# Why doesn't the Merlin 1-D use higher LOX to RP-1 mixture ratios?

So I've been screwing around with CEARUN (https://cearun.grc.nasa.gov/), which is the chemical equilibrium with applications program created by NASA. I've been using it to calculate theoretical ISP's for rocket engines using RP-1 and LOX for a variety of LOX to RP-1 mixture ratios. I've been using the 165:1 nozzle expansion ratio that the Merlin 1D vacuum uses. If we use LOX to RP-1 ratio of 2.327 (about what the Falcon 9 uses based on tank mass numbers then we get a theoretical vacuum ISP of 370.5 seconds. If we use a LOX to RP-1 ratio of 2.9 we get a theoretical vaccum ISP of 381.5 seconds. Note these are all at a chamber pressure of 9.3 MPA, which is what the merlin's use.

In reality I know the Merlin 1D vacuum has an ISP of 348 seconds, which is 93.93% the theoretical ISP. So it follows that by using this mixture ratio we'd get around 358.3 seconds of ISP after efficiency losses. So my questions pretty simple; does anyone know why spacex doesn't use a higher LOX to RP-1 mixture ratio? Doing so would seem to see a 3% increase in ISP. This is nothing to sneeze at, as that would correspond to a ~ 6% increase in orbital energy that the upper stage could achieve.

• How's the density specific impulse for those ratios? – Russell Borogove Jan 17 '19 at 0:48
• Density for ratio of 2.327 is 4.2911 kg/m3, throat velocity is 1196.4 m/s, and fuel flow per throat area is 4.2911*1196.4 = 5133.87 kg/(m2*s) For ratio of 2.9 we get a density of 4.5206 kg/m3, throat velocity is 1159.5, flow per throat area = 4.5206*1159.5 = 5241.6357 kg/(m2*s). So the mass flow, hence thrust, would be greater if you used a higher ratio. – chuckstables Jan 17 '19 at 0:54
• 4 kg per cubic meter doesn't sound right for RP1 and LOX. I'm asking about tanked propellant density versus impulse, but now that think about it LOX is denser, so higher O:F ratio yields better density as well. – Russell Borogove Jan 17 '19 at 1:12
• What does your proposed change do to the combustion chamber temperature? – Organic Marble Jan 17 '19 at 1:56
• Ah. That would probably be it. It raises chamber temperature by about 150 degrees kelvin, but it changes the exhaust temperature by about 400 degrees kelvin! This would mean that more regenerative cooling would be needed. Also exhaust pressure goes up by about 25%, which means that a thicker nozzle extension would be needed. – chuckstables Jan 17 '19 at 2:26