and hope it's successful (by no means assured*), the capacity and price of the Delta IV Heavy aren't as impressive as it seems. Being able to put 48K pounds to LEO is about 10% more than the base model Proton (20.3 metric tons, or about 44,660 pounds) to LEO. That's with the Proton being handicapped by being launched at much higher latitude, meaning a significant performance hit. The lift difference would be quite a bit less if Protons were launched from a lower latitude. The Protons have been launched since the 1960s and after a lot of development troubles, the 3 stage model has a pretty good reliability record. The Proton costs about 1/3rd or less of the cost of a Delta IV Heavy, too. Admittedly, the Proton uses some very nasty chemicals for propellants compared to the environmentally clean Delta IV, but I'd hope we'd be able to do better than this given the time and money spent on the ELV development program.

*Boeing has spent months developing the procedures to process all three modules and have them ignite simultaneously. Given the lateral separation between the engines, regulating the thrust levels is critical to maintaining control. There can also be unknown gotchas waiting in the wings, like the software glitch that destroyed the first Ariane V launch.