America’s space agency, the National Aeronautics and Space Administration (Nasa), has placed a $1.79-billion contract with US rocket manufacturer Aerojet Rocketdyne for an additional batch of RS-25 rocket engines. The new order is for 18 more of the engines, and adjusts a previous contract (awarded in November 2015) for six new RS-25s, to take the total of new engines to be acquired to 24. The RS-25 is used to power to core stage of Nasa’s Space Launch System (SLS) – the agency’s most powerful ever rocket. Four RS-25s are fitted to each SLS core stage. The RS-25 was originally developed to power Nasa’s Space Shuttle, and the agency has 16 of the rockets left over from that programme. These existing rockets have been fitted with new controllers, upgraded to provide the higher performance needed by the SLS, and successfully tested.