Now, leaving aside the hypothetical supersonic jet planes, we still have this problem with the existing airplanes. If we want to decarbonize aviation, then how do we go about this?
On one hand, maybe 747 is an old plane and not that fuel optimized. I checked for 787-9, and it burns only 1.5 liters per second, which translates into 55 MW. It can only carry about 300 passengers vs 520 for a 747, so overall there is an improvement by about 33%. It's not nothing, but it's not that impressive either.
Anyway, with whatever fleet of commercial airplanes the world has now, some old, some new, we consume about 100 billion gallons of fuel per year. That's about 300 million tons.
Let's say we switch to hydrogen somehow. Which has more than double the energy density of jet fuel. We would need about 150 million tons H2 per year. According to the DOE [1], a 1GW nuclear power plant can produce about 150 kT of H2 per year, so we'd need 1000 of them. The whole world produces about 400 GW from nuclear reactors, we'd need to multiply the current fleet by a factor of 2.5, just to decarbonize air travel.
Yep. It's better than nothing, but we'd need to find orders of magnitude from somewhere. I don't think they're there to be found, personally: keeping big lumps of metal off the ground with aerodynamic lift runs into fundamental physical constraints relatively quickly (you can do the maths on glide slope and gravitational potential energy - just by rough order of magnitude it's mindboggling that it works at all).
And you're right, we do have this problem right now. Anything that wants to keep our travel patterns as they are today needs to confront the reality that its energy requirements make any other consideration a rounding error. From my point of view the big picture is fusion or bust, but that's, uh, a bit of a long shot.
The good news is that air transport emissions are only about 2% of the total, so there are much more productive lower-hanging fruit to tackle.
Thinking a bit more, there's nothing special about airplanes. The world consumes about 100 million barrels of oil per day. I guess most is transportation, but in the end it does not matter. There are about 7.5 barrels to a ton, so about 13 million tons daily, which is very nearly 5 GT per year. Each ton of oil produces about pi (3.14) tons of CO2 (of course pi is just a coincidence, but it's easy to remember). So about 15 GT of our worldwide 50 GT greenhouse gas emissions come from burning oil.
How are we going to replace that?
5 billion tons per year means about 150 tons per second. At about 40 megajoules/kg, that's 6000 GW. Six thousand large (1 GW) power plants.
You say it's fusion or bust. What if we had fusion today? Commercial, economical fusion. As easy to build as natural gas power plants. How are we going to build 6000 large such power plants?
It is sobering.
Now, leaving aside the hypothetical supersonic jet planes, we still have this problem with the existing airplanes. If we want to decarbonize aviation, then how do we go about this?
On one hand, maybe 747 is an old plane and not that fuel optimized. I checked for 787-9, and it burns only 1.5 liters per second, which translates into 55 MW. It can only carry about 300 passengers vs 520 for a 747, so overall there is an improvement by about 33%. It's not nothing, but it's not that impressive either.
Anyway, with whatever fleet of commercial airplanes the world has now, some old, some new, we consume about 100 billion gallons of fuel per year. That's about 300 million tons.
Let's say we switch to hydrogen somehow. Which has more than double the energy density of jet fuel. We would need about 150 million tons H2 per year. According to the DOE [1], a 1GW nuclear power plant can produce about 150 kT of H2 per year, so we'd need 1000 of them. The whole world produces about 400 GW from nuclear reactors, we'd need to multiply the current fleet by a factor of 2.5, just to decarbonize air travel.
[1] https://www.energy.gov/ne/articles/could-hydrogen-open-new-m...