Not even rockets. Lose too much from the low density leading to a larger structure and isolation required to handle it as a liquid, and not freeze the oxygen.

Not even rockets. Lose too much from the low density leading to a larger structure and isolation required to handle it as a liquid, and not freeze the oxygen. Meaning worse performance than the pure numbers would imply. Not to mention cost of handling it. Case in point, SpaceX going larger rocket with less efficient kerosene first and now methane to increase efficiency and reduce soot, while still keeping everything simpler than hydrogen.