Solar Airship Design Parameters

For the solar airship design, the principal guiding decisions should be efficiency.
On the criterion of absolute thermal efficiency, the first choice is to use solar thermal
technology rather than solar electricity, perhaps surprising to many.
But the typical efficiency of solar cells ranges around 5%,
while a design based on heat engine technology can aim for a throughput efficiency
closer to 30%. Six times the travel for your watt of sunlight.
 

The number one fact about airships: airships are big.
It helps to keep that one word, big, always in your mind when designing airships.
So our engineering models should not be drawn from transportation vehicles,
which achieve typical energy efficiencies of 12-15%, but from static prime movers,
from power plants, which boast a throughput efficiency closer to 40%.
This measures Carnot thermal efficiency, the kind that really counts.
It makes most sense to use a steam engine, for that reason.
A double-expansion recondensing steam turbine, to get technical.
 

We recondense the steam to gain thermal efficiency, and to simplify our mass balance.
Sure the engine would be lighter and less complex if you blew the steam out the back.
But you'd have to come down pretty quick to pump in more water for the boilers.
If you condense your steam, you use the same distilled water over and over.
The mass of your vehicle remains stable, not changing all the time, so it's just better.
Notice that nothing is being consumed, with a recondensing solar steam turbine.
No fuel gets burned, and no steam is lost, not much, anyway.
Your ship weighs the same at the end of the day that it did in the morning.
 

Obviously, for a solar steam engine you need solar concentrators.
I have a design available for spherical tracking solar concentrators.
They make steam and don't weigh much.
So if your main airship envelope is transparent plastic, you can string out a bunch
of these sunlight collecting balls inside the envelope, six or seven spheres
tracking the Sun to make steam.
Since the spheres track the Sun independently, the direction of travel of the airship
doesn't matter relative to the direction of the Sun at the time.
Whatever direction you travel, the spherical collectors will still track the Sun.
 

So what about propellors? Surely those are standard by now.
Everybody knows what a propellor is supposed to look like.
Well, I've got an idea on that.
One basic principle of airship engineering I haven't mentioned yet is lightweight design.
This carries implications for engineering; one way to decrease weight is to simplify.
The fewer moving parts you have, the less you have to go wrong.
Decreasing the number of moving parts is also just good engineering practice.
My idea is to combine the steam turbine, the rotary part, with the propellor.
So the turbine which is whirled around by the expanding steam,
and the propellor which whirls around to push air back, are combined in one unit.
 

A ducted fan design has an efficiency advantage over an exposed propellor,
because it eliminates the end losses of the air slipping over the propellor tips.
So I propose a centerless ducted propellor, driven from the rim.
Outside the propellor rim, not in the path of air flow, are the separate vanes
of the steam turbine, driven by the expansion of superheated steam.
Inside are the propellor blades, without a hub: the center is empty to ease air flow.
The turbine wheel and the centerless propellor are the same unit, outside and in.
It requires no lubrication, being held in place dynamically by the steam flow.
This is a type of fluid bearing, until now unused in engine design.
In this engine it is combined with labyrinth seals to minimize escape of steam.
 

The solar engine functions in this fashion.
The mirror globes are decoupled from the inner shroud of the envelope
by gas flow from the vertex; differental structured flow allows positioning.
Rotation and fine positioning control is provided by angled gas jets at the bottom.
These are a triple attachment to the gas exhaust port beneath the mirror globe.
The steam boiler tube (mirror weight is negligible) is counterbalanced
by use of thicker material on the clear hemisphere; yet the spherical shape
is not appreciably affected by the thickness differential, because
the spherical solar collector is of fixed size. Stretch is not used in this case.
 

The boiler tube is a half radius extending from the center of the mirror.
For most of its length it is surrounded by a concentric shield,
and that by a chiller tube, to maintain the concentrator's temperature.
At the end it bells out into a small boiler, the superheated water returning around
and preheating the water in the central feedwater tube.
A flex steam line of slightly less than one-fourth of the circumference
of the collector is required, for tracking the Sun in any quadrant.
 

The superheated water is ducted through Dewar insulation
to the first expansion mill, the first turbine, where it expands against the blades.
It expands the second time in a counter rotating turbine.
The second mill drives a centerless propellor meant to unwind
the vorticity induced by the first centerless propellor.
This counter rotation compensates for gyroscopic effects on handling
produced by a single handedness of rotation. It affects maneuverability.
 

The air drawn in through the front first cools the ship.
It is the primary heat sink for ship processes, and also waste engine heat.
It is compressed and spun one way by the first centerless ducted fan,
and compressed and spun back the other way by the second one.
It then leaves the ship much faster than it came in, a little warmer.
Air flowing through a tunnel is a very useful general design for an engine.
Boost may be provided at will, simply by steam injection,
diverted from the feed to the condenser.
The ship may travel twice as fast during this interval.
The loss of feedwater is deprecated, but sometimes you gotta hurry.
 

If you're really in a hurry,
you might wish to diffuse some fuel hydrogen in through a porous
catalytic section of the air column.
Combustion occurs on the surface when entering hydrogen is exposed
to the oxidizing air column at the catalyst doped interface.
The end result is that it makes steam, at a local overpressure.
The engine is now operating only partially from direct sunlight,
and partly from stored chemical energy, as a reaction engine.
This provides much more boost than steam injection.
Such an option adds complexity and is not required in the
most basic airship design.
 

What is required in the most basic airship design,
for West Florida certification,
is complete pressure control in flight of the lifting volumes.
An airship meeting our standards must have facilities for not only
compression of the lifting gases into tanks, but also for their liquifaction.
We expect working cryogenics on flying ships.
In general, the availability of compressed gases is not considered
aqequate margin to ensure safety of the ship.
Elsewhere, your safety precautions may be strictly up to you.
But if you're flying here, it's up to us whether we net you
and yank you out of the sky as a safety hazard, for flying without your chillers.
If you have abandoned your chillers and cryotanks to save weight,
you have left behind an essential safety consideration
integral to your ship's design.
What have you done with the weight you saved?
Was it worth putting those around you at risk?
 

Storing propulsive energy on an airship, for shaded flight,
may be done in a myriad of ways.
The most generally useful of these is electromechanical storage in a flywheel,
the Faraday gyro. These flywheels are tapered discs of copper chromium,
rotating sandwiched between coils of a superconductive magnet.
At center and rim, they float on liquid sodium potassium alloy.
The liquid metal bearings form electrical contacts.
The Faraday gyro is thus an integrated motor generator unit.
Electrical resistance heaters in the boiler plenum replace solar heating
to drive the centerless ducted propellors, when sunlight cannot be concentrated.
Valves then close off the flex steam lines leading to the unused solar boilers.
The steam formed by electrical heating then drives the rim of the ducted fans,
the turbines working the same way they do as with solar generated steam.
 

Other energy storage obviously includes the pressurization of gas,
which is not used for motive power, and the chemical energy of hydrogen.
Hydrogen can be used to produce motive power by combustion,
but generally this power is harnessed as electricity in the ship's fuel cells.
Power from the fuel cells can be routed to the resistance heaters,
in the steam boiler plenum, to supplement or replace the motive energy
stored in the rotation of the Faraday gyros.
This capability is generally not needed, but held in reserve,
for the fuel cells are usually harnessed to provide operational power
for the crew's quarters, and for the many tasks which involve pumping.
 

The favored material for fabricating the inner and outer envelope shrouds
and also transparent internal gasbags, such as the mirror globes,
is perfluourinated polyacetylene.
This material is adequately hard, tough, and resistant to rips
to provide reliable durability, at light film weights, for airship service.
It has some transparency in the infrared, to allow heat rejection
from the mirrors and reduce heat buildup in constant operation.
The film material is treated with a blocking layer, to retard UV damage.
Then it is given an antireflection coating to allow as much sunlight through
as can be expected from three or four layers of plastic.
This material does not show excessive stretch in normal use.