This is a question I've been pondering for the last few weeks after I back-calculated the details of the Merlin 1D engine. Just as a thought exercise, I wanted to see what upgrades the Merlin 1D would need to reach aircraft gas turbine engine time between overhauls. After backing out some dimensions from the published data about the design, I came up with a series of upgrades. I made the following assumptions to get the dimensions:
- Sensor ports, ignition fluid injection ports, and tight bends in piping (such as on the turbine exhaust or propellant exit) generate a stress concentration factor of 3.
- In places where the material is not published, aluminum alloy 2021-T6 is assumed on low temperature components, and Rene 88 is assumed on high temperature components. (High temperature is anything above 400 K).
- The mixture ratio is stoichiometric for the propellants.
- The fuel is assumed to be made up of $C_{12}H_{26}$.
- The parts are assumed to be crack free.
From those assumptions, the following: static pressure/static temperature/wall temperature/passage dimensions/wall thickness/average stress/maximum stress/cycle lives were calculated using the published performance specifications (wikipedia and spacenews.com).
Combustion Chamber
- Pressure: 6.77 MPa
- Static Temperature: 3006 K
- Wall Temperature: 566 K
- Diameter: 0.609 meters
- Wall Thickness: 0.004 meters
- Average Stress: 233 MPa
- Maximum Stress: 700 MPa/1,000,000 cycles or 54,000 hours of engine time
Throat
- Static Pressure: 3.9 MPa
- Static Temperature: 2613 K
- Wall Temperature: 529 K
- Diameter: 0.223 meters
- Wall Thickness: 0.004 meters
- Average Stress: 105 MPa
- Maximum Stress: 316 MPa/Infinite cycles, below endurance limit of 600 MPa
Exhaust
- Static Pressure: 101 kPa
- Static Temperature: 1697 K
- Wall Temperature: 388 K
- Diameter: 0.6077 meters
- Wall Thickness: 0.004 meters
- Average Stress: 7 MPa
- Maximum Stress: 10 MPa/ Infinite cycles, below endurance limit of 600 MPa
Oxidizer Pump
- Exit Pressure: 10.6 MPa
- Casing Diameter: 0.116 meters
- Wall Thickness: 0.005 meters
- Average Stress: 123 MPa
- Maximum Stress: 320 MPa10,000 cycles or 544 hours of engine time
Fuel Pump
- Exit Pressure: 16.1 MPa
- Casing Diameter: 0.173 meters
- Wall Thickness: 0.011 meters
- Average Stress: 128 MPa
- Maximum Stress: 383 MPa/8,000 cycles or 435 hours of engine time
Based on these results, the weak point in terms of static endurance is the turbopump, specifically the fuel turbopump casing. To reach 1000 hours between engine overhauls, the average stress on the casing needs to come down. Nesting the fuel pump inside the oxidizer pump would reduce the net stress experienced by the fuel pump by using the pressure developed by the oxidizer pump to compress the fuel pump casing. And the oxidizer pump increases its wall thickness to 0.009 meters to reach 1000 hours between overhauls.
However, this is based on the data that I could find out. In my opinion, the real weak points are the impellers, shafts, and bearings in the turbopump. Since they spin at 20,000 rpm, they can quickly go through fatigue lives if they have an imbalance occur due to cavitation, debris, or external vibration. I'm sure SpaceX and Barbor-Nichols (the folks who designed the turbopumps) are aware of this and maybe have some solutions. Outside of increasing the damping inside the pump by using a ferrofluid/magnet and having magnetic bearings, I don't know how one would increase the overhaul time.
Thus, I ask the community: How would you do it? What would you change in the engine design to reach 1000 hours between engine overhauls?
