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Comment by yborg | Hacker News Reader

>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...

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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/197200...

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.

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.

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

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.

I think for the fire to start, there has to be titanium fragments, not smooth, solid titanium

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."

> 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.

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.

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.

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.

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.

Then why does the Space Shuttle use Titanium with high pressure NTO? You have to understand the context of that statement.

I guess it's either management didn't listen or sleep deprivation...

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..

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