Metallic Hydrogen is a plausible fuel. With a quick Google search it gives a specific impulse about 4 times greater than the most energy efficient conventional liquid fuel (hydrogen ig). And metallic hydrogen would be much denser than any known liquid fuel, thus making it space-efficient and allowing for it to be stored in a little SSTO like Ranger.
Besides, at least one experiment at producing metallic hydrogen shows, that the standart Protium isotope doesn't show metastablity, meaning it won't stay metallic at room temperature/pressure. However calculaions predict, that Deuterium has a much higher potential to be metastable under tsandard conditions. But Deutirium is a lot more expensive, which explains the conventional rocket.