Goddard’s rocket, the V2 rocket and Redstone all used water in their Alcohol fuel for combustion temperature control. To the best of my knowledge, water has not been used in RP1 fueled rockets for temperature control. Instead, RP1 engines rely on extra rich fuel in their propellant mixtures, not water, to keep temperatures controlled.
Water will mix with ethanol in any proportions, but not with RP1 (kerosene). If water is to be added to a RP1 propellant mixture, it would need to be injected separately.
If an RP1 engine runs 12% rich (ratio 2.33 instead of stochiometric 2.65) then 12% of the fuel mass is being used for temperature control, not combustion. Water, with twice the heat capacity of unoxidized RP1, would provide the same cooling effect with half the mass.
Doesn't it make sense to inject water in the combustion chamber for temperature control in RP1 engines, instead of injecting twice as much mass of never-to-be-combusted fuel? Has this ever been done?
Running RP1 engines fuel-rich provides 2 benefits over the "correct" stochiometric ratio:
- Reduced combustion temperature reduces heat damage to the engine
- Pyrolytic products of unoxidized RP1 provides small molecular weight molecules which increase Isp.
Injecting water provides both of these benefits while saving propellant mass .
- Specific heat of water is 2.08 times that of RP1 and heat of vaporization is 2.15 times higher. https://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html
- H20 is a low molecular weight molecule suitable for high Isp (it works for SSME!)
Notes:
A stochiometric fuel/oxidizer mix of RP1 gives the highest thermodynamic efficiency, but can produce combustion temperature above the working limit of available engine materials. For instance, Monel has a melting point of 1350C https://www.lenntech.com/monel.htm . The stochiometric ratio for RP1 is ~2.65. But even a fuel-rich 2.30 ratio of LOX/RP1 will produce Adiabatic Flame Temperature over 3500C (assumed combustion chamber pressure 75 atm). http://www.braeunig.us/space/comb-OK.htm Cooling rocket engines is a challenge!
The excellent answer to Merlin engine stoichiometry points out some of the complexity in choosing an optimal mix ratio.
Thermal breakdown of large fuel molecules (RP1 is a 12 carbon chain) produces small molecules (H2, CO) which are then oxidized into slightly larger molecules (H2O, CO2). Fuel-rich conditions leave some small molecules un-oxidized, reducing the average molecular weight of combustion chamber gas. This is good for Isp. Extremely rich fuel mix will leave intact carbon chains (visible as soot) which increases the average molecular weight of chamber gas. This is bad for Isp.
In a somewhat related application, water injection has been used in gasoline internal combustion engines to lower combustion temperature and prevent pre-ignition. This allows higher boost pressures in turbocharged engines. It is also very effective at preventing and removing carbon deposits (“coking”) from combustion areas.
