> Arianespace has prototypes of fully reusable engines ready now as well, and is moving into early testing.

The engine was started in 2017 and is in very, very early stages of development. Saying that it is 'ready' is basically just lying. They made some drawing and they have an idea of what they want to do, that's about it.

Their knowlage on reusability is extremly limited, like SpaceX when they started. It took SpaceX many iterations until they managed an engine that was easly reusable. Engine development takes multible years, Promotheus is a project that has just started and given the history of European engine development, I don't expect that it will even be test fired for a long while, if they are lucky the 2020.

Furthermore the Promotheus engine is a traditional gas generator cycle. This means that it is more comparable to the SpaceX Merlin eninge both in terms of performance and reusability. BE-4 and Raptor are much more performant cycles that get much better efficancy out of the fuel. BE-4 uses a Ox-rich staged cycle, while Raptor uses the even more advanced Full-Flow Stage Cycle. One should also mention that both of those enignes are far further in devlopment compared to Promotheus.

So the very best case for the Promotheus engine is that it will be ready in 3 years and the it will require at least 3-5 years more of iterative testing to get out the full performance ASSUMING they can actually regularly fly these engine in real condition. If they do that they have build an engine that is comparable to current SpaceX Merlin engine.

Even beyond that, because the Promotheus engine uses methane, it can not easly be usesed on the Ariane 6 rocket. A rocket needs to be designed around the engine and the fuel. This means that practical use of the Promotheus would require a completly new rocket devlopment program, or at least a major redesign of the Ariane that amount to the same thing (but better marketing). This would also include redesigning the whole lower part of the rocket so that it could fly back with the main engine.

Given how Europe Space operates, none of this will happen quickly. Changes like that require a years long politcal process and to even get such a project started and that would only happen if the Promotheus is actually a successful program.

Arianespace will not fly a reuable rocket until mid/late 2020s. Even once they do, their rocket still depend on soild boosters that can not be econmically recycled. Because they are planning to throw most of the rocket outside of the eninge and a few core system. The Ariane 6 have a far lower % of reusabilty.

I wish it was different as I am one of the people paying for this rocket. The reality is that Arianespace was totally hostile towards SpaceX and all these ideas. They were literally making fun at them and constantly throwing European style vailed insults at SpaceX and 'the American way' even while the Russian and everybody else started to change their tune. That's why they are now 5 years behind and and they are losing ground quickly.

The predicted price of the Ariane 6 is already not competitve with the Falcon 9 right now. That is their own announced price. Even worse, the devlopment cost of 2-3 billion $ does not have to be amortised by the flight cost. If they had to devlop their own technology like SpaceX mostly does, then they would be even further behind.

Unless a lot of things change, Arianespace will be a dedicated launcher for European govenrments. France will try to use its political power with the EU to force all EU countries to use Arianespace rather then flying commercial rockets so they can give Arianespace enough launches to make them 'close' to competitive. This would make Europe globally uncompetitive both in launch and in space in general.