>Apparently the normal 30,000 feet cruising altitude is not higher because the reduced oxygen levels at higher altitudes impact the gasoline combustion too much

>Apparently the normal 30,000 feet cruising altitude is not higher because the reduced oxygen levels at higher altitudes impact the gasoline combustion too much. If this is true, wouldn't it make more sense to bring liquid oxygen up with you to burn with the fuel? That would only reduce your fuel energy density by a factor of 2 or 3, instead of the factor of 30-100 being claimed for batteries. Of course that would not make sense, and rockets don't become the most efficient way to travel until you are a few times higher than 30,000 ft. But it's useful to put the idea of batteries in perspective.