One Page 40K Rules are a set of simple rules for the 40K Universe. The authors write:
One Page 40k is a single-page ruleset designed to be fast to learn and simple to play. It serves as a great introduction to the Warhammer 40.000 universe for people who are new to the game, and as a way to play quick games for veterans of the franchise.
The internet archive has a digital copy of the old SPI science fiction and fantasy gaming magazine Ares. Issue #11 features a full board game called “Albion Land of Faerie.”
Looking for an alternative to Warhammer 40K? Who isn’t? This may be it. Even better: these wargames rules are free. Ivan Sorensen’s Fast and Dirty rules also would work for modern combat.
Here’s a tutorial on scratch build a void shield generator. While intended for Warhammer 40K, a model like this could be used in a lot of science fiction games.
Tom’s Spaceship Miniatures / Games List offers a nice resource for people getting into this type of gaming. FWIW, Silent Death is one of my all time favorite games.