Today’s scheduled Atlas V launch illustrates one of the problems with a heavy lifter that people fail to recognize. If that launch fails today, we will lose not one, but two lunar science missions critical as a precursor to lunar bases. If they were going on separate rides, we’d have a high level of confidence that at least one would be successful.

If you have a heavy lifter, unless it’s carrying mostly propellant, its payload is going to be hugely expensive, because space hardware tends to cost thousands of dollars per pound to manufacture. It’s an all or nothing throw of the dice, with an expendable vehicle, which will never be reliable in any sane sense of that word. Putting up smaller pieces might increase the chances that one of the pieces doesn’t make it, but you won’t lose billions of dollars on a single launch. And if it’s carrying mostly propellant, there are lots of ways of getting cheap payload up, and propellant is almost infinitely divisible onto smaller vehicles.