So I'm trying to calculate delta v, however, with this you need the starting and ending mass. I have the starting mass as 56,892 t and on Mechjeb the ending mass is 47,862 t. The problem is that I can't calculate final mass because if I drain both tanks i get around 42,000 t. How can I calculate this as the Oxidizer does not drain the same rate as the liquid fuel. Btw I am using R.A.P.I.E.R. Engines. (305 ISP).