How significant was Robert Goddard's concept of liquid-propellant rocket models?

   Robert Goddard made an extremely valuable contribution to the field of rocketry with his liquid-propellant models. In fact, most of the rockets used in spaceflight have been liquid-propellant rockets. They contain a mixture of liquid fuel and liquid oxidizer. The liquid oxidizer replaces gaseous oxygen and allows the fuel to burn. Types of liquid fuel include alcohol, kerosene, liquid hydrogen, and hydrazine. Two common liquid oxidizers are nitrogen tetroxide and liquid oxygen.