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Comment by emilecantin | Hacker News Reader

Starship is methane-powered. There are ways to generate methane sustainably (from C02 and water); that's even an essential part of the plan for Mars. I strongly suspect they'll want to test it on Earth before, and having an appliance on the platform that generates the needed propellant for passenger flight would be a very good test (developing something that works reliably with minimal / no human intervention).

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> Starship is methane-powered. There are ways to generate methane sustainably (from C02 and water);

It's not about the generation. Burning methane results in a 1:1 emission of CO2 [0]; literally the equivalent of just dumping pure carbon dioxide into the atmosphere.

https://www.quora.com/What-are-the-products-of-methane-burne...

In the process that would likely be used the CO2 would be sourced from the atmosphere.

https://en.wikipedia.org/wiki/Sabatier_reaction

Ya, but that isn't happening. Look at where liquid hydrogen for rockets is sourced. It could be made from water. In reality, it is made from hydrocarbons because that is cheaper. And even if CO2 is pulled from the air, a huge amount of energy is need to compress/store that rocket fuel, energy that could be put to far better uses. Instead of launching a passenger rocket to london, I'd rather divert that energy into shutting a coal-fired powerplant for a week or two. Once all those powerplants are shut, then we should look at passenger rocket services.

I don't doubt you're right for most existing rockets, but I think the main difference here is that SpaceX has every reason to test the hell out of their propellant-producing appliances here on Earth, even if it's not economical at the beginning. They depend on these appliances working perfectly on Mars for the return trip, so they need to be _very_ reliable.

If these vehicles were from anyone besides a company whose explicit goal is to develop an interplanetary colony ship whose duty cycle will require the launch infrastructure on the other planet to convert atmospheric CO2 into methane using solar power, I’d agree — it hasn’t been done, it’s not economical to do it that way yet.

But Musk needs this process to become economical so that Mars can become economical.

(The same tech may, ironically given Tesla, compete with batteries for the energy storage PV calls for).

If that CO2 was taken from the air in the first place, it's a neutral process overall:

Take CO2 from air => Turn it into methane => Put it into rocket => Burn it back into the air

This process requires far more energy than just using it to power normal modes of transportation. There's no free lunch.

Doesn't matter if it is solar.

That doesn't matter if you made the methane by extracting CO2 from the atmosphere. Then you are just putting the CO2 back from where you got it.

The bad part with burning fossil fuels is that you are putting carbon in the form of CO2 into the air which was previously locked under ground in the form of coal/gas/oil. If you are burning carbon that you extracted from CO2 in the atmosphere, there is no such problem, assuming you didn't use fossil fuels to extract that carbon.

>That doesn't matter if you made the methane by extracting CO2 from the atmosphere. Then you are just putting the CO2 back from where you got it.

It's not that simple. What you're describing is free energy. We're still burning fossil fuels to extract the methane. Maybe that could be powered by renewables one day. But as of now, it is not.

The point is you could do it with renewables without burning fossil fuels. There would be losses in the step between renewables and methane sure, but it doesn't require "free energy".

Indeed, it's more like molecular assembly & disassembly powered by renewables. Here's a good explanation for a well-tested ethanol process:

https://www.sciencemag.org/news/2019/07/former-playwright-ai...

The fuels in the rocket. The fuels in the support vehicles. The fuels for the platform. The fuels in the helicopters. Even if all of them are produced from solar power, a 747 needs less than 1% of the same total fuel per passenger. No matter how it is fueled, a ballistic rocket needs orders of magnitude more energy than conventional flight. It is not an energy-efficient means of realworld transport.

Then price in carbon neutral fuel production to the ticket. There will be passengers rich enough to pay it.

If you've got a problem with that I imagine your real issues are about status envy not environmentalism.

Lol, I'm no communist but I do appreciate that market forces alone will never address the carbon/energy problem, let alone the wider problem of pollution overall. Just because someone has the money do to so doesn't mean that we should support them burning away the energy to light a small city simply to get to their vacation spot a few hours faster than on a private jet.

How about reaching a dying father a few hours faster? A middle class person might judge the extra $5-10k (from savings) worth it.

If SpaceX/energy company deploys a new nuclear power plant (and one day fusion) to provide the energy, why should you care? It's a new energy source and not competing away your personal idea of worthwhile energy use.

Not sure where you're getting these figures. Here's an analysis which estimates CO2 emissions from Starship for E2E will be roughly similar to those from a 777 flight based on fuel consumption. Depending upon how the methane is generated it could be a carbon-neutral flight. This doesn't include support vehicles, but that would be quite speculative at this stage.

https://www.nextbigfuture.com/2019/12/spacex-rocket-emission...

Look deeper at those numbers. Net fuel consumption/emissions is different than "rocket emissions". Rocket fuel isn't diesel. The emissions from the actual rocket in flight do not capture the total emissions generated in the creation/preparation/compression of rocket fuels. Some rockets produce literally zero carbon during flight (liquid hydrogen + liquid oxygen = water) but that doesn't mean plenty of coal/oil wasn't used in the creation of those fuels.

Yep - pushing air away for 12+ hours at high sub sonic speeds might indeed be less efficient than a sub orbital flight where you exit the atmosphere in a couple minutes and cover most of the distance without meaningful air resistance in the vacuum of space.

Do you have any numbers to back that up? A 747 burns 10-11 tons of fuel per hour and carries about 360 passengers. For a 12 hour flight, that is about 350kg of fuel per passenger. Starship's tanks hold approximately 250 tons of fuel and 950 tons of liquid oxygen. In order to match the per passenger fuel efficiency of a 747 they would need to carry about 700 passengers. Starship has about 20% more internal volume as a 747, so it seems likely that they would be able to fit at least the same number of passengers. That puts them at about 50% of the fuel efficiency of a 747.

In addition, methane combustion produces 2.75kg of CO2 per kg of fuel, while kerosene produces 3kg CO2 per kg of fuel. If they are able to fit 20% more passengers than a 747 due to the larger internal volume then they would be within about 66% efficiency on a CO2 emissions per passenger basis.

As I replied to others, rocket fuel is very different. The total carbon emissions include not just the fuel used by the rocket but the fuels used to create those fuels before the flight. Liquid oxygen is not carbon-free as creating liquid oxygen requires lots and lots of energy. So too methane which is heavily refined/compressed/cooled, processes that take more energy, create more emissions, than burning the fuel itself during flight. The fuel for a 747 is not nearly as energy/carbon-intensive to create.

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