NASA has signed a contract with Aerojet Rocketdyne to resume production of the RS-25 engines for the agency's Space Launch System (SLS) — the strongest rocket in the world — and produce a certified engine.

Under the contract, worth $1.16 billion, Aerojet Rocketdyne will modernise space shuttle engines in order to make them more replaceable and affordable for the SLS.

The modernised RS-25 engines will have fewer parts and welds, and will be certified to a higher operational thrust level. They will also benefit from improvements in materials and manufacturing methods like five-axis milling machines, 3D manufacturing and digital X-rays.

The contract that runs from November 2015 and expires on 30 September, 2024, will revamp Aerojet Rocketdyne's production ability, including furnishing the necessary management, equipment and materials needed for this effort, incorporating modern fabrication methods, producing hardware necessary for development and certification testing, and more.

Furthermore, the contract will also allow for a possible revision in future that would make possible for NASA to order six flight engines.

The SLS will utilise four RS-25 engines to transfer NASA's Orion spacecraft and launch explorers on higher space missions, including to an asteroid situated in lunar orbit and eventually to Mars.

A part of NASA's strategy to reduce the costs of developing the SLS rocket was to control the assets, capabilities and experience of the Space Shuttle Programme. Therefore, the first of the four space missions will be flown using 16 existing shuttle engines that have only been improved.

The first of the four missions are test flights known as "Exploration Mission-1" and "Exploration Mission-2", reported The Verge.

The testing of the shuttle engines will be conducted at NASA's Tennis Space Centre in Mississippi, and the SLS will be launched from the agency's Kennedy Space Centre in Florida.