September 2020
Artemis I Rocket Moves Closer to Hot Fire Test

Tracy McMahan, NASA News
 

SLS test
The Space Launch System (SLS) rocket is completing the Green Run test for the rocket’s core stage, shown installed on the test stand at NASA’s Stennis Space Center, Mississippi. The SLS core stage is being tested for the first time prior to its use on the Artemis I lunar mission. Credits: NASA/Stennis

The Space Launch System (SLS) rocket core stage for the Artemis I lunar mission has successfully completed its first four Green Run tests and is building on those tests for the next phase of checkout as engineers require more capability of the hardware before hot-firing the stage and its four engines.

“We are methodically bringing several complex systems to life and checking them out during the first seven tests,” explained SLS Stages Manager Julie Bassler. “Then it is show time for the eighth test when we put it all together and fire up the rocket’s core stage, just like we’ll fire it up for the Artemis I launch to the Moon.”

On August 5, engineers at NASA’s Stennis Space Center where the stage is loaded into the B-2 Test Stand, completed the fourth of eight planned tests of the 212-foot-tall core stage. For Test 4, engineers performed the initial functional checkout of the main propulsion system components to verify command and control operability (valve response, timing, etc.) and performed leak checks on the core stage-to-facility umbilical fluid and gas connections.

“With test gases flowing through this many parts of a complex rocket stage, we expected the test team to encounter some issues,” said Jonathan Looser, who manages the SLS core stage main propulsion system. “Historically, there’s never been a NASA human-rated launch vehicle flown without one or more full-up tests before flight, and they have all encountered first-time issues. As expected, we found a few with valves and seals and addressed them, and now we’re ready to complete the next four Green Run tests.”

The Green Run testing series formally started in January with modal testing to verify computer models and support guidance and navigation control systems. In March, the test series was interrupted by a shutdown related to COVID-19 cases in Mississippi. When testing resumed in May with appropriate safety measures in place, the team completed Test 2, activation of computers, data collection health monitoring and other “avionics” that make up the brains and nervous system of the core stage. Test 3 was a check of the fail-safe systems that shut down the stage in a contingency situation. Each test builds on the prior test and is longer than the previous one, adding new hardware activations to those already completed.

For Test 4, functional and leak checks of the stage main propulsion systems and engines lasted three weeks. Engineers were able to conduct the test with gaseous nitrogen and helium, which is more efficient than using liquid hydrogen and oxygen propellants, which are only needed for the actual hot-fire test. As these gases flowed through systems, special instrumentation monitored for any leaks or poor connections.

Next up for the Green Run team is Test 5. It will ensure the stage thrust vector control system works correctly, which includes huge components that steer the four RS-25 engines, called actuators, and provides hydraulics to the engine valves.

Test 6 simulates the launch countdown to validate the countdown timeline and sequence of events. This includes the step-by-step fueling procedures in addition to the previous test steps of powering on the avionics and simulated propellant loading and pressurization.

As one final checkout before the full firing test, Test 7 is called the “wet dress rehearsal,” meaning it builds on the simulations in Test 6 and includes fueling the rocket. After once again powering on the avionics, hydraulic systems, fail-safe systems, and other related systems that have been checked out in the prior six tests, the team will load, control, and drain more than 700,000 gallons of cryogenic, or super cold, propellants.

Only after passing these seven tests will it be time for Test 8, a full countdown and hot fire test for up to eight minutes. During the test, all four RS-25 engines will be firing at a full, combined 1.6 million pounds of thrust just as they will on the launch pad. Test 8 will be the final checkout to verify the stage is ready for launch. Afterward, engineers will prepare the stage for its trip to Kennedy Space Center in Florida.

“We want to find any issues here on the ground at Stennis, where we’ve added hundreds of special ground test sensors to the stage for Green Run,” said Ryan McKibben, one of the Stennis Green Run test conductors. “We have great access to the stage on the B-2 Test Stand and have engineers and technicians on hand who are familiar with this stage.”

By the time all eight Green Run tests are complete, Boeing, the prime contractor for the core stage, estimates it will collect 75-100 terabytes of data, not including voice and video data collected. And that’s a lot of homework considering that all the data in the Library of Congress amounts to just 15 terabytes.

NASA is working to land the first woman and next man on the Moon by 2024. SLS and Orion, along with the human landing system and the Gateway in orbit around Moon, are NASA’s backbone for deep space exploration. SLS is the only rocket that can send Orion, astronauts and supplies to the Moon in a single mission.

In addition, some missions, such as Nasa's Dawn and the European Space Agency's Rosetta probe, made flybys of Mars en route to their "real" destinations. Should those be counted too?

It's not difficult to see why some space-watchers have settled on saying "about half" of all missions have succeeded while half have failed – without going into more detail than that.

The poor hit rate has led to jokes about a "Mars ghoul" that swallows unfortunate planetary probes. But the reality is more mundane.

The causes of mission failures generally trace back to engineering oversights, software errors and, sometimes, sheer bad luck.

But Nasa's Mars Climate Orbiter mission, launched in 1998, became the ultimate "metric martyr" when calculations in metric and Imperial standards got mixed up, putting it on the wrong trajectory as it was due to enter Mars orbit.

The spacecraft is thought either to have burned up in the Martian atmosphere, or to have bounced off, causing it to re-enter an orbit around the Sun.

There was a view that the mission was trying to do too much on its modest budget. And, at the time, Nasa was operating under a philosophy dubbed "Faster, Better, Cheaper". Scientists jokingly came up with an addendum: "...you can only pick two". In other words, the system can make the best of only two parameters, at the expense of the third.

Despite this - and a handful of other losses - the US has by far the best record of success.

All Nasa spacecraft launched to Mars since 1999 – from the Spirit, Opportunity and Curiosity rovers, to orbiting craft such as Mars Odyssey and Mars Reconnaissance Orbiter – have not only succeeded but have operated long past their prime missions.

So it is that we bid good luck to Perseverance. Here's hoping it becomes part of that vague half of all missions that find success.

Source: NASA News


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