Worse, as the vehicle ascends the atmosphere gets thinner and thinner, and the benefit of not having to carry O2 for that stage vanishes, so the amount of O2 th

Worse, as the vehicle ascends the atmosphere gets thinner and thinner, and the benefit of not having to carry O2 for that stage vanishes, so the amount of O2 that you save not carrying isn't worth the weight cost of the engines.