Hopefully Crew Dragon proves itself as a capable platform for Earth-orbit transportation, but this fuel issue is a hard one to counter.
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.
> 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.
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.
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.
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.
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.
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.
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 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.
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.
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.
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.
This deserves a separate reply.
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.
Fuels are extensively well studied and hundreds of thousands of man-hours are spent validating these choices
Given that we hopefully will soon see Starship flying, that may be rather moot point though.
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.
Good to know thanks.
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.
https://www.vox.com/2018/4/11/17227036/flight-spacex-gwynne-...
Not a prototype or demonstration, a real commercial flight service.
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.
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.
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.
The robustness of the system is hard to give up. And fueling and recovery has always been a large production requiring attention to detail.
Has NASA or DARPA put out any requests for a next-gen RCS fuel replacement?
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.
"I don't think its a good reason to use a thing if we ignore the reasons to use the thing"
I also think general history of technology - including space technology - supports this point of view.
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.
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.