. Its newer variant is said to be named the SR72 and it and other, civilian hypersonic aircraft in development are expected to travel at speeds of up Mach 6.

Is there currently any heat shielding and paint for repeated use at speeds of up to Mach 6, and travelling at the cruising altitude of the Concorde?

$\endgroup$
  • 1
    $\begingroup$ how high? for how long? $\endgroup$ – user3528438 Feb 8 '18 at 19:34
  • 5
    $\begingroup$ The space shuttle regularly entered the atmosphere at Mach 25. $\endgroup$ – Ron Beyer Feb 8 '18 at 21:43
  • 4
    $\begingroup$ the speed limit of the SR-71 was set by the maximum allowable air temperature at the inlet face of the compressor section of its engines, which was 800F. flying faster would cause this limit to be exceeded, which would start softening the compressor blades. $\endgroup$ – niels nielsen Feb 9 '18 at 0:39
  • 1
    $\begingroup$ Titanium is used on leading edges and other hot spots ( replacing Al ). I don't know the temperature /speed limit. $\endgroup$ – blacksmith37 Apr 30 '18 at 0:38
10
$\begingroup$

I'm reasonably certain the speed limit (so to speak) on the SR-71 wasn't to prevent its skin from melting.

The hottest the skin got during flight was less than 600 C. That's definitely hot--but it's a long ways short of the melting temperature of titanium (1668 C).

Early supersonic aircraft often had control problems, because the leading edge of the air foil would cause a shock wave that separated the air flow. The control surfaces at the trailing edge little enough air flowing smoothly that they lost authority.

In the X-15, they combated this by building a vertical stabilizer that was basically a V-shape--thin at the front, but much wider at the rear:

enter image description here

This would induce a lot of drag at low speed, but for the x-15, low speed wasn't really a major concern.

The SR-71 took a different approach, using a pair of vertical stabilizers.

enter image description here

The leading edge of each stabilizer produces a V-shaped shock wave. At (approximately) the normal cruising speed, that V comes back from the leading edge of one stabilizer, and hits against somewhere close to the trailing edge of the other. Thus, we still have a solid air flow across the trailing edge (and rudder) of each, and maintain nice directional stability.

For that to work, however, the angle from the leading edge of one stabilizer to the trailing edge of the other has to (approximately) match the angle of the shock-wave formed at the leading edge. Outside of the designed speed range, that no longer happens.

On the SR-71, the vertical stabilizers are angled somewhat. This gives a (still fairly narrow) range of speeds at which the aerodynamics "work", rather than having only one specific speed. Nonetheless, the difference in separation between the top and bottom of the stabilizers isn't very large, so the range of speeds at which it works is fairly narrow. If you try to exceed that range, your stabilizers no longer work, and your control over the aircraft quickly deteriorates.

Having said all that, however, the answer is a clear "yes", if heat were to become a problem, there are materials available that can withstand substantially higher heat than titanium. One obvious example would be the Inconel X that was used as the skin for the X-15 (which flew at a bit over mach 7). Another possibility would be carbon or ceramic tiles, like those used in the Space Shuttles (or some of the other heat shielding it used, such as flexible blankets).

Those have some fairly serious shortcomings so they'd probably be avoided unless absolutely necessary. Inconel X is quite a bit heavier than titanium, and while the ceramic tiles were quite heat resistant, they were fragile mechanically, which led to a lot of maintenance work on the shuttles.

$\endgroup$
  • 1
    $\begingroup$ Be careful when talking about melting point :). This property is never considered during design because mechanical properties drop long before that. Also you are talking about the melting point of the pure titanium, which is never used in aerospace industry, because alloys have higher mechanical properties. And 600°C is high temperature, even for the best titanium alloys. If you want high-temperature titanium based compound, check titanium aluminide $\endgroup$ – BambOo Apr 29 '18 at 16:23
  • 1
    $\begingroup$ 600 C is a limit for long term exposure of titanium . It absorbs oxygen and embrittles above 650 C.At slightly higher temperatures nitrogen does the same. $\endgroup$ – blacksmith37 Sep 23 '18 at 21:15
1
$\begingroup$

The SR72 is not a variant of the Lockheed SR71 but an entirely new, unmanned high speed reconnaissance and strike vehicle being proposed by Lockheed Martin. The designation is obviously intended to suggest a connection with the earlier aircraft, but the SR72 is intended to have a different, turbojet/scramjet propulsion system and a different role, not as yet clearly defined. The SR72 is intended to make use of composite materials such as carbon fiber reinforced carbon, previously used in missile nose cones and/or ceramic materials to withstand the high temperatures generated by air friction at the intended Mach 6 speed at 80,000 ft. The use of ablative materials or coatings on the aircraft would create a problem in that the aircraft would be enveloped in a layer of plasma at it's operating speed, which would tend to degrade the performance of on board sensors as well as radio communications.

$\endgroup$
1
$\begingroup$

The top speed achieved by the North American X15 was Mach 6.72 according to what I read. The heating effect on the airframe caused by air friction at this speed will depend on the altitude. Obviously the higher the altitude, the less dense the atmosphere and the less the kinetic heating effect at a given airspeed.

$\endgroup$
  • 2
    $\begingroup$ "according to what I read" asks for the reference. Moreover, keep in mind that the 15 flew almost in space (almost no air friction except for reentry, in which case it was comparable to a space shuttle) $\endgroup$ – Manu H Jun 18 at 7:43
  • $\begingroup$ Wikipedia gives a figure of Mach 6.70 at 102,000 ft, which is well within the atmosphere. The maximum altitude reached during the X15 program was 354,000 ft or 95.9 km which is beyond the Karman line according to both the NASA/USAF and FAI definitions, and therefore in space. $\endgroup$ – J. Southworth Jun 20 at 14:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.