A new way to tweak fuel tanks

[possum]

While playing around with space planes, I realized something that would make building such vessels easier. I was tweaking the oxidizer and thought I could build a smaller lighter space plane if I could replace the removed oxidizer with fuel. Doing that I could use a smaller tank and still have all the fuel I need, it is a lot since I tend to over shoot the landing by a couple hundred Km. I am sure there could be other uses for this but space planes are the reason I thought of it.