Extremely huge dirigible/airplane hybrid feasibility

Question

Hybrid dirigible airplane with an extremely large surface area on the top and bottom. Talking at least 100 km2. It would likely be several modular pieces put together, given its size. It would need to stay aloft for months on end. Stay aloft above most of the weather, ie 40,000 to 50,000 feet, for more access to solar power. Some ability to maneuver with propellers or other means, though I do not expect much considering it's massive size. Maybe 10 mph or so laterally or in addition to the winds direction. Ability to station keep, or at least slow significantly down. See issues with scenario below for specific questions about feasibility, reasonably close to hard science please.

Purposes for this behemoth:

• To block out the sun in the east-central pacific to disrupt El Nino. Including sea water drawn up for increased cloud formation.

• To block out the sun near wildfires, droughts, lethal record temperature areas. In order reduce the temperature and potential create rain.

• To collect solar power for propulsion, ship functions and for inhabited areas below for use.

  • Via tether or microwave.
  • Not intended for large habitation, space for the crew, research and a hotel for the great views of Earth and the night sky.
  • Possible transportation of goods and people but not main purpose.
  • Launch platform for rockets to orbit.
  • Test bed for tether system for future potential space elevator/tether.

Issues with scenario:

• Is there enough lifting gas, Hydrogen, Helium, even pure Nitrogen? the hybrid design should help with lift significantly.

• How much temperature drop can be expected with a couple days of shading with this size structure?

• Is it maneuverable? Can it station keep (or slow down) at all? Can it stay aloft for months? Can it be powered by solar power alone?
• Is a tether feasible from this altitude with current technology?

Answer 1

This question has several facets to it, I'm going to try to answer as many as I can, edits to follow.

Lifting Gas Availability

Helium is a limited resource, although a lot of it is currently produced as a side effect of natural gas extraction. 2008 production was ~169 million standard cubic meters (SCM) of helium. This would not even be close to enough for your mega-structure. 100km x 100km x 100m = 1x10^12 cubic meters or 6000 times the yearly production. (I assumed 100m thickness it could easily be bigger depending on the density of your craft)

Hydrogen however is not very limiting at all, as it is abundantly available in water, requiring only electricity to extract it. So despite the flammability issues and diffusion problem, you are going to have to use hydrogen. On the plus side it is the most effective lifting gas available.

You could also use hot air, which simply requires heat.

Temperature Drop

So I'm going to do a bunch of big approximations to get a back of the envelope calculation for this using a quick and dirty energy balance model.

So the Sun provides ~1000 W per square meter, but only when it's shining and there is a lot of variance over day, night, seasonally, and with latitude. Average all of that variability out over a year and most places on Earth's surface will see an average of around 250 W/m^2. (Using this lower number will underestimate cooling rate during the day)

Some relevant properties of Air:
Specific Heat: ~1.0 kJ/kg K
Density:~1.0 kg/m^3
(varies a lot with altitude and temperature but a rough average for a quick calculation)

So if we remove 250 W/m^2 from a column of air 15 km tall we can figure a rough temperature drop.

Mass of the column of air = 15km * 10km * 10km * 1kg/m^3 = 1.5 x 10^12 kg of air

Rate of energy removal = 250 W/m^2 * 10km * 10km = 2.5 x 10^10 W

E = mcT to power P = mcT/s

solve for T/s = P/(m*c) = .0000167°K/s or .001°C/minute or 1.44°C/day

To get the final coldest temperature you would need to factor in convective heat transfer involving the mixing with warm from air outside of the shade zone (this cooling would cause winds to develop. Anti-cyclone!) as well as radiative heat transfer and heat from geothermal sources, and water motion for oceans or large bodies of water, in general it would be a really complex set of calculations and very specific to the location and timing of the shade placement.

But to simplify it you are essentially creating a stable stationary cold front, which can have temperature drops of up to 30 °C (54 °F) so this would be a likely upper bound for temperature drop from ambient levels in the region under the shade.

Balloon Tethers

Balloon tethers are definitely possible and in common usage at lower altitudes.

Barrage Balloons were used to raise nets of metal cables into the air to impede enemy aircraft. They were raised to ~4,500 m (15,000 ft.), modern tethered balloons are used primarily for military surveillance and reconnaissance, as well as communications. Some current military systems operate at altitudes of 15,000 ft. with 25,000 ft. long tethers.

Longer tethers and higher altitudes are likely possible with existing materials and even better for possible near future materials. For tether material strength over those distances the important factor is known as Specific strength or breaking length, how long a tether can be to support its own weight. Modern existing materials can be quite long Kevlar and Carbon fiber have a breaking length of ~250 km (for reference 50,000 ft is ~15 km).

The bigger issue for tether strength would be if it was used to resist the force of the wind on the balloon structure, but if it is a powered structure capable of some maneuverability, or the tether is not secured to the ground these forces could be mostly negated.

Flight Characteristics

Most of these issues would be an engineering design problem, but not impossible, blimps and zeppelins have been shown to function so it definitely can be done, it just needs to be scaled up a lot.

It won't be fast, will need to be aerodynamically streamlined to resist winds, and will likely not be very maneuverable. Your best bet on travel would be to find favorable winds, by changing altitude, and going with the wind.

As for solar power, the Hindenburg used 4x Daimler-Benz DB 602 (LOF-6) diesel engines each 890 kW (1,200 hp), so total power of 3500 kW.

The Hindenburg had a diameter of 41m x 245m long; so using only the top half of the surface for solar cells (π * D * L)/2 provides roughly 16000 m of surface. Using solar cells (20% efficient) to provide 200 W/m then provides 3200kW.

So comparable instantaneous power, but it would likely be underpowered if requiring continuous thrust for any length of time. And it would be worse when considering storage losses required for night running. As for energy storage hydrogen fuel cells would seem like the obvious choice over batteries, given the likely use of a hydrogen lift gas.

Answer 2

Feasible enough that if I ever become a multi-billionaire I will build one.

Your station will need:

• Airbags that don't catch fire
• Rain
• Lots of solar panels

Your station alternates between above the weather for most of the time, and ducking in to rainstorms to collect water to electrolyze into hydrogen (for lift). The oxygen is vented as far away from the station as possible. You don't want liquid oxygen anywhere near your hydrogen gasbags. I slightly doubt the ability for solar to provide enough production rate to replace hydrogen lost through the gasbags, so you may need a nuclear reactor of some kind.

Over vigorus hairbrushing is a crime. DC brushed motors are illegal, and many other minor rules have to be enforced to prevent fire.

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Because it isn't hanging beneath one big gasbag (I envisage lots of small ones), each individual part of your station is effectively zero weight. This is why blimps can be bigger than aircraft. An aircrafts wings have to support it's fuselage, but most of an airships structure is self supporting. As a result, you could probably come up with some sort of farm/ecosystem on board, to make a truly self sustaining system.

However, I doubt that a space elevator would be any more practical if anchored(?) to a flying platform. Space (near vacuum) is only 100km up. Geostationary is 40,000km, so you still need thousands of kilometres of cable (and cable length is the important factor in space elvators).

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To block out the sun, the platform needs to appear sun-sized. My thumb (1cm) can block out the sun at arms length (100cm). Thus the rule of thumb is that an object that is sun-sized will be 100 times further away than it is large. (For proof, the sun is 1.4 million km diameter at 150 million km). As a result at your operational altitude of 10km, your platform will need to be a 100m disc. That's not unfeasible as the Hindenburg was 245 meters long. If you bolted three hindenburgs together (they are only 40m wide), you could block out the sun from 10km altitude.

How much effect will that have on the temperature? Unfortunately, no-one seems to have studies on 100m shadecloths at high altidude. However, this study found that the shade of a tree tree dropped the air temperature by 0.6 - 2.5 degrees Celsius at mid-day, and the ground temperature by a massive 3.3 - 8.1 degrees. This implies that a 100m area of shade may well have a larger affect than I anticipated in earlier versions of this answer. However, to keep the spot of shade in the same place, your platform will have to move very rapidly to counter Earth's rotation.

Your question states you want 100kmx100km. How big will that appear if it is at 40,000ft? It's only 10km up, so it's going to appear freaking massive. The sun appears with a 0.5 degree angular width. Your platform will have a ... 160 degree angular width. In other words, if you are directly under it, there will be a 10 degree ring of sun on the horizon. I could well accept that that will have significant affects on climate, considering that it's larger than most mountains.....

Or if you mean 100 square kilometers (ie 10kmx10km) then your station is 'only' 60 degrees wide. Still pretty significant.

Using it as a launch launch platform for rockets to orbit is not unfeasible either. There is a company doing airship to orbit, and they plan to have a station at 140,000 ft altitude. I wouldn't want rockets anywhere near my hydrogen gasbags, but I can't see why you couldn't have reasonably large areas without gasbags to permit this sort of thing.

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Your station will not have a very high speed. It will probably travel similar to balloons: find an altitude where the wind is blowing the direction you want to go.

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Side note: The upcoming Mortal Engines movie is likely to feature airhaven - a flying city using airship-type technology.

Answer 3

I think a blimp of that size could be done, but I do not see any way to have it stand against upper atmosphere winds.

This behemoth should be relatively flat and, if round, it would have a diameter exceeding 11km; giving it a (very conservative) height of mere 100m you end up with a cross-section exceeding 1km2. To remain static against winds would require enormous power.

OTOH, given the enormous dimensions and fact you don't really need a rigid structure I think it could be done without special gasses, an old-fashioned hot-air balloon could suffice, probably with just solar power; During the night it would loose a bit, but not enough to really drop and next morning sunshine should be in position to restore lost heat (this in the hypothesis we are able to take advantage of the square/cube law and thus have a huge mass of hot air which can lose heat through a (relatively) small surface (i.e.: do not make your balloon too flat) which is also what weighs down (i.e.: the structure is relatively lighter than equivalent smaller installations, so you need air "less hot").

I also am not real sure about effectiveness of such a beast as sun-shield because I think it is too small (by orders of magnitude) to have a real effect on climate, but I might be wrong.

Answer 4

Monster LTA designs have been postulated, most famously by Buckminster Fuller and his "Cloud 9" flying city. This is actually a rigid airship built from a giant geodesic dome, and utilizing the squad/cube law.

A half mile (0.8 kilometer) diameter geodesic sphere would weigh only one-thousandth of the weight of the air inside of it. If the internal air were heated by either solar energy or even just the average human activity inside, it would only take a 1 degree shift in Fahrenheit over the external temperature to make the sphere float. Since the internal air would get denser when it cooled, Bucky imagined using polyethylene curtains to slow the rate that air entered the sphere.

Cloud 9 in flight

While a sphere is the most efficient shape for a Cloud 9, there is no reason that an oval, disc or other shaped object would not work, so long as the weight of the fabric and structure are always increasing at a much lower rate than the mass of entrapped air as the structure grows in size.