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“Given the demands of the Artemis program between now and the late 2020s,” he said, “it’s going to be very difficult to squeeze a science mission in that time frame.”
While NASA has a goal of being able to launch three SLS missions in a 24-month period, and two in 12 months, the supply chain is currently limited to one SLS per year. That will change by the early 2030s, he said, growing to two per year and thus creating opportunities for additional SLS missions beyond the Artemis program. That will be enabled by changes to at the Michoud Assembly Facility to increase core stage production and a “block upgrade” to the RS-25 engine used on that core stage that will be cheaper and faster to produce.
NASA also expects to shift to the Block 2 versions of the SLS by the late 2020s. The Block 2 will be based on the Block 1B version, with the larger Exploration Upper Stage, to be introduced on the fourth SLS mission, but will replace the existing five-segment solid rocket boosters with a new design that will further increase the vehicle’s performance.
The performance of the SLS is of interest to scientists proposing missions to the outer solar system in particular. The SLS Block 2 will be able to send payloads of nearly 10 tons directly to Jupiter, and nearly as much to Saturn with a Jupiter gravity assist. The use of additional stages, such as versions of the Centaur, can double that payload, as well as enable direct missions to Uranus and Neptune.
NASA is continuing to study various SLS upper stage configuration options to support such missions, he said, along with what would be needed to certify the SLS for carrying the radioisotope power sources required for missions in the outer solar system. However, Stough said that if proposed missions wanted to use SLS, they needed to start discussions with the Human Exploration and Operations Mission Directorate (HEOMD) now to secure a spot on the manifest in roughly a decade.
“While the manifest for SLS is not fully established for the 2030s or the late 2020s, I would say right now is the optimal time to engage with HEOMD to make sure that these missions get on the docket,” he said.
That may be difficult since it’s not clear what missions NASA will pursue that would require, or could benefit from, an SLS launch. The ongoing planetary science decadal, which will provide recommendations on the highest priority missions for the next decade, won’t be completed until the spring of 2022, and NASA will take some time to decide which recommended missions to implement and when.
Stough said NASA’s Jet Propulsion Laboratory has shown an interest for using SLS for the Mars Sample Return campaign, but the next mission in that effort, the Sample Retrieval Lander, is scheduled for launch as soon as 2026.
The experience of Europa Clipper offers a cautionary tale for those seeking to launch missions on SLS. Congress for several years directed NASA to use SLS for the mission, allowing the spacecraft to get to Jupiter several years faster than if launched on alternative vehicles. NASA fought that directive, arguing that using a commercially procured launch vehicle would be less expensive and free up the SLS for the early Artemis missions.
Congress relented in the fiscal year 2021 appropriations bill, but only after NASA warned of a potential torsional loading issue if the Europa Clipper spacecraft was launched on SLS. NASA is now in the process of buying a commercial launch for Europa Clipper.
That issue came up during the steering committee meeting, particularly after Stough emphasized the “benign launch loads” of the SLS. He said later that, because of work already underway to analyze the initial Artemis missions, engineers decided to use “very conservative” limits when examining Europa Clipper to streamline the analysis.
“We didn’t understand that that was going to cause a problem for Europa Clipper,” he said, but could have been corrected. “It really was a nonissue at the end of the day.”
Another issue for those considering SLS is the cost of the vehicle. Stough took issue with some cost estimates for the vehicle. “The cost numbers you hear in the media are typically inflated,” he said, by taking into account fixed costs. He didn’t give specific examples, but some estimates assume an SLS cost of $2 billion each, based on the program’s annual budget and flight rate.
Asked for his estimate of SLS costs, he said “we are close to $1 billion per launch right now.” He projected that to decrease by 20 to 30% by the early 2030s as the flight rate increases.