SpaceX made history in February, when it successfully launched the Falcon Heavy, its massive and heavy lift-launch vehicle, and then retrieved two of its rockets back to Earth.
According to some estimates, what NASA spends each year for Space Launch System development costs is equivalent to 17 to 27 Falcon Heavy rockets, but NASA doesn't plan on going to SpaceX for its rockets.
NASA Not Keen On Buying Falcon Heavy Rockets
During a committee meeting of NASA's advisory council, Bill Gerstenmaier, the space agency's human spaceflight chief, finally addressed the issue. Instead of discussing the low-cost Falcon Heavy, he highlighted the vehicle's payload capacity. Essentially, the Falcon Heavy can't carry as much as the Space Launch System will be able to.
As Gerstenmaier explained, SpaceX's massive lift-launch vehicle has a small payload capacity to trans-lunar injection ratio, or the amount of mass a rocket is able to hold en route to the moon. The Space Launch System will be able to carry 70 metric tons in low-Earth orbit and 26 metric tons to the Moon. The Falcon Heavy can only do 63.8 and 16 metric tons, respectively.
Will NASA even need that extra space? Well, yes, said Gerstenmaier:
"I think it's still going to be large-volume, monolithic pieces that are going to require an SLS kind of capability to get them out into space."
The only problem with Gerstenmaier's statement is that NASA doesn't currently have any "large-volume, monolithic pieces" that requires only the Space Launch System and not any other space launch vehicle. In fact, NASA is still in the stage of soliciting designs for its Lunar Orbiting Platform-Gateway plans in the 2020s, a small space station to fly around the moon. In theory, components for this space station could be designed in a way that would fit inside the Falcon Heavy's payload. This could potentially save NASA billions of dollars, which could be allocated to other aspects of space exploration.
NASA Might Still Use Falcon Heavy Rockets
He did note that NASA might still purchase Falcon Heavy rockets for use in certain situations. Not just the Falcon Heavy either — but also launch systems by Amazon CEO Jeff Bezos and United Launch Alliance.
"[F]or routine servicing and bringing cargo, maybe bringing smaller crew vehicles other than Orion, then Falcon Heavy can play a role."
Overall, though, NASA and SpaceX have a good relationship, and the space agency says there's nothing to worry about since both the Space Launch System and Falcon Heavy will be used, but for different purposes.