That's a pretty hard problem. F-1 took unimaginable efforts to make right - event though the only unusual, untested parameter for this engine is its size. RD-17

That's a pretty hard problem. F-1 took unimaginable efforts to make right - event though the only unusual, untested parameter for this engine is its size. RD-170 took all the genius of Glushko - including his previous somewhat successful experience with RD-270, which deserves a whole story for itself - to get working properly. With all hindsight, only the forth Merlin of SpaceX is actually pushing technical limits - and that's in XXI century. Then, there is the issue of time - to build an engine from scratch is easy to delay a rocket for a year - if you're lucky and do that right from the first attempt. So Orbital words can be taken seriously and for the face value. NK-33 is a pretty good engine.