Re: Fast Green Transport?

Assuming the main concern is anthropogenic greenhouse effetct there is
no problem: any old jet plain will do as long as the fuel has been
manufactured in sustainable manner - i.e. not using fossil fuels.

Not using fossil fuels is not the only condition that needs to be met.
In recent years we have seen an increased demand for biofuels, which
in turn has led to both food shortages (because farmers sell their
crops to biofuel companies) and soil exploitation. Maybe you have
noticed the price hike yourself in your local supermarket. So far just
a few % of the world fuel demand is covered by biofuels. This amount
cannot be much increased without triggering real major famines
throughout the less well-to-do regions of the world.
In short, bio-fuel powered jets would also only be able to operate on
a small-scale basis. So it's back to the drawing board.

Hydrogen of course would be wonderfully clean and green, expect it is
a bitch to store. One cubic metre of liquid hydrogen is just about
75kg, while a tank holding the cryogenic hydrogen will be a lot
heavier.
Kerosene has a pretty good energy density (higher than TNT actually),
so the big airliners have internal tanks large enough for >10.000km
nonstop flights. The same plane with hydrogen tanks would have huge
internal tanks, tiny payload, and laughable range.
If you want some cold hard numbers, the energy density of liquid
hydrogen by volume is just 27% of gasoline (kerosene will be similar).
Bigger tanks will be heavy and large. You'll need to use external
tanks or a bigger airframe, either increases drag, which means you
need more fuel to move it... even today with kerosene, about half of a
drop tank's extra fuel is eaten up by the drop tank's own drag and
mass.
You can see where this is going. A pretty vicious circle. I'm afraid
hydrogen is not a viable fuel for air travel.

I quite like the idea of laser-powered external propulsion. You build
an aerodynamic airframe with air-rams, plus relatively small water
tanks. Propellant is air and water in different ratios. The flight
profile will be almost sub-orbital: after take-off, a steep climb,
powered by the airport's ground-based lasers. The laser in turn can be
powered by anything; solar, geothermal, fusion, whatever you want for
your setting. The plane would ascend to about ~75km and gather enough
momentum to just glide across - literally! - half the globe.
If you wanted a truly suborbital flight, i.e. altitude >100km, your
descent will be so fast that you must have some way of decreasing your
descent rate. It is much easier to just ascend to about 70km, where
there is enough atmosphere left to provide aerodynamic lift at the
attained airspeeds, and then keep gliding and gliding. In the end you
will even have to aerobrake.
You can get virtually anywhere within about one hour.

We-e-e-ell... of course there is also some downside to this: the laser
will need cooling. Even at a beam efficiency of 60%, every 3MW of beam
power leaves 2MW of waste heat. And laser power will have to be on the
order of gigawatts. Maybe you can recycle some of the waste heat, i.e.
put it into a heat engine and generate fresh electricity. But in the
end you will always have to dump some. You can't just put it into the
nearest natural body of water or environmentalists will go nuts. Maybe
the best bet would be to direct it deep underground - I don't know.
.

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