A Convair illustration from a magazine article in 1959 depicting a solar powered spaceship. In this case, a spherical mylar “balloon” is used as a reflector. This would be light weight, but since it would need to maintain some level of internal pressurization to continue to hold its shape, it’s unclear just how long it would work before micrometeoroids turn it into leaky Swiss cheese. Additionally, a reflective hemisphere does not have a true optical focal point, rather a focal “line,” so a lot of the sunlight this thing would capture would be lost of inefficiently used. Presumably this design uses the sunlight to boil a working fluid such as mercury; the superheated pressurized gas blows past a turbine to generate electrical power. The gas then flows through a radiator to cool off and recondense back to a fluid. But given that there are no visible radiators, perhaps the idea was to use the sunlight to directly heat a fluid such as hydrogen to an extreme temperature to be used as a propellant. If your materials are up to it and your reflector is good enough, specific impulses in the area of 800 to 1200 seconds should not be unreasonable. But even here the illustration seems lacking… if hydrogen if the fuel, where are the huge tanks? In all likelihood, this illustration was never meant to depict a solid engineering study, but was merely propaganda art.
