Manufacturing and assembly development work for the Space Launch System (SLS) Exploration Upper Stage (EUS) is starting to ramp up at the Michoud Assembly Facility (MAF) in New Orleans. NASA’s SLS Program and EUS prime contractor Boeing are preparing to begin construction of the test and flight articles that will be needed to certify and fly the new upper stage’s first launch.
A structural test article (STA) will be assembled at MAF and tested at the Marshall Space Flight Center (MSFC) in Huntsville, Alabama. The first flight article will then be completed and travel from Michoud to the relatively nearby Stennis Space Center in southern Mississippi for a Green Run test campaign similar to the one that the first Core Stage is going through.
Standing up EUS manufacturing, production for first flight.
Following its Critical Design Review in December, NASA and Boeing are finalizing weld parameters to assemble the structures for the new Exploration Upper Stage. Friction-stir welding of weld confidence articles is in work ahead of welding full-scale structures later this year.
“We start fabrication of our structural test article this June, this summer,” Steven Wofford, manager of NASA’s new Block 1B/Exploration Upper Stage Development Office, said in a February interview.
Based on current forecasts, the structural test article (STA) would be delivered to Marshall at the beginning of 2023, where it would be placed in Test Stand 4697. “The structural test article will be tested at Marshall, and we are scheduled to wrap up that testing in October of 2023,” Wofford added.
(Photo Caption: Updated NASA renders of the Exploration Upper Stage in development by the SLS Program. The design of the stage was refined over the last couple of years to establish a minimum of 10 metric tons of secondary, “co-manifested” payload could be delivered with an Orion spacecraft and crew to the Moon.)
Much of the tooling at Michoud used to assemble Core Stage hardware will also be used in assembly of the EUS. “We’re inheriting a lot of the facilities, we’re inheriting a lot of the tooling, we’re certainly inheriting trained and experienced personnel,” Wofford noted.
“So we’re in really good shape in terms of manufacturing facilities and capabilities.” The different structural elements are fabricated using different types of aluminum and aluminum-lithium alloys.
“It’s a mixed bag across the different [elements], [the] hydrogen tank, the different structural beams, LOX tank, thrust structure, propellant lines,” Scotty Sparks, deputy manager of NASA’s Block 1B/Exploration Upper Stage Development Office, explained. “It’s complicated. You would think that we would select one alloy and go with it.”
“But what we can do is we really can optimize the design by selecting specific alloys that give us specific properties. So from the technical perspective, sometimes we select a certain alloy because of its performance. Sometimes we select it because of its availability or its cost or its availability in terms of where it is. But for the most part, what we’re doing here is for performance.”
Some of the elements like the LOX tank use the aluminum 2219 alloy that is employed extensively on the Core Stage; other stage structural elements use aluminum-lithium alloys like 2050, 2070, and 2195. One of the development tasks is to certify how to weld the different materials together.
“If I’m not mistaken, there’s like six different joints of different alloys that we’ve got to go qualify,” Sparks noted. “In other words, when you match up a 2219 to a 2195 piece of material, and you try to weld that, that’s what we have Boeing, and that’s what we have our engineering folks at Marshall [doing] is to go off and develop and then qualify those weldments.”
Before welding the STA hardware, the weld schedules are being refined for the different tools, different materials, and different thicknesses used in the structures for the EUS and the interstage element that connects EUS with the SLS Core Stage.
Most recently, test panels for an interstage weld confidence article were welded in the Vertical Weld Center (VWC). The VWC performs the vertical welds to join panels into a barrel assembly; the tooling is one of several at MAF that are being modified for the dual purpose of assembling both Core Stage and EUS hardware.
The weld confidence test panels have the same key dimensions as full-scale articles in order to perform a full weld of two of them together. After the weld is completed, large parts of the weld lands are cut out to perform destructive and non-destructive engineering evaluations of the welded material.
“In terms of manufacturing readiness, I’m really pleased with the progress we’re making there. We’re well on schedule in that area,” Wofford said. “The biggest top-level reason for that is we weren’t starting from scratch.”
“With Core Stage, everything was new. The factory had been shut down essentially, it was a new design, it was the first article of a new design, it was new people — it was new everything. So Core Stage startup manufacturing was understandably difficult.”
“EUS, it’s not brand new,” he added. “EUS leverages all of those lessons learned and all of that ground we already paid for with Core Stage.”
(Photo Caption: A weld confidence article of two test interstage panels is lifted out of the Vertical Weld Center at MAF in February. The two isogrid panels were friction-stir welded together, and then large sections of the weld were cut out for engineering evaluations.)
EUS is the major upgrade from the SLS Block 1 vehicle that will fly the program’s first three launches to the Block 1B configurations. On the SLS Block 1B vehicle, EUS will replace the Interim Cryogenic Propulsion Stage (ICPS), which a close derivative of the upper stage of United Launch Alliance’s (ULA) Delta IV launch vehicle.
The new upper stage is an in-house development effort between NASA’s SLS Program and Boeing and is sized to SLS dimensions. EUS is the same 8.4-meter/27.6-foot diameter as the SLS Core Stage, and the hydrogen-oxygen stage uses four Aerojet Rocketdyne RL10 engines.
EUS will increase the overall payload performance of SLS from about 27 metric tons to 38 metric tons when inserting the Orion spacecraft on a trans-lunar trajectory. On Orion missions, Block 1B has enough performance to “co-manifest” a large secondary payload of approximately 10 metric tons.
In its Cargo configuration, Block 1B is expected to be able to insert payloads of greater than 40 metric tons on a trans-lunar trajectory.
While the Critical Design Review for the stage is complete, Block 1B development efforts continue at Marshall as EUS manufacturing is slated to begin at Michoud in the Summer. “EUS is kind of the long pole in the tent, but it’s not the only part involved in flying the Block 1B variant of the SLS vehicle,” Wofford said.
“There’s the Universal Stage Adapter which is being managed by the SPIE (Spacecraft Payload Integration and Evolution) organization, that’s the adapter that goes between the Orion vehicle and the EUS, so that’s new and under development. The biggest area besides EUS and the Universal Stage Adapter that’s being worked on is in terms of avionics and software [development] and the vehicle engineering tasks, the analysis tasks like trajectories and loads and environments and performance projections and that sort of thing.
“So [there’s a] big integration effort involved in certifying and developing the new vehicle,” Wofford explained. “We are continuing to mature the EUS and Block 1B designs towards the next steps in their maturity.”
“There’s a series of gate reviews, design reviews that are at maturity states beyond the CDR that we just completed, so we’ll be marching through those in the months and years to come. The Block 1B vehicle-level CDR will be in June of next year.”
STA and flight article will help verify, certify design for first launch
The EUS structural elements will be an integrated assembly, in contrast to the over 200-foot long Core Stage, whose structural test articles were divided into separate test assemblies. The flight article structures will be integrated with flight subsystems such as computers and navigation, electrical power, main propulsion system, and its four rocket engines.
The EUS computer system will control the whole SLS vehicle through countdown, launch, ascent, and injection burns to destinations beyond Earth. The flight computers and redundant inertial navigation unit that fly with the Core Stage during Block 1 launches will move up to the EUS for the longer mission; the vehicle flight software will also evolve to support the hours-long mission from the minutes-long Block 1 flights.
Additional avionics will also fly on the stage for EUS-specific functions like the Engine and Propulsion Integrated Controller (EPIC) box for EUS Main Propulsion System (MPS) and RL10 command and control. “It’ll be how flight software commands the [RL10] engines to start and stop as well as the propellant utilization control as well,” Dan Mitchell, NASA’s Technical Lead for SLS avionics and software engineering, said.
(Photo Caption: An expanded view of the Block 1B vehicle showing the Orion and SLS elements that are assembled in the Crew configuration of the launcher. A notional secondary payload module is shown in the drawing; the co-manifested payload would ride on top of the EUS below Orion housed in a Universal Stage Adapter being developed by Dynetics and NASA.)
During the redesign of the EUS, the interstage element that connects EUS with the SLS Core was also modified. “We changed the interstage configuration to make it longer to accommodate future evolution of the engine sizes, the RL10 nozzles,” Todd Holloway, the Contracting Officer Representative for NASA’s Block 1B/EUS Development Office, said.
The RL10 is used by different launch providers in their upper stages, with the main hardware difference being the number and length of nozzle extensions. For example, the RL10B-2 variant used on the ICPS has a three-cone nozzle extension which is partially deployed in-flight; the RL10C-3 for initial EUS use will have a fixed, two-cone extension that eliminates the deployment mechanisms.
Aerojet Rocketdyne is developing a next-generation version of the engine called the RL10C-X that will make extensive use of additive manufacturing, also known as “3-D printing.” For the Block 1B vehicle, NASA is leaving extra room in the interstage for a C-X engine configuration with a longer nozzle than the C-3.
“The idea then is we could just plug and play,” Wofford added. “When the C-X becomes the existing product line sometime in the future, then we can just plug and play within our existing outer mold line with that new engine.”
Public renderings of the interstage were recently updated to match the real interstage’s appearance. It has the same 8.4-meter/27.6-foot diameter to the SLS elements it connects, the aft adapter of the EUS, and the forward skirt of the Core Stage. The interstage baseline currently only has a coat of primer for corrosion protection and a coat of white paint on the outside; the only thermal protection spray-on foam insulation would be applied at the flanges of the two connection points with the EUS at the top and the Core Stage at the bottom.
EUS Green Run planned at Stennis before first launch
Manufacturing, assembly, integration, and testing of the first flight article at MAF is currently forecasting completion near the end of 2023. “We deliver the completed stage ready for Green Run to Stennis in November of 2023,” Wofford said.
Some of the early phases of work on the first flight article, such as welding and assembly of structures, will overlap with STA work. “I took [the STA] off my critical path intentionally,” Wofford explained.
“So I’m doing structural test article stuff in parallel with the flight article, and that was a completely acceptable thing to do from a programmatic risk standpoint and offered substantial scheduling advantage to do that.”
The first flight article will first be barged over to the relatively nearby Stennis Space Center in southern Mississippi for a “Green Run” test campaign similar to the one that NASA and Boeing are currently trying to finish on the first SLS Core Stage in the B-2 Test Stand.
Once the Core Stage finishes its Green Run test campaign in the B-2 position of the test stand, modifications will be made to test the first flight EUS unit there in 2024. The EUS Green Run campaign will follow a similar outline of systematic checkouts, integrated testing, and a test-firing of the stage in the stand.
(Photo Caption: The B Test Stand at Stennis Space Center; the B-2 position on the right will be modified to perform a Green Run test campaign on the first EUS flight article prior to launch. The new upper stage will be outfitted with special test equipment versions of RL10 engines to conduct a sea-level test on engines designed to fire in a vacuum.)
EUS is an upper stage that will operate in high-altitude/vacuum conditions; however, it was decided to configure the stage and the RL10 engines in particular for a sea-level test-firing at Green Run, which reduced the scope of the modifications necessary to convert B-2 to support EUS.
“We’re going to run Green Run with sea-level RL10s,” Wofford explained. “The RL10 is an upper stage engine that’s designed and built to be run at altitude in a vacuum, so the nozzle is optimized to run in a vacuum, and you can’t run it at sea-level.”
“However, if we take the nozzle off and run it with a so-called ‘stub nozzle,’ then you can test the engine at sea-level with no problem.” Aerojet Rocketdyne tests single RL10 engines in a vacuum chamber at their Jupiter, Palm Beach County, Florida facilities. Still, Wofford noted it’s a bigger challenge to fire a cluster of them in a vacuum.
“When you put four of them together, then being able to construct a diffuser that can evacuate that gigantic volume on the gigantic structure at B-2 becomes problematic at best,” he said. “So we made the decision some time ago that we were going to run sea-level engines — special test equipment engines, not the flight articles — for the Green Run.”
“Then we would ship the stage back to MAF, and we would swap the sea-level engines out for the flight article RL10s and then ship that to KSC.”
“Aerojet Rocketdyne, the contractor, is very experienced in analytically reconciling the sea-level test with the flight engine performance. [It’s] a pretty well-known engineering exercise,” Wofford added.
Sparks also noted that currently, the plan for the EUS Green Run hot-fire test would be to perform a burn of approximately 350 seconds to collect the data needed for design verification objectives and to help certify the stage for its first flight.
Lead image credit: NASA.