Agree. And those Merlin-1D seem to be very efficient from economical point of view. Technically they are also good - record setting for open cycle engines. How

Agree. And those Merlin-1D seem to be very efficient from economical point of view. Technically they are also good - record setting for open cycle engines. However, NK-33 are closed cycle and have better Isp. Economically they also made a lot of sense for Orbital, since they were bought as already made for another project. It can be justified.