[eloff]

We can accomplish the same by producing the jet fuel using electricity. Realistically this will likely involve methane (natural gas) extracted from the ground and water, but it could one day be carbon neutral using captured carbon. The trick is to use clean energy like nuclear of renewables. There was an article on HN just recently about doing that with portable nuclear generators like those used on ships, this could be done right in the airport.

We already have a fantastic dense energy battery called kerosene. All we need to do is make it carbon neutral and our existing aviation tech won't even need to change.

[SEJeff]

Methane can be created via carbon capture entirely. It is in the SpaceX TODO list to create a factory to literally create metholox (methane + liquid oxygen is what they use as fuel for the new Starship / Superheavy rockets they want to use to go to mars) via carbon capture. Elon has already discussed this before. The end goal that the rocket launch is carbon neutral when you are using carbon already captured from the atmosphere.

Here is a good overview of the plan: https://cleantechnica.com/2019/10/30/no-you-dont-have-to-wor...

[hoseja]

The goal is not being carbon-neutral. The goal is Mars ISRU.

[SEJeff]

Elon has explicitly stated in interviews that one of the benefits of the design is that here on earth, it means they can be carbon neutral. I'm just relaying what he's publicly stated already. Yes, Mars ISRU is a HUGE benefit of this design, but it also means they can make their own fuel here on Earth with nothing more than some big machinery and solar panels.

I believe his exact quote was:

""" Sometimes I get criticism for, ‘why are you using combustion in rockets and you have electric cars’. There isn’t some way to make an electric rocket. I wish there was. But in the long-term, you can use solar power to extract CO2 from the atmosphere, combine it with water, and produce fuel and oxygen for the rocket. """

[8bitsrule]

"There isn’t some way to make an electric rocket. I wish there was."

That Musk is no Tom Swift. Tom even had an electric rifle, not just a flamethrower.

which he used to great advantage in colonizing Africa

[WalterBright]

The whole "carbon neutral" think is a crock. Coal is, after all, just dead trees, so coal plants run on "biofuel" and are "carbon neutral".

[m-i-l]

> The whole "carbon neutral" think is a crock. Coal is, after all, just dead trees, so coal plants run on "biofuel" and are "carbon neutral".

The difference is in the timeframes for capture and replacement - with coal it is millions of years, and with wood it is years. That means burning coal releases carbon which was trapped millions of years ago, and it would take millions of years to trap equivalent amounts of carbon with the formation of new replacement coal deposits. Burning wood on the other hand releases carbon that has been trapped within our lifetime, and it is easy to replace with new trees grown in our lifetime. Bear in mind that if we reverse millions of years of carbon capture we would revert the atmosphere to what it was like millions of years ago, and time travellers visiting that era are likely to need breathing equipment.

[ppf]

True, but what is a sustainable level of tree harvesting to feed the world's energy needs?

[eloff]

That's not possible, our energy needs are too great for that now. But nobody would seriously suggest wood (or other biofuels) be our principal energy source anyway.

[ppf]

That's my point. Any scheme involving burning wood to create aviation fuel is an indicator that the wrong side of the energy usage balance is being addressed.

[ben_w]

Technically correct, but about as unhelpful as saying “solar isn’t renewable because the sun will eventually go out”.

Our biology and our biosphere are not adapted to the atmospheric CO2 levels we would have if we burned all the fossil fuels.

[WalterBright]

My point is calling something "carbon neutral" if it is burning wood is equally unhelpful. Nothing is gained by burning a tree you just chopped down instead of burning a tree buried for a million years - the same amount of CO2 is released.

[mkl]

But the tree you just chopped down got its carbon from the atmosphere just a few years ago. That's what makes it carbon neutral. The coal (/gas/oil/etc.) got its carbon from the atmosphere in the far distant past, so from our point of view it's new additional carbon that wasn't in the system before. It can only be considered carbon neutral over a time span of millions of years, which is pretty meaningless.

[WalterBright]

> That's what makes it carbon neutral.

Nope. You're emitting exactly the same CO2.

Want to be green? Burn less. Changing your point of view won't lessen CO2 emissions. Burning less carbon will.

[DagAgren]

"Unhelpful"? We have a specific problem we are trying to solve. That problem is: Too much CO2, right now. The term is obviously useful in solving that problem.

Nobody is worrying about CO2 on geological timescales. It is simply not relevant to any discussion anyone is having, or any problem anyone is trying to solve.

This isn't complicated? How are you managing to miss the point with such vigour?

[WalterBright]

> How are you managing to miss the point with such vigour?

I could ask the same of you - how are you helping matters at all by chopping down a tree and burning it over digging up a tree and burning it? You're emitting the same amount of CO2.

Want to reduce global warming?

1. plant a tree

2. burn less

If you're wondering what to do with the tree after it grows, you can:

1. use it for lumber

2. bury it

[Tagbert]

The carbon neutral part is when you plant another tree in place of the tree you just cut down. The new tree then starts taking up the equivalent carbon from the tree you cut down and burned. That is what makes tree farms somewhat carbon neutral. You can’t do that with coal.

[dboreham]

There's an implicit time window on the sum. Burning fossil fuels is evaporating all the carbon captured in the past billion years, over a few hundred years.

[jjoonathan]

Or biofuel. Plants are good at capturing atmospheric CO2 and the resulting biomass can be turned into fuel using well established technology.

If it's nearly suitable for cars I suspect it's even more nearly suitable for aviation on account of the lower volumes and higher margins.

[munificent]

Now I'm just imagining some sort of steampunk wood-burning airplane.

[rdtsc]

> Now I'm just imagining some sort of steampunk wood-burning airplane.

Almost there https://en.wikipedia.org/wiki/Wood_gas

"Flight attendants will resume service in just a moment, after they throw another tree trunk into the wood gas generator"

[konspence]

Already underway[1]. It's completely viable, it just needs investment and commitment.

[1]: https://www.alaskaair.com/content/about-us/sustainability-re...

[WalterBright]

> Plants are good at capturing atmospheric CO2 and the resulting biomass can be turned into fuel using well established technology.

I.e. burning coal.

[_Microft]

Nope, that's outright wrong (and from other replies you should have learned that by now, so why keep reiterating something false?). Anyways...

The total amount of carbon in circulation is what matters because that part can end up in the atmosphere as carbon dioxide where it shows its heat capturing effects. Trees fix carbon while growing and emit it while decomposing but do not change the total amount of carbon in circulation in the long term.

Digging up carbon that has been locked away for aeons is what is so damaging. There would be nothing bad about burning oil IF that oil were produced by capturing carbon dioxide. We do not do that. We dig up loads of fossil resources and add them to the cycle.

If that explanation was too complicted, here is an easier one: fill a bucket to the brim with water, take a cup and start scooping and pouring back to the bucket. Everything is fine. Now get a second bucket full of water and start pouring into the first (material so far not in circulation is added to it). You see the problem?

If you still insist on your view with burning coal and trees being equivalent, then I challenge you to conduct the water-bucket experiment in the middle of your living room. I mean it wouldn't do no harm, right?

[WalterBright]

Which releases more CO2 into the atmosphere:

1. chopping down a tree and burning it

2. digging up a tree and burning it

Both are the same. Calling (1) "green energy" is outright wrong. Want to be green? Burn less carbon. The source of the carbon you're burning is irrelevant.

[philhippus]

Yes, but it takes 100's of thousands of years for the planet to form fossil fuels such as coal. We can't wait that long.

[WalterBright]

Just stuff the tree into the furnace. It'll burn just fine.

(The only reason the industrial revolution switched to coal was because the landscape was denuded of trees. The Jamestown colony in America was tasked with making glass, glass needs lots of fuel, and England ran out of trees. "Connections" by James Burke)

[blacksmythe]

You have to convert the wood to charcoal to burn hot enough for a blacksmith forge: https://en.wikipedia.org/wiki/Charcoal

I highly recommend a visit to Plimouth Plantation to anyone interested in 1700's era technology: https://www.plimoth.org/

Or Old Sturbridge Village for 1800's era technology: https://www.osv.org/

[hnick]

I don't really know where else to ask, so I'll ask here.

Another topic of research is very rapid sub-orbital passenger flight. Are there any numbers on proposed fuel efficiency for that? I'd expect you gain a lot by removing hours and hours of wind resistance, but getting up to speed and out of the atmosphere probably (gut feel) uses more.

[davedx]

I also haven't seen numbers, but I imagine the energy usage even to reach sub-orbital altitude would be pretty huge compared to a jet plane takeoff -> cruising altitude. Sub-orbital means you also have to bring your own oxygen too (rocket vs jet engine), which can approximately double your fuel mass...

[umeshunni]

No numbers, but you might have seen the SpaceX proposal / video here: https://www.youtube.com/watch?v=zqE-ultsWt0

[mrpopo]

> All we need to do is make it carbon neutral and our existing aviation tech won't even need to change.

Except making it carbon neutral will require enormous amounts of energy, shift the price up, and make aviation available only to the richest among us.

Aviation industry may keep existing, but will need to scale down.

[nine_k]

If the fuel is synthetic anyway, uh can be made better than kerosene.

Also, we have a huge amount of free nuclear energy, daily delivered from Sun, about 1 kW / m². It is nit stable enough for electric baseload, but, depending on the process, may be adequate for fuel synthesis.

North of Sahara may become a primary source of jet fuel, given its vast amounts of insolation, cheapness of the land, and access to seawater. Not exactly tomorrow, though.

[nradov]

Better than kerosene how? You're not going to get higher energy density without increasing safety risks.

[mrpopo]

Not exactly tomorrow, but we're facing a problem today. What are we supposed to do to reduce CO2 emissions between today and the mythical day we will have unlimited amounts of synthetic fuel available shipped by tankers from Morocco?

[seanmcdirmid]

Well, if we ever figure out fusion, we could get the enormous amount of energy part.

[mrpopo]

Ok; until then, we have to reduce CO2 emissions by 7% each year from now until we reach net zero in 2050. Even if we figured out fusion today (highly unlikely), it won't be deployed at scale by then (a modern fission reactor takes 10 years to build). So there are two choices really: progressively scale down the aviation industry, or ignore the climate crisis and let 100s of millions in poor countries die from climate-related causes.

[seanmcdirmid]

We could actually do that using fission alone, since we have we enough fuel for 70,000 years or so, and nuclear waste is free of green house gases. It would just be very expensive up front, as nuclear plants tend to be.

Anyways, this is the way France is going.

[sansnomme]

If you don't keep the scale, it would never get cheaper. Airlines struggle with profitability as-is, no point imposing additional undue burden.

[alex_duf]

Assuming we have infinite renewable energy supply to produce kerosene from electricity and therefore carbon neutral flights, it still doesn't quite solve the air quality issue.

A step in the right direction, but I hope this doesn't get used as an excuse to fly more.