All of the equipment in the engine section is now installed; hardware moves through the assembly process in iterative “integrate and test” cycles. After the engine section was integrated and tested, it was next integrated with the boattail structure in late March.
After the top of the boattail was bolted to the bottom of the engine section barrel, an additional set of tubes and wiring was to be connected, followed by functional tests.
“We do checkout most of these systems to some extent before we even leave MAF,” Looser said. “So as we’re assembling the bleed system or the liquid oxygen system or the gaseous oxygen system or different pneumatic systems, we’re assembling those, making those connections and welds at MAF, we will do leak checks and pressure checks.”
“We will fill the hydraulic system and do some check on it. We will do a leak test on the helium tanks, so at the component and the individual system level we do have some level of checkout and test on those before we even leave the factory.”
“We will bleed those tanks off to a lower, standby pressure and then they will stay at that lower pressure until we get ready for the next test and then bring that pressure back up,” Looser added. “We do have the opportunity in the equipment at Stennis to service hydraulics if it’s necessary, but we don’t drain that system in between tests.”
(Photo Caption: The engine section is lifted out of Cell A at MAF earlier this Spring after the boattail structure was bolted to its underside. Integration of all the equipment inside the engine section was and is the critical path in the assembly schedule for Core Stage-1.)
The MPS will get its first full workout during propellant loading operations. NASA is still debating where to conduct those first test cryogenic flows, either at the Kennedy Space Center launch site or as originally decided in the B-2 Test Stand at the Stennis Space Center in Mississippi during a comprehensive Stage “Green Run” test campaign.
“There is a series of tests that occur at Green Run,” Looser said. “Green Run is not just the hot-fire test, the hot-fire test is really the culmination of the Green Run time-frame.”
“Even prior to fixing the stage in the stand there’s a modal test that will occur at Stennis with the entire stage on the crane. There are leak checks and functional checks of the MPS and TVC system.”
“We do basically a simulated countdown, what we call a wet dress rehearsal where we will load the tank and then drain the tank and go through the countdown process as if we were going to do a hot-fire and then stopping just prior to starting the engines,” he added. “All of that is building towards that hot-fire because it is the first stage and we want to check it in a methodical way before we get to that hot-fire test, to give ourselves the best opportunity to have a successful hot-fire and find any potential issues or problems prior to firing that stage.”
Although the Core Stage is scaled up in size, propellant loading in preparation for firing the engines borrows a lot from its Shuttle heritage. “A lot of the basics are the same for SLS as it was for Shuttle, the main driver of that is the engine,” Looser explained.
(Photo Caption: The four RS-25 flight engines assigned to Core Stage-1 as “delivered in place” in October, 2017, at Aerojet Rocketdyne’s facility at Stennis. The engines are bolted into the Core Stage MPS, which supplies propellant, hydraulics, and pneumatics to them. While they are firing, the engines return high-pressure gas to the MPS to maintain pressure to the hydraulic systems and to the propellant tanks. Installation of the engines won’t begin until after the engine section is attached to the rest of the stage, planned for the Summer.)
“By definition we try and keep it similar because loading the vehicle was a hard-fought lesson of the Shuttle and so we wanted to build off that heritage and keep it as similar as we can,” Jackson added. “We’ve got the configuration change, so we did have to do some of our subsystem testing with the anti-geyser test that I mentioned before that was performed at Marshall, but in general we try to maintain the operations as similar as we can to Shuttle because our goal is to build off the process that worked previously.”
Similarly for the terminal, pre-start sequence for the stage, getting the engines to their starting point and bringing up the subsystem equipment to support starting and running them is similar to Shuttle. “In general it’s very similar and again that all goes back to the use of these engines,” Looser said.
“You’re trying to mimic what these engines have seen for thirty plus years of operation. The differences being you have much larger volumes in the tanks and so there are some differences in what it takes in terms of the time and pressure, things like that to achieve those conditions. But in general, the process and the timelines and the sequence are very similar to Shuttle.”
“I guess the caveat to that is, that is what we have by analysis,” he added. “We anticipate we may learn some things as we start going through Wet Dress Rehearsals at Green Run that may change some of that, but everything we’ve done today through analysis and models show that we’ll have a very similar process.”
After Main Engine Cut Off (MECO), MPS has less to do on SLS than on Shuttle because the Core Stage is expendable. “That’s one area where from a propulsion standpoint we will have some differences from Shuttle,” Looser said.
“Because of the reuse of Shuttle, there were certain purges and sequences that happened on-orbit in order to maintain that propulsion system for reuse, and obviously to keep it safe. With a expendable rocket, a lot of things that happen on orbit are not necessary and so after main engine shutdown, really just preparing for separation and from a propulsion standpoint there’s really not much involved there.”
Speaking of MECO the engines are pressurizing the hydraulic systems while they run, but that pressure will eventually decay during shutdown which is done hydraulically. “The key there is you get a certain amount of spin down time from the engine where you’re still getting sufficient gas to keep the turbine spinning and then once that trails off we rely on the supply accumulator to keep the pressure up,” Jackson explained.
“Our supply accumulator is quite large and it keeps the pressure up for the engine just to complete the shutdown process.”
The first flight Core Stage will also be a ground pathfinder for demonstrating recycle capabilities and comparing that test experience against existing analytic models. “During the wet dress rehearsal and Green Run testing we hope to able to validate those models and show how long you have to hold, how long it would take to recycle, and that timeline,” Looser noted.
“I would say it’s been part of the design,” Jackson added. “Our intent with the vehicle and how we set it up was to enable the hold windows. That was kind of proven to be necessary based on Shuttle experience [and] it factored in the design as we’re building the rocket.”
Most of the initial SLS launches are assigned to lunar missions, which will have an approximately two-hour long launch window on a given day. This may be enough time to recycle from a late countdown hold to a second attempt at launch.
(Lead Image Credit: NASA/Eric Bordelon.)