Since the background was completely white, the answer was actually very easy. I copied the image of the aircraft to the clipboard (Command/Ctrl A and then Command/Ctrl C) and then pasted it on top of the photo of the explosion (Command/Ctrl V).
In the layers palette, I pulled down the blend mode submenu (where you see the word 'normal') until I found 'multiply'. When this was selected, miraculously the white sky disappeared and all I was left with was what you see here.
In my Creative Techniques in Photoshop online course here at BetterPhoto.com, I discuss how to do techniques like this, and as part of the critique process I talk about technical know-how as well as the artistry that goes into making images with impact.
Article by Jim Zuckerman. To learn more about photography, explore the many online photography and Photoshop classes offered here at BetterPhoto.com.