WASHINGTON — NASA is stopping work, at the request of Congress, on human-rating the initial upper stage for the Space Launch System, even as the agency argues that its funding projections require it to use that upper stage on crewed missions.

At issue is the future use of the Interim Cryogenic Propulsion Stage (ICPS), an upper stage derived from the Delta 4’s upper stage. The ICPS is intended for use on at least the first SLS launch, which will not carry a crew.

NASA confirmed Feb. 18 that it has instructed teams to stop work on efforts to human-rate the ICPS for later, crewed SLS missions, following instructions from Congress in the report accompanying the 2016 omnibus spending bill. News of the stop-work order was first reported by NASASpaceFlight.com

“Based on the direction in the FY 2016 appropriations bill, NASA has directed the exploration systems programs to stop work on engineering activities related to human rating ICPS,” NASA spokeswoman Kathryn Hambleton said. She added those programs have been instructed to archive their work, and to defer any decision on using the ICPS on the first crewed SLS mission until later this year.

NASA plans to eventually replace the ICPS with the Exploration Upper Stage, a more powerful upper stage that will be human-rated from the outset. However, Congress and the administration can’t agree on when the EUS should be ready.

Report language accompanying the fiscal year 2016 omnibus spending bill passed in December directed NASA to spend at least $85 million of the $2 billion appropriated for SLS for an “enhanced upper stage,” a reference to the EUS, that would be ready by Exploration Mission 2 (EM-2), the second SLS launch. The report also directed NASA not to spend any money human-rating the ICPS for EM-2.

However, NASA requested only $1.3 billion for SLS in its 2017 budget request released Feb. 9. That lower funding level, agency officials said, would not support using the EUS on EM-2, which could launch as soon as 2021 but possibly not until 2023.

“To protect that launch date at our request level, we are assuming an ICPS upper stage for EM-2,” David Radzanowski, NASA’s chief financial officer, said in a conference call with reporters to discuss NASA’s budget proposal Feb. 9.

He added that NASA would continue work as directed by Congress on the EUS this fiscal year, including holding a preliminary design review. “We do, as an agency, want to execute Exploration Upper Stage. We need it for future Mars missions down the road,” he said. “However, at our funding request level, and to maintain the EM-2 launch schedule as proposed in our request, we are assuming an ICPS for EM-2.”

The potential uses, and risks, of flying the ICPS on crewed SLS missions caught the attention of the NASA’s Aerospace Safety Advisory Panel (ASAP), the agency’s independent safety review board, in its annual report issues Jan. 13. “Since the ICPS was not designed as a human-rated stage, it is not clear to the Panel whether NASA will require modifications to the ICPS for use on a crewed mission or accept the risk of flying ‘as is,’” ASAP noted in its report.

Of particular concern, it said, is the lack of shielding to protect it from micrometeoroids and orbital debris. On a typical mission, the upper stage is jettisoned only a few hours after launch, limiting its exposure to any debris impacts that could jeopardize the mission.

However, for EM-2, ASAP recommended the Orion spacecraft remain in Earth orbit for an extended period to test its life support systems before heading into cislunar space. Extending that time in Earth orbit, and thus the time the upper stage must remain attached, increases the risk of the ICPS suffering damage. One solution, ASAP concluded, would be to add shielding to the upper stage.

An alternative included in the report would be to replace the ICPS with the EUS, as Congress is pressuring NASA to do, since it will be designed from the beginning for human missions. “Using the EUS for EM-2 is one potential solution, but NASA does not currently have the funding to make that commitment,” ASAP stated in its report.