SpaceX has announced an upgrade to their Falcon 9 rocket, achieving greater thrusts with, as far as I can tell, the same rocket motors (at least, they apparently still call the motors "Merlin 1D"). See this question, which had no conclusive answer on engine changes. The change takes the Merlin 1D from 147K lb thrust at sea level to 170K lb, an increase of 15.6%.
The increased thrust is apparently due to switching to using extremely chilled fuel (RP-1 kerosene) and oxygen. Not just liquid, but barely above the temperature at which it would freeze solid. This increases the density of the propellants, presumably allowing more mass of fuel to be contained in the tanks and increasing the flow rate (mass per unit time) through the turbopumps and rocket nozzles.
My question is, why are they only doing this now? 15% more thrust from the same engines, and increased fuel capacity (even though it also increases launch weight), seem like highly desirable changes. What are the risks of this approach that led to it not being used previously?
A few guesses:
- Is the change in propellant temperature sufficient to significantly increase the risk of failures in components that must handle cryogenic fluids?
- Is it significantly harder to build spacecraft-usable tanks that can hold near-frozen O2 (and RP-1), instead of "merely" liquid O2?
- Does increasing the mass/time flow rate through the motors (assuming that's actually what's happening) pose a greater risk to the motors? If so, does the fact that the motors can take it imply they were overbuilt for the old propellant density? (Merlin 1D already had the highest T/WR of any production liquid-fueled rocket motor.)
- Are there other changes that are required to support this upgrade, and was engineering those changes difficult or very costly?
While I'm obviously hoping to get as full a picture as possible, feel free to post partial answers (i.e. one thing you know of that poses a significant challenge) as answers rather than comments.