
SpaceX says Crew Dragon capsule exploded due to exotic titanium fire - aerophilic
https://www.teslarati.com/spacex-crew-dragon-explosion-titanium-fire/
======
inamberclad
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
>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...

~~~
semi-extrinsic
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.

~~~
yborg
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.

------
accrual
> To do this, helium is rapidly pushed through a check valve - designed with
> low-molecular-mass helium in mind - to physically pressurize the propellant
> systems. Unintentionally, the NTO that leaked 'upstream' through that valve
> effectively was taken along for the ride with the high-pressure burst of
> helium. In essence, picture that you crash your car, only to discover that
> your nice, fluffy airbag has accidentally been replaced with a bag of sand,
> and you might be able to visualize the unintended forces Dragon’s check
> valve (the metaphorical airbag) was subjected to when a "slug" of dense
> oxidizer was rammed into it at high speed.

This paragraph really drove the point home for me. I was able to easily
visualize the event.

~~~
wizardforhire
Classic Water Hammer!

[https://en.wikipedia.org/wiki/Water_hammer](https://en.wikipedia.org/wiki/Water_hammer)

~~~
lutorm
Well "classic" water hammer doesn't involve the water igniting with the pipe
material... ;-)

~~~
wizardforhire
Totally! You get a water hammer and thermite reaction all in one go! Eeeek!

~~~
gpm
Is metal burning with a non-metal-oxide oxidant a thermite reaction?

~~~
TheSpiceIsLife
Aren't all these thermite oxidisers metals or metalloids?

 _Bismuth(III) oxide, boron(III) oxide, silicon(IV) oxide, chromium(III)
oxide, manganese(IV) oxide, iron(III) oxide, iron(II,III) oxide, copper(II)
oxide, and lead(II,IV) oxide._

[https://en.wikipedia.org/wiki/Thermite](https://en.wikipedia.org/wiki/Thermite)

------
mholt
Direct link to SpaceX press release:
[https://www.spacex.com/news/2019/07/15/update-flight-
abort-s...](https://www.spacex.com/news/2019/07/15/update-flight-abort-static-
fire-anomaly-investigation)

------
delinka
I've taken these from other commenters, but can someone rectify these
apparently contradictory statements for me?

"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]

"Titanium is practically the only metal that can store NTO for long periods of
time over a reasonably large temperature range." [1]

[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)

[1]:
[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/197200...](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720019028.pdf)

~~~
delibes
I think levels of impurities and surface treatment of the metal may cause
variation in whether something is considered suitable or not?

For example, from [0] (which is declassified from 1974!) :

"Titanium can react violently with strong oxidizers if the oxi-dizer
penetrates the oxidized surface layer."

"Free chloride, ions in nitrogen tetroxide can cause an accelera-tion of
intergranular corrosion in titanium."

------
black6
FTA: “...instead of a mechanical check valve (simple but still not 100%
passive), the barrier between pressurant and oxidizer (as well as fuel, most
likely) will be replaced with something known as a burst disk.”

A burst disk comes from a different concept than a check valve. With a check
valve, one presumably desires a fluid flowing in one direction—in this case
the pressurant for the oxidizer vessel—and no back flow. In the rocket engine
industry this setup tends to be used to maintain cleanliness. A burst disk,
however, embodies the same concept as a relief valve—it prevents the
pressurization of a system above its rated capacity. I fail to see how
replacing a check valve with a burst disk fixes the problem and maintains the
same system capability.

~~~
lutorm
I think the idea is that the system is never intended to be depressurized once
it's pressurized, so you don't need a check valve, you just need something to
prevent backflow before the system is pressurized.

~~~
black6
Aha. I have a better understanding after reading the actual SpaceX presser[0].
The SuperDraco thrusters are to be used _only_ for a launch escape scenario,
so they would only be used once before a ground refit of the capsule. That
being the case, a burst disk does make sense as a replacement for a check
valve.

0: [https://www.spacex.com/news/2019/07/15/update-flight-
abort-s...](https://www.spacex.com/news/2019/07/15/update-flight-abort-static-
fire-anomaly-investigation)

~~~
Mvandenbergh
The reason why they have a check valve is that the design was originally
designed to be re-lightable for purposes including but not limited to abort.
At this point, NASA only wants them used for abort so a burst disk is a
simpler design but I guess they never updated the design to reflect the
restricted purpose. That would fit with SpaceX's general design philosophy of
building re-usability in well before the systems are actually due to be re-
used.

------
RyJones
Too bad "Some Birds Don't Fly"[0] is so expensive these days; interesting read
on the early days of misslecraft

[0] [https://www.amazon.com/Some-birds-dont-Gary-
Paulsen/dp/B0006...](https://www.amazon.com/Some-birds-dont-Gary-
Paulsen/dp/B0006BU29W)

~~~
DuskStar
Ooh, I'll have to keep an eye out! Have you read Ignition! [0] to give a
comparison?

0: [https://www.amazon.com/Ignition-informal-history-liquid-
prop...](https://www.amazon.com/Ignition-informal-history-liquid-
propellants/dp/0813507251)

EDIT: archive.org appears to have it:
[https://archive.org/details/somebirdsdontfly00paul](https://archive.org/details/somebirdsdontfly00paul)

~~~
caf
Relevant to this story is this passage from _Ignition!_ :

 _On December 29, 1953, a technician at Edwards Air Force Base was examining a
set of titanium samples immersed in RFNA, when, absolutely without warning,
one or more of them detonated, smashing him up, spraying him with acid and
flying glass, and filling the room with NO₂. The technician, probably
fortunately for him, died of asphyxiation without regaining consciousness.

There was a terrific brouhaha, as might be expected, and JPL undertook to find
out what had happened. J. B. Rittenhouse and his associates tracked the facts
down, and by 1956 they were fairly clear. Initial intergranular corrosion
produced a fine black powder of (mainly) metallic titanium. And this, when wet
with nitric acid, was as sensitive as nitroglycerine or mercury fulminate.
(The driving reaction, of course, was the formation of TiO₂.) Not all titanium
alloys behaved this way, but enough did to keep the metal in the doghouse for
years, as far as the propellant people were concerned._

(RFNA is a mixture of NTO and anhydrous nitric acid).

------
vikramkr
Ooh a titanium fire - that would be really cool to see. I wonder if it's
possible to make titanium thermite as easily as aluminum thermite?

~~~
jandrewrogers
Most metals can be used to successfully make thermite when paired with
appropriate metal oxides. It is a very simple thermodynamics equation that
determines which combos work well. The main considerations are the net energy
release of the reaction, which can vary widely, and the amount of energy
required to bootstrap the reaction, which can be extremely high. Classic
thermite -- aluminum and iron oxide -- is a middling combo but has the large
benefits of being dirt cheap and low density.

Designing exotic thermites is a literal direct application of the elementary
reaction thermodynamics that you learn in basic chemistry classes.
Bootstrapping ignition is a bit more involved and often multi-stage.

~~~
DonHopkins
The article says: "Rather, the cause lies in a more exotic and unanticipated
chemical/material interaction between a plumbing valve, liquid oxidizer, and a
helium-based pressurization system."

How can chemical interactions be "exotic"? Or did they mean "exothermic" or
"exciting" or "esoteric"? Is "exotic" a technical term, or did the chemicals
simply come from a distant foreign country, and how does that affect their
physical properties?

[https://www.quora.com/What-are-exotic-compounds-in-
chemistry](https://www.quora.com/What-are-exotic-compounds-in-chemistry)

Is there such a thing as non-baryonic titanium?

[https://en.wikipedia.org/wiki/Exotic_matter](https://en.wikipedia.org/wiki/Exotic_matter)

>Hypothetical particles and states of matter that have "exotic" physical
properties that would violate known laws of physics, such as a particle having
a negative mass.

I can understand why you'd want to fill a rocket with exotic matter of
negative mass: to make it lighter! ;)

~~~
jandrewrogers
The term "exotic" in a chemistry context means one of two things.

First, it can mean observed chemistry that is seen neither in nature nor the
lab under ordinary conditions. In other words, it is theoretically possible
but the required conditions are so atypical that the expected probability of
it occurring unintentionally are very small. I'm guessing this is the case
that applies here.

Second, "exotic" can mean engineered chemistry that is far outside the
standard industrial spectrum, typically because there is a unique application
with very special requirements. Industrial chemistry is all about _price
performance_ of the chemistry not absolute performance, but some use cases are
more performance sensitive than price sensitive so people will design bespoke
and unusual ("exotic") chemistry for the purpose. It is a curve. Everyone uses
the cheapest chemistry that still does the job adequately.

------
phkahler
Doesn't their solution preclude restarting the engines? Maybe that was never
intended.

~~~
manicdee
The Super Dracos are only intended to be ignited once, either for a launch
abort or a propulsive landing.

~~~
jessriedel
Is it the case that, if used for propulsive landing (which is speculative at
this point), each of the Super Draco engines would be turned on and then left
on -- presumably, with variable non-zero thrust -- until landing has
completed? It's not obvious to me that they wouldn't need to be throttled down
to zero. Or is that not relevant so long as the pressure from the helium
remains on throughout engine usage?

~~~
ansible
The latest I've heard on /r/SpaceX is that the propulsive landing is
completely off the table for this vehicle.

I suppose they might retain the ability in case of a chute failure, but that
would be it.

I'd still like to see them try it.

~~~
jessriedel
Musk says it's still on the table as a backup plan "pending NASA approval".
Different commenters on /r/SpaceX have greatly varying estimates about how
realistic that is.

[https://twitter.com/iamkostmos/status/1104510464627625984](https://twitter.com/iamkostmos/status/1104510464627625984)

I hadn't heard any thing from official channels since then.

~~~
skgoa
> Musk says it's still on the table as a backup plan "pending NASA approval".

read: NASA says it's not happening.

------
Kye
Exotic Titanium Fire would make a great band name.

~~~
thrax
Too many syllables.

~~~
Agathos
So, album then.

------
thrill
"Rockets are tricky."

------
PhantomGremlin
Based on what just happened, this is somewhat alarming:

 _For the time being, NASA has published a tentative target of mid-November
2019 for Crew Dragon’s first crewed launch to the International Space Station_

I'd rather fly 1,000,000 miles in a Boeing 737 MAX than risk one ride in this
contraption. It surely needs quite a bit more unmanned testing before it can
honestly be considered to be "man rated".

Of course, NASA has a long and sordid history of pushing spaceflight while
playing down many potential catastrophic problems. The Shuttle problems prove
that.

And there's no need to belabor Musk's shortcomings. As someone on HN recently
quipped, in response to Musk's claim of full driving autonomy by 2020: _Tesla
has an advantage here in that they don 't feel the need for their autonomy to
be particularly safe._

~~~
mikeash
Airliners are the safest form of transport ever devised. You’re safer in an
airliner than you are in the shower.

Space travel is one of the most dangerous forms of transport ever devised.
Even the most insane, wildly optimistic estimates for the safety of the
Shuttle were orders of magnitude more dangerous than an airliner.

Comparing the two does not make sense. If airliners are your standard of
safety for space travel, you might as well take a shortcut and just say “don’t
launch humans into space.”

~~~
PhantomGremlin
_Comparing the two does not make sense. If airliners are your standard of
safety for space travel, you might as well take a shortcut and just say “don’t
launch humans into space.”_

It most certainly makes sense. A lot of sense.

The Dragon capsule spectacularly blew itself to smithereens. Now is not the
time to bring out some duct tape and baling wire to slap the thing back
together so that it can attempt to fly astronauts into space before the end of
the year.

Now is the time for careful and considered testing. Not rushing. This thing
needs to be safe.

I can guarantee you one thing. If American astronauts die this year because of
inadequate testing, that will be the last time you see American astronauts
being launched into space on a NASA sponsored platform.

If Americans die because this was rushed, you can kiss NASA's manned space
efforts goodbye. We'll pay the Russians what it takes to keep going to ISS.
And when ISS ends, NASA will be out of the manned space flight business. The
operative word going forward will be "robotics".

~~~
lutorm
Space travel is not "safe", period. If you decide to go into space, you should
be aware that you're exposing yourself to significant risk. The question is:
what risk are you willing to accept?

You may still be right about your prediction, though. But that just says
something about people having unrealistic expectations.

