NASA is officially still deciding whether or not to take the time to conduct the big, integrated test of its Space Launch System (SLS) Core Stage. “This is why we test” is an oft-quoted philosophy in this phase of development of a new system; in order to keep hopes alive for a first launch within the 2020 calendar year, the agency revisited whether or not to continue to apply that philosophy to SLS.



Up until recently the “Green Run” or acceptance test of the stage was viewed as a critical development step in the program for the newest and most complicated part of the launch vehicle, but the test campaign is expected to take many months to complete and coupled with time lost during Core Stage assembly would likely to push launch of Exploration Mission-1 (EM-1) into 2021. The Stage Green Run would culminate in the one and only planned hot-fire test of a Core Stage before EM-1.

Delivery of the first Core Stage by prime contractor Boeing has been delayed several times, and NASA was hoping to both play catch up and speed up plans for a U.S. crewed lunar landing in 2024. The agency conducted a 45-day study that concluded in mid-April to again look for any means possible to recover some of the time lost and try to keep the EM-1 launch date target within 2020.

NASA is reportedly leaning towards keeping the test, which an outside safety panel strongly recommends, but a final decision has not been announced. Although the mission is uncrewed, it is crucial for SLS to fully deliver in its first launch on EM-1 because most of Orion’s lunar flight test objectives on its first solo flight depend on it.

Green Run is for the rookie stage, not the veteran engines

“Green Run” in this case is short for Core Stage Green Run and the test campaign is focused on testing the stage, not the engines. A Green Run is an acceptance test of new hardware and for liquid propulsion system testing at the Stennis Space Center in Mississippi the term can be applied to everything from acceptance testing new components to new engine builds to new rocket stages like the SLS Core Stage.

The Aerojet Rocketdyne RS-25 engines are one of the only Core Stage systems that are ready to fly today because they finished their testing a long time ago. The four flight engines (Engines 2045, 2056, 2058, and 2060) that will be installed in the first Core Stage have each demonstrated safe performance on multiple Space Shuttle launches when they were called Space Shuttle Main Engines (SSME).

(Photo Caption: The four RS-25 flight engines assigned to EM-1 as “delivered in place” in October, 2017, at Aerojet Rocketdyne’s facility at Stennis.)

Two ground-only “development” engines (Engines 0525 and 0528) conducted numerous development tests that demonstrated the Shuttle design as adapted to SLS in both nominal and off-nominal conditions. Development hot-fire testing of the RS-25 “adaptation” configuration that will fly on the first four SLS launches concluded eighteen months ago.

Those single-engine tests from 2015 through 2017 qualified the Shuttle design to fly under SLS operating conditions, including their new engine controller units (ECU). The single engine testing in the A-1 stand at Stennis typically uses the development engines instead of flight hardware to test them out to the edges and corners of its operating envelope.

Using development engines avoids both general wear and tear on the flight engines from extra operating cycles and the additional demands of margin testing on the engine hardware; however, two RS-25 flight engines that didn’t fly during the Shuttle Program were standalone engine Green Run tests. Those acceptance tests further demonstrated the safe operation of RS-25 flight hardware to SLS requirements.

A third flight engine was fired during development testing in a calibration test of the A-1 stand.

In addition to the RS-25 engines, the SLS Boosters and the Interim Cryogenic Propulsion Stage (ICPS) upper stage also have an extensive ground test history, flight history, or both.

(Photo Caption: The SLS Core Stage and its elements. Although designed around the Space Shuttle Main Engines and using some other Shuttle Main Propulsion System (MPS) hardware, many of those components have either been modified or redesigned. The rest of the stage is new, including the arrangement of its systems.)

In contrast to the other elements of SLS that are flight-proven or ground-tested, the Core Stage is the new piece of the launch vehicle. It started development behind designs and hardware already evolved from Space Shuttle or the cancelled Constellation program and no working version has yet been tested.

When finally completed Core Stage-1 will not just be the first flight article, but also the program’s first working stage article. The Green Run of this first working unit is the only planned opportunity to demonstrate a full-duration test run of a Core Stage before the program’s first launch.

For the integrated hot-fire test, the RS-25 engines are essentially test support equipment to evaluate how well the new Core Stage meets the requirements to run four of them at the same time for more than eight minutes. In addition to meeting the demands of the engine cluster during mainstage, new SLS flight software will also be seeing its first full use.

The flight software running on the stage’s new flight computers is in charge of the overall control cycle of vehicle management, monitoring systems health, and commanding the stage through the Green Run test sequences via a brand new avionics system.

NASA and Boeing are qualifying the different Core Stage subsystems individually and testing some of them together in pairs and small groups, but the tests during the Stage Green Run will be the first time all stage systems are operated together.

Nothing official as ASAP makes recommendations

As a part of initiatives to shore up the EM-1 schedule, NASA began an internal 45-day study in early March looking for any other ways possible to shorten the schedule to launch readiness. Final assembly of Core Stage-1 is still the critical path, and a revised plan was recently put into effect to rearrange the order in which the three remaining pieces were attached and do both those mates horizontally.

The hope is that this will buy back a few months of schedule and stage assembly will be complete by the end of the year; however, by past scheduling that would still keep a launch before the end of 2020 in jeopardy. The 45-day study was looking for more schedule options and the Stage Green Run would stand out for anyone looking at content to cut.

The study was due on April 15 and after a review that week at NASA Headquarters in Washington, D.C., the SLS Program held an All-Hands meeting on April 22 where it was reported that the test was kept on the schedule. Ars Technica also reported on an internal memo NASA Human Exploration and Operations Directorate (HEOMD) Associate Administrator Bill Gerstenmaier sent the same day.

NASA says nothing has been decided yet. “A final review of the assessment is still underway and no decisions have been made,” NASA spokesperson Mike Curie wrote in an email following news of the All-Hands meeting.

(Photo Caption: The mated Core Stage-1 engine section and boattail are driven into the Final Assembly area on a recently-furnished tool at the Michoud Assembly Facility (MAF) on April 1. Integration and checkout of the duo was moved into this area and the final assembly sequence was reworked to advance the completion date for whole stage, perhaps before the end of the year.)

The Aerospace Safety Advisory Panel (ASAP) emphasized their recommendation about the Green Run test in a public statement made on April 25 at the conclusion of their meeting at the Marshall Space Flight Center in Huntsville, Alabama. “There is no other test approach that will gather the critical, full-scale, integrated propulsion system operational data required to ensure safe operations,” Dr. Patricia Sanders, ASAP Chair, said during the public meeting.

“Shorter duration engine firings at the launch pad will not achieve an understanding of the operational margins and could result in severe consequences if conducted in a much less controlled environment than Stennis and if the margins are exceeded. I cannot emphasize more strongly that we advise NASA to retain this test.”

The Core Stage production schedule has been a critical path concern for a long time and there were reports as far back as early Summer 2018 of studies to look at substituting a brief Flight Readiness Firing (FRF)/static fire on Launch Pad 39B at the Kennedy Space Center instead of the Green Run in hopes of getting back some of the time lost. Although static fire tests at the launch site have often been run before the first flight of a new program or a new vehicle, typically those were preceded by more thorough acceptance tests.

The Space Shuttle Program ran twenty-second long FRFs on new Orbiter Vehicles prior to their first flight, but not before putting a Main Propulsion Test Article (MPTA) through several ground test-firings in the late 1970s and early 1980s in the B-2 test stand. SpaceX still performs flight-duration acceptance tests of stages in Texas before the pre-launch first stage static fire on their launch pads in Florida.

The Delta 4 program used the same Stennis B-2 stand to hot-fire a Common Booster Core (CBC) first stage prior to both the program’s first launch and a pre-launch FRF. The Saturn program also performed flight-duration acceptance test firings of their stages at sites around the country in the 1960s, including what is now called the Stennis Space Center.

(Photo Caption: The Space Shuttle Main Propulsion Test Article was used to conduct long-duration, integrated propulsion system test firings at Stennis. The left image shows the orbiter aft fuselage built and integrated for the MPTA being lifted into the B-2 Stand in 1977. The aft fuselage, analogous to the SLS Core Stage engine section, was mated with a truss structure so it could be attached to a Shuttle External Tank. The figure on the right shows the integrated MPTA test configuration; for SLS these same basic elements were merged into a bigger, inline design.)

An FRF on the pad at KSC would be limited. The water supply for sound suppression and deluge of the flame trench and launch platform at KSC is designed and sized for the vehicle to liftoff and clear the area relatively quickly. It will run out after only seconds of engine run time.

As with Shuttle, SLS doesn’t throttle up to full power until after liftoff; whereas the Stennis test plan is to throttle the engines up to 109 percent of rated power level (RPL) and try to replicate a flight-like throttle profile, it’s unclear if KSC can support full power during a hypothetical FRF.

During the ASAP’s April 25 public meeting to summarize their private discussions with NASA earlier in the week, Dr. Sandra Magnus may have alluded further to the “understanding of the operational margins” that Dr. Sanders stated in her opening remarks.

In a summary of ASAP’s review of the status of NASA’s Commercial Crew Program (CCP), Dr. Magnus, a former Shuttle astronaut who flew on the program’s final flight, noted that prior to committing to crewed flights CCP requires enough test data “to ensure that we understand the margins, that we are controlling those margins, and that we’re operating in the environment that those margins require.”