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Ben Blackburn says:
August 5, 2019 at 3:12 pm
The check valves were needed to prevent propellant from going from the propellant tanks back into the unpressurized helium lines.
There is a valve at the helium tank, long pipes taking a twisted path to the propellant tanks, a check valve at the propellant tanks, and then throttle valves between the propellant tanks and the engines.
Once the helium valve is turned on, the pressurization lines are at a higher pressure than the propellant, so no propellant will get back up into the helium lines.
And once the system is activated and pressurized, it will stay pressurized until it lands and is safed, either for abort or for propulsive landing.
The throttle valve is what controls the engines for maneuvering, the fuel system stays pressurized.
In order for propellant to mix, it would have to travel upstream a long distance through 2 long and twisty pipes, and then back feed through 2 pressure regulator valves, and finally to the manifold at the helium tank.
That’s not going to happen!
What did happen is that a small amount flowed back through a check valve and pooled in a low spot in the piping, and then when the system was pressurized rapidly, it was driven at extreme force down the pipe, like water hammer burst the check valve, and under the high pressure and temperature of the impact reacted with the titanium in the valve causing the explosion. It wasn’t propellants mixing that caused it, and as long as the system is pressurized, the helium prevents the back flow.