Since Full-Flow Staged Engine uses preburned fuel/OX-rich gases in combustion chamber, don't those exhaust fumes produced by the preburner somehow unbalance or 'pollute' the pure combustion of fuel and oxygen in the chamber? I know there is still coming a lot of leftover fuel and OX through the preburner but what about the other gases produced by the combustion?

For instance in SpaceX's Raptor Engine which uses liquid methane and LOX, preburner produces some carbon dioxide and water steam. Don't these gases spoil the pure combustion? Or is the amount of these fumes so insignificantly small?

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    $\begingroup$ The preburner produces some carbon dioxide and water steam, but the combustion chamber does the same. Why should adding some reaction products spoil the combustion? Of cause most of the fuel and oxidizer goes into the combustion chamber and only a small part to the preburner. $\endgroup$ – Uwe Jan 17 '18 at 23:14
  • $\begingroup$ Pure carbon dioxide may be used on earth to extinguish a fire by replacing the oxygen of the air with carbon dioxide. But there is oxygen pumped into the combustion chamber, this oxygen could not be replaced by very little carbon dioxide from the preburner. It is a mix of some carbon dioxide with steam and excess fuel or oxygen, not useful to extinguish a fire. $\endgroup$ – Uwe Jan 20 '18 at 22:01

It's not a problem. The thermodynamic and chemical properties of the fuel and oxidizer suffer a very small penalty from the H20 and C02 gases they contain after pre-burning. Meanwhile you get to feed your combustion chamber with hot gasses, ensuring efficient mixing and combustion. To steal a line from Real Genius, "It's like lasing a stick of dynamite." Note that the SSME (RS-25) feeds its combustion chamber with preburned fuel and oxidizer (with low concentration of exhaust products) and it's the gold standard for ISP.

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