DISCUSSION ON AIRSHIPS
Upon looking at today’s Airship proposals, one would assume that the world had stopped turning over the last 100 years. Over-complex, fragile, rigid architectured ships that belong to the early last century seem to be the only solution retained in our modern days when so much has been accomplished in hi-tech material developments in that same span of time!
Polymer Fabrics and Films have exceed performance boundaries in mechanical, densities, longevity and gas permeability that would allow the design of new and novel Semi-Rigid airships and perhaps even Non-Rigid ones previously deemed to be impossible to build.
Non-Rigid airships are after all, the simplest, lightest and most affordable lifting dirigeable vessels that can be built. They contain no rigid structures and no extra aerodynamic fabric skins that would both require additional buoyant volume to lift. They are the epitome of efficiency as Airships are themselves!
But, some will say that they are less safe (which I believe to be totally wrong) and limited to smaller ship sizes (which I would certainly love to challenge with fresh data’s on new modern fabrics and films). This last assertion is based on two basic principles which have legitimate basis but ones that can be challenged as well with new existing technologies… not tomorrow’s dreams!
A Non-Rigid airship, or Blimp as they are affectionately called, are basically built of a continuous fabric “football-shaped” outer shell filled with a buoyant gas. If we are to suspend a notable payload at its bottom center, this elongated balloon then needs to act as a beam so as not to pinch-up like a vertical “pogo*” if it’s membrane is left to distort elastically or worst… perform a banana-split figure in most cases!
Is anyone getting hungry here!
So, how does this spineless hovering creature performs this physical prowess ? … with internal pressure my dears!
As one of our distinguished colleague would say: “gas IS the structure!”. I would’nt go as far but I prefer to state that the interaction of the continuous outer skin WITH the internal pressure exerted upon it from the internal gas makes it a pretty impressive bending resisting structure.
Even though a Blimp must react like a beam with its furthest chord in compression and its nearest one in tension, all of its surfaces are actually in tension. Only, some are more than others!
And that’s precisely the argument that it’s detractors will jump-to when hammering out the inherent limit of the Non-Rigid airship. A larger Blimp cannot extend indefinitely and must always keep a somewhat proportional parabolic profile to maintain its stiffness around the transverse axis (y).
So, for very large payloads (>> 60 Metric Tons), such super-Blimp would have to grow its midddle diameter to humongous figures to maintain its longitudinal bending strength and that would hamper its delicate state of pressure vs tension equilibrium. That would no doubt hamper as well its aerodynamic drag. I personally believe that drag is not such a critical issue for these hovering monsters (or moving mountains) and that speed should instead be reduced to reasonable goals.
Those same detractors will bring as well the material limits in Hoop mode when facing such large diameters.
You see… Hoop stress is proportional to diameter and pressure delta. That argument is nevertheless baseless since the pressure delta between the internal gas and the atmospheric pressure at the top of the ship is always way below 1 psi and Hoop stress is always orders of magnitude lower than the performances offered by modern fabrics and films.
Also, it is quite possible to design multi-lobes Non-Ridid architectured ships as demonstrated by the two brands of Hybrid Airships currently flying.
The second argument used is that such high volume Non-Rigid Airships cannot effectively accommodate the required air pumps necessary to fill the Ballonets at the required flow rate to allow emergency descents. You see, a Non-Rigid Airship contains enclosed pressurized lift cells (bags) that need to adjust their internal volumes to the ever-changing outside atmospheric pressure. Because the gas cell(s) perform the duty of the aerodynamic skins as well, you understand that this outer cover needs to be made constantly taut. Obviously, atmospheric pressure is related to flight altitude and the Ballonet normally have to fill-up to a volume equivalent to around 15% of the total gas cell. That translates to a cruise altitude of between 3000 to 4000 ft.
As you can see, a common Airship doesn’t fly very high!
You can think of a Ballonet has a lung constantly “breathing-in” and “puffing-out” outside air as the ship climbs and dives.
This argument is also limited in scope and assumes that the only way to increase the volume of an open bag is to blow air inside. As we will show in the following pages, there are much more energy efficient ways to expend such 3D volume.
A Rigid-AirShip is otherwise referred to as an equilibrium buoyant dirigible ship because its enclosed gas cells are partially filled at ground level and expand as the ship climbs to its flight altitude and contract again as it descends back to its base. Now, because those gas cells cannot be held taut for a sizeable part of their flight time, a Rigid-Airship needs a separate aerodynamic outer skin held by an additional structure. That means we need more gas volume to lift those two new elements. This is just like tax!
But wait a second… since these gas cells have their internal pressure constantly fluctuating, they cannot either provide bending stiffness for the entire ship so more structure need to be added.
Heck! This is like filling a bottomless bucket !
More gas volume again!
Some old chap named Zeppelin who happened to be a count (let’s call him “Count Z” from now on) came up with a pretty good idea to make the structure required to hold the outer skin part of the overall ship structure… or basically to mount all the rigid stuff outside of those cells. This way you don’t cannibalize the internal volume optimized for gas cells. It’s not a bad idea in itself but that is called an “exo-structure” because it’s built on the outer periphery of the object.
Sadly, an exo-structure grows in area at around 3X the square of the size of the object growth. As the Airship’s payload requirement increases, its volume needs to grow proportionally, but thankfully at the cube of its size.
Basically, all that means is that the outer periphery is definitely the worst place to put hard (and relatively heavy) structures if we want to save weight for gigantic objects.
If we look at nature, we can quickly see that exo-skeletons have thrived in the world of the“incredibly small” such as for insects for exemple because weight is not an issue as such scale. If we look at the world of the “incredibly-large” such as sea mammals, their adoption are non-existant. Long gone dinosaurs and contemporary whales have central spines and they are tall and strong to support such masses… even if buoyant.
In a different angle, Think of a rigid airship as a gigantic egg with a shell infinitely thin as the ship’s diameter increases. That’s a very fragile object… and so were classic Rigid Airships.
What’s even worst is that such shell, to be weight efficient, needs to be made of mostly voids by using an intricate lattice of extremely light beams all interconnected to carry all loads. That’s a very, very non-economic fabrication process!
So, here we are in the 21st century with new polymers and fibre reinforced fabrics that enables us to push the boundaries of light weight flexible structures into space-age applications. Most specialized manufacturing workers in the aeronautical industry are paid on average annual salaries in the fifties and sixty thousands. And we want to develop an industry of giant cargo ships with payloads in the realms of the 100s of metric tons. How do we do this? Seriously!
AirShip's traditional architectures
Below is the conventional classification of AirShip architectures.
Non-Rigid AirShip beam equilibrium
A Blimp is a masterpiece of pressure and tensors equilibrium. It is not as "fragile" as many would like us to think but it must nevertheless maintain this equilibrium to operate as a beam.
Ballonets... giant placeholders!
An AirShip's buoyancy in a given atmosphere is related to its volume and not to its internal pressure. A Ballonet is therefore a device to modify the internal volume of a pressurized gas cell. What we want to achieve in the end is to ensure the gas cell is constantly taut as the AirShip travels between lower atmospheric pressures at altitude and higher ones at sea levels.
Rigid AirShips... early 20th Century marvel
To give you an idea of the complexity of those flying cathedrals... just have a look below at the construction of the Hindenburg! If you look closely, you will see the thousands of kms of lines holding the ship together.
Did you say: "Ultra-light exo-skeleton structures..."
Get a grasp of how delicate the exo-skeleton of those giant AirShips have to be to achieve interesting payload to lift ratio. This picture is from either the USS Akron or its sistership, the USS Macon.