Activation of the B-2 test stand at Stennis Space Center in Mississippi for the “Green Run” test campaign of the first Space Launch System (SLS) Core Stage is almost complete and NASA and its contractor workforce are moving into practice and simulations of the upcoming tests while finishing up remaining tasks. The B-2 position of the test stand has been used for stage testing from Saturn to Shuttle to Delta IV, and NASA is planning to set up the first Core Stage there in the second half of 2019.
A series of tests in the Fall of 2018 verified the working operation of critical cryogenic propellant management and environmental control systems for Core Stage service and support. Over the past six years, the Shuttle-era B-2 test stand structure was restored, construction of additional structure for SLS completed, and outfitting and activation of test stand and test control center systems are almost complete.
The final major structural pieces, the two forward hold downs called “yellow boxes,” were also installed in late 2018. A “form and fit” Pathfinder test article is planned to arrive in the early Spring to practice lifting and handling the real thing when it finally arrives at Stennis.
Wrapping up activation
The systems that will flow hazardous fluids and gases to the vehicle were tested in the Fall following their activation. As noted in the November Lagniappe issue, systems that flow cryogenic liquid oxygen (LOX) and liquid hydrogen (LH2) propellant from storage locations into the stand were tested individually.
The High-Pressure Gas Facility (HPGF) at Stennis was also tested in October to verify recent upgrades to meet the gaseous nitrogen requirements of the Core Stage. “We pretty much to date have finished all the restoration,” Bryon Maynard, Project Engineer for NASA’s SLS Core Stage B2 Green Run Project said during an interview on December 20, just prior to NASA’s closure by the ongoing, partial U.S. government shutdown.
“We’ve pretty much finished all the construction work for the SLS and we’re pretty much down to our last activation test in the fluids area, cryogenics, which is the LOX replenishment.”
LOX and LH2 are supplied to the B-stand by barges, with up to three barges for each commodity that can be docked simultaneously in the area; the stand itself also has a tank for storing LOX. When the stage is in place, the propellant will flow up to the tail service mast (TSM) umbilical plates on the +Z side of the engine section and into the vehicle.
Without the vehicle, special equipment was used for activation testing. “For hydrogen we’re probably within about thirty, forty feet of the umbilical because we have to put activation spools in because we’ve got to send the hydrogen back to the flare stack,” Maynard explained.
“So you’ll see the line come up short of the umbilical maybe by thirty, forty feet and then we hook up an activation spool that goes to our vents. That last thirty feet of pipe that we have to put back in doesn’t affect the overall system. We’ve pretty much got ninety-nine percent of it [activated].”
The equipment that connects directly to the vehicle umbilical plates will be installed after activation. “That’s our special test equipment, so once we finish activation on the facility then we’ll put that last piece in there,” Maynard said.
During fueling, liquid oxygen and liquid hydrogen that boil off or are used for chilling down propellant and engine hardware exit the vehicle through vent and bleed lines. The highly flammable hydrogen is directed to a flare stack at a safe distance from the stand where it is burned off, while the oxygen is dumped out to a pit set away from the stand on the opposite side.
The HPGF supplies commodities such as air, gaseous helium, gaseous nitrogen, and gaseous hydrogen to the different test complexes at Stennis. It was upgraded to support the gaseous nitrogen demands of the Core Stage during tanking operations. As a part of the environmental control system (ECS) for the vehicle, a ground supply of temperature and humidity controlled air is typically run through the dry sections (forward skirt, intertank, and engine section).
During and around fueling operations while the cold, flammable liquids and gases are in the vicinity, that air-conditioning is switched over to heated nitrogen gas and the stage needs a lot of it with various vent doors kept open. “We have to maintain a high flow to keep basically a kind of positive pressure inside the area,” Maynard explained.
“When they go up for flight they always want to make sure they equalize, so there’s these vents open. So you kind of have to feed enough so that the outside air doesn’t come back in and so during cryogenics, we’ll use nitrogen,” he added.
“Yellow boxes” one of the final additions for SLS
In contrast to the single position A-1 and A-2 test stands nearby that have supported testing LOX/LH2 stages and engines in the past and present, the B Test Complex at Stennis has a dual position stand with the B-1 and B-2 positions co-located. The B-1 position in the stand is currently used for acceptance testing Aerojet Rocketdyne RS-68 flight engines for use in United Launch Alliance (ULA) Delta IV Common Booster Cores (CBC).
The B-2 position originally supported development and acceptance testing of Saturn V S-IC first stages in the mid to late 1960s. It was modified for Space Shuttle Main Propulsion Test Article (MPTA) testing in the late 1970s and early 1980s, and then supported Delta IV CBC testing during its development in the late 1990s and the turn of the century.
“The B-2 test stand has always been used for stages, Saturn, Shuttle, Delta 4, and this one,” Maynard said, referring to SLS.
“Around 2009-10, we were given marching orders to do an assessment on refurb-ing (refurbishing) the stand. And then about 2012 we got the go to go ahead and do it.”
“Basically there was a lot of rusted structural steel, a lot of rework,” he added. “The structure left over from the Saturn and the Shuttle Program is all the grey stuff down.”
“Now we did take this grey structure right here, we actually lifted it up and moved it back twenty feet,” he continued. “It saved us a boatload of money and a lot of mass that we would have had to build extra on the backside. And then all the white structure up above, all the way up is all put in specifically for SLS.”
Some subsystem activation work began while construction was winding down, but activation activities have been going on through 2017 and 2018.
One of the final custom structural pieces installed for the SLS Core Stage were the two “yellow boxes” up near the top of the stand. Similar to the Space Shuttle External Tank, the thrust beam that runs through the stage’s intertank takes most of the vehicle thrust loads from the two Northrop Grumman Solid Rocket Boosters (SRB) and the four Aerojet Rocketdyne RS-25 engines.
The yellow boxes are the two forward hold-down structures that will do most of the work holding the stage in place in the stand, attaching to either end of the intertank thrust beam at the forward SRB attach points. “We just finished yesterday actually the last torquing of the bolts, so they are now firmly installed,” Maynard said during the interview in December.
He also noted that one of the hold-downs weighs a little more than the other, but the average between them is about sixty-five thousand pounds.
Remaining activation tests
There were still a few systems going through activation testing at the end of December. At the time of the interview, the other liquid oxygen system was being tested. “On the LOX side we have two systems,” Maynard noted. “One is a very high flow system to do the fast fill on the vehicle and then we have another system to keep the replenishment going.”
“We have barges for hydrogen, three docks, that provides us enough hydrogen to go into the vehicle,” he explained. Then on the LOX side we have three LOX positions and that provides enough for the fill and then we have our day tank which is what we’re activating today for replenishment.”
“You get better quality LOX in there because that’s kind of what you want to do in your replenishment — you don’t want to put a lot of heat back into the system.”
Asked about other services for the Core Stage, he said: “So you’ve got helium — we’ve almost wrapped up all of our helium testing, which is providing the purges and stuff that the engines require from helium through the vehicle. We also have helium to press up the bottles on the vehicle, so we’ve tested that.”
“There’s also what we call the CAPU (Core Auxiliary Power Units) spin-start,” he added. “Before the engine starts we have to spin them up, so that’s what we use the helium [for].”
Maynard noted activation work that’s left for the stand: “Right now we’re doing the LO2 replenishment system and then I think the last fluid system we have to check is our hydraulics, which is the hydraulics that we would connect up to the vehicle to gimbal the engines without the CAPUs. So that’ll be sometime early next year.”
“The rest of the stuff is kind of here and there, we’re really getting down to the end of our punch list.”
Once the stage is set up in the stand for Green Run, the SLS Program based at the Marshall Space Flight Center in Huntsville, Alabama, and Stages prime contractor Boeing will be in charge. “Not to be sly, but the way we’re involved we’re actually a filling station,” Maynard observed.
“We’re going to fill it up, but after we fill it up it is Marshall and Boeing all the way. We’re a gas filling station.”
Stage test firing demands lots of water
Testing and activation of other B-2 subsystems occurred over the last several months. The hot-fire test which culminates the Green Run campaign for the flight stage will require a lot of water to protect the flight and ground hardware.
The flame bucket will be deluged with water, similar to single-engine tests in other stands at Stennis; however, additional measures are needed for sound suppression, to protect the Core Stage from the noise that it makes during the eight-plus minutes it is firing while it is held in place.
“If you leave the pad, you’re basically flying away from that acoustic energy, but we’re staying in the stand and we’re basically beating the vehicle with that acoustic energy for five-hundred seconds,” Maynard explained. “So to knock off that acoustic energy we put in an acoustic ring above the aspirator.”
“And so basically the water is coming across normal to gravity right under the nozzles and that will absorb some of that acoustic energy.”
In addition to the flame bucket deluge and sound suppression water, plumbing for fire extinguisher (firex) water is also installed on the stand. The facility water supply can be routed to the firex system in case it is needed for contingencies with the vehicle and/or stand.
“The way that would work on the deluge is during the test all the water is going to the bucket or the acoustic ring,” he explained. “If there is an event then what they’ll do is shut off the risers going to the bucket, at the same time they would open up the valves that would send the water up into these water cannons and the ring here.”
There are firex rings on the stand at different levels to provide coverage of the vehicle’s exterior.
The Test Control Center (TCC) that will be used with the B-2 stand for the upcoming Core Stage tests is one that had been intended to be used for the A-3 stand instead of the existing control center for the B complex. “Because of B-1 testing and the amount of people and stuff that we use, we had already ‘refurb-ed’ the test center there for A-3, so it was in a really great state so we just said ‘hey why don’t we just add some more copper, connect it to the B-stand?'” Maynard explained.
“And so we’ll run the whole system from what we used to call the DAF, the Data Acquisition Facility.” The A-3 stand was built for the Constellation program; following cancellation of Constellation at the beginning of the decade, the stand was mothballed.
“The control center is pretty much ready to go,” he noted. “That’s part of our activation is to wring out the control center.”