Almost from the inception of NASA’s large and costly rocket program, the Space Launch System, aerospace engineers have questioned the viability of a rocket that will fly infrequently, perhaps as little as once every two to four years. The most influential body to review the rocket, the National Research Council, concluded in 2014 that such low flight rates “will not be sustainable over the course of an exploration pathway that spans decades.”

NASA has steadfastly maintained that it will be able to fly the SLS rocket on an annual basis. However, on Tuesday, the website NASA Spaceflight.com reported on an “all hands” meeting at Kennedy Space Center in Florida where Robert Lightfoot, the agency’s top civil servant, addressed employees. According to the report, NASA officials explained during the meeting that the SLS lacks “booked missions at this time due to tight funding.”

Essentially this appeared to be an acknowledgement by NASA that it lacks funding to build payloads for its flagship rocket, largely because it is spending so much time and money building that rocket. This has been a main contention of SLS critics, who have said it gobbles up so much of the agency’s budget that NASA cannot afford to use it. For this reason the SLS has been derided as a “rocket to nowhere.”

After Lightfoot’s appearance in Florida, a NASA spokeswoman said the space agency still intends to fly the SLS rocket on an annual basis. “Our goal remains to maintain an SLS launch rate of once per year after EM-2,” Kathryn Hambleton told Ars. “This cadence will allow us to demonstrate, in the proving ground of cis-lunar space, the full range of capabilities we need before we are ready to send humans to Mars.”

However, any allusions Lightfoot made to payload issues buttress concerns critics have about the rocket being designed to preserve jobs and infrastructure rather than being designed for actual exploration. “It is fascinating to see an acknowledgment of a problem that has been obvious to most of us for a long time,” said Lori Garver, a former deputy administrator of NASA who left the agency in 2013, in an interview with Ars. “I guess it is a healthy first step.”

A second issue raised by Lightfoot’s meeting concerned further delays to SLS missions. When Congress directed NASA to build the SLS rocket in 2010, the law required an initial test flight by the end of 2016. However, due to delays, the agency is now targeting fall 2018 for “Exploration Mission 1,” or EM-1. Meanwhile, NASA originally chose the year 2021 for a second mission, EM-2, which would become the first crewed flight of the SLS rocket and Orion crew vehicle. However, the agency acknowledged last year that the EM-2 flight could slip all the way to April, 2023.

During the all-hands meeting, according to the report, Lightfoot told employees the space agency is considering moving humans off of EM-2 and onto EM-3. The reason he cited is NASA’s desire to use a more powerful upper stage on EM-2. For the EM-1 first test flight, NASA is using an “interim” upper stage, but, to use the interim stage for a crewed flight, NASA would have to spend $150 million or more to ensure it is reliable enough for humans.

Because NASA may not want to fly crew members on the initial flight of its untested upper stage, EM-2 may have to be re-designated as a non-crewed mission as well. During the Florida meeting Lightfoot expressed his preference for launching a Europa spacecraft. This robotic mission has widespread support in Congress, but, as Ars has exclusively reported, it will not be ready to fly until the end of November, 2023, at the earliest. If that is the case, EM-3, the first mission to carry astronauts into space, would not occur until 2024 or 2025, long after initially promised.

A NASA spokeswoman, Kathryn Hambleton, said that was not the case. “We are proceeding toward a launch readiness in 2021 for a crewed mission of EM-2, which would include the Exploration Upper Stage (EUS)," she said. "There is no requirement for a validation flight before flying crew with an EUS.”

As NASA and its SLS program enter the final year of President Obama’s second term, the agency is feeling pressure to show progress with the rocket. Further delays to an initial launch could imperil the agency’s efforts to sustain the program into a new presidency.

Although space has not been a campaign issue during the presidential primaries, the next president will likely review space policy, as Obama did when he named aerospace veteran Norman Augustine to lead a committee to review NASA’s human spaceflight plans in 2009.

Critics believe that during this review process, the SLS will not come off favorably. NASA’s justification for spending in excess of $2 billion annually to develop and maintain the rocket is that it is necessary to go to Mars. But neither Congress nor the White House has provided the additional funding to build the payloads, such as a lander, habitation module, and myriad other capabilities needed for a Mars mission. This suggests that they support the idea of a Mars mission, but their support doesn't run deep.

Outside of an ambitious Mars plan, it may prove difficult for NASA to justify the need for the rocket, because there would be few, if any, other customers. The US Department of Defense, for example, has told NASA it does not need a 70-ton heavy lift rocket for its payloads.

At the same time, private companies SpaceX and United Launch Alliance are developing new rockets at a fraction of the cost of SLS. These rockets will have the capability to explore the Moon and space near the Moon. It’s possible SpaceX’s Falcon Heavy rocket, which may cost about one-tenth of the SLS, will have flown by the time the next president takes office.

“The next president, any of the candidates who gets elected, will have to look at an expenditure of this size, and will have to look at its value compared to other ways of exploring space," Garver said.