Waiting for Artemis 1 schedule update, official decision on SLS Green Run

by Philip Sloss

A flurry of initiatives announced early in March to find a way to fly Exploration Mission-1 by this time next year were supposed to be complete by now, but NASA has not announced findings or decisions on speeding up schedules and eliminating typical development testing. All of NASA’s internal reviews of the pre-launch assembly and production work content, integrated schedules, and overall cost estimates for the first two integrated missions, now called Artemis 1 and Artemis 2, were advertised to be complete by the end of the Spring.

Major assembly of the Artemis 1 hardware elements could be completed in the next month, with mating of the Crew Module and Service Module of the Orion spacecraft for the mission expected in July and mating of the engine section to the rest of the first Space Launch System (SLS) Core Stage possible around the end of the month. Although NASA’s internal recommendation a few months ago was to ship Core Stage-1 to the Stennis Space Center in Mississippi and conduct the Stage Green Run test, the civilian space agency’s political leadership will make the final decision on schedule vs. testing.

Engineers want to perform the full-duration, acceptance firing on this first-ever working Core Stage article, something typically done on almost all new rocket stages, in part to verify their mathematical modeling of propellant tank thermodynamics.

Mating milestones approaching, but no new schedule yet

The final mates of the major Artemis 1 flight test hardware elements could occur in July. Final assembly of first-of-their-kind Orion spacecraft and SLS Core Stage vehicles are nearing completion at the Armstrong Operations and Checkout (O&C) Building at the Kennedy Space Center (KSC) in Florida and the Michoud Assembly Facility (MAF) in New Orleans, respectively.

Artemis 1 is the second Orion test flight, but the first with a working Service Module. The final mating between the two, in-space Orion modules by prime contractor Lockheed Martin was expected to start somewhere in the last half of July, with the Crew Module reaching the final steps ahead of ready to mate recently. “Just over the weekend we actually weighed it and measured its CG, a detailed procedure so we understand the mass properties very well,” NASA Orion Program Manager Mark Kirasich said on July 1.

The Service Module is just behind after resolution of a couple of issues. “There were two [issues],” Kirasich noted. “The Solar Array Drive Electronics were electronic boxes that we actually pulled out of the vehicle, another example of a first-time build.”

Credit: NASA/Frank Michaux.

(Photo Caption: The Artemis 1 Orion Service Module in the test cell configured for the Direct Field Acoustic Test in May. The module is being readied for mating with the Crew Module. The solar array wings and the Spacecraft Adapter cone will be removed prior to mating and shipment to Plum Brook Station in Ohio in a few months for thermal vacuum and electromagnetic interference testing.)

“When the Solar Array Drive Electronics were installed in the vehicle (by Airbus DS in Bremen, Germany), they were tested, they worked fine,” he explained. “We shipped the Service Module here, we tested, it was fine. And then we qualified a box in Europe and we found a manufacturing anomaly that we looked at really closely because we didn’t want to pull the boxes.”

“But we ended up deciding we needed to, so we pulled the boxes out, shipped them back to Europe, the boxes were opened and the fix was made and the boxes will arrive back here on Wednesday (July 3) and we’ll put them back in.”

Kirasich also reviewed an issue with the Pressure Control Assemblies: “When we tested the pressurization sequence here in the States we saw some things where it didn’t operate exactly as expected,” he said. “So we took the time the past couple of months [and] we actually did some development testing outside of the vehicle at the valve manufacturers shop.”

“We learned a few things and we retested that two weeks ago and the system performed magnificently, so we’re over that hump. Right now it’s getting these [solar array] electronics boxes back in and installed.”

At MAF in New Orleans, the engine section of the first SLS Core Stage is the final element to be attached. All of the hardware is installed in the element and functional testing of the aft engine compartment continues to verify that everything is working to requirements.

Functional testing is expected to be complete by the end of July, after which the engine section will be rotated to horizontal and mated to the rest of the stage. In the meantime, the four Aerojet Rocketdyne RS-25 engines were finally delivered to MAF in June for upcoming installation.

Credit: Philip Sloss for NSF

(Photo Caption: The integrated engine section/boattail assembly surrounded by scaffolding at MAF on June 28. Additional scaffolding is installed on the inside to provide better work access. In the lower right is the tented entrance and exit to the controlled work area inside; technicians crawl in underneath the boattail to reach the internal work locations. Plastic hoses such as the one seen here above the yellow railing middle right are connected to the compartment’s vents to keep the air flow moving from the inside to the outside.)

One item will remain uninstalled with the Core Stage finally ships from MAF, the liquid hydrogen (LH2) feedlines in the engine section that run from the bottom of the LH2 tank to the LH2 inlets on each of the engines; the full installation has to be done while the stage is in a vertical orientation.

Although installation can’t be completed yet, the lines themselves are already staged inside the engine section. “They are currently in there in I’ll call it their ‘flight installed’ position and they are held in there by tools,” Jason Grow, SLS Core Stage Lead Propulsion Engineer for Boeing, explained. “So when we breakover [the engine section], that tooling will hold it in place so that it is ready to go and then we actually use that tooling to do the mate as well.”

Exactly where the Core Stage will next be vertical is still not official.

While Stages prime contractor Boeing is working to complete the stage and have it ready to ship from MAF in December, NASA has still not announced an official decision on whether the destination will be the Stennis Space Center or the Kennedy Space Center. The civilian space agency is still going through a series of reviews of the Artemis 1 and Artemis 2 schedules and cost estimates started earlier this year. Embedded in the Artemis 1 schedule decision is the choice that NASA leadership at Agency Headquarters in Washington, D.C. has to make on whether or not the Core Stage will go to Stennis for the Green Run test campaign or not.

Although the last big picture review of the series was intended to be complete and reviewed by NASA leadership by the end of June, at a June 28 media event at MAF there was still no word on when an announcement would be made about those new schedules or about the final decision on whether or not to skip Green Run to speed up the Artemis 1 schedule. “I can’t tell you the exact date, but I can assure you that it’s high on our list as far as coming to a conclusion so we can keep moving forward,” NASA Deputy Administrator James Morhard said.

Core Stage tank repressurization data and thermodynamics a Green Run priority

In mid/late April following the first two technical assessments, the internal agency recommendation was to stick with the baseline, go to Stennis, and do the Stage Green Run. The test campaign could take six months to complete making a launch date in 2021 more likely, which is why arguments are being made to skip it in favor of improving the chances of Artemis 1 launching in 2020.

“We have a significant amount of analysis that shows that we could potentially not do the Green Run,” Bill Gerstenmaier, NASA Human Exploration and Operations Mission Directorate (HEOMD) Associate Administrator, said during a NASA Advisory Council (NAC) meeting in late May. “My concern is a lot of the analysis has lots of unknowns in it and my experience has been you do these tests not because of the things that you’re trying to test and verify but because of the unknown unknowns that come about and our internal recommendation is we’d like to go do this test. We’ll see where that goes moving forward.”

The planned test campaign culminates in a flight-duration, eight-minute long “hot-fire” of the Core Stage and its engines in the B-2 Test Stand at Stennis. These four RS-25s are flight-proven, but no Core Stage has ever been fired on the ground or in flight; Core Stage-1 will be the first-ever working article for the SLS Program when it is finished.

Gerstenmaier also noted in Congressional testimony in early May that the models for some of the subsystems are complex. As with the Orion Program’s Ascent Abort-2 test last week, a full-duration test firing is desired to anchor integrated propulsion system analytical modeling in reality.

Credit: NASA.

(Photo Caption: The small-diameter tank pressurization/repressurization lines are indicated in this drawing in red and green near the pointers for the element labels.  The red line runs to the top of the liquid hydrogen (LH2) tank, the green to the top of the liquid oxygen (LOX) tank. The moving parts for the repress system are located in the engine section closer to the hot-gas outlets on each of the four RS-25 engines.)

“The engines actually pressurize the tank and that’s a very complex mathematical model of how the cryogenic propellants and things interact and the way to see all that is in the full up test,” he said. “You can test engines and individual components but you can’t test it as an integrated system as well as you can for a fairly long duration test.”

In the case of Orion, the Launch Abort System (LAS) needed to be qualified before a crewed flight but would only be used in an emergency. For SLS, full-duration Core Stage operation is critical to the success of every mission it helps launch.

AA-2 provided test data to help verify the computational fluid dynamics (CFD) modeling; the Green Run test would do something similar for the Core Stage tank repressurization system.

The pressurization/repressurization system within the Core Stage MPS is based on the same concepts as the Space Shuttle MPS that the RS-25 engines flew with Shuttle, but the implementation is significantly different from Shuttle. “We redesigned that whole system, so it’s not very orbiter-like,” John Shannon, Boeing’s Vice President and Program Manager for SLS, noted in a recent interview. “We had those little pop-up valves before (Shuttle), now we have these tank pressurization assemblies that Valcor (Valcor Engineering Corporation) makes for us, much more robust.”

Gaseous hydrogen from each engine’s low-pressure fuel turbopump (LPFTP) outlet and gaseous oxygen from each engine’s oxidizer heat exchanger outlet are tapped off while they run to keep the stage’s large liquid hydrogen (LH2) and liquid oxygen (LOX) tanks pressurized as the engines simultaneously drain them of propellant. “For the repressurization system you have gas coming off of each one of the engines, just like you would do in the Space Shuttle, but you had a poppet that would open and close and you were constantly [cycling between open and closed],” Jonathan Looser, NASA SLS Core Stage Propulsion Lead, explained.

Credit: NASA.

(Photo Caption: A schematic of part of the Shuttle MPS tank pressurization infrastructure. SLS is using four Space Shuttle Main Engines (SSME) instead of three, but gaseous hydrogen (GH2) and gaseous oxygen (GO2) outlets are the same. From there, the repress system equipment to manage the hot-gas flow is different. The SLS MPS is also using the hot-gas from the engine outlets for more purposes than Shuttle, not just managing the ullage pressure in the stage propellant tanks within tolerances, but also keeping the turbines spinning in each of the four Core Auxiliary Power Units (CAPU) that power the four hydraulic systems packed into the engine section.)

“You’ve got a manifold that allows some amount of low-flow, continuous flow through each one of those in the manifold at all times and then a valve that will open and close if you need to go to a higher pressurization flow. “The orbiter pressurization system was very sensitive to changes in flow and things like that and this one should be a little bit more robust.”

“You cycle based on [propellant tank] ullage pressure, so as the ullage pressure demands more pressurant gas then it will command the high-flow legs to send more gas,” Looser added. “You have a continuous flow through the low-pressure legs and you’ll open up a valve [to] send more from each engine.”

“For Shuttle it was open-close, open-close, constantly trying to maintain pressure. I think there was a low-pressure leg that went through and then the rest were three poppets that would open and close for high-flow; each leg on this one has a low-flow and a high-flow pressurization leg.”

The tanks must be kept pressurized to maintain their structural integrity, and getting enough data to verify the modeling of the interactions between the hot gas filling the top of the tanks and the cold, cryogenic liquid at the bottom can only be done in a Green Run hot-fire. “One of the main objectives of the Green Run from a propulsion standpoint is pressurization and understanding that,” Looser said. “Understanding all the thermodynamics that are going on inside that tank is something we really want to do a full-duration hot-fire for.”

As Gerstenmaier noted, NASA and Boeing have modeled the behavior of the integrated system analytically; however, engineers want to get at least one ground test to anchor those models. “The pressurization model is the one that we would like to get the most data at Green Run because you can model tank thermodynamics, but this tank is larger, it’s thicker materials,” Looser explained.

He added what they’re looking for: “Seeing how the four engines flow hot-gas pressurant into the tank and that interaction of the hot pressurant gas and the liquid level surface,” he said. “There’s some uncertainties there with the different sized tank and the different dome shapes and so that’s something that we really want to anchor that model with some test data.”

“If you run for a few seconds you really don’t get down into the lower parts of the tank where you have more warm gas than you have cryogenic propellants in there.” Internal studies were conducted looking at a short Flight Readiness Firing (FRF) on the pad at KSC for a few seconds as a substitute to an acceptance test to speed up the Artemis 1 schedule; however, a short test won’t exercise the repressurization system or get much in the way of data to see the repressurization thermodynamics going on in the tanks.

“We want to get some steady state run time and just understand the thermodynamics inside the tanks and correlate those models,” Looser said. “The data that we want to get for the full duration is running at full power level (109 percent of rated power level), that’s the data that we’re really interested in and we can do that on the test stand.”

“Obviously [we’re testing the] feed system as well, loading the tank, and [seeing] how the feed system drains and residuals after test, those are some objectives but the pressurization is one of the main objectives from a propulsion standpoint.”

Springtime internal reviews supposed to be complete in June

After NASA came back to work following a five-week long partial U.S. government shutdown through New Year’s and January, both the Trump Administration and NASA leadership reacted to schedule estimates that Exploration Mission-1 (EM-1) couldn’t fly until 2021 by starting a new series of reviews to find ways to bring the launch date back well into 2020, which included consideration of existing commercial launch vehicles instead of SLS for launching Orion on its previous mid-2020 commitment date. The Congressional submission by NASA of its original FY 2020 budget proposal in March detailed three different assessments of the schedules and cost estimates for EM-1 and Exploration Mission-2 (EM-2), missions subsequently called Artemis 1 and Artemis 2.

“The goal of this activity is to maintain an early as possible launch date,” the budget document said. Following the first two assessments that ran through mid-April, the budget submission said a third assessment would be an “independent schedule risk review” conducted by the NASA Office of the Chief Financial Officer (OCFO) from mid-April through the end of the Spring.

“The OCFO assessment report…is expected to be completed late spring,” the budget document said. “NASA leadership will review the results of these assessments in late spring 2019 at an Agency Program Management Council, before considering potential updates to the EM-1 and EM-2 launch planning dates.”

(Photo Caption: Some of the test support infrastructure in the B-2 Test Stand built to surround the SLS Core Stages for ground acceptance testing, more frequently referred to as “Green Run” testing. The circular tubing is for fire extinguishing water. The yellow box is one of the forward hold down points that would bolt to the stage’s forward booster attachments in the intertank. The test stand at Stennis provides three-sixty degree access to the stage at several platform levels along its length, which allows technicians to make changes without moving it.  Full access at KSC is located in the Vehicle Assembly Building (VAB), not at the launch pad, and the vehicle is more likely to require rollbacks to the VAB to resolve problems.)

As a part of overall program assessments, the work schedule for the Green Run test campaign was also examined. “We’re looking to optimize it, make it as short as possible,” Bill Hill, Deputy Associate Administrator for Exploration Systems Development, said during a late May presentation to the Human Exploration and Operations committee of the NAC.

“I did a line-by-line walk-through the first part of May to go through and see what was in the schedule. One of the things that we discovered that was a little bit disappointing was their schedule was basically based on a single shift.”

“So we’re looking at making sure we can pretty much at least do two ten-hour shifts,” Hill added. “We’re constrained by the software and avionics; we can only have it powered up for a hundred twenty hours. It’s a qualification limit and we’re looking at if we can exercise that as well.”

“We’re also taking a look at what Stennis and what Kennedy folks we can get there to augment the Boeing and Marshall staff for that, so we’re taking a hard look at it.  We’re trying to get the test time down, we think we can get it down to six months or less.”

At the June 28 media event at MAF, NASA SLS Stages Manager Julie Bassler outlined the current thinking on the pace of events at Stennis. “As soon as we ship from MAF we’re going to Stennis Space Center and we’ll install the Core Stage into the B-2 Test Stand there and about four to six weeks after we install it we’ll be preparing for the Green Run test itself,” she said. “So about three months in, we will be conducting the Green Run test.”

“We are really concentrating on what [the schedule] risks are out in front of us there, we’ll have to plan for weather delays given what seasons that we’re going to be there. Boeing has their test team already being trained at Stennis, so we’re on a very good path and right now we’re really focused on the risk and how we can mitigate any of that risk.”

Lead Render by artist Mack Crawford – for NSF/L2

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