Hey, nice attempt to spaceship design. But I agree with Mr. Crankshaft
. Just few additional thoughts to expand him points:
Your designs looks too retro because of these angular shapes. Exactly like floating boxes (to be honest, not so bad, but, anyway, you get the idea). Start to use more flowy shapes, like this not spaceship, but human being, or, rather, animal. Maybe it's military design and you can get inspiration from some predators from animal kingdom, like form, proportions, colors, etc. Classic example of this approach. Also, Dynamic Sketching+Documentary movies/ Wikipedia/Google search is good way for acquiring useful knowledge about "design" in nature for applying in vehicle design. Also It's good approach because of enlargement of visual library. You get theory then you create visual representation of this conception. See some of my studies for example. But, yeah, about flowy shapes. Except gesture drawings, organic shapes from dynamic sketching is one of best ways to understand smooth motion of surface. Few of my shapes/forms for exapmle (Not best, but...you get the idea :). Another one from google. One more good tutorial about finding fresh shapes (gives good results in conjunction with organic shapes/forms exercise).
About human influences: another good way to add human-made feeling is add some graphics, like logotypes, signs, patterns, fonts, etc. Like on real machines. But maybe not so literally, instead of "Warning " sign you can put just orange triangle, instead of logo - just blue square. And it's already will be more realistic.
Commonly used three levels of detalization, like 70/20/10, but, since your ships are just small sketches, 70/30 are good too (maybe even something more expressive like 80/20 or 90/10). Anyway, classic article about this. Also, if you use multiple values of opacity and size of line - use their advantages. Like: silhouette - fat, solid line; big shapes - middle-sized line with opacity; small details - tiny, almost transperent line (Or just use one-size/opacity line for whole sketch). It will make representation of your design is more clear. And it's all about proportions.
And finally some good examples, from legendary spaceship builder, to illustrate this points: One
. Try to analyze them on your own. Also He have gumroad tutorial
on creation of last one, maybe it will be useful for you (but I didn't watch this one, so I can't recommend).
Hope it will help you and keep it up ;)