SLS Core Stage MPS: more than just a fuel tank

by Philip Sloss

MPS equipment

The Core Stage MPS is functionally similar to Space Shuttle MPS, and the engine section of a Core Stage is packed with much the same type of equipment as a Space Shuttle Orbiter aft compartment was, starting with the large propellant feedlines, manifolds, and valves that direct the liquids into the tanks before launch and then from the tanks to the engines while they are running.

The liquid oxygen and liquid hydrogen enter the vehicle from ground storage into the MPS lines at the bottom of the stage. “All of the fluids come in through the TSM, the Tail Service Mast Umbilical,” Looser said.

“We have two umbilical plates on the engine section, so all the hydrogen, all the oxygen, the helium to fill those tanks all come through individual quick disconnects that are on those plates.”

Credit: NASA/Eric Bordelon.

(Photo Caption: Close up with the -Z side of the Core Stage-1 engine section, showing the section of one of the two LOX feedlines that enter from outside. Most of the outside of the engine section is covered with the light-brown colored cork seen along the bottom. That area will eventually get a top coat of white paint seen already covering other areas of the circumference acreage. The LOX feedline and fittings that attach it to the engine section are covered with spray-on foam insulation (SOFI) that will turn a darker orange color after the stage has been outside for an extended period of time.)

The space at the top of the engine section interior is taken up by the aft dome of the liquid hydrogen tank and the aft manifold at the bottom of the dome which has a fill and drain line and four LH2 feedlines that run from there to each engine’s hydrogen inlet. “The sump on the bottom of the hydrogen tank, that’s how the hydrogen will enter the tank,” Looser explained.

“There’s a fill and drain line that connects to that, so you have four feed lines that come off of that sump and feed the four engines. You also have a fill and drain line that comes into that sump as well and that’s how the hydrogen tank is filled.”

With the change to finish overall Core Stage while in the horizontal, complete installation of the LH2 feedlines won’t occur until the stage is rotated to vertical after shipment from MAF.

The LOX feed system in the engine section is connected to the tank at the top of the stage by two large feedlines, also called downcomers, that run in large part on the outside of the stage.

“Even though the LOX downcomers come down the side of the vehicle, that LOX comes into the vehicle through the engine section through a fill and drain line and then once it enters the system it goes up those downcomers and fills the oxygen tank.”

“The biggest difference is that there are two downcomers coming off of the vehicle from the LOX tank down to the engine section; Shuttle only had one,” Jackson noted. “And that actually changed how we load it a bit. Shuttle would do more of a helium bubbling to prevent geysering in the downcomers.”

“Geysering is a huge ordeal that we go through to make sure you don’t get a water hammer or you get a bubble collapse in your downcomers. Since we have two ducts we actually create a circulation flow. That allows us to bubble some helium up one leg and that gets a flow moving and that’s an effective approach to avoid geysering.”

A semicircular, smaller-diameter crossover line in the engine section runs between the two large LOX feedlines as they enter the engine section from the outside for anti-geysering. The two feedlines then split via a Y-duct into the four LOX engine feedlines that connect to each engine’s LOX inlet.

A prevalve in each engine feedline is used to isolate the propellant from the engines when necessary and the fill and drain lines have a valve to isolate the vehicle from the ground supply.

Credit: NASA.

(Photo Caption: A replica of parts of the LOX feed system of the Core Stage MPS, used for anti-geyser testing at the Marshall Space Flight Center in 2014-2015. The two large diameter LOX feedlines enter the engine section from the outside and branch to each of the four RS-25 engines. The semi-circular, anti-geyser line that runs between the two main feeds that come from the LOX tank up at the top of the stage.)

One Shuttle system that was deleted from the SLS MPS was the LH2 recirculation system. “We do bleed hydrogen and oxygen through the engines at a very low rate to condition those prelaunch, Looser said.

“In the Shuttle days, we had recirculation pumps that kept that hydrogen [in] the system. Early on in this program for cost saving measures we decided we could get rid of those and we bleed that hydrogen overboard. Although it sounds like we just dump it over the side that’s not the case.”

“It goes back through a manifold where we send it back through a separate disconnect on those (TSM) umbilicals and then it will go out to the flare stack on the ground, either at Stennis if we’re testing it for Green Run or to the flare stack at Kennedy during launch,” he added.  Because the hydrogen bleed off the engines is still liquefied as it exits the vehicle, a hydrogen separator was added at Launch Pad 39B to allow it to boil off before it gets the flare stack.

The LOX feed system delivers the oxidizer into the engine section at colder temperatures than the engines were designed for, so heaters are used near the engine inlets. “Heaters on incorporated on the LOX feedlines near the engines, not on the downcomers which are specifically the lines on the outside of the vehicle,” Looser and Jackson noted. “These are used to maintain the start-box temperature, but are not used after that.”

The engine thrust mounts are connected to a large thrust structure near the bottom of the engine section. Much of the hydraulic system equipment is mounted to the top of the thrust structure.

“For each engine there is a, we called it a TVC platform,” Looser explained. “It basically looks like grating material; it’s a platform that sits on the thrust structure in the engine section and the TVC components for that engine are on that platform. And so there’s four identical platforms in the engine section for those four engines.”

Each engine has a hydraulic system to supply its TVC and operating needs. “It all starts around the actuator,” Jackson said. “We have a term we use called ‘direct reuse,’ that’s when we are actually using a part that literally flew on the Shuttle, not just the design but we’re using true parts — similar to the engines. The actuators are one of those parts, so they are pulled right off of orbiters and are going to fly.”

Each engine has two actuators for pitch and yaw. “Everything that we put in the TVC system basically is to feed those actuators, so you have a hydraulic system which means you have a hydraulic reservoir, you’ve got accumulators just so you keep the pressure surges under control, you’ve got a main pump that feeds it, that circulates the system, that keeps the pressure up,” Jackson added.

Equipment for each hydraulic system includes a Shuttle-modified Core Auxiliary Power Unit (CAPU). The CAPU turbines are spin-started with a ground-supply of helium before engine start; as the engines are started six seconds before liftoff, the gaseous hydrogen tap off pressure will eventually bypass the helium to keep the turbines running.

Credit: NASA.

(Photo Caption: A slide from a NASA presentation showing some of the Shuttle MPS hardware being reused in the SLS Core Stage MPS along with the RS-25 engines. There is said to be enough Shuttle MPS/TVC hardware for at least the first two Core Stages.)

The TVC/hydraulic subsystem is one of many going through qualification testing at the Marshall Space Flight Center (MSFC) in Huntsville, Alabama, where the SLS Program is based. “The actuators are the thing that jump out at people but you also have electrical controllers, TACs (TVC Actuator Controller),” Nick Johnston, NASA test conductor at Marshall for the TVC Systems Integration and Components Branch, noted in an interview.

“We also have a reservoir that’s just where you store [or] house your hydraulic fluids. An accumulator also which helps sort of dampen those pressure fluctuations, and that’s the supply accumulator, we also have one on the return side which is underneath,” he said, referring to the subsystem qualification test configuration in the TVC lab at Marshall.

“We also have a filter manifold, that has our filters, that’s where all the plumbing kind of ties together as you can see from all the pipes going to it,” he added. “And a circulation pump; its function is just to circulate hydraulic fluid during the loading of the propellant because the temperatures in the rocket get so cold.”

“We don’t want a cold slug of fluid or something like that going through [the system], so the circ pump just circulates the fluid through, kind of keeps it all thermally consistent.” The TVC circulation pumps are another piece of direct reuse Shuttle hardware.

Each of the four hydraulic systems also have an exhaust gas heat exchanger. As with the Shuttle APUs, the CAPUs also have exhaust gas that is dumped overboard; in the case of the Core Stage, the exhaust is ducted to vents that are located on the boattail.

As a part of subsystem qualification testing, the hydraulic system in the lab will be run through a launch profile. “We’ll have tanks of helium and hydrogen outside and we’ll bring it up on helium just like we would [on] launch day,” Johnston said.

“When we test with these guys, the SIL (Software Integration Lab) facility, they’re commanding. They’ll pretend to do an engine start and at that time we would turn on the hydrogen gas pressure which will overcome the helium.”

“It runs at higher pressure so it’s got a check valve system, so it just pushes against it hard enough, it takes over essentially and then the system will run on hydrogen gas,” Johnston added. “In our world it blows out the exhaust gas heat exchanger and then we run into an exit duct which goes outside the facility and up.”

Looser and Jackson noted that the SLS Program has at least two full Core Stage sets worth of TVC hardware being reused from Shuttle Orbiters before the program will have to build additional equipment. Prior to that, a decision will be made whether to build new units to the Shuttle design or to an evolved design.

“Design reuse” is the other type of reuse from Shuttle. “There are many components across the MPS and TVC systems that benefit from design reuse (using a Shuttle era design with small modifications to meet SLS environments),” they noted.

“The MPS system evolved the heritage designs for numerous components to meet the SLS environments; however, the MPS has new build hardware. For example, the Prevalves are direct evolution of the Shuttle Orbiter Prevalves with changes to strengthen the housing.”

Credit: NASA/Troy Cryder.

(Photo Caption: USA technicians flank the primer green-colored pitch and yaw TVC actuators in Shuttle Orbiter Atlantis’s Main Propulsion System during engine number two installation operations for the STS-132 mission in February, 2010. These Shuttle MPS hydraulic actuators were removed from subsequently retired Orbiters and are being reused in Core Stage MPS hydraulic/TVC systems.)

The five large COPV tanks are also located in the engine section to provide the vehicle’s helium supply for pneumatically actuating MPS valves, critical engine purges, and for a backup engine shutdown capability. “Those five helium tanks are the source of our pneumatic system,” Looser noted. “They provide the helium pressure to operate the large valves that control the flow of LOX and LH2 through the system.”

During engine mainstage, the tapped off hydrogen gas from the engines are routed inside the aft compartment to spin the CAPU turbines. Both tapped off gaseous hydrogen and oxygen are routed from the engines for tank repressurization, with lines that run from inside the engine section outside and up to the top of both propellant tanks.

A sufficient amount of ullage pressure in the tanks needs to be maintained for structural integrity as the engines drain the propellant out of them. Additionally, space in the engine section is consumed by tubing that connects all of the equipment together and wiring bundles that connect the equipment and instrumentation to avionics boxes co-located in the aft compartment and in other areas of the stage.

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