The Raptor is actually a full flow stages combustion engine, the most efficient type we know to date. SpaceX is indeed optimizing for Thrust to Weight as well as efficiency (Isp, how much fuel the rocket needs to provide a certain amount of acceleration). The problem is that rocket engines get dis- proportionally heavier once they surpass a certain thrust. This is why SpaceX is optimizing for thrust to weight as well. If a total of 3 engines that do 230 tonnes of thrust each weights less than 1 that does 600 tonnes, then it is clearly better to have more, smaller engines.paperburn1 wrote:so it looks like Musk is going after that low hanging fruit, Elon Musk stated that SpaceX were instead working towards a potential staged cycle engine. The forthcoming engine currently under development by SpaceX has been named "Raptor". Raptor will use liquid methane as a fuel, and was stated as having a sea-level thrust of 6,700 kilonewtons (1,500,000 lbf). Since the initial announcement of Raptor, Musk has updated the specification to approximately 230 tonnes-force (2,300 kN; 510,000 lbf)—about one-third the original published figure—based on the results of optimizing for thrust-to-weight ratio.
This also gives you some advantages when it comes to engine out capability (losing one of many engines is not as troublesome as losing one of few) and throttling. The latter is particularly important for powered landings. There is also the possibility to build smaller launch vehicles with those lower thrust engines. They might replace the 3 core Falcon Heavy with a single core Raptor based rocket. They recently got a grant for the Airforce to specifically evaluate their raptor methalox engine for use as an upper stage for (presumably) Falcon9 or Falcon Heavy which could significantly increase payload to higher orbits compared to the lower Isp kerolox Merlins.
What does get more complex is the plumbing to get the fuel to that many engines. I am sure that SpaceX knows what they are doing, though.