LH2 tank status:
The aft join will assemble the LH2 tank and the engine section with its boattail assembly into the bottom half of the rocket. Right now the plan is for the LH2 tank to follow the LOX tank through primer and SOFI applications. The tank has now completed the same major milestones as the LOX tank, with it being moved last week out of Cell E in Building 110.
“It’s been recently in Cell E, to finish it’s wash cycle and as soon as the IT structural test article was ready to go we moved it back east to get it out of the way and then we just picked up the hydrogen tank, lifted it over the wall for Cell E and put it in Cell D,” Whipps explained.
The tank was internally cleaned following its pneumatic proof test in Building 451. The tank was run through a series of test cases where it was pressurized with nitrogen gas up to and over flight pressure while hydraulic actuators simultaneously placed representative loads on the structure.
After the tests were completed, the tank was brought back to Building 103 where the welds went through the same ultrasonic NDE inspections that were carried out shortly after the welds were made.
With the post-proof NDE inspections and internal cleaning completed, the LH2 tank will go through a round of sensor installations before TPS applications begin. The first step, though is the work being carried out in Cell D.
“The next activity is kind of a paint rolling technique and put primer in the stripe around top and bottom that will be where the roll rings will go when we horizontally move this tank around in subsequent processing,” he explained.
“Once those rings get put on there, we can’t get primer onto those areas because they have roll rings in the way. So we’ll put that on first even before the rest of the tank sees primer, so that later when we…do other processing, primer is there under the roll rings.”
Engine section remains critical path and biggest challenge:
The engine section is where all the components of the Core Stage come together and it is the busiest and most complicated piece of the rocket. “It is by far the most complex of the sub-elements that make up the Core Stage and there’s just a blizzard of activity going on,” Whipps noted.
“We’ve finished all the structural assembly, meaning everything associated with the barrel, everything associated with the cruciform, and the gimbals that we’ll actually attach the engines to later, that’s all been put together. And then after that we’ve got all the shelves installed and there’s probably miles of plumbing and electrical work associated with all the propulsion-related [hardware] — downcomers that feed both LH2 and LOX into the four different rocket engines, thrust vector control and hydraulic systems are all going in.”
“So it’s extremely complex, but again it’s a little bit of a beehive [of activity] and there’s a complex dance going on,” he continued. “We can only get literally so many people in there [at once], and [the challenge is] how do we keep — at every level — everyone working efficiently. And of course you have to get parts, paper, and people all there at the right time doing the right things and that’s kind of the learning curve that we’re trying to maximize now.”
When fully assembled, the aft dome of the LH2 tank takes up much of the upper volume inside the engine section, and the Main Propulsion System (MPS) hardware alone includes the LOX feedlines (as known as downcomers) that enter the engine section from the outside and branch off inside to each of the four engines. Likewise, LH2 feedlines inside branch from the bottom of the tank to the engines.
The engine powerheads, thrust structure, and all of the hydraulic thrust vector control (TVC) systems needed to move the engines to help steer the vehicle in flight are also co-located with, in many cases, dedicated avionics controllers for each critical piece. Each engine has a hydraulic power unit (called a Core Stage Auxiliary Power Unit or CAPU), and each CAPU has its own controller. Likewise, each engine has its own TVC hardware and avionics controller.
In addition, two Combined Control System Electronics (CCSE) boxes work with the flight computers to handle control of the MPS, commanding and reporting the position of the various MPS valves used for things like propellant management and tank pressurization. A second rate gyro assembly, and two more data acquisition controllers are also located inside.
The miles of wiring that Whipps referred to are necessary to connect power coming from the intertank and network data between the different components in the engine section, the intertank, all the way back up the systems tunnel to the flight computers in the forward skirt.
“Dozens of people working [are] on it all the time, literally 24/7. We’ve got folks working on it at various levels at the same time.”
Across the aisle where the engine section is set up for this integration work, some of the larger elements are being prepared for future installation or are staged, ready when their turn comes. “You might look across the aisle and see some of the propellant lines and elbows that are ready for installation,” Whipps noted.
For now, a controlled work area is set up in and around the engine section. “Almost like a clean room, but not quite to that level of cleanliness,” he explained, “but you’ve literally got people in there that are in [a] bunny suit kind of thing.”
“So all of the parts that we’re going to put in that are larger, like the composite overwrapped helium pressure vessels, things like that where we’ll have to lift them in with a crane, we’ll be waiting until we get all this ancillary work that will be in and around it [done], so that when we lift that in we’ll have to break the controlled work area to do the crane work and then put it back together again.”
“This is part of this choreographed ballet that they’re trying to learn how to do,” he noted.
Around the outside of the barrel, sections of cork are starting to be applied in some areas as time and other work permits. And nearby, the boattail structure that will attach to the bottom of the engine section is coming together. “We’re well along with the boattail, it’s a good success story,” he said.
“It’s not as complex as the other parts but there are still plenty of parts and that’s been going along very well.”
The engine section has been the critical path item for this first Core Stage for a while and since it is the most intricate piece of the rocket, it has the most extreme of the first-time learning curves.
When asked about getting to the end of the integration process for the element, Whipps said: “We have a theoretical sequence, but we’re going to have to determine [how that will occur] whether from a non-conformance standpoint and from a lessons learned standpoint, etcetera.
“I think we expect to have a rev 1, 2, and 3 of that plan before we finally have them in. But obviously we’re thinking hard about it to be sure that we’re as efficient as we can with that manufacturing sequence, but that’s what you do with rev 1, right? That’s when you learn all those lessons.”
“You start with a default plan — and we’re still following that as best as we can — but clearly we’re learning as we go along that what we thought would make sense when we had a bunch of industrial engineers, production engineers, six or twelve months ago put our original work instructions together that a little like your plans for battle they don’t make it through their first skirmish.”