WASHINGTON — NASA has issued a call for proposals to construct a second mobile launch platform that will be used by an upgraded version of the Space Launch System rocket starting in the early 2020s.

NASA issued a solicitation June 29 for Mobile Launcher 2 (ML2), its name for the second platform. The procurement will go in a two-step process, starting with a request for qualifications due at the end of July. NASA plans to invite up to five companies from that initial phase to submit full-fledged proposals, due in November.

In procurement documents, NASA said that Mobile Launcher 2 “is similar in nature to and concept of operations” to the existing mobile launch platform, originally built for the Constellation program and since modified to support the Block 1 version of the SLS. The new mobile launcher, though, will be built for the taller SLS Block 1B rocket, which uses the larger Exploration Upper Stage in place of the Interim Cryogenic Propulsion Stage (ICPS) for the Block 1 version.

The new platform is also intended to support future versions of the SLS. “NASA’s intent is to incorporate flexibility and expandability (modularity) into the ML2 design, necessary to accommodate future modifications for the ‘Block 2’ (B2B) vehicle crewed and uncrewed payload configurations,” the documents state. The specific design of the Block 2 version of the SLS, intended to launch up to 130 metric tons into low Earth orbit, has not been set, nor a date for first launch.

Mobile Launcher 2 will be used, like the first mobile launcher, as the platform for assembling and testing the SLS in the Vehicle Assembly Building at the Kennedy Space Center, transporting the rocket to Launch Complex 39B, and supporting the vehicle’s launch. The platform includes various interfaces for fueling the rocket, providing power and data, and a crew access arm.

NASA expects to award a contract for Mobile Launcher 2 in February 2019, with a period of performance not to exceed 44 months. That schedule would have the platform completed by late 2022. A first launch of the Block 1B version of the SLS would thus not take place until after that, likely 2023.

NASA’s original plans called for modifying the existing mobile launcher after the first SLS Block 1 launch, currently scheduled for late 2019 but widely expected to slip into 2020, so that it could be used for Block 1B launches on the second and subsequent SLS missions. That raised concerns about a long gap between SLS missions, estimated at 33 months, primarily because of the work needed to modify the launch platform.

Despite a recommendation from NASA’s Aerospace Safety Advisory Panel that the agency build a second platform, NASA did not include funding for it in its fiscal year 2019 budget proposal released in February. “We’ve got a funding level, and it’s got to be shared among the various priorities,” Bill Hill, NASA deputy associate administrator for exploration systems development, said at a conference shortly after the budget’s release.

However, the final omnibus spending bill for fiscal year 2018 passed by Congress in March included $350 million to start work on a second platform. “The funds also will allow flexibility for future NASA and other Federal agency missions that will require heavy-lift capabilities beyond those of current launch vehicles as well as enable a sustainable Space Launch System (SLS) launch cadence,” the report accompanying the bill stated.

Building Mobile Launcher 2, rather than modifying the first platform, will allow that first platform to be used for additional flights of the SLS Block 1. NASA now expects to carry out at least one more launch of the Block 1 version, either carrying a crewed Orion spacecraft or another payload, such as the Europa Clipper mission. A crewed mission would require human-rating the ICPS, which was not originally planned when the Block 1 was to fly a single uncrewed mission.

NASA has not provided a formal cost estimate for Mobile Launcher 2. The Senate’s version of a fiscal year 2019 appropriations bill would give NASA an additional $255 million the bill states is needed to complete the platform.

“By providing the funds to complete a second mobile launch platform, NASA will have greater mission flexibility to launch using SLS launch vehicle variants that utilize both the Interim Cryogenic Propulsion Stage and Exploration Upper Stage engines,” the report accompanying the Senate bill states. “This flexibility will allow for SLS to have a more regular launch cadence, enable earlier crewed launches for future lunar missions, and provide further opportunities for scientific missions, such as the Europa Clipper.”