Lifting Gases in a Venus Aerostat Habitat

Previously I went through how you could calculate the buoyancy or lift of an airship. Here we’ll have a short look at using different lift gases on a Venus aerostat habitat.

Depending on what we are trying to accomplish we may want to use different lift gasses or use combinations of them. For each lift gas on Venus I’ll compare with what lift an aerostat habitat (or airship) would achieve on earth using hydrogen as a lifting gas.

In all calculations I assume 20 degrees Celsius and 1 bar of gas pressure.

You can look at my Airship.jl github repository to get hold of the code for performing the buoyancy calculations in Julia. The Julia code I am using to calculate the plots look like this:

len = linspace(200, 800, 10)  # Lengths of airship where
height = len/3               

envelope_thickness = 0.0015  # in meters

a = height/2
c = len/2

function lift(liftgas, atmosphere)
   oblate_spheroid_lift.(a, c,
                         envelope_thickness,
                         MylarDensity,
                         liftgas,
                         atmosphere) / 1e6
end

earth_lift = lift(HydrogenMass, AirMass)
methane    = lift(MethaneMass,  CO2Mass)
ammonia    = lift(AmmoniaMass,  CO2Mass)
oxygen     = lift(O2Mass,       CO2Mass)
nitrogen   = lift(N2Mass,       CO2Mass)
air        = lift(AirMass,      CO2Mass)

Then I use this line to copy to the clipboard a CSV formatted string with the lift for different diameters on the disc shaped aerostat habitat.

export_table(hcat(len, earth_lift, methane, ammonia))

Breathable Air as Lifting Gas

Regular breathable air is a lifting gas on Venus. This is very handy for building a habitat, because it allows us to use all the interior space of the envelope for living space, with the limit that whatever we build isn’t too heavy.

You can see that hydrogen gives about twice the lift as breathable air gives in the denser CO₂ atmosphere of Venus.

Methane and Oxygen as Lifting Gases

Methane is an interesting lifting gas to use. It is marked in blue. As you can see it has slightly better lift than hydrogen on earth when used as a lifting gas on Venus.

So why Methane? It is great for energy storage. Methane can be used as rocket fuel, internal combustion engines or even fuel cells. You could of course use Hydrogen for the same thing but hydrogen is a very impractical fuel. It can’t easily be stored, because it leaks out easily form any storage container due to how small hydrogen molecules are. Methane being a much larger molecule can be store much longer without leaking, which is important from an aerostat habitat which is supposed to last indefinitely.

On earth Methane is in fact used as a lifting gas but considered too dangerous for balloons carrying humans. But on Venus that will be different because the whole surrounding atmosphere is basically one huge fire extinguisher. Should a fire start on the exterior of the envelope leaking methane out, it would die immediately because methane can’t burn in CO₂, it needs oxygen.

Of course we might need to store oxygen is some way, so we can actually burn the methane when needed. Oxygen is the green graph, so obviously it works as a lifting gas, but it might be dangerous to put pure oxygen inside a ballonnet or envelope. Any spark and the oxygen is ready to fuel the combustion of anything flammable. The safest might be to store oxygen in a tank, although that is impractical due to the low boiling point.

Using Nitrogen or Ammonia as Lifting Gas

3.5% of the Venus atmosphere is Nitrogen. Being lighter than CO₂, it can be used as a lifting gas. The benefits of Nitrogen is that it is inert. It doesn’t easily react with anything, which in practice means it is not hazardous to people. That is why Nitrogen is used in e.g. food packaging.

Ammonia (NH₃) is interestingly considerably lighter than Nitrogen gas (N₂), since hydrogen atoms weigh very little. While hazardous to humans it has a number of advantages. It turns into liquid at only -35 °C, while most other gasses of interest I have discussed here, need less than -180 °C to turn into a liquid. That means they are difficult to store in tanks. Ammonia is easy however. That is very practical for controlling the elevation of an airship. Ammonia can be sucked in and compressed when you want to descend and let out into the ballonets when you want to ascend.

Silane, SiH₄

Silane has similar lift to oxygen, but it has the advantage of being combustable in carbon dioxide. That means you save on the weight of oxygen.

That is significant because if you look at the chemical equation for combustion of methane, you’ll notice that we need two oxygen molecules to combust each methane molecule and these are each significantly heavier.

CH₄ + 2O₂ ⟶ CO₂ + 2H₂O

Silane is heavier but as you can see on the left side of the equation, there is nothing besides silane we need to store.

SiH₄ + 2CO₂ ⟶ SiO₂ + 2C + 2H₂O

Let us compare the compare the weight budgets. Lets calculate the total weight of the inputs in atomic mass and find the fraction of this mass which is oxygen.

CH₄ + 2O₂ = 16 + 2*32 = 80
2O₂ / (CH₄ + 2O₂) = 2*32/80 = 0.8

So a whooping 80% of the total mass of fuel and oxidizer would be made up of oxygen. Let’s look at how much weight the silane would be compared to the combination of oxygen and methane:

SiH₄ / (CH₄ + 2O₂) = 32/80 = 0.4

As you can see there is a 60% weight saving from utilizing silane over methane.

Combining Lifting Gases

The optimal might be to simply combine different lifting gases. An airship typically has multiple ballonets inside the envelope. These could contain different gasses. Some could contain ammonia to be able to manipulate the altitude easily, without having to drop ballast or vent gas like airships usually do.

Other ballonets could contain methane, oxygen or silane to generate energy.