The Vinci engine seems very very similar to the RL10B. Both are hydrolox expander-cycle engines with 465 s Isp and extendible nozzles with a ~250:1 expansion ratio. The biggest difference is that the Vinci has a bit less than double the thrust.

Why design a whole new engine rather than just buy two RL10Bs when they are so similar in role, function, and form? Conversely, why wouldn't NASA's Exploration Upper Stage use two Vincis rather than 4 incredibly expensive RL10s?

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    $\begingroup$ I think you are focusing too much on the technical similarities and overlooking the political; you might be dismayed to find how much the Not Invented Here and Not Manufactured Here effects have on purchasing and awarding of contracts... $\endgroup$ – astrosnapper Feb 4 at 1:55

The RL10 is a terribly expensive engine to produce (I've seen estimates of US$25 million per unit), mainly due to its welded-tube nozzle construction. I expect Vinci will be substantially cheaper to produce on a one-for-one basis, let alone two-for-one. Furthermore, export of RL10s is restricted by US law, though I imagine the ESA could get a waiver if the political will was there.

Other agencies have developed upper-stage hydrolox expanders in the general RL10 class for the same reasons: CE-7.5, HM7B

On the flip side, the RL10 family of engines has vastly more flight history than any other hydrolox upper stage engine, so the conservative voices in US launcher development will naturally favor it. While NASA does work with foreign hardware from time to time, there may also be some political pressure to assign contracts to US companies.

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