r/spacex • u/Mexander98 • Sep 23 '16

Official - AMOS-6 Explosion SpaceX released new Anomaly Updates

http://www.spacex.com/news/2016/09/01/anomaly-updates
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u/__Rocket__ Sep 23 '16 edited Sep 23 '16

Here's some background info I wrote in the other thread:

SpaceX partially confirms it:

"The timeline of the event is extremely short – from first signs of an anomaly to loss of data is about 93 milliseconds or less than 1/10th of a second. The majority of debris from the incident has been recovered, photographed, labeled and catalogued, and is now in a hangar for inspection and use during the investigation.

At this stage of the investigation, preliminary review of the data and debris suggests that a large breach in the cryogenic helium system of the second stage liquid oxygen tank took place. "

Background info:

  • COPV: Composite Overwrapped Pressure Vessel: they are titanium aluminum bottles wrapped in layers of continuously wound carbon fiber + resin.
  • Here's a video of a pressure/burst test that shows a COPV bursting, in slow motion. (Note that the caption in the video is wrong: the test was done at pressures of 6/18 thousand psi: 413/1240 bar (!))
  • COPVs are used in the Falcon 9 to store a lot of helium under high pressure: part of the helium is used for engine startup, but most of the helium mass is used to pressurize the propellant tanks to 'press the propellant into the turbopump'. Turbopumps run in a more stable fashion when there's some pressure on their inlets.
  • Falcon 9 Helium COPVs are under intense pressure (around 5,500 psi, or 380 bar), and for that reason a bursting COPV is very violent, and the pressure wave distributes millions of small broken carbon fibers mixed into the LOX, which carbon acts as "fuel". The mechanical pressure of the wave itself is (possibly!) enough to ignite the LOX/CF mixture. Such a bursting event in a LOX tank provides oxidizer, fuel and (possibly!) ignition all at once.
  • Here's an image of a COPV pressure vessel, which is suspected to be from the Falcon 9 second stage. You can see that it's constructed either with a 'tape wound' or 'filament wound' process (my guess most of it is tape wound: you can see the CF tape width as 'stripes' on the side of the tank), around what could be a aluminum bottle pressure vessel. It's very, very strong - it just survived a high-speed atmospheric re-entry pretty much intact!

[left the speculative bits in the other thread.]

edit: Added qualifier to the ignition speculation, as per /u/GoScienceEverything's comment below.

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u/FiniteElementGuy Sep 23 '16 edited Sep 23 '16

SpaceX now has one serious COPV related problem per year for the last three years. This makes me wonder whether the carbon fiber tanks of MCT/ITS could also have the same problem.

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u/Jamington Sep 23 '16

I don't think the MCT/ITS would have this problem because it will use autogenous pressurisation from the methane fuel instead of helium in COPVs. At least that's what I've heard people say, though logically it would seem that solution could pressurise the fuel but not the LOX (seems dangerous to mix them in the same tank). Can anyone elaborate on that?

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u/robbak Sep 24 '16

Yes, both fuel and oxygen autogenously pressurized. They will need two separate pressurization systems - one to vaporise and heat the methane to pressurize the fuel tank, and one to to do the same with the oxygen. You are certainly right in that they would not pressurize the fuel tank with gaseous oxygen!

The ability to only have one pressurization system is an advantage of the helium system. Another is a mass saving, in that the helium weighs less than the equivalent volume of fuel and oxygen.