Looking only at the vacuum specific impulse of plain hydrogen-oxygen
rockets, is it possible to advance their specific impulse to 500, or
even 520?

For example, I recall Mr. Spencer recently mentioning that an ideal
oxidizer:fuel ratio for hydrogen & oxygen was 4:1, but 6:1 was used
because of tankage mass penalties. Would a 4:1 ratio provide an
increase in specific impulse? If so, approximately how much?

Mike Miller, Materials Engineer