The big and annoying issue with the Crew Dragon design seems to be the chosen RCS fuel (AFAIK, monomethylhydrazine and N2O4), which makes both fueling and clearing after landing a delicate process.
Hopefully Crew Dragon proves itself as a capable platform for Earth-orbit transportation, but this fuel issue is a hard one to counter.
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.
Hypergolic fuels have TONS of advantages, including extremely reliable ignition. Basically all crewed orbital spacecraft have used hypergols in some ways. SpaceX's Starship will not use hypergols, although it probably won't be available for years.
> Basically all crewed orbital spacecraft have used hypergols in some ways.
Right, but Soyuz spacecraft is flying since 1960-s, has a half a year orbital storage time, and yet it has no toxic RCS for the capsule. At least that simplifies handling after landing.
And it was long since proven RCS doesn't need to use toxic components. The fact that Crew Dragon still uses them points more to the tradition than to the necessity.
Soyuz still uses hypergols on the service module. SpaceX has no separate service module (just the trunk which has no propulsion), which saves money as the whole propulsion system can therefore be reused. If the Soyuz used peroxide for the service module, it’d weigh far more due to lower performance.
Peroxide has serious limitations in performance (and, like all strong oxidizers and monopropellants, has serious safety concerns by itself and can burn you). SpaceX does nothing out of pure tradition, if that hasn’t been made clear by their approach so far. The future is not hypergols, but there are legitimate reasons for it today.
SpaceX - correctly - speeds up the process by maximally reusing the known solutions for the problems - this not only makes development faster, but saves a whole lot of money on testing the variants which will be found not working.
I disagree that SpaceX does nothing out of pure tradition. In many areas, where invention is hard, unlikely and expensive, SpaceX instead opts to optimize the known solutions rather than invent new ones. Practically everywhere - landing first stages, full-flow combustion Raptor, reusing Dragon, all of that has good precedents in Delta-Cliper, RD-270, Gemini and VA TKS - SpaceX makes a right commercial decision to use proven technologies, which brings us results and doesn't break the bank. Note that truly innovative things, like spinning detonation wave engines, are still in the research labs, and not - at least in a big way - in the SpaceX factories. That's correct way to go considering that we have and know a lot already which we don't use fully.
> if that hasn’t been made clear by their approach so far.
No it hasn't. They just need to be read differently. The fact that they brought - well-tested and beneficial - traditions from computer industry into rocketry doesn't make them doing everything strictly from the first principles.
Kliper is a relatively modern design, which, while admittedly not produced in metal, used much more benign components for RCS. And Kliper also doesn't have a separate service module.
One can always say "but Klipper is a paper spacecraft". Of course, nothing is perfect. Buran can be brought as an example, and arguments against that could be broght up - "oh, it's too old". The point still stands - it's possible to not use toxic components, it's desirable - and it's not done. In my opinion, performance arguments - for a relatively low delta-V system - aren't convincing. After all, Crew Dragon is much bigger and newer craft that Soyuz.
Paper spacecraft can do anything. Buran was designed for the same mission lengths as the Shuttle, on the order of a few weeks at most, not half a year or more. For long mission durations, storable hypergolics are the way to go for now. We don't have good flight time on anything else, and we need that for something we're putting crew on.
> For long mission durations, storable hypergolics are the way to go for now.
Soyuz is a counterexample which flies for decades. Lockheed's engineers working on orbital fuel storage (Advanced Cryogenic Evolved Stage) will also point that it's not a law of physics.
Why weird? Choosing something takes trade-offs - in the case of SpaceX capsules, they chose not to develop non-toxic technoogy, saving some time and money, but making an inferior product. It's not weird, it's their decision - which I'm not sure is good.
Given that we hopefully will soon see Starship flying, that may be rather moot point though.
Your posts use terms like "inferior" to automatically shift the conversation to the notion that hypergolics are inferior solely because they are dangerous chemicals, and ignore engineering decisions that are made.
Its the same as arguing that RP1 engines are inferior to H2 engines because RP1 is based off fossil fuels. It's not relevant to the point.
One might say that Crew Dragon's days are counted already.
Even SpaceX' Gwynne Shotwell (she is SpaceX' long-time COO, in case you do not know her yet) is hoping that SpaceX will fly people on Starship in maybe 3-4 years and she is way more grounded than Mr. Musk.
They plan to achieve this unreasonable sounding feat by producing large numbers of Starships and launching them very often to accumulate massive experience with the craft in very short time in parallel basically.
I wouldn’t say she is that much more grounded: she has also said they are planning on passenger New York<->Shanghai rocket trips for a price between an economy and business class plane ticket by 2028:
So that's longer than from Alan Shepard's flight to Moon landing. And the task - with all peculiarities - still seems simpler.
I don't think this points to the "ungroundness" of Shotwell. Similar projects were discussed and BOTEd for quite some time. The whole suborbital tourism thing is practically rather similar technology, and we're a couple of decades in the implementation time for that.
It is hard to emphasize how much harder that last X% of reliability is to allow something like this for flights of hundreds of civilians. We’re not talking space tourism but a utilitarian service for business travelers and stuff.
Hey don’t even have a settled design and have made radical design changes recently.
SpaceX commercial crew program began in 2010 and is only just coming to fruition.
But principles are long since known, and prototypes are coming more and more often. Delta-Cliper, Space Ship One, New Shepard, unmanned Dragons, first stages of F-9, aerodynamic tests in wind tunnels - all of them contribute, to various degrees, to the technologies needed.
That doesn't seem like a crazy estimate. It's not conservative, but seems doable.
The very first SpaceX launch was in 2008. Since then they have developed and iterated on reliable, reusable space travel. Its plausible that they can adopt that technology to a commercial platform in 8 years.
What's great though is that all the technology, systems, tooling, procedures, experience, etc.. that has been developed for Dragon 2 will roll right into Starship. It's a huge head start.
Earlier this week when they were de-fueling they mentioned that the "escape system" was still active during that process. Is that because the fuel is potentially dangerous? Tangentially related, can the "escape system" be activated even when the rocket itself hasn't left the ground? I watched the escape system test but IIRC it involved launching and then aborting so the pod that carries the crew was already high off the ground before the escape system was triggered. It surprised me that it could triggered from a standstill.
Anything involving fuel is dangerous. Demo-2 is first time that humans are on the rocket while its being fueled. The escape system is on while fueling and defueling since those are high risk times for the rocket.
Even when boarding commercial flights, crew tells you not to fasten seat belts if they’re still refueling. Just in case you need to evacuate (of course it’s just a token concern, I would t expect to survive an airplane bursting in flames even if still docked.)
people have survived even crazier situations, like an airplane cartwheeling down a runway AND THEN bursting into flames. (100+ survivors, out of ~300 IIRC)
The abort system is like ejection seats: the greater the part of the flight envelope it works in, the better, because you have smaller portions of the flight that problems are unsurvivable. The abort system tests were for the most difficult regimes of flight: maximum dynamic pressure, and zero altitude, zero airspeed. Having tested that, they can be sure it'll work in less difficult sections.
Maybe not, but they'd better do. The experience can be changed - SpaceX, for example, convinced them that loading fuel is better done with crew onboard.
The point is, it's better not to have toxic components close to humans, and it also can be done technically. There could be reasons otherwise, but so far I don't see them particularly convincing, robustness and all notwithstanding.
You're not arguing anything. You're saying "I think something else can be good too".
Until you literally give an actual engineering (comparison of performance/reliablity/etc) reason that hypergolics are the wrong choice to make, this is effectively saying that "something should be different because I think so"
> Until you literally give an actual engineering (comparison of performance/reliablity/etc) reason that hypergolics are the wrong choice to make, this is effectively saying that "something should be different because I think so"
As with many discussions, we talk here mostly qualitative differences. Quantitative differences are harder to get by - for example, back of the envelop calculation says that 30 kg of peroxide in Soyuz landing capsule could be, roughly, replaced with 15 kg of hydrazine, so you'd have a 15 kg mass saving here, but in general actual engineering calculations are harder. I wish they wouldn't be such.
Given those very crude estimations and the practice, which shows there are no big unforeseen problems, we have a valid arguments for the alternative. It's an engineering argument, an estimation and not necessarily full calculations, and it's not effectively an arbitrary opinion. You may finally decide the issue using hard numbers, but since we don't have them now, the question stands.
What would be the options? Compressed cold-gas (which don't have much kick, and require compressed gas which carry their own dangers).
You could go with a monoprop Hydrazine which would eliminate half the toxic stuff. There are also some new developments here: hydroxylammonium nitrate (HAN)/water/fuel ? No idea of the trade offs there.
Biprop hydrocarbon/gaseous compressed oxygen? Presumably that'd require an ignition source (and power source). How do you see that trade off?
Do you consider high-test peroxide non-toxic? (It certainly has a different risk profile, but is nasty stuff on contact with about anything that can oxidize). The decomposition would also make storage and rotation a consideration.
Soyuz uses straight hydrogen peroxide, I believe, 82% concentration. For 3 tons capsule it takes 30 kg of peroxide - monopropellant - to land.
Buran is still the example of a high tech option - it used LOX and kerosene for RCS, the biggest trick was to keep LOX liquid for a month-long missions. It was done with mid-1980 technologies.
Kliper planned to use LOX and ethanol. The craft wasn't completed in metal though.
One has to remember that hydrogen peroxide - the rocket concentrations are called HTP, high test peroxide - can be used as an oxidizer, after a catalytic decomposition - and after decomposition HTP is easily hypergolic with fuels. "Hypergolic" doesn't mean "nitrogen-containing, long-storing and toxic", it means "self-igniting" - and it could be non-toxic (or at least HTP toxicity is on the whole different level that hydrazine's one).
To come back from orbit you don't need too much delta-V. So Soyuz uses 30 kg of peroxide, maybe it would use 15 kg of hydrazine for the same effect - you're saving 15 kg in a 3 ton spacecraft but losing on toxicity. Similarly, Crew Dragon is a heavier craft, flying on a bigger F-9 rocket, it can probably allow for some extra mass spent for RCS fuel in order to make it non-toxic.
> You could go with a monoprop Hydrazine which would eliminate half the toxic stuff.
Hydrazine is, I think, a way bigger half. NTO is comparatively benign... not so carcinogenic. But still lethally dangerous :( .
> Biprop hydrocarbon/gaseous compressed oxygen?
Was done at least. Yes, it would require extra systems. You think it can't be done?
> Do you consider high-test peroxide non-toxic?
Comparing to hydrazine - yes.
> The decomposition would also make storage and rotation a consideration.
Yes. Have to chill it, take care while in storage and keep tanks extra clean. But still.
HTP may be (comparatively) non-toxic but that doesn't mean it's necessarily less dangerous. The Kursk (Soviet ballistic missile submarine) was sunk by a malfunctioning HTP torpedo.
Having an oxidizer that easily starts to exothermically decompose sounds extremely dangerous. This was pointed out as the main factor why HTP "lost" to NTO/IRFNA as the storable oxidizer of choice already back in Clarks Ignition (1972).
The toxicity of NTO is bad, extremely so, and everyone knows it. But having your oxidizer tank suddenly go boom on its own accord is even worse.
Crew Dragon carries 3 klbs, or 1.4 tons of hypergols. It doesn't just use those tanks for RCS, but also for the abort system. Also, Crew Dragon is closer to 14 tons at landing.
Indeed. If you want to switch to a less efficient propellant, you also have to explain how to make the launch escape system work with it, otherwise you've traded the extra safety of not having to handle hypergols for a massive extra risk in a less capable escape system.
Hopefully Crew Dragon proves itself as a capable platform for Earth-orbit transportation, but this fuel issue is a hard one to counter.