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Supersonic flight without loud booms? NASA is working on that. //
When earlier designs flew faster than sound, individual shockwaves coming from different features like the nose, canopy, and wings merged into one powerful shockwave as they traveled to the ground. “We designed the X-59 so that those individual shockwaves don’t merge, which makes the boom quieter,” said Mike Buonanno, the X-59 air vehicle lead at Lockheed. //
“This way the vast majority of shocks generated by the X-59 are directed upward, and very few go to the ground,” Richardson explained. But perhaps the most unusual victim of eliminating shock-generating features was the canopy. The X-59 has no front-facing windows. //
And it is quiet—the sonic boom it will make should be around 75 PLdB, roughly like a car door slam from 20 feet away. //
On a New York to Paris flight, Concorde burned through roughly four times more fuel than the Boeing 747 while carrying one-fifth of the passengers.
The problem is that you can’t really do much about both fineness and lift-to-drag ratios—they're limitations imposed by pure physics. If you wanted to keep the right fineness ratio and have enough space for over 500 passengers in a supersonic airliner, you’d just need to make it absurdly long. //
Concorde used 119,600 L of fuel to carry 120 people 7200 km. That's 13.8 L/100km per passenger – almost exactly double the consumption of the Boeing 707, at best; it was worse on shorter flights. The subsonic planes that fly its routes now get 2.2 to 3.4 L/100km per passenger.
That, more than anything, is what killed it. Even if you sold every seat in a Concorde, it had six times the fuel bill per ticket of a similarly sized subsonic plane. It would have become uneconomical even if maintenance were negligible and spare parts were free, which they were not.
Can modern engine technology bring that down? Maybe. I can see 7 L/100km per passenger being potentially achievable with lots of R&D. //
RZetopan Ars Scholae Palatinae
8y
1,222
"At Mach 2.2, air friction heated up Concorde’s fuselage to 121° Celsius"
We see this error a lot, even when describing asteroids entering the atmosphere. The temperature rise is due to the rapid compression of air and not due to an alleged friction. A boundary layer forms near any surfaces parallel to the air flow (just like liquids flowing in a pipe) while the heat is generated at the leading and trailing edges where the surface is not parallel to the flow and the air must undergo a rapid change in speed. Any locations where the flow is forced to quickly change speed is where maximum heat is generated due to the rapid compression. These are the same locations where shock waves form in supersonic flows. Given the large number of errors in this article, we can assume that the author is neither familiar with fluid flows nor supersonic aircraft. Relying on mass market newspaper reports often leads to promoting totally nonsensical claims*.
*Having been subjected to newspaper reporters on multiple occasions, it is astonishing how wrong they can be, both before and after "informing" them. For a trivial example, look at what happened to Irving Finkel's report on a 4,000-year-old cuneiform translation: //
Wickwick Ars Legatus Legionis
14y
33,543
paulfdietz said:
I see your claim fairly often, and I find it very annoying. It seems to equate the heating in supersonic flow with the heating air undergoes in adiabatic compression, for example in a bicycle pump.
But heating at a shock is not an adiabatic (entropy conserving) process. Shocks are irreversible. Entropy increases across a shock as fast molecules slam into slow ones in a region a few mean free paths wide. Compression does occur at a shock, but the heating is higher than is required by the increase in density. Indeed, there is a finite upper limit to the density ratio of a gas going through a shock, regardless of Mach number.
At sufficiently high speed, and particularly for the atmospheric entry case you describe there, almost all the heating is due to dissipation. In that respect it's much more similar to friction (a dissipative process) than it is to adiabatic compression.
You're conflating isentropic (reversible) and adiabatic. Adiabatic is the correct term in this case. In the limit of mild shocks (or low-angle oblique shocks), a shock wave of pressure ratio -> approaches an isentropic process.
Almost all shocks are adiabatic. That just means there's no heat transfer during the process. Which is the case. My compressible flow book has a single chapter discussing non-adiabatic shocks at the end of the book. These are when water condenses out of air, e.g. There's no external heat transfer, but the gas phase loses energy to the liquid phase so it's a non-adiabatic process for the gas.
But that dissipation isn't friction. It's compression heating when the compression is done too fast.
Edit: Let's put some numbers on this. Let's consider a Mach 2.2 normal shock (at the tip of the nose) at 15 km. That's 12 kPa and, -56.5C (216.7K), and 195 mg/m^3 density.
My compressible flow tables put the pressure ratio at 5.48 and the temperature ratio at 1.8569 (for gamma = 1.4 or air). That means the density change is 2.95 (Pressure ratio over temp ratio). So we've got a final density of 575 mg/m^3 at 402.3K (129 C). Isentropic compression to achieve a density ratio of 2.95 would be a pressure ratio of 4.54 (again, gamma = 1.4). And the temperature ratio for an isentropic compression of pressure ratio 4.54 is 1.54. So the isentropic temperature rise (so no dissipation) would result in a temperature of 334K or 61 C. So the isentropic compression heating would be 117 deg. C while the adiabatic heating would be 185.6 deg. C. So, in fact, pure reversible heating would do more than half of the heating that is experienced through a normal shock wave at Mach 2.2.