Many thanks to IPACS for creating the airport (EPTO) in my town . This is my favourite place to tests
In fact, the F15e aircraft has already droptanks built in and all you need to do is add the lines to option.tmc for planes: <[string8][Requirements][drop_tanks]>.
However, this was not made available earlier because the code in dop_tanks folder is broken. But you can fix it by editing drop_tanks/system.tmc file and change <[float64][AspectRatioMultiplier][1.0]> to <[float64][AspectRatioMultiplier][4.0]> (twice) and the model behaves stably.