SpaceX
NASA
Koenigsmann
Commercial Crew Program
the US Air Force
Accident Investigation Team
the National Aeronautics and Space Administration
the Federal Aviation Administration
FAA
the National Transportation Safety Board
NTSB
USAF
Falcon Heavy
NTO
SuperDraco
Hans Koenigsmann
Kathy Lueders
Dracos
Super Dracos
Dracos —
NTO
Earth
the International Space Station
SpaceX
Cape Canaveral Air Force Station
Cape Canaveral
Florida
toEarth
Landing Zone
STP-2
Landing Zones
McGregor
Texas
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The investigators' best hunch at this point, SpaceX said in a statement, is that during the processing of the used spaceship, a "slug" of the liquid oxidizer for the Super Dracos — a toxic substance called nitrogen tetroxide — leaked through the check valve and into the pressurization tubes. "This effect was not expected." Super Dracos are used on SpaceX's Crew Dragon spacecraft as part of the vehicle's launch escape system.SpaceX Although the investigation is ongoing, Koenigsmann said the planned fix is to replace the check valve — which can be turned on and off — with a "burst valve" that opens one-way at a high pressure. Below is SpaceX's full and detailed statement on the matter: On Saturday, April 20, 2019 at 18:13 UTC, SpaceX conducted a series of static fire engine tests of the Crew Dragon In-Flight Abort test vehicle on a test stand at SpaceX's Landing Zone 1, Cape Canaveral Air Force Station in Florida. After the vehicle's successful demonstration mission to and from the International Space Station in March 2019, SpaceX performed additional tests of the vehicle's propulsion systems to ensure functionality and detect any system-level issues prior to a planned In-Flight Abort test. In order to understand the exact scenario, and characterize the flammability of the check valve's titanium internal components and NTO, as well as other material used within the system, the accident investigation team performed a series of tests at SpaceX's rocket development facility in McGregor, Texas. The Crew Dragon spacecraft originally assigned to SpaceX's second demonstration mission to the International Space Station (Demo-2) will carry out the company's In-Flight Abort test, and the spacecraft originally assigned to the first operational mission (Crew-1) will launch as part of Demo-2.
As said here by Dave Mosher