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As someone on reddit pointed out: "The following metals have been found to be incompatible with nitrogen tetroxide and must not be used: Aluminum 2024, Zinc, A

As someone on reddit pointed out: "The following metals have been found to be incompatible with nitrogen tetroxide and must not be used: Aluminum 2024, Zinc, Aluminum 7075, Silver, K-Monel, Titanium, Brass Cadmium, Bronze Hastelloy, Copper, EZ Flow 45 Braze."[0] [0]: Materials Compatibility with Liquid Rocket Propellants, https://apps.dtic.mil/dtic/tr/fulltext/u2/866010.pdf https://apps.dtic.mil/dtic/tr/fulltext/u2/866010.pdf

yborg 7y ago
>Boeing report published 1970 >SpaceX: "It is worth noting that the reaction between titanium and NTO at high pressure was not expected." SpaceX did not expect a reaction known to Boeing almost 50 years ago. My guess would be that this is actually known by a lot of people in rocketry, since UDMH/NTO is a standard propellant - the Proton is entirely fueled with it. I also guess that the titanium component of the valve is not normally exposed to the oxidizer, and only came in contact with it when the valve broke. It would seem that the simple issue of shattered valve fragments flying down the oxidizer line would have been sufficient to destroy the thruster irrespective of a titanium fire...
- russdill 7y ago
  I think for the fire to start, there has to be titanium fragments, not smooth, solid titanium
  - lvs 7y ago
    Not hard to achieve during unscheduled disassembly. But the same report also notes: "Nitrogen tetroxide with a water content in excess of 2.5 percent can accelerate titanium intergranular corrosion to the point of violent reaction."
    - DuskStar 7y ago
      > Not hard to achieve during unscheduled disassembly. I think the assumption is that by the time a RUD starts you no longer care about whether the various parts of your fuel system are going to explode, since the fuel is a far larger concern and it's going to go up anyways.
      - Phillipharryt 7y ago
        With that sort of thinking in mind no one would make any failsafes. "It's fucked anyway" is a constrictive and dangerous way of designing things, and likely to introduce even more failure points.
        sah2ed 7y ago
        No need to overthink this. There’s really no point in designing additional failsafes into a launch vehicle when it disintegrates which can happen for a variety of reasons beyond anyone’s control — RUD is really just an euphemism for an explosion.
        zaarn 7y ago
        It's a reason against failsafes that only work against a RUD. It doesn't mean there are no failsafes against RUDs or to protect the crew and equipment in case a RUD occurs.
    - russdill 7y ago
      In the SpaceX report, it says that the NTO struck the valve at high speed and pressure, causing it to shatter. Before that happened, the NTO leaked though the valve into the helium tank slowly and did not cause a fire even though it was in contact with the valve for a much longer amount of time.
- Bombthecat 7y ago
  I guess it's either management didn't listen or sleep deprivation...
  - Bombthecat 7y ago
    I guess truth hurts. Those guys work 14 to 16 hours a day. While as a ceo and "quick drafting solutions" that works. But when it comes to stuff the like this. You get this. The reaction is known for 59 years. And they still did it..
- sephamorr 7y ago
  This NASA document (pages 62 onwards)[0] (1974) here gives a more in-depth description of NTO materials compatibility than the link above (1970) Titanium is practically the only metal that can store NTO for long periods of time over a reasonably large temperature range. This does come at the expense of shock sensitivity (particularly after a soak/adsorb), but the industry has to use it anyway, while working to mitigate the shock problem. [0] https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720019028.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/197200...
  - baybal2 7y ago
    There are corrosion resistant aluminium alloys that are passivated by washing them with fluoric acids that create a film of fluoride compounds on tank walls.
- semi-extrinsic 7y ago
  To give them benefit of the doubt, it could be that "the reaction (...) was not expected" is trying to say "we did not expect NTO to make contact with this valve". It was on a helium ullage line, after all.
  - bumby 7y ago
    All check valves leak to a certain extent. Not arm-chair quarterbacking here, but (without seeing their fault tree) I would expect them to know that. Expecting them to see the 'water hammer' issue is a different story
  - yborg 7y ago
    I didn't include the followup statement from the SpaceX press release : "Titanium has been used safely over many decades and on many spacecraft from all around the world." The implication being that there's just no way SpaceX could have suspected this problem because structural titanium use in a spacecraft would inform decisions regarding its use in contact with corrosive propellant. Obviously, this was written by a PR person, not engineering management, but it plays to an image of SpaceX being a company to handwave past the hard-earned lessons of decades of "Old Aerospace" companies. They should be more careful in releasing these kinds of statements.
- gettingsnarky 7y ago
  > My guess would be that this is actually known by a lot of people in rocketry, since UDMH/NTO is a standard propellant - the Proton is entirely fueled with it. Nah, the older engineers who would have known got aged out of the system a long time ago. Younger engineers are better than old ones.
- mlindner 7y ago
  Then why does the Space Shuttle use Titanium with high pressure NTO? You have to understand the context of that statement.
raverbashing 7y ago
That's crazy, we tend to forget how "rocket science" is complicated And sometimes you think you're mixing "milk and cereal" and it catches fire and you don't know why.
- delinka 7y ago
  Pyrotechnic Breakfast - I have my next album name
- gettingsnarky 7y ago
  But engineers did know why. For 50 years.
woodandsteel 7y ago
I have no expertise at all in this area. However, I assume that NASA was monitoring SpaceX's design from early on, and that if the design was clearly dangerous NASA would have insisted that SpaceX change it. The reason I say this is that NASA has a strong safety culture, and I assume monitoring from the beginning is what is needed to have a safe design.

yborg 7y ago

>Boeing report published 1970 >SpaceX: "It is worth noting that the reaction between titanium and NTO at high pressure was not expected." SpaceX did not expect a reaction known to Boeing almost 50 years ago. My guess would be that this is actually known by a lot of people in rocketry, since UDMH/NTO is a standard propellant - the Proton is entirely fueled with it. I also guess that the titanium component of the valve is not normally exposed to the oxidizer, and only came in contact with it when the valve broke. It would seem that the simple issue of shattered valve fragments flying down the oxidizer line would have been sufficient to destroy the thruster irrespective of a titanium fire...

russdill 7y ago
I think for the fire to start, there has to be titanium fragments, not smooth, solid titanium
- lvs 7y ago
  Not hard to achieve during unscheduled disassembly. But the same report also notes: "Nitrogen tetroxide with a water content in excess of 2.5 percent can accelerate titanium intergranular corrosion to the point of violent reaction."
  - DuskStar 7y ago
    > Not hard to achieve during unscheduled disassembly. I think the assumption is that by the time a RUD starts you no longer care about whether the various parts of your fuel system are going to explode, since the fuel is a far larger concern and it's going to go up anyways.
    - Phillipharryt 7y ago
      With that sort of thinking in mind no one would make any failsafes. "It's fucked anyway" is a constrictive and dangerous way of designing things, and likely to introduce even more failure points.
      - sah2ed 7y ago
        No need to overthink this. There’s really no point in designing additional failsafes into a launch vehicle when it disintegrates which can happen for a variety of reasons beyond anyone’s control — RUD is really just an euphemism for an explosion.
      - zaarn 7y ago
        It's a reason against failsafes that only work against a RUD. It doesn't mean there are no failsafes against RUDs or to protect the crew and equipment in case a RUD occurs.
  - russdill 7y ago
    In the SpaceX report, it says that the NTO struck the valve at high speed and pressure, causing it to shatter. Before that happened, the NTO leaked though the valve into the helium tank slowly and did not cause a fire even though it was in contact with the valve for a much longer amount of time.
Bombthecat 7y ago
I guess it's either management didn't listen or sleep deprivation...
- Bombthecat 7y ago
  I guess truth hurts. Those guys work 14 to 16 hours a day. While as a ceo and "quick drafting solutions" that works. But when it comes to stuff the like this. You get this. The reaction is known for 59 years. And they still did it..
sephamorr 7y ago
This NASA document (pages 62 onwards)[0] (1974) here gives a more in-depth description of NTO materials compatibility than the link above (1970) Titanium is practically the only metal that can store NTO for long periods of time over a reasonably large temperature range. This does come at the expense of shock sensitivity (particularly after a soak/adsorb), but the industry has to use it anyway, while working to mitigate the shock problem. [0] https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720019028.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/197200...
- baybal2 7y ago
  There are corrosion resistant aluminium alloys that are passivated by washing them with fluoric acids that create a film of fluoride compounds on tank walls.
semi-extrinsic 7y ago
To give them benefit of the doubt, it could be that "the reaction (...) was not expected" is trying to say "we did not expect NTO to make contact with this valve". It was on a helium ullage line, after all.
- bumby 7y ago
  All check valves leak to a certain extent. Not arm-chair quarterbacking here, but (without seeing their fault tree) I would expect them to know that. Expecting them to see the 'water hammer' issue is a different story
- yborg 7y ago
  I didn't include the followup statement from the SpaceX press release : "Titanium has been used safely over many decades and on many spacecraft from all around the world." The implication being that there's just no way SpaceX could have suspected this problem because structural titanium use in a spacecraft would inform decisions regarding its use in contact with corrosive propellant. Obviously, this was written by a PR person, not engineering management, but it plays to an image of SpaceX being a company to handwave past the hard-earned lessons of decades of "Old Aerospace" companies. They should be more careful in releasing these kinds of statements.
gettingsnarky 7y ago
> My guess would be that this is actually known by a lot of people in rocketry, since UDMH/NTO is a standard propellant - the Proton is entirely fueled with it. Nah, the older engineers who would have known got aged out of the system a long time ago. Younger engineers are better than old ones.
mlindner 7y ago
Then why does the Space Shuttle use Titanium with high pressure NTO? You have to understand the context of that statement.

raverbashing 7y ago

That's crazy, we tend to forget how "rocket science" is complicated And sometimes you think you're mixing "milk and cereal" and it catches fire and you don't know why.

delinka 7y ago
Pyrotechnic Breakfast - I have my next album name
gettingsnarky 7y ago
But engineers did know why. For 50 years.

woodandsteel 7y ago

I have no expertise at all in this area. However, I assume that NASA was monitoring SpaceX's design from early on, and that if the design was clearly dangerous NASA would have insisted that SpaceX change it. The reason I say this is that NASA has a strong safety culture, and I assume monitoring from the beginning is what is needed to have a safe design.