The Prometheus rocket engine is an ongoing European Space Agency (ESA) development effort begun in 2017 to create a reusable methane-fueled rocket engine potentially for subsequent technology use in the Ariane 6 or its successor rocket.By 2020, the development program was fully funded, and is being developed for the ESA by Ariane Group.
The engine is aimed to be reusable with substantially lower costs than traditional engines manufactured in Europe. The cost goal is to manufacture the Prometheus engine at one-tenth the cost of the Ariane 5's first-stage engine.
This engine is to cost no more than 1 million euros.
By June 2017, Patrick Bonguet, lead of the Ariane 6 launch vehicle program at ArianeSpace, indicated that it was possible the Prometheus engine could find a use on a future version of the expendable Ariane 6 launcher. In this scenario, a "streamlined version of Vulcain rocket engine called Vulcain 2.1 would have the same performance as Vulcain 2. The expendable Ariane 6 was then expected to make an initial launch in 2020.
By June 2020, the ESA was onboard with this plan, and had agreed to completely fund the development of the Prometheus precursor engine to bring the "engine design to a technical maturity suitable for industry." The objective of the overall program as stated in June 2020 was to utilize Prometheus technology to eventually "lower the cost of production by a factor of ten of the current main stage Ariane 5 Vulcain 2 engine."