NASA is making good progress on developing the core stage of its Space Launch System rocket, as directed by Congress in 2010. Per the request of legislators, the agency is using legacy hardware such as the space shuttle's main engines to propel this core stage and solid rocket boosters to give it an initial kick off the launch pad.

Meanwhile, the agency hasn't yet settled on an architecture for the upper stage of the massive rocket, which is used to boost payloads beyond low-Earth orbit and into deep space. For at least the first, uncrewed flight in 2018, NASA will use an interim upper stage. But Congress has been pressing the agency to settle on a new, permanent upper stage, dubbed the "Exploration Upper Stage," in time for the second launch of the SLS rocket in 2022 or 2023.

According to an insider website, NASASpaceflight.com, the agency had been considering nine proposals for the upper stage, but it has now chosen an architecture that uses four RL-10 engines as it moves forward. The RL-10 engine, a workhorse developed in the late 1950s and first flown in 1963, runs on liquid hydrogen/liquid oxygen fuel.

In documents explaining the decision, NASA says it prefers the RL-10 engine because it is reliable, long-tested, and could be most quickly integrated into a flight-ready upper stage for a rocket that's already six years into development. NASA's decision comes even as other companies, such as XCOR and Blue Origin, have been developing modern, more versatile upper-stage engines. NASA also spent more than $1 billion in the last decade on its own powerful upper stage engine, the J2X, before shelving that project in 2014.

The RL1- engine is manufactured by Aerojet Rocketdyne, a long-time NASA contractor that has also received a $1.16 billion to restart production of the space shuttle's main engines used to power the SLS core stage. While the space shuttle reused those engines on multiple flights, they will now be flown once on the expendable SLS rocket.