On 6/8/2020 6:24 PM, JF Mezei wrote:
But this brings a new question (sorry :-)
The SN4 test firings includded intially only the pre-burners (allegedly,
from the commentators).
That seems possible to me yes.
How can they run only the pre-burners? Wouldn't that run turbopumps that
would push a lot of unburned methane and oxygen out of the engine bell ,
risking ignotion when it sees a spark or a falme at the landing site?
Yes I would assume so. There is a risk of ignition outside the engine
until the methane gas dissipates to below about a 6% methane/air ratio.
Below that it isn't methane rich enough to ignite.
Or can they adjust mixes so that they have full burn of methane in both
the pre-burners so the the exchaust no longer contains combustible gases?
They have a full burn of methane in the LOX turbopump preburner,
remember it's output is oxygen rich, no methane left.
Theoretically you could do a run on the oxygen preburner only with no
risk of explosive gases being ejected out the engine nozzle. Only
oxygen. Not sure the engine is designed to do that however.
They are probably limited on what kind of fuel mixtures they can pass
through the pre-burners. Tinkering with the fuel and oxidizer ratios can
drastically alter the temperatures inside the pre-burner and can effect
the turbines in the turbopumps in highly negative ways. For years it was
thought among US rocket companies and NASA that the effort to design and
operate an oxygen-rich preburner was just too difficult to justify
because of the high temperatures involved.
I believe The SpaceX Raptor was the first serious US attempt at it on a
commercial rocket engine, followed by Blue Origins BE-4.
Yes, this Wikipedia article gives this detail:
https://en.wikipedia.org/wiki/Staged_combustion_cycle
The Russians came out with first with oxygen rich preburners starting
with the S1.5400 and the NK-33, then the RD-170 of which the RD-180 used
in the Atlas-V is a derivative.
Dave
Suggest you watch Tim Dodd's video to also help answer some of your
questions.