NASA and prime contractor Boeing have reworked the final assembly plan for the first Space Launch System (SLS) Core Stage to bring the scheduled completion date back to the end of 2019.
With the engine section at the bottom of the rocket pacing the overall schedule into the middle of the year, changing the sequence of the last two major connections allows more work to be done on the rest of the rocket and reduces the list of tasks to complete when everything is joined together.
The rest of the stage without the engine section will be connected horizontally in the next month at the Michoud Assembly Facility (MAF) in New Orleans. Connecting the top of the stage first forces the final connection with the engine section also to be done horizontally, and orphans a sequence of tasks that must be done completed the next time the stage is rotated to vertical.
The new plan should save several months of time versus the few weeks required to finish those last assembly tasks at the stage’s next destination.
NASA is still debating internally where Core Stage-1 (CS-1) will go when it leaves MAF, with a 45-day study due today, but Boeing is hoping to have the stage fully mated in the middle of the Summer and ready for transport to its next venue by the end of the year. If the space agency decides to skip performing a full Core Stage ground test at the Stennis Space Center to gain back additional schedule, CS-1 would go immediately to the Kennedy Space Center in Florida for launch.
Horizontal work makes better use of time, with one exception
The latest change in plans rethinks one of the basics of assembly, which was that Core Stages would be vertically assembled into two half sections first. A final join would then be done horizontally, because at essentially two hundred feet long without its engines installed, the stage is significantly taller than the Vertical Assembly Building at MAF, Building 110.
“We had major join 1, that’s the forward skirt, the LOX tank, and the intertank,” Craig Williams, Boeing’s Core Stage Integrated Product Team Director, said in an April 8 interview. “That one independently is unchanged, that was all stacked vertically and it’s just about complete.”
“The second join was originally the LH2 (liquid hydrogen) tank to the engine section and that was originally planned to be done vertically here at MAF. So what we had come up with to expedite this plan was a horizontal approach.”
The Core Stage is the new piece of SLS and building the first one has gone slowly; the engine section is the most complicated part of the stage and was the slowest to finish. With some estimates that the stage wouldn’t be ready for another year and NASA pressing hard to return Americans to land on the Moon by 2024, the agency brought in additional oversight and after worked with Boeing to recover schedule.
“Our new plan then would be taking the LH2 that we just talked about that was planned to be a vertical mate to the engine section, not do that but mate the LH2 initially to the forward join horizontally in our Area 47/48,” Williams explained. “So that would be our second mate.”
“Meanwhile, this new plan allows the engine section to go along being final integrated and then functionally tested,” he added. “Then when it was done with its life as an independent volume we’d lastly take that engine section, turn it horizontally, and mate it to the [top] four-fifths of the rocket and then make the rocket complete.”
The engine section finally completed its standalone integration work in mid-March, was then mated to the boattail assembly, and moved into the area on April 1 in a newly built tool. The connected engine section and boattail assembly still has months of functional testing to do before it is ready for the now final connection, so the other two pieces will be connected first in mid-May.
By late July, Boeing hopes to have the engine section and boattail ready to mate to the bottom of the LH2 tank. The new plan allows work to continue on the rest of the rocket in the meantime, rather than forcing it to be deferred until the Summer.
The engine section was designed to be integrated to the LH2 tank vertically, so this late change in the final assembly plan will leave one set of Main Propulsion System (MPS) assembly tasks incomplete when work on the stage is finished at MAF.
“What that does is we have a few connections internal to the engine section join to the hydrogen tank with our four feedlines and our fill and drain line that can’t be worked until the vehicle is vertical again,” Williams explained. “There would be some vertical work inside that volume to tie in those last commodities if you will and then make those final hookups and then do some testing and then we’d be ready.”
Under the new plan, the stage remains horizontal for the remainder of its time at MAF and the stage is transported horizontally by barge, so the LH2 MPS work would need to be done after arrival at its next destination. Currently that is still penciled in to be the B-2 Test Stand at the relatively nearby Stennis Space Center in Mississippi; however, NASA is now prioritizing and emphasizing the schedule for the first SLS launch, Exploration Mission-1, and the barge may instead go directly from MAF to the Kennedy Space Center in Florida.
The five connections come off of the LH2 tank’s aft manifold that is at the bottom of the tank and which sits inside the engine section volume. Four feedlines run from the aft manifold through prevalves to the LH2 inlets on each of the RS-25 engines in the stage. A fill and drain line runs from one of the Tail Service Mast Umbilical (TSMU) plates on outside of the engine section into the aft manifold for ground loading and offloading of the cryogenic propellant.
“We’d mate them and then we would do our leak checks on them and we would close them out internally with the pour foam closeout process,” Williams noted. He said the work should take about three weeks and that similarly deferred portions of the stage’s final integrated functional test (FIFT) that are related to the MPS pieces would then be completed.
Engine section / boattail work moved out to open area
After completion of engine section standalone integration and testing, the next cycle was to integrate with the boattail and then begin functional testing of the all the engine section elements. The pieces were moved into the VAB at MAF and structurally bolted together in Cell A, also known as Area 39.
The original plan was to do all of the integration and testing between the engine section and boattail there in Cell A and then bring in the LH2 tank for the next integration and test cycle; however, another aspect to the updated final assembly plan was to move the current work to an area with better access and controls.
“We did that boattail-engine section stacking in [Area] 39 as originally planned and then that area there is probably the least environmentally controlled area in the factory and it was cramped for space so we came up with an opportunity to build that engine section boattail transportation tool that enabled us to come over here to [Area] 47,” Williams said.
“[Area] 47 is the north part of the high bay there and 48 is the south part of the Final Assembly bay, so it’s parked in the 47 area and it enabled us to do a few things. It increased our access, it gave us better controls of our environment, particularly around ESD (electrostatic discharge) concerns with temperature and humidity and there’s quite a bit of electrical work to complete in that installation volume.”
“And then it allowed us to utilize the Final Assembly test areas and all the utilities, so we burned down some extra risk with our earlier deployment of that test system there,” he added. “In the current schedule we should be testing I believe in early June, late May is when we’ll start testing for the engine section and like I said just utilizing those utilities early.”
The boattail assembly is a streamlined extension for the bottom of the engine section and also includes a downward-facing base shield that provides protection from the heat generated during launch and ascent. After the mated engine section/boattail moved into the Final Assembly area on the new tooling, work resumed making the tube welds and wiring connections between the two.
“We moved over to there to 47 on April 1st and the team immediately started working and I think we’ve probably done five or six of the orbital tube welds already,” Williams said. “So that’s one of the first operations we’re doing, that they’re engaging in.”
“There was a lot of coiled, stored wire harnesses in the engine section independently and in the boattail, now we’re uncoiling all these and then they’re running up the length to their mated endpoints. So if you were to envision we had to get those guys mated together before we could complete routing of those wire harnesses so the team is off and running and engaged in that work as well.”
“There’s two levels internal to the engine section, the upper level if you will above the SRB ring the access is the exact same, the internal access platforms have never come out so that remains as is,” he added. “We had to provide some additional access with an internal access tooling kit we’re building that fits up inside the boattail and allows us to get up to the height below the SRB ring so that kit is actually going to be here Monday and we’ll start installing that in there.”
In addition to the better environmental controls in Final Assembly, the area allows more of the support staff better access to the technicians working on the hardware than in VAB. “We have seen particularly an increase with the ship side support teams if you will,” Williams noted. “If you were to go to engine section now you’d see a team of about seventy people out there, all disciplines, engineering, technicians, and the test folks, so a much better footprint to support the build in 47/48 than Area 39.”