Would pressurised air holes on supersonic aircraft mitigate the heat problem?

Would air holes or tubes of 50mm diameter blowing pressurised air outside the surface of a hypersonic or supersonic aircraft mitigate the problem of heat? They would be placed such that when they blow, there is no surface of the aircraft skin colliding directly with the air outside when the aircraft is travelling at supersonic or hypersonic speeds.

In areas where lift and control is required, they'd blow at a lower pressure and leave just enough enough air colliding with those areas for them to function. Would this mitigate the sonic boom?

• Where would this air come from? – Dan Pichelman Feb 15 '18 at 14:59
• @securitydude5: While the idea is not frivolous, that doesn't mean an answer can be provided without months of research. Unfortunately industrialization of ideas is not a simple task as you seem to assume most of the time. – mins Feb 15 '18 at 15:16
• Are you innovative? It seems like you are not familiar with aero-engineering (how do flight controls work? how are wings configured? what does the yoke do?) making up non-existent problems, throwing out random, unsubstantiated ideas, and asking other people to do the work of figuring out if they're any good. – abelenky Feb 15 '18 at 15:19
• You've asked 30 questions in the last 15 days, and most of them have been heavily downvoted. Perhaps you should reconsider the questions you post? – abelenky Feb 15 '18 at 15:25
• There should have been a system-generated Question Ban on this user days ago. That he is still asking questions is clear evidence that the parameters for when a question ban goes into effect need to be revised. – Ralph J Feb 15 '18 at 16:00

The idea of using small air holes to cool a surface is not completely 'out there'. In turbine engines, the turbine blades are cooled by blowing bleed air from a compressor stage over the blades.

Here, cooling with bleed air makes sense, because the air in the turbine is hotter than the compressed bleed air, due to the added energy from fuel combustion.

However, it would not work to cool from a supersonic shock. The reason for this is the ideal gas law. Specifically, we're going to look at a reversible adiabatic compression. This is the compression of a gas, without any heat exchange with the environment. Due to adiabatic compression, the temperature increases as follows:

$$T_{end}=T_{env}+\mu ( P_{end}-P_{env})$$

with $µ$ the Joule-Thompson coefficient, which for air has some positive value. The key point from this equation is that the final temperature is only dependent on the pressure increase.

A supersonic aircraft creates a shockwave in front of the aircraft. This shockwave is a sharp increase in pressure, resulting in a sharp increase in temperature. This rise in temperature is what heats the aircraft.

The 'protective' air film you think of, must have a higher pressure than this shockwave - else, air is going in rather than out of the holes. Using the above equation, this means that the air going out of the holes must have a higher temperature than the shockwave. Your idea only makes your airplane toastier!

The only solution is to change from an adiabatic compression to a more complicated thermodynamic cycle: most likely, adiabatic compression in the aicraft engine, followed by isobaric (constant pressure) heat exchange with the environment. Heat exchangers, however, are heavy, bulky structures, and are avoided as much as possible. In fact, passenger airplanes first compress the air to a much higher pressure than needed, to artificially increase the temperature; all in an effort to reduce the heat exchanger size.

Assuming you have a heat exchanger made from handwavium, we can turn to your control surfaces. The outside air doesn't care whether they hit a solid surface or a film of air - the shockwave creates just as much of a pressure differential, which is what creates the force on your control surface. In fact, a shockwave is often situated a little bit in front of the surface, because there is some 'trapped air' behind the shockwave. You may need to increase the pressure of your air holes at the pressure side of your control surface, so the system must include some control valves.

The sonic boom is still exactly the same. A sonic boom is just 'air couldn't get out of the way quickly enough'; it doesn't matter whether it needs to get out of the way of a solid airplane or a thin air film.

I can't imagine why your holes should be 50mm - that's huge. You want the holes to be as small as possible. This way, you can put them as close together as possible, so that each bit of aircraft skin is close to a hole, and protected by a film from that hole. Think of it like this: what does the middle of a hole contribute to the cooling of the neighbouring skin?

• Well, theoretically you can compress the cooling air in stages with intercoolers between the pressurization stages. This even reduces the work needed for pressurization and produces (relatively) cool, pressurized air. Obviously, this now shifts the problem to finding a heat sink onboard. – Peter Kämpf Feb 17 '18 at 10:43
• @Peter An intercooler would need a heat exchanger as well, and a relatively big one as the temperate at intermediate stages is less high. But of course everything is possible at a military budget... – Sanchises Feb 17 '18 at 11:29

I disagree that a "heat problem" even exists.

The Concorde, a supersonic passenger jet, faced a peak temperature of 153°C (307°F), and that is only over a tiny part of the nose. (source)

Even though that sounds like a lot, it is about the right temperature for baking muffins in a kitchen; a typical pizza bakes at 425°F. No one would say my kitchen has a "heat problem".

Even for extreme planes like the SR-71, heat is pretty easy to manage, and not really a concern that needs an entirely new system of airholes, tubes, inlets, valves, sensors and controls to deal with.

You have a solution in search of a problem. The problem does not exist.

• No heat really is a problem at high speeds as it degrades the yield strength of the airframe structure. This requires the aircraft to be overbuilt, adding hundreds if not thousands of extra pounds of structure onto the aircraft, decreasing its useful load. The other solution is to use exotic alloys and composites, which drive the costs through the roof and have other problems with an immature material. Past Mach 2.5 heat is a major factor for aircraft design, especially getting into hypersonic or runway to orbit aircraft. – Carlo Felicione Feb 15 '18 at 16:05
• To cope with the heat, special heat-resistant aluminium alloys were used on the Concorde. This should prove that heat was indeed a problem. – Peter Kämpf Feb 17 '18 at 10:46
• The SR -71 had to use titanium, expensive and hard to machine, which was very novel at the time are required considerable research. Also, the thermal expansion of it is so great, it leaks fuel on the ground. So, saying "heat was pretty easy to manage" on the SR-71 shows you haven't read about it much at all. – Penguin Feb 17 '18 at 11:27