A Falcon Heavy with an expendable core costs $95M and lifts 2.5x as much to GTO as the Atlas V. It's Pluto injection orbit payload is nearly half of the Atlas V's GTO payload. In the answer to this question
PearsonArtPhoto posts graphs showing that an Atlas V has the ability to accelerate more payload to high (outer system) velocities than the reusable Falcon Heavy, and not much less than the expendable Falcon Heavy (and is equal at 60 km/sec).
Launching a Falcon Heavy with an expendable core should perform roughly 90% as well as the fully expendable FH, so if it's performance curve was placed on the second chart it would drop to the Atlas V curve even sooner (a little over 50 km/sec).
I'm having trouble understanding why this is true. I understand that Falcon Heavy's upper stage performance is sub-par, and I think because
- The vacuum Merlin doesn't have a great ISP, and
- The FH first stage completes it's burn at a lower velocity than non-reusable competitors, forcing the second stage to fire sooner, so first stage boosters won't require heavy heat shielding to be recoverable.
I can conceptualize why both of these would hurt FH performance to GTO, but once we reach GTO it seems to me that FH payload performance beyond GTO would be roughly proportionate to it's GTO performance. IE if it can accelerating 2.5x as much mass to GTO insertion orbit as an Atlas V, then it should be able to accelerate something like 2.5x more to Pluto insertion orbit.
Clearly from the charts this isn't true. The Falcon Heavy relative performance continuously degrades vs. it's competitors as the required DeltaV increases. I don't understand why, or the the forces driving these curves, can someone explain them?