One of the main elements of NASA’s Space Launch System (SLS) Core Stage that makes it more than just a big fuel tank is the Main Propulsion System (MPS). All the equipment for the care and feeding of uprated Space Shuttle Main Engines (SSME), adapted for SLS by Aerojet Rocketdyne as the RS-25, is repackaged from the backend of Space Shuttle Orbiters into a more traditional inline rocket stage.



SLS upsized the Shuttle elements — the big fuel tanks are now longer, and there are more engines in the bottom of the stage. Along with the engines, the MPS is packed into the complicated engine section of the Core Stage.

Pulling everything together into the first unit has been a struggle for civilian space agency and prime contractor Boeing; the current target is to complete the first article by the end of the year, well behind early schedule estimates.

The MPS supplies the propellant to run four RS-25 engines for over eight minutes during launch while also supplying the hydraulics to throttle and point the engines based on commands from the vehicle’s flight computers, managing valve positions to control supply rates, keeping the propellant tanks pressurized, and then ensuring a safe and orderly shut down at the end of powered flight.

MPS also works in tandem with ground systems to control the pre-launch loading of propellant and the chilldown of the engines so they are ready to start at the appointed time.

Even with fuel tanks assembled, Core Stage still incomplete

The big fuel tank part of the first Core Stage flight article was recently assembled at the Michoud Assembly Facility (MAF) in New Orleans, with the liquid hydrogen (LH2) tank attached to the forward join that was assembled earlier this year. The second of three major joins for Core Stage-1 started with bolting the two sections together over the Memorial Day weekend.

Assembly of the remaining sections was revised to do the last two of three major joins horizontally, as opposed to just the final one. The revised plan was recently put into effect to allow more work to be done in parallel in order to complete stage assembly by the end of the year. The sections now joined in the Final Assembly area at MAF encompass four-fifths of the stage’s major elements, but those don’t make a rocket stage any more than the Shuttle External Tank did by itself.

(Photo Caption: Major join 2 of Core Stage-1 in progress at MAF on May 29. The mated assembly is four-fifths of the stage elements, but is missing its fundamental propulsion elements located in the engine section, such as the MPS, engines, and other components.)

The remaining piece of the Core Stage to be joined is arguably the most complicated one in the launch vehicle, the engine section, which houses much of the MPS equipment and is where the engines are attached. The engine section isn’t the only part of program development behind early estimates, but it has ended up taking the longest to complete for this first flight article and is what prompted the change in final assembly plans.

The same types of equipment bunched in the aft compartment of the Shuttle orbiters similarly creates a cramped space in the engine section, with SLS adding a fourth set of equipment to support an additional engine. The stage is the ground-started sustainer lifted by two large Solid Rocket Boosters (SRB) for the first two minutes of launch.

The Main Propulsion System is designed around the Space Shuttle Main Engines (SSME) inherited from the Space Shuttle Program; for the Core Stage, SLS transformed the other launch elements from Shuttle into a bigger, in-line rocket. “A lot of the basics are the same for SLS as it was for Shuttle,” Jonathan Looser, NASA SLS Core Stage Propulsion Lead, said in an interview. “The main driver of that is the engine.”

“We’re using the RS-25 which is the old Space Shuttle Main Engine and so start box, pressures, temperatures, all of those types of parameters have many years of history and so your starting point for your loading sequence, your timelines, your conditions that you’re going for are all based in thirty-plus years of Shuttle history, and so we do the best we can to replicate a lot of that.”

The flight engines, spare units, and parts in service at the end of the Shuttle were transferred to the SLS; additionally, MPS makes use of both Shuttle hardware and designs. Some MPS components are “direct reuse,” parts that flew on Shuttle orbiters; other components are “design reuse,” where the Shuttle design was evolved to meet SLS requirements.

(Photo Caption: The Space Shuttle Main Propulsion Test Article (MPTA) was used to conduct long-duration, integrated propulsion system test firings at Stennis Space Center. SLS merged the MPTA aft fuselage and External Tank elements into an inline rocket stage. The left image shows the orbiter aft fuselage built and integrated for the MPTA being lifted into the B-2 Test Stand in 1977. The figure on the right shows the integrated MPTA test configuration.)

“Main propulsion is four different areas that we kind of break up. We include main propulsion systems and thrust vector control, your thrust vector control is the system that helps steer the rocket, so your hydraulic systems that power the pumps that are necessary to gimbal the engines, that entire system is your thrust vector control.”

“The liquid oxygen feed system, the liquid hydrogen feed system — both of those obviously, you have your two main propellant tanks,” he added. “And then from a main propulsion standpoint it’s all lines, ducts, valves that go to provide those propellants from the tanks to the engines.”

“The other two main parts of the main propulsion system is your pressurization and pneumatics,” Looser continued. “So the pressurization system, pressurizing both the liquid oxygen and liquid hydrogen tanks with helium and with oxygen and hydrogen once the engines are running, that’s all part of our propulsion system and the valves and the lines necessary to do that are all included.”

“If you had a chance to see inside the engine section you saw a lot of those helium lines installed and small valves as part of that. And then pneumatics is everything that is required to operate those valves, the large valves. So the five large helium tanks are the source of our pneumatic systems, so those five provide the helium pressure to be to operate those large valves that control the flow of liquid oxygen and liquid hydrogen through the system.”

“An easy way to think of it is main propulsion pretty much is there to fill the tanks and feed the engines,” Collin Jackson, Boeing Propulsion Technical Lead Engineer, explained in the interview. “So if there’s a tank on the rocket that we need to fill, it’s usually main propulsion that owns it and if we’re filling it we’re trying to give it to the engine, it’s either to give the oxygen or the hydrogen or the helium that the engine needs to do its purging, so that’s MPS. And then thrust vector control (TVC) is really to point the nozzle, anything it takes to steer the engine and point it so you can steer the vehicle.”

Coordination of components

MPS works in conjunction with equipment from other subsystems located both inside the engine section and outside, but then some of its systems are also interdependent.

The hydraulic systems provide the power to the thrust vector control actuators to control the position of all four engines, and for operation the RS-25 engines themselves. Valves in the engines are hydraulically actuated to get them started, adjust power levels, and shut them down.

The pneumatic system is used to actuate the MPS valves, all the components to fill the propellant tanks and feed the engines during launch and ascent. “They also provide a critical purge gas to the engine that it needs during flight,” Jackson noted.

(Photo Caption: The engine section and its boattail extension in Cell A of Building 110 at MAF. A part of a series of ‘integrate and test’ cycles to build up the stage, the boattail structure was first bolted to the bottom of the engine section. As a part of new final assembly plan, they were moved to Area 47/48 in Building 103 to first integrate additional fluid and electrical connections and then functionally test the mated element.)

Five large composite overwrapped pressure vessels (COPV) store the gaseous helium used by the pneumatic system. Helium also provides an alternate way to close the engine valves to pneumatically shutdown an engine in a contingency case where a running RS-25 has gone into hydraulic lockup.

“The thrust vector control system provides hydraulic actuation pressure to the engine valves and there are two different ways to shut down an engine,” Looser said. “There are pneumatics so you have helium pressure to those engine valves and then the Core Stage TVC system provides hydraulics to those engine valves as well.”

“It’s the same engine that we flew on Shuttle so the supply and operation of those Shuttle valves is very similar for SLS.”

As in Shuttle, the engine operation provides pressure to keep the propellant tanks pressurized to maintain their structural integrity as they are emptied during powered flight. Ground-supplied helium is used before liftoff for pressurization, but once the engines start gaseous oxygen and hydrogen is tapped off of all of them and routed back to the top of their respective propellant tanks.

For SLS, some of engine’s hydrogen gas tap off also branches to the hydraulic system to spin the auxiliary power unit turbines after the RS-25 engines are up and running.

“We use the auxiliary power units out of the Shuttle, but we’ve modified them a little bit,” Jackson noted. “We took the actual turbines out of the Shuttle, but they were using hydrazine because the Shuttle you’ll recall had to act as an airplane on the way down.”

“Since SLS doesn’t, what we do instead of carrying the hydrazine [is] we just tap off the press gas going to the hydrogen tank and use that to spin the turbine that we use to power the main pump that gives you your hydraulic pressure.”

(Photo Caption: Two United Space Alliance (USA) technicians carry a Shuttle auxiliary power unit (APU) to one of the aft compartment access doors of Shuttle Orbiter Endeavour in June, 2008, for installation ahead of the STS-126 mission. The APU hardware was removed from Shuttle Orbiters after their retirement and modified for use in the TVC/hydraulic system of the Core Stage MPS. Now called Core APUs or CAPUs, the turbines are spin-started by helium prior to engine start; after the engines start, gaseous hydrogen from operation is used in part to keep the CAPU turbines spinning.)

The two propellant tanks have a series of sensors that are a part of the Cryogenic Level Sensing System (CLSS) that is used to pace pre-launch propellant loading and let the vehicle computers know when it is about to run out of fuel in-flight. The liquid level sensors in both tanks are wired to the CLSS avionics box located in the intertank.

“Depending on what level we are, where we are in the loading process, there are sensors inside each one of the tanks that will show us how full that tank is,” Looser said. “As we get towards the top of the tank, we slow the fill process down to a topping and a replenish and we just maintain a full tank during the hours of loading and propellant conditioning and engine conditioning at the end of the tanking process.”

The MPS equipment works in concert with other equipment in the engine section from other subsystems. Several avionics boxes in the engine compartment are used by the SLS flight computers for vehicle management which covers everything broadly, but there also dedicated controllers for propellant management, the hydraulic power units and systems, and TVC actuators. There is also a Rate Gyro Assembly and two Data Acquisition Controllers.

The RS-25 engines themselves are managed as a separate subsystem and have their own dedicated avionics, an engine controller unit that is integrated with the powerhead.

The hazard gas detection system equipment monitors concentration levels in the dry sections of the vehicle — the engine section, the intertank, and the forward skirt. “In terms of what we consider the classic propulsion system it is separate, but it is very integral to the operation of the Core Stage,” Looser said.

“That hazardous gas detection system is looking for any leaks from any of the valves, any of the lines, it’s measuring the atmosphere inside that compartment looking for hydrogen leaks, oxygen leaks, or maybe helium leaks from any of the helium tanks as well. And not just the dry compartments but we do have sensors inside some of the umbilical plates, so at those quick disconnects if there are leaks at those joints that system is looking for any potential for leaks there as well.”

(Photo Caption: In the center of the image are the two tail service mast umbilical plates on the +Z side of the engine section. The vehicle-side plates will be connected with ground-side plates that provide the different fluid, electrical, and environment control services to the vehicle. Hazardous gas detection sensors are deployed at this ground/vehicle umbilical connection and other parts and other parts of the vehicle looking for leaks.)

While the hazard gas detection system is looking for signs of a leak, the ground environment control system (ECS) is mitigating the risk of a fire in case of a leak. During propellant loading operations, a flow of nitrogen gas is run through the dry sections to purge open areas of flammable vapors. During non-hazardous periods, a dry, conditioned air purge is run through the sections.

Heaters are also used directly to manage the temperatures of some of the MPS equipment in the engine section. “For hydraulic fluid lines, we use tape and rope heaters,” Looser and Jackson noted.

“The circulation pump is integrated into the system and circulates the hydraulic fluid directly – this is another Space Shuttle Orbiter direct reuse part. Heaters are also used on the gaseous helium bottles in order to maintain the necessary helium temperatures.”