SLS: Crucial test for Nasa's 'mega-rocket'

The orange core stage was filled with more than 700,000 gallons of rocket fuel
The core of a giant NASA rocket that will bring astronauts back to the moon has undergone a crucial test.
For the first time, engineers fully loaded a core stage of the Space Launch System (SLS) rocket with supercold liquid propellant, controlled it, and then emptied the tanks.
This propellant is fuel and an oxidizer - a chemical that causes the fuel to burn.
The engineers wanted to check that things were working as expected before the SLS made its maiden flight in about a year.
It was part of a testing program called the Green Run that is being conducted at NASA's Stennis Space Center near Bay St. Louis, Mississippi.
This assessment, known as Wet Dress Rehearsal (WDR), was the seventh of eight tests at the core level. NASA said the missile responded well to the charged propellant. However, the test terminated unexpectedly a few minutes earlier than planned.
With the completion of the WDR, however, the eighth and final test - a "hotfire" - was to be initiated in which all four RS-25 engines at the base of the core are fired together for the first time.
Nasa's new "Megarocket" set for critical testing
What is Nasa's giant SLS missile?
Artemis: To the moon and beyond
This missile segment will trigger the first mission in Nasa's lunar exploration plan known as Artemis. This mission, slated for November 2021, will put the next-generation Orion spacecraft on a loop around Earth's only natural satellite.
There is no crew on board for this test flight. But the third Artemis mission in 2024 will land the first humans on the lunar surface since the Apollo 17 mission in 1972.
The Green Run will help ensure that any issues are addressed before the complex missile segment is transported to Florida to prepare for launch.
In a matter of hours, Stennis engineers loaded the core stage with more than 700,000 gallons (approximately 2.6 million liters) of liquid hydrogen and oxygen.
SLS graphics
"This is an incredibly exciting time," said Jim Maser, senior vice president of Aerojet Rocketdyne, which builds the RS-25 engines. "We're really getting to some of the most important aspects of the testing program."
The rocket section is anchored to a giant steel structure called the B-2 test stand that was used in the 1960s and 70s to test engines for the massive Saturn V rocket that carried astronauts to the moon.
The propellant was brought to the construction site on six ships via the waterway that meanders through the grounds of the Stennis Space Center.
The barges were tied up near the test stand while the supercold (cryogenic) propellants on board were directed into the core stage.
Hydrogen and oxygen are gaseous at room temperature, but gases take up a lot of space. By converting it into liquids, a corresponding amount can be stored in a smaller tank.
This requires the hydrogen fuel to be cooled to minus 253 ° C (minus 423 ° F) and the oxygen (the fuel's oxidizer) to be cooled to minus 183 ° C (minus 297 ° F).
The SLS core stage was lowered to the B-2 test bench in the Stennis Space Center in January
After filling, the tanks had to be refilled and topped up continuously, as liquids boil off over time at such low temperatures.
During the test, fluids flowed through the turbo pumps - which supply propellant to the engine combustion chambers - and the engines themselves. This helps prepare the systems to be started.
It's all designed to mimic as closely as possible what would happen in the hours before a real flight. "We're just trying to get as much data as possible so we can get further on the next run. And we want to find anything that could be improved during this wet dress to prepare for the hot fire," said Ryan McKibben, Green Run test director for Nasa, said BBC News.
The SLS core stage at their New Orleans facility before being transported to the Stennis Space Center for testing
On its Artemis blog, NASA said: "First glances at the data show that the phase during the loading and refilling process of the propellant worked well."
While all of this was happening, teams from Nasa and Boeing - the prime contractor for the SLS - ran a simulated start countdown. You should count down to the T-Minus (remaining time) of 33 seconds.
But NASA said the test ended a few minutes before the scheduled countdown time. The teams evaluate the data to determine the exact cause of the premature shutdown.
In October, Boeing Vice President and SLS Program Manager, John Shannon stated, "We will spend approximately two weeks reviewing the data to ensure that all systems are performing as expected.
"We will inspect the vehicle and make sure there are no surprises."
The SLS in numbers
The missile will be 98 meters high in its original configuration, or Block 1 configuration
The Block 1 SLS can send more than 27 tons (59,500 pounds) in lunar orbits - the equivalent of 11 large sport utility vehicles (SUVs).
A future version of the SLS called Block 2 Cargo will take 46 tons (101,400 pounds) to the moon - that's 18 large SUVs
The SLS will produce 8.8 million pounds (39.1 meganewtons) of thrust in its Block 1 configuration
Four RS-25 motors are located at the base of the core stage. They are the same ones used in the space shuttle
If the data looks good, the engineers move on to the "hotfire".
The SLS missile consists of a huge core stage with two smaller boosters on the sides. The four powerful RS-25 engines at its base are the same as those used by the now retired space shuttle orbiter.
The launcher provides the raw power needed to send Orion into space and then hurl it towards the moon.
Last month, Stennis engineers removed and replaced a component called a clutch on one of the four pilot valves that supply liquid hydrogen to the RS-25 engines. The pilot valve had shown inconsistent performance during testing.
Officials have planned to ship the giant core to its launch site at the Kennedy Space Center in Florida by Jan. 14 to keep the SLS on course for its November flight.
Last week, NASA said the Artemis-1 mission was on track to launch in November 2021
The green run went largely smoothly; There was a five-week stop due to Covid-19. In addition, due to the particularly active hurricane season, work at the site had to be stopped six times due to tropical weather.
"It was extremely important for us to stick to the schedule," said John Honeycutt, SLS manager at NASA, during a press conference in October.
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