NASA’s Space Launch System (SLS)

The other big rocket being built at the moment is NASA’s Space Launch System (SLS) and it is a very impressive rocket that is coming together. It’s going to be a monster at just over 98 metres high, that’s shorter than the legendary 111 metre high Saturn V but the SLS will produce 8.8 million pounds of thrust versus the Saturn V’s 7.5 million. Later versions will even surpass the huge 118 tons of cargo capacity for the Saturn V by enabling a massive 130 tons to be put in orbit. Of course, the aim of the rocket is to get astronauts back to the Moon, and then beyond under the current plans that NASA has.

The rocket is essentially the same fundamental design as most rockets with a core and subsequent stages bolted on top of each other and a crew vehicle sitting on the nose. In the SLS’s case the crew module is the Orion spacecraft which is also undergoing development in tandem with the SLS. At the heart of the core stage is the venerable RS-25 rocket engine that has seen refinement over 30 years of space shuttle operations. The engine has had 3000 starts with 100% reliability. Though, unlike the space shuttle, the RS-25s on the SLS will not get used again, as once the core stage is finished lifting the rocket out of the atmosphere the rocket engines do not get recovered. There are currently 16 RS-25s in the inventory and they will be used for the first four launches. NASA wants to get the production line reopened for the RS-25 so the next generation can power the SLS beyond the first four missions.

RS-25 engine (Credit: NASA)

Helping the enormous rocket get off the pad and into space are the two solid rocket boosters (SRB) that get attached to the sides of the core. These are similar to the SRBs used on the space shuttle but with an added fifth segment to give more power adding up to 3.6 million pounds of thrust per SRB. The boosters operate for about 2 minutes after launch before separating off the assembly and falling back to Earth. These are big rockets in their own right, standing at about 54 metres and weighing about 725 tons each. The boosters have completed a couple of tests and passed the critical design review so are well on their way to being ready for the first flight. The manufacturing of the actual boosters for the first flight are nearing completion with the 10 segments of the two boosters already complete.

Testing a mock up of the Orion module to make sure it stays upright in the water (Credit: NASA, Bill Stafford)

Of course there is way more to a rocket than the huge boosters and the powerful fist stage engines. There’s also the upper stages, the spacecraft itself and the bits which join the rocket together, one of these pieces is the join between the core and the cyrogenic propulsion stage which is also nearing completion. This bit is very important as it enables the first stage to break away and be clear of the rocket so the second stage can ignite safely. 2/3 of the huge core stage of the rocket is made up of a liquid hydrogen fuel tank. The test version of this tank has been produced and is about to undergo strength testing. Basically they make a structural copy of the flight version tank to test to see if it can handle all of the pressures the tank is likely to be under during launch. The huge tank will hold just over 2 million litres of liquid hydrogen.

SLS Liquid Hydrogen Tank Test Article Moved at NASA’s Michoud Assembly Facility (Credit: NASA)

Exploration Mission – 1 is the name of the first launch of the SLS and it will carry an unmanned version of the Orion spacecraft, planned for June next year. The aim is to send the spacecraft off to the Moon where it will orbit for 6-10 days and then come back, if all goes well the Orion module will splash down in the ocean. The overall mission will be about three weeks and the spacecraft will go further than any other spacecraft designed for humans – the orbits around the Moon will be significantly larger than the Apollo mission profiles. The orbit around the Moon is called a deep retrograde and will be about 70,000km from the Moon’s surface. If all goes well with Exploration Mission -1 then the next flight will be crewed.