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Comment by lujim | Hacker News Reader

Subsonic flight is cutting through cheese with a sharp knife. Supersonic flight is cutting through cheese with a hammer. There is no way it will ever be commercially economical with gas turbine engines and afterburners. Even super cruise without afterburners is incredibly inefficient and has very few uses outside of tactical aviation.

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Drag in supersonic flight is not nearly as big an issue as you would think. The Concorde was inefficient mostly because it used afterburning turbojets that had poor efficiency at low speed.

https://en.wikipedia.org/wiki/File:Qualitive_variation_of_cd...

That is coefficient of drag. You have to remember that goes into the drag equation which includes velocity squared.

force_of_drag = .5 * mass_density * (velocity^2) * coefficient_drag * area.

As that coefficient starts tapering off after transonic drag decreases velocity squared is there to ramp up the drag.

That's true, but supersonic planes also travel higher (Concorde -> 18 km, XB-70 -> 22 km). The real problem is that L/D decreases at supersonic speeds. For example, the XB-70 had a L/D of about 7, while the 707 had a L/D over 18...

As lift is also proportional to density, area and square of velocity, that whole term cancels out from the equation of fuel usage per distance, basically only L/D or cL/cD matters.

In practice it might be a bit more complicated.

EDIT: indeed here we can see that Concorde had lift to drag ratio of only 7.5 at Mach 2, while Boeing 747 has 17.7 at Mach 0.9 or so. https://en.wikipedia.org/wiki/Lift-to-drag_ratio

But you should also factor in their comparative passenger capacities (e.g. impact per passenger-mile).

Perhaps better would be aircraft weight to number of passengers ratio. Revenue wise the supersonic airliner flies faster, meaning you get more seat miles per day per seat.

It's worth pointing out that Concorde didn't cruise using its afterburners; it only lit them briefly to punch through the transonic flight regime.

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