Why a "Sweet Spot"?

[sugmulllun]

Hi:
In reading about the amazing performances advertised for SpaceX's Merlin 1D I keep coming across the statement that 1400(and some) psi is a "sweet spot".
Can anyone give me a little insight as to what is magical about this chamber pressure? I assume that it's more than just a flattening of a statistical curve made
from parameter variables such as material tolerances vs abuse vs thrust....

[mutleyeng]

i was just looking at the figures for 1D funnily enough.
so if i may tack on a question to the O.P, how on earth do they get from 96 thrust to weight ratio for the 1C, to 160 for the 1D?

[cjameshuff]

The Merlin is a gas generator cycle engine, burning some of the fuel to drive a turbine for running the pumps, and then dumping the exhaust. Power generated by the turbine is thus unavailable for producing thrust. Operating at higher pressure gives better performance due to higher exhaust velocity, but requires more pump power, which increases those pump losses. At some point, pump losses exceed the gains of increasing pressure...for the Merlin, that's around 9.7 MPa.

i was just looking at the figures for 1D funnily enough.
so if i may tack on a question to the O.P, how on earth do they get from 96 thrust to weight ratio for the 1C, to 160 for the 1D?

Further optimization using data on real-world pressures and temperatures, perhaps. The 1C was their first engine with regeneratively cooled combustion chamber and nozzle, and was to be used on their first Falcon 9 launches, so it's reasonable for them to have been a bit overly conservative.

[Jerry]

...the nozzle is also designed to be most efficient within a fairly small range of psi. Less pressure, and the gas expansion works less efficiently at the end of the nozzle; more pressure, and the design limits come into play. The 'sweet spot' might also refer to a pressure at which there is less danger from dangerous resonant effects - but that is just a ooma guess.

[ugordan]

so if i may tack on a question to the O.P, how on earth do they get from 96 thrust to weight ratio for the 1C, to 160 for the 1D?

As already pointed out, 1C was their first regen engine so they obviously identified improvements as lessons learned. 1D is said to have significantly fewer parts (solenoids, etc.) and they also removed some weight where applicable. For example, if you look at the recent test firing video you can notice the thrust attachment points (the old quadrupod) are different than any of the earlier Merlins. While the earlier Merlins had Falcon 1 in mind, this engine looks solely designed for the Falcon 9 core. Further proof of this is that they're also redesigning the thrust structure for F9 v1.1 and the engines will no longer be arranged in a 3x3 pattern, but apparently an octagon with a central engine. This obviously affects thrust structure attachment points, but in the end will most likely result in reduced thrust structure mass.