As a follow-up question to What is the typical temperature of an airliner's hull during flight? I wonder how very fast airplanes, such as the SR-71 mentioned, avoid overheating with a total air temperature outside of more than 400 degrees Celsius.
To avoid overheating, the usual trick is to select the right material:
- The Concorde used a special aluminium alloy, called Hiduminium, which had higher strength at elevated temperatures and allowed the Concorde to cruise at Mach 2.02. Aluminium melts at 660°C.
- The MiG-25 used stainless steel instead of aluminium to make its top speed of Mach 3.1 possible (top speed without engine damage was Mach 2.83). Titanium would had been even better, but was expensive and difficult to work with at that time. Stainless steel melts between 1400°C and 1450°C.
- The XB-70 used titanium in addition to stainless steel. Its cruise speed was also Mach 3.0. Titanium melts at 1668°C.
- The SR-71 used titanium for 85% of its structure to allow a cruising speed of Mach 3.2.
- The X-15 was built from Inconel-X, a nickel-chromium alloy which has excellent strength at high temperature (it starts to melt at 1393°C) and allowed it to go up to Mach 6.5.
- The Space Shuttle used a ceramic insulation on top of the load-carrying aluminium structure which kept the heat away from the structure during descent. Note that the Shuttle had to be connected to a heat sink to cool the on-board systems after landing.
The next trick is to fly higher up where air is less dense. Lower density does not reduce the air temperature, but it reduces the heat flow so the airframe will settle at a lower temperature. Remember that the eventual heat is the combination of convection, radiation and heat conductivity. Flying high up allows especially the top surface to radiate heat freely into the black of space above.
The hottest aircraft ever was the North American X-15 A-2. For a speed record attempt, the whole aircraft was covered with pink, ablative paint so the process of sublimation would carry some more heat away. To protect the paint from liquid oxygen, a white topcoat was applied also.
X-15 A-2 shortly after being dropped from the B-52 carrier (picture source). Note the small barrel under the lower fin: This was a ramjet which was test-flown on this occasion.
Damage to the ventral stabilizer. The ramjet separated prematurely due to frictional heating (picture source)
A third trick is to fly fast only for a short time. A heat-seeking air to air missile will easily reach Mach 3, but only for less than a minute. To keep the sensor cooled, a pressurized gas (argon or nitrogen) would be expanded (the AIM-9X even uses a stirling cryo-cooler). By heating the structure or using an internal heat sink, the limited heat load can be tolerated, albeit only for a very limited time.
I'd like to answer with a focus on the SR-71, since I happen to have a book that gives details on its design.
Ben Rich was the lead of the propulsion and thermodynamics group for the SR-71, and Kelly Johnson's successor as head of the Skunkworks for later programs. In the "Faster Than a Speeding Bullet" chapter of his memoir Skunkworks (pg 203 in the first paperback edition, 1994), he writes:
... I volunteered some unsolicited advice about how we could use a softer titanium that began to lose its strength at 550 degrees. My idea was to paint the airplane black. From my college days I remembered that a good heat absorber was also a good heat emitter and would actually radiate away more heat than it would absorb through friction. I calculated that black paint would lower the wing temperatures 35 degrees by radiation. But Kelly [Johnson, head of Skunkworks and the then-A12 project] snorted impatiently and shook his head... Overnight, however, he apparently had second thoughts..."On the black paint," he said, "you were right about the advantages and I was wrong." He handed me a quarter. It was a rare win. So Kelly approved my idea of painting the airplane black, and by the time our first prototype rolled out the airplane became known as the Blackbird.
The chapter goes on into greater detail on various unique material selections:
stainless steel hydraulic lines
Hastelloy X ejector flaps
Elgiloy control cables
Gold-plated plumbing lines
Titanium screws and rivets
Special rubber for the landing gear wheels, which were then inflated with nitrogen
Jet fuel with a higher flash point (JP-7)
The fuel acted as an internal coolant. All the heat built up inside the aircraft was transferred to the fuel by heat exchangers. We designed a smart valve -- a special valve that could sense temperature changes -- to supply only the hottest fuel to the engines and keep the cooler fuel to cool the retracted landing gear and avionics.
We designed the cockpit air-conditioning to bleed air off the engine compressor and dump it through a fuel air cooler, then through an expansion turbine, into the cabin at a frigid minus 40 degrees F, which lowered the ovenlike 200-degree cockpit to a balmy Southern California beach day.
So I guess there's three generic design principles embodied here:
- selection of materials to withstand high temperatures
- rejection of as much heat as possible--here by radiation from the aircraft's surface and convection out the engine exhaust via the fuel
- sequestration of heat away from critical areas, as much as possible
There are multiple ways to avoid overheating.
The simplest method is to use a high temperature material- like Titanium in SR-71 or steel alloy in Mig-25; in Concorde, a special aluminum alloy (AU2GN) was used. In case of other aircraft, steel or Ti is used in stagnation points (like wing leading edges), while the other exposed surfaces are made out of other materials.
In SR-71, the fuel was used as a heat sink to dissipate the heat generated in the airframe.
In X-15, an ablative insulation was used to overcome heating problems:
The airplane was covered with ablative insulation designed to permit flights to Mach 7.4. A silicone elastomeric ablator was sprayed on in variable thickness appropriate to the local heat loads. Leading edges were protected by a related erosion-resistant material applied in preformed sections
- Aerothermodynamic design is important for high speed aircraft. For example, when NASA tested a dummy ramjet in X-15, the mounting pylon nearly failed due to interference heating, with leading-edge heating rates of the order of seven times compared to those without interference being estimated.
Failure of pylon due to interference heating from dummy ramjet, image from history.nasa.gov
- To help reflect and radiate the high amount of heat produced during supersonic flight, the Concorde had a high-reflectivity white paint that was about twice as reflective as the white paint on other jets.