Nasa’s giant SLS rocket: a guide

SLS leaves the launchpad

SLS leaves the launchpad

Nasa has been developing a huge rocket called the Space Launch System (SLS) to launch astronauts to the Moon – and eventually Mars. Set to make its debut in late 2021, the SLS is the most powerful launch vehicle built since the 1960s.

Nasa has plans to send a man and woman to the lunar surface this decade, in what will be the first landing with humans since Apollo 17 in 1972.

In the last 20 years, astronauts have been making routine trips to and from the International Space Station (ISS).

But the Moon is nearly 1,000 times further than the ISS; getting astronauts there requires a monster rocket.

The SLS is the modern equivalent of the Saturn V, the huge launcher built during the Apollo era. Like the Saturn, it is split into segments, or stages, stacked on top of each other. But the rocket also incorporates technology from the space shuttle.

The first version of the SLS will be called Block 1. It will undergo a series of upgrades in coming years so that it can launch heavier payloads to destinations beyond low-Earth orbit.

The Block 1 SLS will tower 23 storeys above the launch pad – making it taller than the Statue of Liberty.

“It is truly an immense rocket. It is just jaw-droppingly big,” said John Shannon, vice president and program manager for the SLS at Boeing, the rocket’s prime contractor. He told BBC News in 2019: “When you see the SLS put together, you just haven’t seen anything like it since the Saturn V.”

The rocket will launch astronauts in Nasa’s next-generation crew vehicle – Orion, boosting it to the speeds necessary to break out of low-Earth orbit and travel onwards to the Moon.

SLS graphic

SLS graphic

How the rocket works

The SLS consists of a giant core stage flanked by two solid rocket boosters (SRBs). The core houses two large storage tanks: one for liquid hydrogen, the fuel, and another for liquid oxygen, an “oxidiser”, which makes the fuel burn.

Together, these are known as propellants.

At the base of the core stage are four RS-25 engines, the same ones that powered the spaceplane-like shuttle orbiter, retired in 2011.

Workers inside the SLS hydrogen tank use a technique called friction stir welding to plug holes

Workers inside the huge SLS hydrogen tank use a technique called friction stir welding to plug holes

When liquid hydrogen and oxygen are fed into the engine chambers and ignited with a spark, the chemical reaction produces vast amounts of energy and steam.

The steam exits engine nozzles at speeds of 16,000 km/h (10,000 mph) to generate thrust – the force that propels a rocket through the air.

The SRBs give the rocket extra power to escape gravity’s clutches. These twin boosters stand more than 17 storeys tall and burn six tonnes of solid propellant each second. They provide 75% of total thrust during the first two minutes of flight.

The most powerful rocket ever?

If we use thrust as a measure, the SLS will be the most powerful rocket ever when it flies to space in 2021. The Block 1 SLS will generate 8.8 million pounds (39.1 Meganewtons) of thrust at launch, 15% more than the Saturn V.

In the 1960s, the Soviet Union built a rocket called the N1, in a bid to reach the Moon. Its first stage could produce 10.2 million pounds (45.4 Meganewtons) of thrust. But…

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