What @vcharng said.
It works best with rockets.

There's a link in the desc of the underwater camera mod, it tells you how to do it

Use connection editor.
attach the rocket to the releasing end of a pylon, then attach the rocket AGAIN to part of your plane.
When the pylon's AG is triggered, the rocket will ignite, but it's still connected to your plane so you'll still receive boost.

Thank you so much!

Here's how I did it a long time ago. Basically trapping the missile very tightly between small fuselage pieces.