Since we only have the title, it's difficult to determine the main character. However, it might be a spaceman who had a unique experience, perhaps someone who made a significant discovery in space or overcame great odds during his space missions. It could be a pioneer in a particular area of space exploration, but without more details, it's all speculation.
There's no way to tell for sure who the main character is just from the title 'Spaceman Based on a True Story'. It could be a well - known astronaut like Neil Armstrong or someone less famous but with an equally interesting story.
I don't know who the main character is without further information. It could be any astronaut who had a remarkable story to tell.