NASA finished stacking the twin solid rocket boosters for Artemis III on September 30, 2026, inside the Vehicle Assembly Building at Kennedy Space Center, and all four RS-25 engines are now installed on the Space Launch System core stage. The next milestone is a tanking test, in which the partial stack rolls to Launch Pad 39B and is loaded with propellant. NASA gives that test only a loose window of the coming months, and it has not happened yet.
The day in tech, in one email: TLDR is the daily newsletter with links and TLDRs of the most interesting stories in startups, tech and programming.
We may earn a commission if you sign up.
The hardware milestones come ahead of an Artemis III flight with four astronauts that NASA has described as planned for next year.
Boosters stacked in High Bay 3
According to NASA’s October 13 mission blog, engineers finished attaching the nose cones to the two boosters on Wednesday, September 30, in High Bay 3 of the Vehicle Assembly Building. That step closed out the boosters’ stacking work, which had begun in early July. By NASA’s August 27 update the crews were already more than halfway through, working with processes streamlined from earlier Artemis launches.
The boosters matter because they supply more than 75 percent of the rocket’s thrust at liftoff, so the stack is largely built around them. Teams have also integrated the final booster motor segments onto the mobile launcher for what NASA calls the short stack, the configuration that stands before the core stage is lifted in.
The sequence is easier to follow in order, because the NASA blog moves between hardware and dates. Booster stacking began in early July and ended September 30. Engine installation began August 24 in a separate high bay. The core stage still has to be joined to the boosters, and only then does the stack go to the pad. None of the later steps carries a calendar date in NASA’s account, and all of them remain ahead as of this writing.
Four RS-25 engines on the core stage
The engine work happened in parallel in High Bay 2. NASA’s August update said technicians began installing the four RS-25 engines on August 24, and the October blog now reports all four in place. Each engine weighs about 7,700 pounds and produces nearly 500,000 pounds of thrust, according to the August account, which also named the units as E2054, E2057, E2048 and E2052.
One of the four has a flight history. Engine 2048 previously flew on STS-129 and STS-95, a reminder that the RS-25 design descends from the space shuttle main engine and that SLS reuses surviving shuttle-era hardware. Four engines at nearly 500,000 pounds each add up to close to 2 million pounds of thrust from the core stage, which is the smaller share beside the boosters’ more than 75 percent. A separate account of the installation, from Starlust, gives the same 7,700-pound weight.
The tanking test at Pad 39B is the next step
With the engines in and the boosters stacked, NASA says the core stage will join the boosters in the coming weeks, completing the short stack. Only after that does the partial stack roll out to Launch Pad 39B for the tanking test, which NASA places in the coming months. The agency has given no date for it. A tanking test fills the stage with propellant on the pad to check the ground systems and the vehicle together before anyone is asked to fly on it.
Rolling out to the pad is also where the crew comes in. NASA’s blog says astronauts Randy Bresnik, Frank Rubio and Andre Douglas, with Luca Parmitano of the European Space Agency, plan a launch day rehearsal and an emergency exit walkdown at the pad. The four were assigned to the mission on June 9, and training so far has covered Orion systems, a water survival class and a post-landing emergency exit course.
What Artemis III is meant to do
The mission is narrower than its number suggests, and the stack under construction in the Vehicle Assembly Building is built for a flight that stays in Earth orbit. Reporting by the Eastern Herald describes Artemis III as an orbital test in low Earth orbit that would send four astronauts to rehearse rendezvous and docking with one or both human landing systems, with the lunar landing left to Artemis IV. The same account says NASA has not said whether a lander will be ready for Orion to dock with.
Timing remains loose. Space.com described the preparations as a ramp toward a 2027 astronaut launch, and NASA’s own blog gives no launch date. What can be said is the order of events: boosters done, engines in, core stage to be joined, then the pad test. Until the tanking test is scheduled and carried out, the fueling step stays a plan rather than a result, and readers should expect NASA to announce its timing in a later update, as it did with the stacking and engine milestones reported here.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
The day in tech, in one email: Get TLDR →
We may earn a commission if you sign up.
More from Morning Overview
- Lake Powell sank to a record 3,517 feet, nearing the level that stops Glen Canyon Dam’s turbines
- Seven of Earth’s nine planetary boundaries are breached, and all seven are worsening
- The NSA says three phone features should be off whenever you aren’t using them
- Hurricane Hunter radar shows four warning signs that a tropical cyclone is about to strengthen, a University of Miami study found
