The sketches above were done using the digital equivalents of a medium pencil and various shades of oil pastels. I used a light gray background rather than white just to reduce the contrast a little, then I completely shaded each ship with a 50% gray. From there, I used a 75% gray for the light areas and a 25% gray for the shadows (100% being white and 0% being black). Finally I created some highlights using pure white, which is nicely set off against the off-white background. The oil pastel brushes in Painter allow you to create some very nice blends between shades and are easy to work with.
These are rough shape studies with relatively little detail, but they are fast and easy to produce and do a good job of getting the concept across.
Cool sketches, and thanks for the technique description!
Related question - what would you do if you wanted to add some color to these, start over fresh with a different technique, use this as an overlay to a color layer, or what?