Airship Overview

Aerostat engineering has at least two glaring, conspicuous gaps.
There evidently has never been any attempt to design and build a high-altitude airship, and there evidently has never been any attempt to design and build a high-speed airship.
For the most efficient means of transportation on the planet, that's a shame.
It almost seems to indicate some spell in operation, turning the minds of engineers away from clearly productive paths.
Less imaginatively, conspiracy theories could undoubtedly prove the polluting influence of economic and political interests to deflect efforts to develop a technology so obviously in conflict with the aircraft industry.
Whatever the history of the matter, it's time to catch up.

The efficiency advantage of the aerostat vehicle over the airplane is the elimination of that fraction of the propulsion energy used to generate lift.
That fraction is quite large.
Less obviously, the aerostat vehicle also eliminates the need for that fraction of the propulsion energy used to haul those great huge wings through the air.
The drag produced by the wings of the airplane is eliminated.
At the very outset, then, the airship starts out with at least twice the thermal and mechanical efficiency of the corresponding airplane.
Engineering intuition points strongly toward looking more deeply into this matter.

The equations of hydrodynamic drag strongly support this intuitive response.
All right, so an airship must be big.
But in the first order of calculations, the amount of drag is proportional not to the volume of the vehicle, but to its cross-sectional area.
This says that making an airship long and skinny will reduce drag.
That answer is acceptable, and has determined the traditional cigar shape of the airship.

A point well worthy of stressing, is that the airship basic design is scalable, although the airplane basic design is not scalable.
Airplanes have critical design components, the airfoil components, which must be redesigned from scratch for any appreciable change in size.
If you make an airship bigger, it will work in ways which are predictable by linear extrapolation from known behavior.
If you make an airplane bigger, it will break apart and crash.

When economies of scale are considered, it is quite clear that our society has committed a massive blunder by not arranging for shipping by aerostats.
And, with the engineering imagination unfettered on this topic, it very shortly becomes clear that entirely new effects could be produced by very large aerostats, possibilities that were not within our power to do in any other way.
Moving large buildings without disassembly springs immediately to mind.

We had established that wind drag varies with frontal area.
The effect of this drag can be diminished, however, if much of that frontal area comprises a scoop for the input of air into a reaction engine.
In this case the air in front of the vehicle is no longer trapped into stagnation, that is no longer compressed against the front of the vehicle.
It is instead in motion.
lowering the pressure, resulting in decreased drag in the same frontal area.

A turbojet engine is not the automatic selection for high-performance airships.
It has some problems: it is heavy, made of metal, requires machining to high tolerances, needs frequent maintenance, and blows itself apart when a bird hits it.
Its efficiency (Brayton cycle) is not great.
It will do when better alternatives are not available.
The ramjet is the preferable alternative, and has the advantage that it's a lot quicker.
It doesn't have the altitude ceiling of a turbojet, it can go a lot higher.

The ramjet has some drawbacks.
Its thermal efficiency isn't that great either, in its simplest form.
It needs a starter motor to get the vehicle up to speed.
A ramjet-powered airship would not have full power available when drifting, nor when maneuvering at low altitude.
The tradeoff is favorable when ramjet advantages are accounted.
It can be built of ceramic, such as silicon carbide, or of silicon, saving weight while improving strength and thermal resistance.
It doesn't need many moving parts, and especially doesn't need a lot of headaches like precision tolerances and dynamic balancing.
Older ramjet designs show massive flameholder structures to prevent flame blowout, but these should not be needed with advanced techniques of forcing combustion such as electric arc ignition or focused laser ignition.
The ramjet has one overriding characteristic: it is quick.

The starter motor for the airship, to get up to ramjet speed, or for fine maneuvering in close quarters, should be a rocket engine.
It is quick.
It can be throttled to very low thrust levels for careful motion, with very high thrust levels on call immediately.
It does not intrude the dynamic complexity of gyroscopic stabilization, a gratuity of propellors.
It can use the fuel-delivery components of the ramjets, with favorable design.
The rocket engine has less weight than the piston-driven or turbine engine which would be needed to drive a propellor.
Most importantly, it doesn't have a propellor, thus no concern about how to get the propellor out of the way for high-speed travel.

Combining aerostatic lift with the ramjet engine may form the mainstream of aerospace development in the twenty-first century.
All performance records for aircraft are well within reach of such a design.
In fact, speed and altitude records may be routinely surpassed in test flights, and endurance records should also be fair game.
One militarily significant feature of such vehicles, featured prominently in the novel, may contribute more than any other single factor to making the combat jet airplane obsolete.
This is the design which places a deep anechoic structure inside an outer envelope transparent to radio waves, such that no radar reflection is returned.
The appearance of the anechoic chamber is familiar from the acoustics lab, with closely spaced cones or pyramids forming pockets to trap the impingent waves.
The same principle applies to radio waves, when the conductivity is graded to be highest at the bottom of the pockets.
At whatever frequency, all the energy of the incoming wave is converted to heat, and none is returned.
This makes the vehicle perfectly black to active radar imaging, a condition which cannot be achieved by any airplane.


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