The SLS moon rocket will make its maiden flight by the end of the month, NASA says

At the Kennedy Space Center in Florida, NASA's $4.1 billion Space Launch System moon rocket was transported overnight to pad 39B, setting up a long-awaited maiden flight to send an unpiloted Orion crew ship around the moon.

The 3.5-million-pound, 322-foot-tall SLS rocket, and its 10.5-million-pound mobile launch platform began moving out of the VAB at 9:55 p.m. Eastern Daylight Time, cheered on by hundreds of spaceport workers and their families.

The rollout started about an hour late Wednesday due to nearby thunderstorms. Upon lowering the mobile launcher onto pedestals atop the pad, engineers will connect power, data, propellant lines, water lines, and other systems to prepare the rocket for pre-launch tests.

The countdown will begin at 10:23 a.m. Eastern Daylight Time on Monday, Aug. 29, kicking off a 42-day journey to send an unpiloted Orion crew capsule to the moon.

The Earth and moon's positions continually change, and spaceport propellant supplies need to be replenished, so backup launch opportunities are available on Sept. 2 and 5. To launch the SLS later this fall, NASA must haul it back to the VAB for battery service and other repairs.

As part of the Artemis 1 mission, the SLS will be tested in deep space as well as Orion's solar-powered heat shield will be tested during the rocket's descent into Earth's atmosphere.

NASA plans to launch four astronauts on SLS' second flight in 2024 on Artemis 2 - followed by the first woman and the next man to the moon in 2025-26 if the test flight goes well.

As the world's most powerful operating rocket, SLS uses two extended shuttle-era solid-fuel boosters and four upgraded shuttle-era RS-25 engines to generate a combined 8.8 million pounds of thrust at liftoff, 15% more than NASA's legendary Saturn 5 spacecraft.

As configured for block 1, the SLS is capable of launching nearly 30 tons to the moon. A more powerful upper stage and advanced boosters will allow a single flight to carry nearly 50 tons to the moon.

As part of its new Super Heavy-Starship program, SpaceX is building an even bigger, more powerful rocket with double its capabilities, but it cannot fly it in one go. Starships are designed to refuel in Earth orbit before leaving for deep space.

At the Stennis Space Center in Mississippi, NASA tested a Boeing SLS core stage and four Aerojet Rocketdyne RS-25 engines last year. United Launch Alliance's second stage and Lockheed Martin's Orion capsule were attached to the booster in Florida.

As part of the countdowns for launch, engineers performed four tries, but ground-system issues and a stuck helium valve spoiled the run. Two were located near the core stage and one near the main engine cooling fitting.

A hydrogen leak is notoriously difficult to identify and fix since it only occurs when the ultra-cold propellant is flowing through the parts and lines. The repairs must be performed at room temperature.

The umbilical fitting was successfully repaired, and fueling tests showed no problems. Despite the VAB repair after the recent countdown rehearsal, the engine 'bleed' line hasn't been tested under cryogenic conditions yet. The launch of SLS will not occur until Aug. 29.