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> In cruise mode Concorde was as efficient as a Boeing 747. Yes - a plane that carries 550 passengers vs a plane carrying 100. Also, the game has changed a fa

> In cruise mode Concorde was as efficient as a Boeing 747. Yes - a plane that carries 550 passengers vs a plane carrying 100. Also, the game has changed a fair bit - no one is building 4 engined planes anymore - because while it was about as efficient as a 747, it’s going to be less than half as efficient as a 777. That carries 400 passengers.

credit_guy 4y ago
By efficiency I meant the lift-to-drag ratio [1]. The wiki page I linked quotes an L/D ratio for Concorde (in cruise mode) of 7 to 7.5 and for Boeing 747 of 15.3 to 17.7. Taking the mid-points of 7.25 and 16.5 we get that the L/D ratio for Boeing is 2.275 higher than of Concorde. But Concorde's speed was 2.41 times higher than Boeing's (Mach 2.05 vs 0.85). There are 2 things to go from here to actual fuel per passenger*mile. How much fuel do you need to produce the same thrust, and how much aircraft do you need to carry a passenger. On both counts, supersonic airplanes have a problem: supersonic engines have a lower bypass ratio, so they are inherently less efficient, and supersonic planes need to be sturdier, and therefore heavier than subsonic ones. These problems are hard. But the difficulty of building a supersonic aircraft does not come from the air resistance being the square of the speed. By the way, the L/D ratio decreases with the speed, but it does not tend to zero in the limit. This fact is actually stunning, if you think of it. As the speed goes up, at some point the L/D ratio stops decreasing, so you end up burning significantly less fuel per mile. That's the reason some militarizes invest in hypersonic missiles: they have much longer range than their size alone would make you think. [1] https://en.wikipedia.org/wiki/Lift-to-drag_ratio https://en.wikipedia.org/wiki/Lift-to-drag_ratio
- smachiz 4y ago
  Lift to drag is interesting - but without dividing by seats or tons is incomplete. You’re not really measuring work being done (actually transporting goods or people). These problems are hard. Lift induced drag goes down as velocity decreases, parasitic drag goes up as velocity increases. There’s there’s wave drag to contend with. Yes there’s weird places where going faster is helpful, but I doubt we’re going to see real net efficiency gains going faster than Mach .8-9 anytime soon.
  - credit_guy 4y ago
    Glad we got to this point. So, can Boom compete with subsonic airplanes. Where can they have an edge? They have two actually: 1. they are a startup. Boeing and Airbus are old and ossified. Smaller manufacturers are probably not as ossified, but they already have a niche, so their risk appetite is different: they prefer to put their research money to work towards incrementally improving their current designs. Boom is in a life and death situation, they have a huge fire under their feet, and so they'll get more research done for the same amount of dollars 2. Clean slate. All current civilian airplanes are cylinders with flat wings. It's a proven formula. Manufacturers don't want to move too much from a proven formula. But Boom does not have this type of fetters. In particular, they can design a body that follow the Whitcomb area rule [1]. Which is what they are actually doing [2]: Contoured fuselage: According to the principle of area-ruling, Overture’s fuselage has a larger diameter toward the front of the aircraft and a smaller diameter toward the rear. Boom has applied this design technique to minimize drag and maximize fuel efficiency at supersonic speeds. In addition to that, Boom plans to use wings that are not flat (they call them gullwings). Presumably, they know what they are doing, they say they used 26 million-core hours of compute between the first and the second iteration of their Ouverture. Will the manufacturing costs explode? I guess they should be higher than for traditional geometries. But that works to their advantage: it keeps the current manufacturers from trying these non-standard geometries, and keeps them in a local optimum from which they find it harder to move. But it won't be a dealbreaker for a premium aircraft. That's basically the Tesla Roadster gameplan. [1] https://en.wikipedia.org/wiki/Area_rule https://en.wikipedia.org/wiki/Area_rule [2] https://boomsupersonic.com/news/post/boom-supersonic-reveals-refined-overture-and-announces-landmark-market-expanding-alliance-with-northrop-grumman-and-new-tier-one-suppliers https://boomsupersonic.com/news/post/boom-supersonic-reveals...

credit_guy 4y ago

By efficiency I meant the lift-to-drag ratio [1]. The wiki page I linked quotes an L/D ratio for Concorde (in cruise mode) of 7 to 7.5 and for Boeing 747 of 15.3 to 17.7. Taking the mid-points of 7.25 and 16.5 we get that the L/D ratio for Boeing is 2.275 higher than of Concorde. But Concorde's speed was 2.41 times higher than Boeing's (Mach 2.05 vs 0.85). There are 2 things to go from here to actual fuel per passenger*mile. How much fuel do you need to produce the same thrust, and how much aircraft do you need to carry a passenger. On both counts, supersonic airplanes have a problem: supersonic engines have a lower bypass ratio, so they are inherently less efficient, and supersonic planes need to be sturdier, and therefore heavier than subsonic ones. These problems are hard. But the difficulty of building a supersonic aircraft does not come from the air resistance being the square of the speed. By the way, the L/D ratio decreases with the speed, but it does not tend to zero in the limit. This fact is actually stunning, if you think of it. As the speed goes up, at some point the L/D ratio stops decreasing, so you end up burning significantly less fuel per mile. That's the reason some militarizes invest in hypersonic missiles: they have much longer range than their size alone would make you think. [1] https://en.wikipedia.org/wiki/Lift-to-drag_ratio https://en.wikipedia.org/wiki/Lift-to-drag_ratio

smachiz 4y ago
Lift to drag is interesting - but without dividing by seats or tons is incomplete. You’re not really measuring work being done (actually transporting goods or people). These problems are hard. Lift induced drag goes down as velocity decreases, parasitic drag goes up as velocity increases. There’s there’s wave drag to contend with. Yes there’s weird places where going faster is helpful, but I doubt we’re going to see real net efficiency gains going faster than Mach .8-9 anytime soon.
- credit_guy 4y ago
  Glad we got to this point. So, can Boom compete with subsonic airplanes. Where can they have an edge? They have two actually: 1. they are a startup. Boeing and Airbus are old and ossified. Smaller manufacturers are probably not as ossified, but they already have a niche, so their risk appetite is different: they prefer to put their research money to work towards incrementally improving their current designs. Boom is in a life and death situation, they have a huge fire under their feet, and so they'll get more research done for the same amount of dollars 2. Clean slate. All current civilian airplanes are cylinders with flat wings. It's a proven formula. Manufacturers don't want to move too much from a proven formula. But Boom does not have this type of fetters. In particular, they can design a body that follow the Whitcomb area rule [1]. Which is what they are actually doing [2]: Contoured fuselage: According to the principle of area-ruling, Overture’s fuselage has a larger diameter toward the front of the aircraft and a smaller diameter toward the rear. Boom has applied this design technique to minimize drag and maximize fuel efficiency at supersonic speeds. In addition to that, Boom plans to use wings that are not flat (they call them gullwings). Presumably, they know what they are doing, they say they used 26 million-core hours of compute between the first and the second iteration of their Ouverture. Will the manufacturing costs explode? I guess they should be higher than for traditional geometries. But that works to their advantage: it keeps the current manufacturers from trying these non-standard geometries, and keeps them in a local optimum from which they find it harder to move. But it won't be a dealbreaker for a premium aircraft. That's basically the Tesla Roadster gameplan. [1] https://en.wikipedia.org/wiki/Area_rule https://en.wikipedia.org/wiki/Area_rule [2] https://boomsupersonic.com/news/post/boom-supersonic-reveals-refined-overture-and-announces-landmark-market-expanding-alliance-with-northrop-grumman-and-new-tier-one-suppliers https://boomsupersonic.com/news/post/boom-supersonic-reveals...