Re:Musk explains why SpaceX prefers clusters of small engines
by Obi_Juan_Kenobi on Thursday February 08, 2018 @04:26PM
(#16335726)
Isn't this largely offset by the reduced efficiency of smaller engines?
Larger combustion chambers are highly desirable because you can increase the maximum heat in the center of the combustion chamber. By varying the fuel mix, the outer portions can burn cooler to avoid damaging the chamber, while the center burns extremely hot for better efficiency. Basically, the better the ratio of cross-sectional area to circumference, the better the engine.
There's also the issue of having to replicate the turbopumps for each engine, all of the plumbing and gimbaling mechanics (though only the center engine on F9 has full gimbal control), etc.
Of course, there are downsides to large chambers such as combustion instabilities, some scaling like you've mentioned, production and economics, overall vehicle design, etc.
Basically, there are many design tradeoffs in rocket engine design. After all, it's rocket science!
Larger combustion chambers are highly desirable because you can increase the maximum heat in the center of the combustion chamber. By varying the fuel mix, the outer portions can burn cooler to avoid damaging the chamber, while the center burns extremely hot for better efficiency. Basically, the better the ratio of cross-sectional area to circumference, the better the engine.
There's also the issue of having to replicate the turbopumps for each engine, all of the plumbing and gimbaling mechanics (though only the center engine on F9 has full gimbal control), etc.
Of course, there are downsides to large chambers such as combustion instabilities, some scaling like you've mentioned, production and economics, overall vehicle design, etc.
Basically, there are many design tradeoffs in rocket engine design. After all, it's rocket science!