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Comment by joakleaf | Hacker News Reader

Indeed these numbers are impressive, but can it even be done?

Is it even possible both in terms of climate change/overall environmental impact, and in terms of how much methane (fuel) we actually have.

Elon Musk's vision is getting 1 million people to Mars (if I remember correctly) within this century. 100 per starship will require 10000 launches. Distributed over 70 years, that's about 1300 Martian voyages per year.

So approximately 3 per day. Each of those launches, however will require launches of refuelling tanks (is it 6 each?)... So we are talking 19 launches per day just _people_ going to Mars!!!

Sorry, but isn't that completely impossible!??

Not just for the required fuel and environmental impact, but also given how often they just postpone basic launches due to weather.

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> Sorry, but isn't that completely impossible!??

> Not just for the required fuel and environmental impact, but also given how often they just postpone basic launches due to weather.

The environmental impact of a rocket launch is roughly in the same category of the environmental impact of a airliner doing a single long-distance trip. (I did this calculation for F9, SS/SH would be higher but not more than two orders of magnitude higher.) Pre-covid, there were approximately 100 000 airliner flights a day. You do the math.

And that's before we consider that they are fueled with methane, and that there are sources of methane that are potentially not just GHG-neutral, but GHG-negative. (Collecting agricultural methane emissions and burning them is dramatically better than releasing them directly.)

As for weather, it is possible to launch rockets in very bad weather, for example the Russians frequently launch in literal blizzards where no winged aircraft could fly. It's just that ability to fly in inclement weather is something you need to design for -- rocket bodies are generally not strong against transverse loads, and this is made worse by having a high fineness ratio. SS/SH, being much more stubby than most current American rockets, will be much less impacted by weather than them.

Methane can be synthesized from CO2 and hydrogen. Going this route could potentially make the rocket fuel carbon negative, since the rocket will carry some of the carbon outside the atmosphere. Supply is basically unlimited as long as we have CO2 and water.

Currently we postpone many launches due to an abundance of caution. As spaceflight gets more routine we will increase our risk tolerance and launch in worse weather.

> we are talking 19 launches per day […] isn't that completely impossible!??

The US uses over 20 million barrels of petroleum every single day. We have 44,000 airline flights every day.

I think 19 starship flights won’t even move the needle.

https://en.wikipedia.org/wiki/SpaceX_Starship says a starship uses 3,400,000 kg (first stage) + 1,200,000 kg = 4,600,000 kg of propellant, both methane + liquid oxygen.

Let’s guess making the fuel takes 5,000,000 kg of oil (corrections welcome. I guess that’s a reasonable estimate. Less than 100% of fuel is methane, but the specific energy of methane is about 20% higher than that of oil, and creating liquid oxygen takes energy, too)

Multiply by 19 gives you 95,000,000 kg, or about 700,000 barrels. That’s 3% of the oil usage in the USA.

And that excludes construction, maintenance, and ground operations.

⇒ I think it would move the needle, more so given that we should work hard on getting that number down.

Why count oil as the energy source for liquefying oxygen? Why not PV? Why not use natural gas instead of cracking oil?

Also, separate point: Starship return flight to Mars requires large-scale manufacture of methane from atmospheric CO2, using solar (or technically nuclear, but I don’t see that happening in this case).

It’s the only way of getting back — their tanks are empty when they finish landing on Mars — but it works fine here too.

> corrections welcome.

Methane is not made from oil. It's either directly pumped out of the ground as natural gas, or collected from organic decay processes. Methane is more carbon-efficient, as it has almost 2x more hydrogen per carbon compared to longer hydrocarbon fuels, and most of the energy in the molecule is in the hydrogen, to the tune of 40% more energy per ton of CO2. One ton of methane burnt produces ~2.7 tons of CO2 (and 2.2 tons of water), compared to one ton of octane producing 3.1 tons of CO2 (and 1.4 tons of water).

Of the propellant in the rocket, less than a quarter is methane, and more than 3/4ths are LOX. The stoichiometric ratio would be ~1:4 (by mass), but for various reasons most rocket engines are more fuel-efficient when burning fuel-rich, so the real ratio is probably somewhere between 1:3.6 to 1:3.8. The energy cost of producing LOX is really, really low, less than 1% of the energy content of similar mass of methane. Also, this process is done with electricity, and is very amenable to intermittent production. (So the CO2 impact is effectively zero).

Coming from the other direction, according to the department of energy, the US yearly CO2 emissions from all anthropogenic sources are ~6.7B tons of CO2 equivalents, while the launch of a single SS/SH produces about 2700 tons. If you were launching a thousand of them every year, they would account for 0.04% of emissions.

Emissions calculation for Starship:

- each starship burns 4600 tons of methalox fuel

- the mixture ratio of the propellent is 78% O2 and 22% CH4 [1]

- that means that each 100 tons of methalox results in 53.6 tons of CO2 and 2.5 tons of CH4

- generally in the climate science it is accepted that one ton of CH4 is equivalent to 25 tons of CO2

- overall, each 100 tons of propellent will produce 116 tons of CO2-equivalent emissions

- a single launch of 4600 tons propellent will result in 5.34 kT CO2 emissions

- if we get to 5000 launches per year we end up with about 25 MT of CO2-equivalent emissions

- the current worldwide level of emissions is 45 GT CO2-equivalent as of 2017 [2]

- that means 5000 launches will increase our global emissions by 0.06%

[1] https://twitter.com/elonmusk/status/1258580078218412033

[2] https://en.wikipedia.org/wiki/List_of_countries_by_greenhous...

This doesn’t take into account that the methane used in fuelling can be extracted from the atmosphere and that much of the fuel / oxidizer mixture actually is removed from Earth entirely.

> the propellent is O2 and CH4 [1]

> that results in CO2 and CH4

The chemical reaction of O2 + CH4 -> CO2 + CH4 does not compute.

More likely is the reaction 2 O2 + CH4 -> CO2 + 2 H2O.

So no methane emissions, that's the actual fuel. Only CO2 (and water) emissions.

Sorry, I skipped some steps in my calculation.

The chemical reaction is indeed CH4 + 2 O2 -> CO2 + 2 H2O. So, for each 16 grams of CH4 you get 64 grams of O2, that's a stoichiometric mass ratio of 1:4.

The fuel ratio for the Raptor engine is 22:78, according to Elon Musk's tweet. So, for each 100 g of propellant mixture you get 19.5 g of CH4 to burn using 78 g of O2 and you are left with 2.5 g unburned CH4.

I simply neglected the H2O resultant from the reaction because, while H2O is a very potent greenhouse gas, it has a fast cycle in nature (rain, etc). Also, I have no idea how to account for it as a greenhouse gas.

From what I've heard there is a significant fraction of carbon monoxide in the exhaust, so there will be a lot less CH4 in the exhaust than you have calculated. CO is only a weak greenhouse gas.

That is very interesting. Do you happen to have a link? I'm not doubting you, I think you are right, but a link that goes into this type of detail would be a very interesting one to read.

I probably heard that here: https://everydayastronaut.com/rocket-pollution/

Otherwise it would have been in a comment on an Eric Berger Ars Technica article.

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