You need martin Wrights DXT program.
The alphacnannel is basically a greýscale copy of the texture layed behind the coloured texture.
This greyscale contains the opacity information.
Normally all is visible = alphatexture is just white.
- Find the texture with the droptanks on it.
- Make a copy of the texture, paint all white except the droptank area which is painted black.
- Save as XYZ_Alpha.bmp or whatever.
- Load the original texture with DXT program.
- Load the corresponding ALPHA bmp also.
- Now save the main bmp in a format, that includes the alphachannel feature - DXT1 should do it for you.
(DXT3 does work also, but it will not only blend the droptanks out, but every other part of the plane behind)
http://www.mwgfx.co.uk/programs/dxtbmp.htm
http://www.michael-reimer.com/CFS2/CFS2_Tips.html
(I need to update it to the new site of Martin)