Looking for a young adult book written in the 70's or 60's where a young man invents a spaceplane?

I am looking for a novel written in the 70's or 60's that had a young man who invented a space plane that had a reflective or diamond coating and used a rare and special element to break the speed of light. With the help of aliens he stops a russian plot to build more space planes and take over the world. thanks for your help.