Great game that gives you lot of content to play a d&d campaign in space. It uses largely the rules from OD&D (not only the white box but also the supplements, which means ability scores have several different uses, you can multi-class etc...). You can get the first book for free if you're not convinced, but it will surely encourage you to buy the two others. To go more in detail, you have 5 races : humans, alphas, half-alphas (space elves and half-elves), deltas (space dwarves) and androids. There are also 5 classes : trooper (fighter), templar (paladin), rogue, medic (cleric), psi-sensitive (magic-user). Psionics don't feel like re-skinned magic, but really like something between magic and psionics from od&d. Medics don't have spells and can only heal. Other than that, rules feel familiar to anyone who knows old-school d&d, simple enough to be matered quickly by someone who never played d&d before, complete enought to resolve most of the situations that will present to you with satisfying results, and can be easily mixed with other sci-fi osr rpg like White star or Warriors of the red planet. Love it !

Space Dungeon offers an interesting variation: rather than a language for each alignment, the game, or the setting, has a language specific to each class, most efficient to discuss matters of professional interest.