About payload capacity, since this is the purpose for the thread. I find it amazing that one vehicle can win such a wide range of payloads. As we have seen the F9 can be a beast to LEO and yet still has flights for payloads under a few thousand kilos.
I wonder how much of the cost of those low-mass launches is the Merlin that’s expended on the upper stage. The MVac is much harder to make than the SL version.
Why do you think the Merlin Vacuum is “much harder to make“? Do you mean it costs more? Or that it takes more labor? Or what?
I wonder if eventually they might fit an older SL Merlin on F9US with very low mass payloads instead?
Vacuum rocket engines work differently than sea level ones, so even if the expansion ratio of the nozzle wasn’t an issue (it is), then a sea level Merlin engine likely would fail while trying to operate in space.
Plus, if this were at all a possibility, then SpaceX likely would have tried it already…
The new SL engine added to the rotation would be cheaper to make, and could serve multiple launches.
You seem to assume that a Merlin Vacuum costs significantly more than a sea level Merlin 1D. Where is this assumption coming from, because in some aspects the Merlin Vacuum is a more simple engine.