As someone on reddit pointed out:<p>"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]<p>[0]: Materials Compatibility with Liquid Rocket Propellants, <a href="https://apps.dtic.mil/dtic/tr/fulltext/u2/866010.pdf" rel="nofollow">https://apps.dtic.mil/dtic/tr/fulltext/u2/866010.pdf</a>
> 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.<p>This paragraph really drove the point home for me. I was able to easily visualize the event.
Direct link to SpaceX press release: <a href="https://www.spacex.com/news/2019/07/15/update-flight-abort-static-fire-anomaly-investigation" rel="nofollow">https://www.spacex.com/news/2019/07/15/update-flight-abort-s...</a>
I've taken these from other commenters, but can someone rectify these apparently contradictory statements for me?<p>"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]<p>"Titanium is practically the only metal that can store NTO for long periods of time over a reasonably large temperature range." [1]<p>[0]: Materials Compatibility with Liquid Rocket Propellants, <a href="https://apps.dtic.mil/dtic/tr/fulltext/u2/866010.pdf" rel="nofollow">https://apps.dtic.mil/dtic/tr/fulltext/u2/866010.pdf</a><p>[1]: <a href="https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720019028.pdf" rel="nofollow">https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/197200...</a>
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.”<p>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.
Too bad "Some Birds Don't Fly"[0] is so expensive these days; interesting read on the early days of misslecraft<p>[0] <a href="https://www.amazon.com/Some-birds-dont-Gary-Paulsen/dp/B0006BU29W" rel="nofollow">https://www.amazon.com/Some-birds-dont-Gary-Paulsen/dp/B0006...</a>
Based on what just happened, this is somewhat alarming:<p><i>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><p>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".<p>Of course, NASA has a long and sordid history of pushing spaceflight while playing down many potential catastrophic problems. The Shuttle problems prove that.<p>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: <i>Tesla has an advantage here in that they don't feel the need for their autonomy to be particularly safe.</i>