When reading On the Cusp: What’s NextDARPA Perspective on Space Briefing prepared for 53rd Robert H. Goddard Memorial Symposium March 12, 2015 I saw the slide shown below. The slide's heading is Notional X-Plane Upper Stage Trades and it mentions
- Propellant = 176.5K lbs (LOX/RP)
- ISP (vac) = 310 sec
- PMF = 0.84
which does sound like a SpaceX Merlin 1D engine. I believe this is uses as simply a nominal engine spec for this notional, conceptual discussion.
But it made me wonder if there has been more serious discussion or tests involving the use of either SpaceX's engines or other private launch-providing liquid fuel engines on totally different vehicles.
I'm guessing there may be examples for solid rocket boosters being adapted in new ways since they are mostly self-contained, so the focus should probably be on the more complicated liquid propellant launch-enabling engines where the challenges are greater and the likelihood lower.