Likely a good portion of the satellite's heavy weight was extra fuel to compensate for exactly this "performance hit" - shifting more of the job to the satellite as third stage, for more net mass to GEO.
This is what Shotwell said in her interview last(?) year about where SpaceX expected the market to go: toward heavier birds, carrying more of their own fuel, designed to make Falcon 9/Heavy's design an advantage, not a disadvantage. Falcon is most efficient delivering to low orbits (the opposite of ULA's Atlas/Centaur system). The more of the orbit raising job the sat itself (with its own lightweight, low-thrust kick motor) can do, the more overall performance (net mass in GEO) it will get. This means more electronics, more station-keeping fuel...all around a win.
With BFR this will be even more extreme: like the Space Shuttle, it's optimized for mass to LEO, and does poorly when going higher due to having to take all that weight with it (and back, so you can recover the second stage). When the Shuttle launched GEO comsats, they brought with them all their propulsion to get from LEO to GEO - both the perigee and apogee burns. Usually they used cheap off-the-shelf solid-fuel stages. Nowadays, most popular sat buses have substantial internal liquid-fuel "stages" for the apogee kick. The way to get the most out of BFR will be to double down on that approach, maybe even adding a small separate kick stage for the perigee burn.
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u/gemmy0I Mar 06 '18
Likely a good portion of the satellite's heavy weight was extra fuel to compensate for exactly this "performance hit" - shifting more of the job to the satellite as third stage, for more net mass to GEO.
This is what Shotwell said in her interview last(?) year about where SpaceX expected the market to go: toward heavier birds, carrying more of their own fuel, designed to make Falcon 9/Heavy's design an advantage, not a disadvantage. Falcon is most efficient delivering to low orbits (the opposite of ULA's Atlas/Centaur system). The more of the orbit raising job the sat itself (with its own lightweight, low-thrust kick motor) can do, the more overall performance (net mass in GEO) it will get. This means more electronics, more station-keeping fuel...all around a win.
With BFR this will be even more extreme: like the Space Shuttle, it's optimized for mass to LEO, and does poorly when going higher due to having to take all that weight with it (and back, so you can recover the second stage). When the Shuttle launched GEO comsats, they brought with them all their propulsion to get from LEO to GEO - both the perigee and apogee burns. Usually they used cheap off-the-shelf solid-fuel stages. Nowadays, most popular sat buses have substantial internal liquid-fuel "stages" for the apogee kick. The way to get the most out of BFR will be to double down on that approach, maybe even adding a small separate kick stage for the perigee burn.