Following internal conversations regarding the upper stage design for the forthcoming second flight of the Space Launch System (SLS) – named Exploration Mission -2 (EM-2) – NASA has confirmed a move to change not just the upper stage of SLS for EM-2, but also to change the iteration of SLS that will fly the mission.

The original plan and evolution of SLS:

Under the original plan designed for SLS phased inauguration, SLS Block I was set to debut the SLS rocket in the first quarter of the U.S. Federal Government’s 2018 Fiscal Year – December 2017.

The Block I design – as still baselined – will use an Interim Cryogenic Propulsion Stage (ICPS) – a modified Delta Cryogenic Second Stage that is currently in use on United Launch Alliance’s Delta IV rocket.

The desire to use an ICPS for SLS Block I instead of developing the actual upper stage to be used for full-up SLS missions stems primarily from a Congressional mandate regarding SLS development and system implementation.

In the U.S. 111th Congress’s “National Aeronautics and Space Administration Authorization Act of 2010,” a specific provision was enacted into law that required NASA to “achieve full operational capability for the transportation vehicle developed pursuant to this subsection by not later than December 31, 2016.”

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This led directly to the desire to create SLS in a multi-Block evolutionary format, with the first Block flying SLS with Solid Rocket Boosters from the Space Shuttle Program and left-over RS-25D main engines from the Space Shuttle Program for the rocket’s first and core stages, and a modified Delta IV rocket’s second stage for the upper stage.

Thus, the Delta IV’s second stage was dubbed the Interim Cryogenic Propulsion Stage (ICPS) because it was never intended to be the final upper stage used for SLS.

Under the phased implementation plan, the actual upper stage for SLS, the Exploration Upper Stage (EUS), was slated for introduction on the Exploration Mission -3 (EM-3) cargo flight of SLS, thus allowing the EUS to fly on an uncrewed SLS mission before being used to ferry astronauts on subsequent SLS flights.

However, as all things do with new rockets in spaceflight, the inaugural flight of SLS quickly slipped beyond the NASA 2010 Authorization Act requirement of December 31, 2016 to No Earlier Than (NET) December 16, 2017.

With that new date, and an increasing understanding of SLS development and funding environments, the first three missions of SLS were announced, all classed as Exploration Missions (EMs)

EM-1, the uncrewed, debut flight of SLS in its Block I configuration, was (and still officially is) slated for December 2017.

EM-2, the first crewed flight of SLS (still in its Block I configuration with the ICPS) would then follow in 2021.

EM-3, the first flight of the EUS, would then see SLS fly an uncrewed cargo mission in 2023.

But as the evolution of SLS continued to take shape and political and Congressional pressures/directions changed, NASA announced two Block I variants of SLS: the Block IA (with no Upper Stage) and the Block IB (with a Dual Use Upper Stage which was then later confirmed to be the EUS).

At the time, and still now, it was unclear what the two Block I iterations would be used for (if used at all) or if the Block IB would replace the Block II for the foreseeable future based on political and funding realities from Congress.

The new SLS Block IB design:

In April 2014, it was first reported by NASASpaceflight.com that NASA was considering advancing the SLS Block IB into production for use for the EM-2 and EM-3 missions.

The decision created some uncertainty as to what upper stage this Block IB variant would use for the EM-2 mission – which was, until May, still slated to use the ICPS.

It is now confirmed that EM-2 will use an SLS Block IB with EUS — thus eliminating the ICPS on EM-2 and moving forward development and production of the EUS by two years and one SLS flight.

According to information obtained by NASASpaceflight.com, available on L2, the SLS Advanced Development Office has officially put forth a recommended Point of Departure (POD) for the Exploration Upper Stage (EUS) based on “guidance for the Block 1B vehicle (EM-2). It now has the Exploration Upper Stage.”

Under this EUS POD, the new upper stage for SLS Block IB will be a four (4) RL-10-C1 engine stage (a switch away from the J-2X engines) with a maximum propellant load of 285,000 lbm.

The EUS will carry a length NTE (Not To Exceed) 60 ft, an LH2 tank diameter of 8.4 m, and a LOX tank diameter of 5.5 m.

The NTE 60 ft length requirement does not relate to performance needs of the EUS but rather to the already baselined launch tower service arms.

Particularly, exceeding a 60ft length of EUS would impede the crew access arm’s adjustability limits to provide safe and necessary access to Orion.

The NTE length requirement also appears to have formed the now-understood LH2 and LOX tank sizes – as larger propellant tanks would have necessitated an increased length for EUS.

What this means for EM-2’s understood objectives:

While it is understood that this switch from ICPS to EUS on EM-2 will not have an effect on SLS’s Critical Design Review, the one potential schedule impact that this decision does carry for SLS is the content of the EM-2 mission.

While there is no reason to think that the EM-2 Block I to Block IB/ICPS to EUS change will have any impact on the EM-1 mission in 2017, the fact that ICPS will now not be flying on EM-2 would seem to eliminate the mission’s ability to fly crew.

The NASA Safety Office and the Astronaut Office are on record as being against flying crew on an unflown stage – which is what EUS will currently be for EM-2.

This would mean, if there are no changes in the stance of these two offices or in SLS funding and/or flight schedule, that EM-2 would have to be an uncrewed test flight (like EM-1) in 2021.

This would have the knock-on effect of pushing NASA’s first human spaceflight on a NASA rocket from EM-2 to EM-3, no earlier than 2023.

However, there are claims that a cargo flight could be inserted between EM-1 and EM-2 in the 2020 timeframe that would provide the ability to launch a NASA outer solar system probe on the SLS Block IB with EUS variant – thus fulfilling the need to fly EUS prior to EM-2 and crew.

However, the likelihood of being able to insert such a flight would mean a drastic acceleration of both the EM-2 rocket – which would need to be advanced by a full calendar year to meet a 2020 cargo flight timetable, and the EM-3 rocket -which would need to be accelerated by more than two years to meet the EM-2 launch date of 2021.

Moreover, as improbable as that is at this stage, it would be even more improbable for the outer solar system probe in question – Europa Clipper – to meet a 2020 launch date.

At present, Europa Clipper is a proposed mission under study by NASA’s Jet Propulsion Laboratory for a proposed 2025 launch on either a 551 variant of the Atlas V rocket or a SLS rocket.

Nonetheless, whatever the fallout might be, the decision to advance EUS to EM-2 and bring the Block IB SLS iteration into production and flight earlier than anticipated is now the track that SLS and NASA will pursue.

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