RCS engines are practically always hypergolic since you need to be able to pulse on and off very easily/quickly, and propellents need to be storable (non-cryo). If you look at the choices remaining, they're all various levels of extremely toxic.
It's not a question of choices - there are choices. I suspect it's a question which is considered less important so far, so SpaceX didn't pay that much attention to it, considering they had to do a lot of other things already. But I hope we'll get there sooner rather than later.
Dreamchaser with its propane/N2O thruster is a better counterexample. They avoided toxic components so it could theoretically land at any airport, making it a strong selling point. There are also new(-ish) hydroxylammonium nitrate based propellants which are storable, better than hydrazine and aren't toxic either.
HAN and ADN are also interesting options.
That being said, they should be tested and flown a lot more on cargo launches before they're ready to be put on crafts carrying humans.
They had insulation and active stirring system, that was enough for 15-20 days. To fly up to 30 days they needed a microcryocooler.
> Crew Dragon can be rated for hundreds of days
Isn't Soyuz rated for 200+ days?
> Dreamchaser with its propane/N2O thruster is a better counterexample.
Agree, a good example.
Mass is often an issue with spaceflight. We're arguing about what's better overall, not what's better by all possible criteria. Crew Dragon I'd say has more mass to spare than earlier spacecrafts - because at least of economy of scale and more mass-efficient subsystems.
> so SpaceX didn't pay that much attention to it
That's quite an assumption. They didn't decide as you would have, so they must not have thought about it much?
> I suspect it's a question which is considered less important so far, so SpaceX didn't pay that much attention to it, considering they had to do a lot of other things already.
They had a lot of other things to work on, so they - because of that, I suspect - didn't pay that much attention to it.
I don't think it's the same as what you wrote.
https://en.wikipedia.org/wiki/KTDU-80#Pneumatic_Pressurizati...
It's not saying Soyuz is perfect, or near that. It's about Crew Dragon - the spacecraft could use already existing ways to avoid toxic materials in operations, and it wasn't done. And it's hard to do after the design is complete. So we can reasonably hope for the next spacecraft to be better - fortunately trying to make Starship refuelable on Mars and limiting propellants to LOX and methane works for the goal of reducing toxicity.
Right, but at least Soyuz doesn't have them in the returning capsule. Crew Dragon could be just as good in this aspect, but unfortunately isn't. So when moving the capsule from the sea the team has extra dangers.