SpaceX launched the most powerful rocket in the world back in February when its Falcon Heavy sent Elon Musk's car into space - but it still isn't powerful enough for NASA.
Following the successful launch of the Falcon Heavy rocket, NASA has come under increased scrutiny over its decision not to save money by pursuing it as a commercial option for launches.
NASA's Space Launch System (SLS) vehicle is currently being developed to fulfil the space agency's planned deep space exploration missions - including a manned mission to Mars.
The SLS will be the most powerful rocket in history for deep-space missions, currently penciled in for an asteroid and ultimately to Mars.
However its development has been slow and very expensive.
On Monday, this prompted questions about whether it might be better for the agency to use SpaceX's commercially available craft instead of spending billions developing the SLS.
During a NASA Advisory Council meeting, Wayne Hale - the former manager of the space shuttle program - broached the topic.
"So there are a lot of folks who ask why don't we just buy four or five or six of those [SpaceX Falcon Heavy launch vehicles], and do what we need to do without building this big, heavy rocket and assemble things like we did with the space station?"
The Falcon Heavy side boosters return to Earth in unison, 06-02-2018. Image: SpaceX Flickr
Bill Gerstenmaier, who is in charge of human spaceflight at NASA, responded that the Falcon Heavy was simply too small.
Referring to the rocket's playload capacity to transport something to the moon, Mr Gerstenmaier said the Falcon Heavy was "a lot smaller than any of [the SLS craft]" which NASA is developing, and which would be used to bring "large-volume, monolithic pieces" of kit into space.
He said the SLS had unique capabilities which nobody else would be able to match, although they could contribute to missions.
"For routine servicing and bringing cargo, maybe bringing smaller crew vehicles other than Orion, then Falcon Heavy can play a role," he continued.
The Falcon Heavy lifts off from Cape Canaveral, 06-02-2018. Image: SpaceX
And this was not limited to SpaceX, as Mr Gerstenmaier added: "What's been talked about by Bezos can play a role. What United Launch Alliance has talked about can play a role."
While potential contracts for smaller spaceflights seem to be on the table, some astrophiles have criticised Mr Gerstenmaier's position.
NASA is yet to develop any of the "large-volume, monolithic pieces" of kit which require it to develop the SLS.
NASA's stance has annoyed SpaceX fans as well as those who have spotted potential wastage in the public sector and who would prefer some time to be spent considering whether a Falcon Heavy or similar vehicle could be part of NASA's space programme.