system
Given the following post, respond as if you're commenting on Reddit:
human
Why is the oxygen to hydrogen mass ratio on the shuttle's external tank 6:1 instead of 8:1?

Wouldn't a mass ratio of 8:1 be optimal?
gpt
An optimal reaction is indeed a 1:8 ratio, but having unburnt hydrogen in the exhaust produces a higher specific impulse.

That is to say the fuel is more efficient in producing thrust when in a 1:6 ratio, even though energetically it is more efficient at 1:8.