THREAD: 1/ We got lots of good questions today on why #FalconHeavy was needed for #USSF44 instead of Falcon 9. You can actually see the reason (yay maths!!!) by comparing the Hotbird 13F & USSF-44 stage separation velocities. 
2/ This is the center booster's shutdown velocity (2nd stage starting velocity) for USSF-44: ~14,270 km/h. That's 51% of low Earth orbit velocity. Now, compare that to the geostationary transfer orbit (GTO) mission of #Hotbird13F just a few weeks prior...
3/ On that GTO mission, the #Falcon9 booster (while still recovered) got the 2nd stage to a starting velocity of ~8,210 km/h. That 2nd stage had quite a lot more work to do (more deltaV needed) to haul the payload into the parking orbit before burning for GTO than USSF-44's did.
4/ But that was OK because Hotbird 13F was designed to raise its orbit from GTO to full GEO on its own. So Falcon 9 is all that was needed to launch it.
5/ For USSF-44, the payload(s) needed to go directly to GEO on the rocket itself. And there was no way a Falcon 9 could have gotten the 2nd stage/payload to its needed ~14,270 km/h starting velocity. Which was needed because...
6/ ...the 2nd stage needed to have enough propellant to get into the initial low Earth parking orbit & then burn for GTO & then burn to circularize at GTO apogee. So #FalconHeavy was needed. And...
7/ This mission almost needed more umph from the side boosters - which would have necessitated a dual ocean landing on both drone ships - due to the original payload mass of USSF-44, which, when lowered slightly, allowed for the dual return to launch site (RTLS) landings today.
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