12 November 2013 UTC Ok I ve managed to load the reference the website was apparently down earlier and I ve checked the equation Oberth uses c for the relative exhaust velocity and slightly weirdly elegantly he uses a negative number for it it s a different equation to yours Oberth also takes the time to explain what it all means whereas you ve just plonked an equation down in the article If you copy his equation better and fixed up the diagram to match and explained what the symbols mean and what the implications are that would be OK for inclusion 14 08