Getting a plant running at scale should enable a much bettter assessment of costs of e-SAF.

My present understanding is that even that has only a partial impact on GHG from jets, but I have not looked into it in any detail, and would hope to do so over the next couple of days,

If anyone has info and links to hand, that would be great!

Thanks Carl.

Trying to get my head around your link, I note that at the very edge of the possible they show a reduction in GHG of 108%, which presumably means very slightly carbon negative.
They also say:

' When integrating the FT fuel production process with corn ethanol production, the WTW GHG emissions of FT fuels are 57–65% lower compared to petroleum counterparts.;

Which as far as I can work out is telling us that the process they favour, or perhaps a more readily realisable on, would cut GHG by around half or a bit better.

I was trying to find out what else you need apart from hydrogen for 100% SAF

The nearest I came was this:

https://afdc.energy.gov/fuels/sustainable_aviation_fuel.html

This is the DOE, but refers to blends at up to 50% SAF.

It would appear that biological sources are the thing.

I have no idea how rampable they are, although there are no present plans it would seem to do anything with any significant impact on ever growing emissions for the foreseeable future.

I know little about biological sources, or whether with hydrogen there are other alternatives.

AFAIK DAC is a nonsense, which since we are trying to limit direct emissions of CO2 from power plants etc, would, if true, make pretty much of a nonsense of longer range plans for SAF production.

It would appear however that planes will be, or at least can be, built capable of running on 100% SAF, so maybe we are not simply turning out umpteen new planes which have no conceivable chance for their decades of use of doing anything but emitting vast amounts of GHG.

You can run a new jet engine on 100% but not an engine that has previously run fossil jet fuel the aromatics swell the seals and you need to keep putting some in otherwise they shrink and you get leaks.

@SJC:

I was wondering how much long haul fleet expansion with very inadequate carbon reduction available will build in obsolescence for decades, as that is the life of a plane.

Following on from your remarks about one you have used not SAF fuel, the engine is basically screwed for SAF, I wondered how long engines last.

https://aviation.stackexchange.com/questions/20745/what-is-the-lifespan-of-an-airline-engine

' The most important concept in engine life is the Time Between Overhaul(TBO), which is basically the manufacturer recommended time period after which the engine is stripped down,checked thoroughly and required parts replaced. After overhaul, the engine is usually cleared till the next TBO.

An overhauled engine is theoretically as good as a new one and comes with the same life (TBO) and warranty, if applicable. Airline engines (for example the Rolls Royce Trent series) usually have TBOs of over 15000 hours. The record for maximum time for an engine on wing (i.e. use in aircraft before removal for overhaul) is well over 40,000 hours.'

So how many years is that, on average?

https://privatejetcardcomparisons.com/2017/07/11/how-many-hours-a-year-do-private-jets-fly/

' The Top 20 Part 135 charter operators by blight hours averaged 448 hours of flying per aircraft compared to the 3,000 to 4,000 hours per year commercial airliners fly, based on the 2016 hours flown and fleet size as reported by ARGUS. '

So for the long haul jets we are interested in as being the major source of GHG, they will strip out and refurbish/replace the engines in around 4 years or so.

If I have not dropped any spanners, then it looks as though at least as far as the engines are concerned we are not building in an excessive degree of obsolescence from the get-go.