6 ms·

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 clear

by avmich 6y ago

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.

Robotbeat 6y ago
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.
- avmich 6y ago
  > 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.
  - Robotbeat 6y ago
    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.
    - avmich 6y ago
      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.
      - chipsa 6y ago
        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.
        avmich 6y ago
        > 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.
    - avmich 6y ago
      > SpaceX does nothing out of pure tradition 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.
  - NikolaeVarius 6y ago
    This is such a weird take on things. They don't just choose to use random chemicals for no reason, there are engineering tradeoffs made. Fuels are extensively well studied and hundreds of thousands of man-hours are spent validating these choices
    - avmich 6y ago
      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.
      - NikolaeVarius 6y ago
        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.
        avmich 6y ago
        Dangerousness is relevant, and it's in this aspect which inferiority is mentioned.
        NikolaeVarius 6y ago
        If you're going to change your argument like this, your post becomes "Hypergolics are less safe for humans than non-hypergolics" Good to know thanks.
sephamorr 6y ago
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.
- avmich 6y ago
  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.
  - catalogia 6y ago
    For Kerosene/Gaseous Oxygen like Buran used, you'd need some sort of sparkplug right? That seems inherently less reliable. > 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?
    - avmich 6y ago
      I think the whole quote is in order here. > 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.
      - catalogia 6y ago
        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.
_Microft 6y ago
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.
- cma 6y ago
  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: https://www.vox.com/2018/4/11/17227036/flight-spacex-gwynne-shotwell-space-ted-conference-interview https://www.vox.com/2018/4/11/17227036/flight-spacex-gwynne-... Not a prototype or demonstration, a real commercial flight service.
  - avmich 6y ago
    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.
    - cma 6y ago
      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.
      - avmich 6y ago
elliekelly 6y ago
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.
- phkahler 6y ago
  A couple years ago they did a pad abort test. Not from a rocket, but from the ground. It got out to sea just fine.
- NikolaeVarius 6y ago
  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.
  - eecc 6y ago
    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.)
    - dnautics 6y ago
      people have survived even crazier situations, like an airplane cartwheeling down a runway AND THEN bursting into flames. (100+ survivors, out of ~300 IIRC)
- chipsa 6y ago
  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.
rrmm 6y ago
Do you think NASA considers it an issue? They seem pretty ok with it and have plenty of experience with it. 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?
- avmich 6y ago
  > Do you think NASA considers it an issue? 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.
  - NikolaeVarius 6y ago
    Reliability and performance is literally the reason that the risk to use hypergolics are made. "I don't think its a good reason to use a thing if we ignore the reasons to use the thing"
    - avmich 6y ago
      I think you can have adequate reliability performance and win on safety with a different choice of propellants. I also think general history of technology - including space technology - supports this point of view.
      - NikolaeVarius 6y ago
        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"

by avmich 6y ago

Robotbeat 6y ago

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.

avmich 6y ago
> 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.
- Robotbeat 6y ago
  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.
  - avmich 6y ago
    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.
    - chipsa 6y ago
      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.
      - avmich 6y ago
        > 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.
  - avmich 6y ago
    > SpaceX does nothing out of pure tradition 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.
- NikolaeVarius 6y ago
  This is such a weird take on things. They don't just choose to use random chemicals for no reason, there are engineering tradeoffs made. Fuels are extensively well studied and hundreds of thousands of man-hours are spent validating these choices
  - avmich 6y ago
    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.
    - NikolaeVarius 6y ago
      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.
      - avmich 6y ago
        Dangerousness is relevant, and it's in this aspect which inferiority is mentioned.
        NikolaeVarius 6y ago
        If you're going to change your argument like this, your post becomes "Hypergolics are less safe for humans than non-hypergolics" Good to know thanks.

sephamorr 6y ago

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.

avmich 6y ago
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.
- catalogia 6y ago
  For Kerosene/Gaseous Oxygen like Buran used, you'd need some sort of sparkplug right? That seems inherently less reliable. > 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?
  - avmich 6y ago
    I think the whole quote is in order here. > 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.
    - catalogia 6y ago
      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.

_Microft 6y ago

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.

cma 6y ago
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: https://www.vox.com/2018/4/11/17227036/flight-spacex-gwynne-shotwell-space-ted-conference-interview https://www.vox.com/2018/4/11/17227036/flight-spacex-gwynne-... Not a prototype or demonstration, a real commercial flight service.
- avmich 6y ago
  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.
  - cma 6y ago
    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.
    - avmich 6y ago

elliekelly 6y ago

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.

phkahler 6y ago
A couple years ago they did a pad abort test. Not from a rocket, but from the ground. It got out to sea just fine.
NikolaeVarius 6y ago
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.
- eecc 6y ago
  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.)
  - dnautics 6y ago
    people have survived even crazier situations, like an airplane cartwheeling down a runway AND THEN bursting into flames. (100+ survivors, out of ~300 IIRC)
chipsa 6y ago
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.

rrmm 6y ago

Do you think NASA considers it an issue? They seem pretty ok with it and have plenty of experience with it. 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?

avmich 6y ago
> Do you think NASA considers it an issue? 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.
- NikolaeVarius 6y ago
  Reliability and performance is literally the reason that the risk to use hypergolics are made. "I don't think its a good reason to use a thing if we ignore the reasons to use the thing"
  - avmich 6y ago
    I think you can have adequate reliability performance and win on safety with a different choice of propellants. I also think general history of technology - including space technology - supports this point of view.
    - NikolaeVarius 6y ago
      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"