> NASA and ESA are phasing out use of hydrazine because of its unfortunate handling characteristics.
I read about various 'green propellant' efforts over the years, but it seems in practice hydrazine is still used for orbital manouvering systems. Of course for those the volumes are small compared to the launch rockets themselves. For launch vehicles it seems only old Russian and Chinese designs still use hydrazine.
> Although toxic, it is a lot lighter than air, so if it leaks it goes up, and does not hang about poisoning people in a broadening area.
Eventually yes it will dissipate upwards, but typically ammonia accidents result in a vapour cloud traveling close to the ground. E.g. https://www.youtube.com/watch?v=qIi4_Poo2HY
> The main problem with hydrocarbons as synthetic fuel is that you need the carbon, which in air is at below 0.05% concentration. It is certainly possible, but seems unlikely to approach ammonia in cost.
There's certainly a largish cost to concentrating CO2 from the atmosphere. Carbon Engineering, one of the companies in the DAC space, claims 8.8 GJ/ton. Just some back of the napkin comparison to the enthalpy of formation for CO2 and H2O (+ adding an assumed 70% efficiency for CO2 dissociation and water electrolysis), and assuming we're building hydrocarbons with a 2:1 H:C ratio, that would mean a roughly 40% energy penalty compared to starting with a concentrated CO2 feedstock.
One the plus side you get a fuel with cheaper and safer handling, better energy density, and compatibility with existing equipment. Hard to say which approach will win. Might well be as you said, that for large industrial users that can take appropriate precautions like maritime shipping or peaker power plants ammonia will be a better solution, but for other smaller scale usage synthetic hydrocarbons will win.