I know Boeing proposed Methane turbofans, and they were estimated to have 60% less carbon emissions and cheaper than jet fuel - after eating the large development and infrastructure costs. You could take this even further, producing Hydrogen from clean electric energy is about three times more expensive than methane for the same energy content, leading to a moderate increase in the cost of even a low cost ticket.

But as with all radically new technology, there needs to be a strong industry, government and consumer push. The program that produced the Methane turbofan concept (SUGAR Freeze) has lost any political appeal.

Costs aside, wouldn't replacing carbon emissions with methane be drastically worse? Doing a casual search says that methane is 2000-3000% more potent as a GHG.

I assume the methane would be the fuel, not the byproduct.

I imagine you won't combust 100% of the fuel. So wouldn't it be both.

Pure methane is a bunch of molecules with just one carbon, which simplifies things a bit vs. a cracked and heterogeneous mix of longer chain (10+) molecules.

You can incompletely combust a 10-carbon molecule and wind up with other hydrocarbons but if you burn up CH4, you're definitely just getting one CO2.

>The problem is, those trips can't be made by train.

Transcontinental flights (one side of the continent to the other, see [1]) can usually be taken by train. You probably mean intercontinental flights.

[1] https://en.wikipedia.org/wiki/Transcontinental_airspeed_reco...

You're being pedantic. One can travel from New York to San Francisco (well, Emeryville) by train but it's going to take multiple days and, even if you handwave the existence of high-speed rail through two continental divides among other things, it's a very long trip. That's never going to be a mainstream replacement for even a subsonic 5 to 6 hour flight between a multitude of east coast-west coast city pairs.

If there were high-speed trains that could make cross-US trips in, say, 15 hours, and at comparable or cheaper cost vs. airplanes, I can imagine there might be demand. I would be willing to take those overnight.

But even if we could replace most <500 mile flights along the most popular routes (starting by connecting cities up and down the coasts), it would be a huge win as far as fuel economy is concerned.

And you're being parochial. Who said "continent" was limited to North America?

The claim was that the trip can't be made, not that it can be made but is undesirable as compared with alternatives.

> The problem is, those trips can't be made by train.

They can't now, but they could be. All that is required is a level of capital investment that can't ever be recovered while in competition with air and sea-surface travel...

The transatlantic tunnel has been a sci-fi fantasy since 1895. It could cost as little as $500 billion today.

> Not as great as train, but better than driving solo in a gasoline car.

It was my understanding that driving is better for shorter trips, because the overhead of taking off and landing is significant. I don't have a source. IIRC I read this in "Doing Good Better."

Is it still competetive vs car for supersonic flight?