Upper stage added to SLS stack in Vehicle Assembly Building

The Interim Cryogenic Propulsion Stage for NASA’s first Space Launch System test flight was stacked on top of the rocket July 5. Credit: NASA/Kim Shiflett

The upper stage for the first flight of NASA’s Space Launch System was installed on top of the heavy-lift rocket earlier this month, moving the agency one step closer to liftoff of the Artemis 1 test mission to the moon.

Teams inside the Vehicle Assembly Building at NASA’s Kennedy Space Center lifted the Interim Cryogenic Propulsion Stage on top of the SLS rocket stack July 5. The addition of the upper stage completed stacking of the propulsive elements for the first SLS mission, known as Artemis 1.

Last month, ground crews mounted the SLS core stage between the rocket’s two side-mounted solid-fueled boosters, which were stacked on a mobile launch platform inside the VAB earlier this year. Then teams added the Launch Vehicle Stage Adapter, a conical structure that tapes from the larger diameter of the core stage to the smaller upper stage.

The Interim Cryogenic Propulsion Stage, or ICPS, was built by United Launch Alliance and is based on the upper stage used on the company’s Delta 4-Heavy rocket. The ICPS will provide the boost to send NASA’s Orion crew capsule out of Earth orbit toward the moon on the Artemis 1 test flight.

No astronauts will fly on the Artemis 1 mission, but the test flight will pave the way for future piloted Artemis lunar missions, beginning with Artemis 2 scheduled for launch in 2023.

The five-meter-diameter ICPS contains liquid hydrogen and liquid oxygen propellant tanks to feed the stage’s Aerojet Rocketdyne RL10B-2 engine. The ICPS has a slightly larger liquid hydrogen tank than the Delta 4 upper stage, features a second hydrazine bottle for additional attitude control propellant, and has electrical and mechanical interfaces for attaching the Orion spacecraft, according to ULA.

Before transporting the upper stage to the VAB, teams loaded hydrazine fuel into the ICPS to feed maneuvering jets used to point the rocket in space.

During the Artemis 1 launch, the solid rocket boosters will fire two minutes, doing most of the work to lift the Space Launch System off pad 39B at the Kennedy Space Center. The core stage’s four hydrogen-fueled RS-25 engines, leftovers from the space shuttle program, will burn more than eight minutes to place the rocket into space with a velocity just shy of that required to reach a stable orbit.

The ICPS main engine will fire two times on the Artemis 1 mission, first for a Perigee Raise Maneuver to place the Orion spacecraft into around Earth, followed by a Trans-Lunar Injection burn lasting nearly 20 minutes to send the capsule toward the moon, ULA said.

The upper stage will then deploy the Orion spacecraft, which has its own service module for course correction burns and maneuvers to enter and exit lunar orbit, then return to Earth for a splashdown in the Pacific Ocean. The ICPS will deploy more than 13 CubeSats to explore the moon, asteroids, and other destinations in deep space.

The Interim Cryogenic Propulsion Stage is raised on top of the Space Launch System rocket inside the Vehicle Assembly Building. Credit: NASA/Kim Shiflett

NASA has ordered three ICPS units from United Launch Alliance to power the first three Artemis missions to the moon. Boeing is developing a more powerful Exploration Upper Stage with four RL10 engines for later Artemis launches.

The stacking of the SLS upper stage sets the stage for lifting of the Orion Stage Adapter, the attachment ring connecting the rocket with the Orion spacecraft.

Then a structure will go on top of the rocket to simulate the weight of the Orion capsule. The mass simulator will be atop the Space Launch System for testing to verify the propellant lines, fluid connections, and other umbilicals running between the mobile launch platform’s tower and the rocket can safely release and retract as they will at liftoff.

Then teams will move into structural resonance testing, or modal testing, of the fully-stacked launch vehicle. Once that is complete, teams will move the real Orion spacecraft — which will already be integrated with its launch abort system — to the VAB for attachment to the top of the Space Launch System, an event that sources say is now expected no sooner than September.

NASA will roll the fully-assembled 322-foot-tall (98-meter) Space Launch System from the VAB to pad 39B for a wet dress rehearsal this fall. The rehearsal is essentially a practice countdown, during which the launch team will load cryogenic propellants into the Space Launch System.

After the practice countdown, the SLS and Orion spacecraft will return to the Vehicle Assembly Building for final closeouts, inspections, and ordnance connections.

NASA Administrator Bill Nelson said Wednesday that the agency is holding to a target launch date for the Artemis 1 mission before the end of this year.

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