NASA SLS Core Stage ready for high-stakes Green Run static fire - NASASpaceflight.com

NASA’s Space Launch System (SLS) program and Core Stage prime contractor Boeing are finally ready for the big moment of the rocket’s Green Run campaign and the biggest moment in the 10-year old program, a static firing of the stage.

Boeing is conducting the SLS Core Stage Green Run design verification campaign with the stage bolted into the B-2 Test Stand at Stennis.

Core Stage-1 arrived at the test site almost a year ago in mid-January, 2020; the campaign is now ready for the culmination of testing with a flight-duration, nearly 500-second test firing of the stage.

(Photo Caption: Core Stage-1 during its first full tanking test performed as a part of the Green Run Wet Dress Rehearsal (WDR) on December 20. The liquid oxygen tank vents at the top of the stage through a port on the circumference of the forward skirt. Additional venting of oxygen from the four RS-25 engines can be seen near the bottom of the stage.).

“It’s not a development test series, where you bring a new stage [or] a new test article out there and run it on all the corners of the boxes and see what it’ll do and see what it won’t do and then go make changes,” Marc Neely, Core Stage Green Run Test Operations and Execution Manager for NASA, said in 2019.

The Green Run Hot-Fire is predominantly a test of the new rocket stage as opposed to the engines.

For this Stage Green Run, the engines are essentially test support equipment to evaluate how well the new Core Stage and its new Main Propulsion System meets those well-established but SLS-adjusted engine requirements

The new SLS flight software is in charge of vehicle management’s overall control cycle, monitoring systems health, and commanding the stage through the Green Run test plan via a brand new avionics system

The B-2 position of the B Test Stand at Stennis was rebuilt and refurbished to support SLS Core Stage test firings

The test stand also provides the sound suppression water to protect the stage from the acoustics of an eight-minute long static firing and a water deluge to protect the flame bucket while the stage is firing its engines into it

In addition to critical stage operating parameters during the engine firing, the water supplies are also required to continue running the test to full-duration

Boeing provides the ground control computer system to orchestrate the test, called the Stage Controller

As the test team monitors from the test control center, the Stage Controller will control vehicle purges, load the propellants from barges docked at the test stand into the Core Stage, manage heaters in the vehicle, manage the terminal countdown sequence, safe all the elements after engine shutdown, and capture many continuous data streams from start to finish to capture the results of the tests

The Hot-Fire test, test case eight, is the eight minutes of the months-long campaign at Stennis, often referred to as “The Green Run.” During the eight-minutes it takes for the four engines to empty the stage of propellant, the stage will be commanded through different test sequences to meet design verification objectives

The Core Stage flight computers running the Green Run Application Software (GRAS) take over primary control of the vehicle for the final half minute of the countdown and throughout the firing

The flight computers take over enforcing the large set of vehicle criteria needed to continue the countdown through ignition, but the Stage Controller also continues to monitor critical vehicle parameters and remains in charge of the health and operation of critical test stand systems

(Photo Caption: Sound-suppression water flows through an acoustic ring in the B-2 position of the test stand. The Core Stage engines will fire down through this spray of water throughout the test.  The flat plate of water created by the continuous spray will help deflect and dampen the sound waves from the firing engines that rebound off the test stand’s flame bucket back towards the vehicle.)

The Wet Dress Rehearsal (WDR) performed on December 20 was planned to proceed all the way through the terminal countdown from T-10 minutes to T-33 seconds, stopping a few seconds short of the point where the Stage Controller hands over authority of the vehicle during the test to the three Core Stage flight computers

The vehicle computers will begin the Automated Launch Sequence (ALS) at T-30 seconds and will run the stage through the eight-minute long test-firing.  The Stage Controller will run test stand systems and it will maintain water flows to the B-2 position’s acoustic ring and flame bucket throughout the test

The water flows will suppress some of the sound energy of the firing engines that might rebound off the flame bucket back up towards the Core Stage and protect the flame bucket from the engine exhaust.  At T-12 seconds, the Stage Controller will start the hydrogen burn-off igniters, which prevent a build up of hydrogen gas that occurs during the hydrogen-rich engine start sequence

“In broad strokes once we get into ALS there’s some final comparisons that the Stage Controller will do to look at the data to assure that we’re getting into the engine start box and the Stage Controller will then give that data to ALS that yes, we’re in the start box,” John Cipoletti, Boeing SLS Green Run deputy test director, explained in an interview early in 2020

Like the Exploration Ground Systems launch computer system in Florida, the Stage Controller will give the Core Stage ALS a “go for main engine start” at T-10 seconds if everything is still operating within expected ranges

“Then [ALS] will start working with the Core Stage engine controllers to get the engines ready for ignition and so they’ll ignite and they’ll build up to a hundred percent throttle at the zero point then they’ll immediately ramp up,” he noted

During the final countdown and throughout the firing of the stage, the Stage Controller will continue to control and monitor ground-side test stand systems

With the stage locked down in the stand for the test firing duration, NASA and Boeing plan to experiment with the interplay between the engines and stage systems

Without the need to steer the stage in a static test, the NASA Green Run software running on the vehicle flight computers will use the stage’s hydraulic TVC system to move the engines in test patterns and collect engineering data that will be used to verify and calibrate analytical model predictions and also help to certify the vehicle for its first launch

(Photo Caption: A water spray is seen activated in the flame bucket for the B-2 position of the B Test Stand at Stennis during the Core Stage Green Run WDR. The water deluge will protect the test stand flame bucket from the stage’s engine exhaust while a simultaneous high-rate water flow will help to dampen sound waves generated while the engines are firing as they rebound towards the stage.)

There are four hydraulic systems in the stage, one for each engine, and the engines and the hydraulic systems are interdependent

The frequency response test will measure the dynamic response of the Core Stage structure to the experimental TVC actuator movements.  “As we continue at about two minutes and thirty seconds there’s what’s called a sine sweep of those nozzle engines so they’ll start moving in a sinusoidal pattern to check the structural response of those rocket engines moving,” Shannon said

(Photo Caption: A presentation slide showing how the frequency response test (FRT) in the middle of the Green Run hot-fire is used to refine one of the SLS Program’s analytical models of vehicle dynamics. Three separate gimbaling tests of the four Core Stage engines will be conducted during the planned Hot-Fire test; the FRT would be the second test, starting about two and a half minutes into the engine firing.)

The FRT gimbaling lasts for about two minutes.  “The FRT is to see how much the tail wags the dog, as the gimbals and the big engines are moving around how much [are] the forces on the Core Stage going to be,” Shannon noted in an earlier interview

There is no plan to intentionally stop the test early; if the vehicle and ground systems perform within limits the stage will run to a nominal shutdown

This test would provide another data set showing how the stage balances the demands on the gas being tapped off the running engines to keep the propellant tanks pressurized as they near empty while simultaneously supplying power to the CAPU turbines under a high TVC gimbal load

“One of the main objectives of the Green Run from a propulsion standpoint is pressurization and understanding that,” Jonathan Looser, NASA SLS Core Stage Propulsion Lead, said

Liquid oxygen is fed to the engines through two, long feedlines also called “downcomers” that run from the LOX tank at the top of the stage down to the engine section

Following MECO, the Core Stage flight computers will close its prevalves and hand control of post-cutoff safing of the combined vehicle and ground systems back to the Stage Controller