I did some back-of-napkin calculations on the Falcon 9's 1st stage numbers here: http://www.spaceflight101.com/falcon-9-v11.html http://www.spaceflight101.com/

I did some back-of-napkin calculations on the Falcon 9's 1st stage numbers here: http://www.spaceflight101.com/falcon-9-v11.html http://www.spaceflight101.com/falcon-9-v11.html Including the rest of the dry mass fraction of the vehicle, (the tank, the structure to hold the tank etc), each Merlin's dry thrust-to-weight ratio drops to 43.05. (654kN * 1000 N / kN / 9.8 (kg * m/s2)) / (Inert mass / 9.0). The GE F414 in the high-thrust configuration is the dry-mass comparison using COTS. Let's say it's 8.0 when factoring in some control system and inlets. Isp of a jet engine burning liquid methane is ~2000 even in full reheat. Isp of rocket engine is ~330 for methane-lox in the newer full-feed Merlin. The impulse to fuel weight ratio is almost 10x inverse in favor of the air-breathing system. The dry mass thrust-to-weight is decisively in favor of the rocket engine. The wet mass thrust-to-weight is decisively in favor of the jet engine. Affect of these factors on cost is inconclusive from the napkin. How can air-breathing engines potentially lower the TCO? 46 reusable RS-25 were flown on 135 shuttle missions, so the average lifetime of each engine was...8. The lifetime of a gas turbine is measured in thousands of hours, so let's say 200 missions and without so many major overhauls. Even if Space X is pulling out some cool voodoo for Merlin reuse-ability, gas turbines are in a different league in terms of both reliability and lifetime. While a 1st stage rocket body has to re-enter, land, and be transported back to the launch site, an air-breathing 0.5 stage can fly itself back from its short distance downrange, thereby incurring zero transport cost back to the launch site. An extreme application of air-breathing might include ram-jet inlets that opened after mach ~1.8 and took over air-breathing duties up to Mach 4.0 as the 414's idled. Even in that case, the whole system is not that far downrange and can use altitude to make up quite a bit of the journey back and use some vertical landing system if it saves weight. The fuel efficiency lets you do this with 414's in spite of their lower T:W. Does using air-breathing assist pay off by making a lighter 1st stage or heat-shielding the 1st stage to make a faster release point? Only extensive analysis of the various options can answer these questions. I believe the mechanisms are there, and that's what justifies the design research.