4 ms·

>conventional aircraft still leave a huge amount of efficiency on the table Go on. I’m listening and I’ll bet Airbus and Boeing are too

wmf 5y ago
Cost is the missing element there. They know how to make more efficient planes (e.g. Dreamliner) but the industry can't afford to apply that efficiency to the entire fleet.
credit_guy 5y ago
Not the GP, but I'll take a stab. Roughly speaking, you can increase the efficiency of an airplane in 2 ways: increase the thermodynamic efficiency of the engine, and make the engine push more air at a slower speed. For the first one you need to make the engine burn hotter. Since metals melt at a certain temperature, you need to use ceramics if you want to exceed that temperature. Such as Silicon Carbide [1] which "melts" (or decomposes) at 2830 degrees Celsius. The efficiency of an ideal Carnot engine is 1-T_cold/T_hot, so if T_cold = 300K and T_hot = 3000K, you end up with 90% efficiency. Of course the jet engine is not quite the Carnot engine, but the idea stands that higher burning temperature leads to higher efficiency. Now, if manufacturers decide to use Hafnium Carbide one day, that one melts at 1000 degrees Celsius higher. The second method is due to the relationship between momentum and energy. Energy is m v^2/2, which means that for a given energy (think quantity of jet fuel burned in the engine), you can either push a certain amount of air at a given speed, or 4 times as much air at half the speed. But momentum is m v, so the second choice gives you twice the momentum. In other words, twice the thrust. To push more air, you want to grab more air, so you want turbines with higher diameter, and a higher bypass ratio. General Electric and Pratt&Whitney do exactly that with their engines. Engines with higher bypass ratios and using ceramics (more precisely, ceramic matrix composites). In fact GE's engine is so fantastic, that Boeing decided to put it on the 730-Max, and the rest is history ... So, yes, Boeing and Airbus know what needs to be done, but it's just not that trivial. [1] https://en.wikipedia.org/wiki/Silicon_carbide https://en.wikipedia.org/wiki/Silicon_carbide
- Robotbeat 5y ago
  You missed the big one: improve the lift to drag ratio. A Cessna does like 10. State of the art airliner does like 20. But the Virgin Globalflyer did 35-40 and modern sailplanes do 50-70+, and it’s possible to do up to 100.
  - credit_guy 5y ago
    I did skip that one, as the topic was engines (gas fueled or electric), not overall efficiency. But since you mentioned this, here's yet another big one (known by very few people): just go faster. Much faster. The lift-to-drag ratio goes down, but the time spent in the air goes down more, so you might end up burning less fuel overall. In cruise mode, a Concorde quite efficient, but it was horrible at takeoff and climbing; because you need fuel to carry fuel, the problem was compounding and overall Concorde was not economical. If someone can solve the takeoff problem (e.g. by using catapults) and climbing (maybe using detachable and reusable boosters?) then a Mach 5 jet could be very competitive. I think this is what the Boom guys had in mind when they say they aim to get to where anyone can travel to any destination on Earth in 4 hours for only $100 [1]. [1] http://edition.cnn.com/travel/article/boom-supersonic-four-hours-100-bucks/index.html http://edition.cnn.com/travel/article/boom-supersonic-four-h...

wmf 5y ago

Cost is the missing element there. They know how to make more efficient planes (e.g. Dreamliner) but the industry can't afford to apply that efficiency to the entire fleet.

credit_guy 5y ago

Not the GP, but I'll take a stab. Roughly speaking, you can increase the efficiency of an airplane in 2 ways: increase the thermodynamic efficiency of the engine, and make the engine push more air at a slower speed. For the first one you need to make the engine burn hotter. Since metals melt at a certain temperature, you need to use ceramics if you want to exceed that temperature. Such as Silicon Carbide [1] which "melts" (or decomposes) at 2830 degrees Celsius. The efficiency of an ideal Carnot engine is 1-T_cold/T_hot, so if T_cold = 300K and T_hot = 3000K, you end up with 90% efficiency. Of course the jet engine is not quite the Carnot engine, but the idea stands that higher burning temperature leads to higher efficiency. Now, if manufacturers decide to use Hafnium Carbide one day, that one melts at 1000 degrees Celsius higher. The second method is due to the relationship between momentum and energy. Energy is m v^2/2, which means that for a given energy (think quantity of jet fuel burned in the engine), you can either push a certain amount of air at a given speed, or 4 times as much air at half the speed. But momentum is m v, so the second choice gives you twice the momentum. In other words, twice the thrust. To push more air, you want to grab more air, so you want turbines with higher diameter, and a higher bypass ratio. General Electric and Pratt&Whitney do exactly that with their engines. Engines with higher bypass ratios and using ceramics (more precisely, ceramic matrix composites). In fact GE's engine is so fantastic, that Boeing decided to put it on the 730-Max, and the rest is history ... So, yes, Boeing and Airbus know what needs to be done, but it's just not that trivial. [1] https://en.wikipedia.org/wiki/Silicon_carbide https://en.wikipedia.org/wiki/Silicon_carbide

Robotbeat 5y ago
You missed the big one: improve the lift to drag ratio. A Cessna does like 10. State of the art airliner does like 20. But the Virgin Globalflyer did 35-40 and modern sailplanes do 50-70+, and it’s possible to do up to 100.
- credit_guy 5y ago
  I did skip that one, as the topic was engines (gas fueled or electric), not overall efficiency. But since you mentioned this, here's yet another big one (known by very few people): just go faster. Much faster. The lift-to-drag ratio goes down, but the time spent in the air goes down more, so you might end up burning less fuel overall. In cruise mode, a Concorde quite efficient, but it was horrible at takeoff and climbing; because you need fuel to carry fuel, the problem was compounding and overall Concorde was not economical. If someone can solve the takeoff problem (e.g. by using catapults) and climbing (maybe using detachable and reusable boosters?) then a Mach 5 jet could be very competitive. I think this is what the Boom guys had in mind when they say they aim to get to where anyone can travel to any destination on Earth in 4 hours for only $100 [1]. [1] http://edition.cnn.com/travel/article/boom-supersonic-four-hours-100-bucks/index.html http://edition.cnn.com/travel/article/boom-supersonic-four-h...