(C.37)

That’s a rather nice result! The gross transport cost of this idealized

airship depends only^{E} its speed *v* and length *L*, not on the density *ρ* of the

air, nor on the airship’s frontal area *A*.

This cartoon also applies without modification to submarines. The

gross transport cost (in kWh per ton-km) of an airship is just the same

as the gross transport cost of a submarine of identical length and speed.

The submarine will contain 1000 times more mass, since water is 1000

times denser than air; and it will cost 1000 times more to move it along.

The only difference between the two will be the advertising revenue.

So, let’s plug in some numbers. Let’s assume we desire to travel at a

speed of 80 km/h (so that crossing the Atlantic takes three days). In SI

units, that’s 22 m/s. Let’s assume an efficiency *ε* of 1/4. To get the best

possible transport cost, what is the longest blimp we can imagine? The

Hindenburg was 245 m long. If we say *L* = 400 m, we find the transport

cost is:

If useful cargo made up half of the vessel’s mass, the net transport cost

of this monster airship would be 0.06 kWh/t-km – similar to rail.

The ekranoplan, or water-skimming wingship, is a ground-effect aircraft:

an aircraft that flies very close to the surface of the water, obtaining its lift

not from hurling air down like a plane, nor from hurling water down like a

hydrofoil or speed boat, but by sitting on a cushion of compressed air sand-

wiched between its wings and the nearby surface. You can demonstrate

the ground effect by flicking a piece of card across a flat table. Maintaining

this air-cushion requires very little energy, so the ground-effect aircraft, in

energy terms, is a lot like a surface vehicle with no rolling resistance. Its

main energy expenditure is associated with air resistance. Remember that

for a plane at its optimal speed, half of its energy expenditure is associated

with air resistance, and half with throwing air down.

The Soviet Union developed the ekranoplan as a military transport ve-

hicle and missile launcher in the Khrushchev era. The Lun ekranoplan

could travel at 500 km/h, and the total thrust of its eight engines was

1000 kN, though this total was not required once the vessel had risen clear

of the water. Assuming the cruising thrust was one quarter of the maximum;

that the engines were 30% efficient; and that of its 400-ton weight,

100 tons were cargo, this vehicle had a net freight-transport cost of 2 kWh

per ton-km. I imagine that, if perfected for non-military freight transport,

the ekranoplan might have a freight-transport cost about half that of an

ordinary aeroplane.