text: Metastable metallic hydrogen may have potential as a highly efficient rocket propellant, with a theoretical specific impulse of up to 1700 seconds (for reference, the current most efficient chemical rocket propellants have an  less than 500 s), although a metastable form suitable for mass-production and conventional high-volume storage may not exist. Another significant issue is the heat of the reaction, which at over 6000 K is too high for any known engine materials to be used. This would necessitate diluting the metallic hydrogen with water or liquid hydrogen, a mixture that would still provide a significant performance boost from current propellants.