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Comment by philipwhiuk2 | Hacker News Reader

Alternatively, methane is easier to create from Carbon Dioxide. Burning methane in an engine is practical. https://climate.miami.edu/renewable-energy/methane-as-a-fuel...

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Methane doesn't seem very practical for an airplane, storing the pressurized gas in tanks seems heavy. Would you store liquid methane? Liquid methane and liquid oxygen are used as rocket propellants, not sure if that makes sense for an airplane compared to conventional liquid (at ambient temperature/pressure) fuels.

You're correct: methane would not work well as a replacement for conventional aviation fuel both for the reason you mention (weight of storage system) as well as the lower energy density per unit volume: 35 MJ/L for jet fuel vs. 9 MJ/L for compressed natural gas at 3600 PSI. (https://en.wikipedia.org/wiki/Energy_density)

I think LNG would be more likely candidate , and doesn’t kg matter more than L here?

kg matters a lot, of course, but most airplanes are out of space for additional tankage as currently designed. (They are designed for a given mount of fuel capacity volume, in relation to the mass of that fuel volume. None would have a 4x multiple.)

Scaling volumetric storage by a factor of four produces about 2.5x the wetted frontal area (4 ^ (2/3))

If I understand correctly LNG is 20MJ/L so not a 4x difference (but much colder, with all that implies...)

I would agree there’s a likely range reduction, but I don’t have any stats on current normal fueling as a percent of max capacity (iow: are there many routes that are running more than half full today?)

Liquid actually makes a lot of sense for airplanes. You can even use simple styrofoam insulation, as the consumption rates would still exceed the evaporation rates. The drawback is that if for some reason the plane can't fly but has fuel, it would have to be bled off to not build pressure.

One of the reasons liquid methane works well as a rocket propellant is that rockets need to carry their own oxidizer with them. Because methane is a smaller molecule, less oxygen is required to oxidize a given amount of methane.

With an airplane, which gets its oxidizer from the atmosphere, methane's lower oxidizer requirement is basically irrelevant.

The amount of oxidizer on a molar basis shouldn't depend on the size of the molecule, just the number of C and H atoms that need to be oxidized to carbon dioxide and water, respectively. Some molecules (ex. ethanol) also have some oxygen already in them which reduces oxidizer needed but also reduces the energy density of the fuel (its basically partially burned already).

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