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The Falcon 9 uses a helium system to pressurize the tanks. Does the Heavy first stage also use helium? How big would the explosion be in case of a failure assuming the boosters would go too?

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The Helium per se does not explode. What happened in the AMOS-6 incident is that when fueling the upper stage which has Helium COPV (Carbon Overwrap Pressurized Vessels) that are submerged in the subcooled LOX. The Helium as it was being pumped in, faster than usual, cooled even further and they think caused oxygen to solidify and work its way into the carbon fiber mix and finally burn.

Then the COPV burst and since it is very pressurized (many thousands of atmospheres of pressure) it blew out the tanks and now you have tens of thousands of pounds of LOX and RP1 mixing in midair, where it is kind of warm which is a recipe for burning.

If all three boosters were to burst and burn it would be pretty ugly. Usually a fuel explosion is limited by the amount of oxygen to burn around it. This however brings its own oxygen supply to the party which would burn quite heavily and quickly. Not being contained it is likely more of a conflagartion than an explosion, but at this scale that is just semantics.

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The Falcon Heavy cores are essentially 3 Falcon 9 first stages, with only a few minor differences to distinguish them.

If an explosion somehow resulted from any circumstances, all 3 cores would explode, likely causing an explosion about 3x from the AMOS-6 incident.

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