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Were there specific breakthroughs which allowed SpaceX to get such an incredible thrust to weight ratio? The next closest American engine was almost twice as heavy (199 vs 82), and the next closest overall was still quite far behind (199 vs 159).

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    $\begingroup$ I wonder how much of it is just that most engines are not built by the organization building the rockets. Engine developers produce proposals for engines that they are confident they can deliver, thus with conservative weight specifications, and once they achieve the weight specification, there is little reason for them to reduce the weight further. $\endgroup$ – Russell Borogove Mar 12 '18 at 3:05
  • $\begingroup$ I wonder if thrust-to-weight has become more of an issue lately, as numbers of stages have dropped and re-use enters the picture? If the first stage doesn't run for long and is then dumped, the weight of the first stage engines is relatively unimportant. $\endgroup$ – Steve Linton Mar 12 '18 at 13:28
  • $\begingroup$ @SteveLinton - Not sure I follow you on this? T/W is based on the weight of the engine, not the stage, right? $\endgroup$ – David Mar 12 '18 at 18:01
  • $\begingroup$ @david Yes, but how much it matters depends on how the engine is used $\endgroup$ – Steve Linton Mar 12 '18 at 19:00
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    $\begingroup$ Also meant to say thrust fur upper stage engines is also relatively unimportant, Isp is what matters there $\endgroup$ – Steve Linton Mar 12 '18 at 19:01
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As I understand the two major items are continuous refinement of the design, and 3-d manufacture of key components of the engine. The Merlin is the most recently designed engine used on a large scale, and thus can better take advantage of modern design of such components.

Looking at some of the other top contenders, it seems that most of the very efficient engines were developed in the last decade or so. Likely computer aided design, complex simulations, and other such technology has allowed more careful design then in the past.

It also seems like the Densified Liquids was a major contributor. That was the major difference between the Merlin D and the Full Thrust, which gained about a 30% ratio.

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    $\begingroup$ Also lots of the original engines were designed like 40 years ago during the space race, computer-based design and simulation has come a long way along with sensing and optimization procedures $\endgroup$ – Dragongeek Mar 12 '18 at 7:19
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    $\begingroup$ I agree that continuous refinement and modern design/manufacturing are major factors but they aren't particularly specific. As far as specific features go Face Shutoff and the Densified propellants helped. The Face Shutoff reduced the weight of the engine by removing a number of valves and the Densified propellants increased the thrust. $\endgroup$ – Evan Steinbrenner Mar 13 '18 at 20:37
  • $\begingroup$ Specifically, densified propellant increases thrust without making the engine heavier with bigger pipes and pumps. $\endgroup$ – Saiboogu Mar 14 '18 at 1:07
  • $\begingroup$ I think an important part of mass savings for the -vacuum version is lack of cooling. The bell nozzle is made of material that heats red hot but doesn't lose structural integrity; that's a big mass saving. $\endgroup$ – SF. Mar 14 '18 at 1:13

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