boneh3ad said:
To be fair, I didn't do any calculations. I merely factored in that a 747 needs a 10,000 foot runway to operate...
Actually, I've seen one put down at
http://www.airnav.com/airport/KPOB", and take off again. Pope's primary runway is 7,501 feet long.
Actually, this isn't at all unusual, as Evergreen's (and other carriers') 747's landed there all the time, primarily as TRANSCOM-contracted air cargo transport for various units in the 18th Airborne Corps out of Ft. Bragg, NC (adjacent to Pope).
What was unusual about this particular flight is that it was the http://en.wikipedia.org/wiki/Shuttle_Carrier_Aircraft" , and with the Space Shuttle on top of it, no less! It sat for about two days on yellow ramp, then departed in the wee hours.
...all while using presumably more powerful engines and being a fraction of the size. Unless Boeing planned to come up with 4 turboprops that put out more thrust than four CF6's or four RB211's.
The SCA 747s both use 4× P&W JT9D-7J turbofans, 50,000 lbf (222 kN) each.
At this point in time, I am unaware of a turboprop that can put out that kind of power.
No doubt. Neither do I. It's certainly conceivable they could be designed and built, however, and in that slower flight regime (250 kts) a turboprop, even very large, multi-bladed ones, would be more efficient than a high-bypass turbofan.
There is a third reason: versatility. A conventional aircraft can fly to a lot more meaningful places than a GEV. Sure, 75% of the Earth is water, but 60% of the world's population doesn't live within 100 km of a coast.
You're correct, and GEV's are not the panacea of air transport, that's for sure. They're best for long, overwater hauls, although they've also proven their worth in island-hopping air taxi service, as well, as they are indeed cheaper to operate than traditional floatplanes, and can pull up to pretty much any dock (which are many). Airfields are less common. For that matter, they can also be used as diving platforms, and can simply be beached at remote locations.
It seems to me that GEV's are more of a supplement to the shipping lines than they are to aircraft, since they have about the same useful areas they cover.
I agree, as we're primarily talking about coastal ports. However, because Boeing's concept aircraft included flight up to 20,000 ft and conventional landing gear, it's likely the designers intended to fly it well inland, possibly over mountainous terrain (transition altitude in CONUS is 18,000 ft).