A vacuum airship, also known as a vacuum balloon, is a hypothetical airship that is evacuated rather than filled with a lighter than air gas such as hydrogen or helium. First proposed by Italian monk Francesco Lana de Terzi in 1670, the vacuum balloon would be the ultimate expression of displacement lift power. …
An airship operates on the principle of buoyancy where air is the fluid in contrast to a ship where water is the fluid. The density of air at standard temperature and pressure is 1.28 g/L and 1 L of displaced air has sufficient buoyant force to lift 1.28 g. Airships use an airbag to displace a large volume of air; the bag is usually filled with a lightweight gas such as helium. The total lift generated by an airship is equal to the weight of the air it displaces, regardless of the materials used in its construction or the gas used to fill the airbag; However for flight it is necessary for the total lift capacity to exceed the ship’s weight, which includes the weight of the gas used to fill the airbag
Vacuum airships would theoretically replace the helium gas with a near-vacuum environment and would theoretically be able to provide the full lift potential of displaced air. The main problem with the concept of vacuum airships however is that with a near-vacuum inside the airbag, the outside pressure would exert enormous forces on the airbag and causing it to collapse if not supported. Though it is possible to reinforce the airbag with an internal structure, it is theorized that any structure strong enough to withstand the forces would invariably weigh the vacuum airship down and exceed the total lift capacity of the airship, preventing flight …
via Vacuum airship – Wikipedia, the free encyclopedia.
I gave up on trying to make hydrogen non-flammable and instead have moved to the idea of building a hull that is super light and strong to support a vacuum inside it. If 1 L of displaced air can lift 1.28 g, my hull will need to contain 708,738 (call it 709,000) liters of vacuum in order to lift 1 ton (2,000 lbs). The place to start is small. Can you make something that fits in your hand that is strong enough not to collapse when all of the air is removed from it? It will need to withstand 14 lbs per square inch. That’s a lot.
What is the best shape?
Years of submarine design experience gives what seems to be the best answer: A sphere (or tubes with spheres on the end). Pressure hits from all sides so a sphere is the strongest structure against air pressure.
Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus respectively. This strength results from the covalent sp2 bonds formed between the individual carbon atoms. In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 gigapascals (GPa). (For illustration, this translates into the ability to endure tension of a weight equivalent to 6422 kg (14,158 lbs) on a cable with cross-section of 1 mm2.) Further studies, such as one conducted in 2008, revealed that individual CNT shells have strengths of up to ~100 GPa, which is in agreement with quantum/atomistic models. Since carbon nanotubes have a low density for a solid of 1.3 to 1.4 g/cm3, its specific strength of up to 48,000 kN·m·kg−1 is the best of known materials, compared to high-carbon steel’s 154 kN·m·kg−1.
Under excessive tensile strain, the tubes will undergo plastic deformation, which means the deformation is permanent. This deformation begins at strains of approximately 5% and can increase the maximum strain the tubes undergo before fracture by releasing strain energy.
Although the strength of individual CNT shells is extremely high, weak shear interactions between adjacent shells and tubes leads to significant reductions in the effective strength of multi-walled carbon nanotubes and carbon nanotube bundles down to only a few GPa’s. This limitation has been recently addressed by applying high-energy electron irradiation, which crosslinks inner shells and tubes, and effectively increases the strength of these materials to ~60 GPa for multi-walled carbon nanotubes and ~17 GPa for double-walled carbon nanotube bundles.
CNTs are not nearly as strong under compression. Because of their hollow structure and high aspect ratio, they tend to undergo buckling when placed under compressive, torsional, or bending stress.
Nanocomp Technologies Inc. of Concord, New Hampshire has managed to make the largest sheet of carbon nanotubing ever, rekindling the long-standing dream of a fantastical space elevator that lifts us into orbit along an ultra-light yet ultra-strong carbon nanotube cable. Sure, at 18 square feet, the sheet is smaller than a beach blanket but it contains a billion billion nanotubes, which makes it 200 times as strong as steel and 30 times less dense.
Moreover, it’s flame retardant and conducts electricity, which would make it useful in tiny electronic devices. Ironically, the problem with most carbon nanotubes is that they’re too small, or rather, too short—on the order of tens of microns long. Short nanotubes are difficult to incorporate into existing manufacturing processes and lack the high performance properties of long carbon nanotubes. They also tend to be delivered in powder form (think of graphite pencils). By contrast, Nanocomp’s tubes stretch a few millimeters and the sheets are specially treated to keep them from shedding black specks of carbon.
Sadly, the tubes tend to snap when molded into long tethers, as was the case last year with the MIT-Nanocomp team’s entry in the NASA “Centennial Challenges” space elevator games. The more probable application is for making lightweight composite coatings for airplanes, maybe even space planes. So not as cool as a space elevator, but we’ll take it.
How much does that sheet weigh?
It would be super light! CNT’s have a density of 1.4 g/cm3. Compare to titanium which is much heavier at 4.50 g/cm3 (0.163 lbs/in3). Interestingly, mylar (Polyethylene terephthalate) the material used to make balloons has about the same density, and therefore the same weight per amount of material as CNTs, that is, 1.4 g/cm3).
There are different kinds of strengths of materials. What we want is something that is both strong and rigid. Can CNTs be made into a rigid hollow sphere? Has anyone done this?
One paper I read used Liquified Petroleum Gas (LPG) to provide a cheap carbon source for large scale production of carbon nanotube arrays on a sphere surface. The spheres made were only 1100 to 1200 micro meters, however. I want something big big big!
Let me know if you find anyone who has done this. I want a hollow sealed rigid CNT ball with a vacuum inside! Anti-gravity, baby.
Or would the sphere collapse because CNT isn’t so strong in terms of compression?
Update: 4/11/2017, four years after I wrote the above, NASA is working on a vacuum airship, supposedly for use on mars.
Writing about the vacuum airship, NASA says, ‘This concept is similar to a standard balloon, whereas a balloon uses helium or hydrogen to displace air and provide lift, a vacuum airship uses a rigid structure to maintain a vacuum to displace air and provide lift.
‘Mars appears to have an atmosphere in which the operation of a vacuum airship would not only be possible, but beneficial over a conventional balloon or dirigible.
‘Through a more in-depth analysis of the vacuum airship model, it can be shown that the vacuum airship may theoretically carry more than twice as much payload as a modeled dirigible of the same size, a 40-meter radius, in the Martian atmosphere.’
Each of the 22 early stage projects will get $125,000 (£100,000) of funding over nine months.