# Why is SpaceX's Raptor's design so simple in comparison to SSME?

What is especially striking to me is how simple Raptor is in configuration compared to SSME.

1. The Raptor doesn't have an LP fuel or oxygen pump. What's even more surprising, if we are to compare the oxygen's pressure gain before and after the pump, the SSME has a $$100\,\mathrm{psia}=6.89\,\mathrm{bar}$$ inlet pressure from the tank. The LP pump raises it to $$421\,\mathrm{psia}=29.03\,\mathrm{bar}$$, and the HP pump further raises it to $$4025\,\mathrm{psia}=277.51\,\mathrm{bar}$$, finally an additional boost pump raises it to $$6939\,\mathrm{psia}=478.42\,\mathrm{bar}$$, and the fuel-rich oxygen HP pre-burner burns at $$4812\,\mathrm{psia}=331.77\,\mathrm{bar}$$. On the other hand, the Raptor intakes at $$3\,\mathrm{bar}$$, and raises it to $$633\,\mathrm{bar}$$ to burn in an oxygen-rich pre-burner at $$564\,\mathrm{bar}$$ with just a single centrifugal stage! What's SpaceX's secret sauce for such large single-stage pressure ratio without surging?
2. The Raptor's oxygen pump sits directly on top of the main combustion chamber, while the SSME's two pumps are on the opposite sides of the main combustion chamber. Why couldn't the SSME use a similar configuration with oxygen-pump pre-burner on top and HP oxygen impeller discharge directly on top of the main combustion chamber?
3. Why Raptor opts to use the methane discharge fresh out of the fuel pump instead of the aft of the nozzle cooling circuit to use in the oxygen-rich pre-burner, as the latter is a supercritical fluid (methane goes supercritical at $$45.99\,\mathrm{bar}$$ and $$190.56\,\mathrm{K}$$) and has better combustion than the former which is a true liquid?
4. And last but not least, what's SpaceX's solution for the oxygen-rich environment at $$377\,\mathrm{bar}$$, $$748\,\mathrm{K}$$ injector and $$546\,\mathrm{bar}$$, $$811\,\mathrm{K}$$ pre-burner?

Does anyone know the starting sequence of the Raptor?

• On point 3: The pressure at the aft of the nozzle cooling is lower then the pressure inside oxygen-rich combustor. They'd need another pump or increase the pressure of the methane pump. You probably want to avoid both. – Christoph May 13 at 6:44
• This question is very broad, it's actually about 8 questions in 1 once you factor in starting sequence and others. You might get more traction if you split this up uhoh stylie. – GdD May 13 at 8:21
• I'd ask rather "Why was SSME so ludicrously complex?" As for point 3, you want the preburner "propellant" to have stable characteristics, not whatever comes out of cooling the bell nozzle, colder or hotter, more or less dense depending on cooling demands and momentary heat output of the engine. – SF. May 13 at 10:26
• Let's wait until Raptor accumulates more than a few minutes of runtime before making odious comparisons :) – Organic Marble May 13 at 11:57
• Also keep in mind the SSME diagram is derived from real blueprints, but the Raptor diagram is a string of educated guesses based on a tiny handful of social media images and random Musk/Mueller/Shotwell comments. Comparisons based on this data must be taken with a grain of salt. – Saiboogu May 14 at 13:29