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.
Buran had non-toxic components for RCS. Kliper was planned with non-toxic RCS. Soyuz spacecraft at least has non-toxic components for capsule RCS, so when the capsule lands, it doesn't need to be approached in a chemical protection suit.
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.
Buran was using LOX, storing it for up to two weeks with a cryocooler and a complicated cooling system, Crew Dragon can be rated for hundreds of days and doesn't have the volume/mass/cost to spare.
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.
Propane/N2O is an interesting combination. It needs to be pressurized quite a lot (the N2O in particular) to stay liquid, ruling it out for large systems like launchers. But for small thrusters, it means the system is self pressurizing, meaning (potentially) less mass and complexity for the entire thing.
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.
> Crew Dragon can be rated for hundreds of days and doesn't have the volume/mass/cost to spare.
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.
> 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 strongly suspect they've given the matter orders of magnitude more engineer-hours of consideration than you have. Yes, they have many other things to consider as well; that's why they have a lot of engineers.
I think for some questions you don't need to measure who spent more time to determine who's more correct. I also think that SpaceX realized they'd need to give many more efforts to develop this particular superior technology - I agree that this could be expensive and would take time. Maybe the Starship plans also affected their decision.
From a quick search their main RCS is still a dimethylhydrazine which is quite toxic. They're just on the engine part of the craft instead of on the capsule so it doesn't come down to Earth.
Right, but at least the capsule, which goes through atmosphere and keeps people inside, doesn't have the toxic substance.
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.
The way to do that requires throwing away the entire engine section which it a lot of equipment when you're trying to make a commercially viable option and need to reuse as much as possible.
Soyuz still uses UDMH and nitrogen tetroxide as its propellants. It simply jettisons the propulsion module before reentry so the reentry module has no thruster system and therefore no (toxic) fuel.
With the return of a SpaceX vehicle, the recovery crew don't have suits on, that would be very unwieldy at sea, and they would just back off if trace gas detectors measure a toxic leak.
> Soyuz still uses UDMH and nitrogen tetroxide as its propellants.
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.
Isn't the crew dragon using those hypergolic thrusters only in an abort scenario? IIRC they did test using them to do a propulsive landing, but the current plan is to use parachutes and land in the sea, no? And those thrusters had some ports to prevent seawater ingress, so I would guess that if they haven't been fired and there's no leak there's no danger.
