As far as I can tell, the role of the 707 was transport for the average family and that role has already been taken by the 737 etc and airbus alternatives. What is specific about the 707 that other aircraft have not already been doing? With that in mind, what will be the next 737 class of aircraft be and what configuration?
The 707 made long range air travel
affordable by the average family, instead of the particularly rich.
And because it's used by the average family instead of the rich, it means that airlines are diving for the cheapest aircraft to operate per seat-mile.2 engines versus 4 to have half the maintenance costs, all the different winglets to save a few more % in drag/fuel. This does give the fancy new truss-braced transsonic wing a chance, as the much thinner wings should be more efficient. Even if I hate how it looks, and am highly suspicious that the UDF or whatever they're calling it this go-around will be unacceptable to the airline execs unless it's overwhelmingly more efficient than a big turbofan... "it's just a propeller, and that's old technology!"
I do see synthetic fuel being used, the trick is to find a process to get it from air and water without too much exotic chemistry, so that you can pull CO2 from the air (and a lot of it) then chain those carbons together with either OH or simple H at the ends. 100% carbon neutral there, then the question is how carbon neutral is your power source.
Blended wing-bodies have three significant challenges to their adoption by airlines.
- One is the evacuation standards. Need to get all pax off the plane in 90 seconds, which may be doable with some fancy work.
- The second is passenger comfort. Even the wide-bodies have their passengers less than 10 feet from the center of rotation for the aircraft. But the BWBs have their passengers a lot farther out from the center, which means that they're going to get moved a lot more as the plane banks or gets thrown around in turbulence.
- The last one is constructing the pressurized volume. A cylinder is very easy to pressurize. A rugby ball less so, and a flattened rugby ball even less so. It would take heavier and stronger ribs to make the passenger section sustain even a mild 10psi differential.
That said, I do see BWBs getting used in air freight, where the only pressurized area is up front for the cockpit.
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High speed rail isn't particularly viable in the US, mostly just due to sheer size of the country. It may be viable to run a high-speed line up the west coast and east coast, Florida to Texas, and from NYC to Chicago or so. But that's about it. And even then it would be a multi-
trillion-dollar project for the shortest lines, since many would have to be either elevated above the road grades or put into tunnels beneath the streets. Because Americans are morons and step in front of trains all the time.
But you're not going to find it easy or even quick to make a "high-speed" rail run from California to Texas, or Los Angeles-Salt Lake City-Denver-points east. Too many mountains, and way too tall. Roughly 200 miles from LA is Donner Pass. It's at 7500ft elevation, which would require a continuous 1% climb from LA to the pass. And then it drops down to 4500ft at Reno, less than 50 miles later, a continuous 1% drop. Reno to Winnemucca (4300ft) to Elko (5100ft), and then you're climbing up and over another mountain range to Salt Lake City. Then you get to do that a
third time to get to Denver, going north into Wyoming at 7500ft and down the face of the Rockies from Cheyenne to Denver. No train is going to do more than 80kph on that kind of ground, even assuming very straight rail cuts that tunnel through mountains instead of twist around them. No way, no how. San Diego to Dallas has the same kind of mountains in the way.
So it's roughly 3 days to Denver, and then 1 day from Denver to the east end of Tennessee, where you get to climb up another set of mountains before dropping down to the coast. That may or may not slow you down as much, the Appalachian mountains are nowhere near the monsters that the Rockies are in the west.
The US is a damn big country. It's farther from New York City to LA than it is from London to Moscow. In fact, it's almost twice as far (1550 miles from London to Moscow, 2550 miles from NYC to LA). It's 2800 miles from NYC to Seattle.